From 36a83f25a228bff466e425487b5e361d84ed17bc Mon Sep 17 00:00:00 2001
From: ShikiSuen <shikisuen@outlook.com>
Date: Fri, 14 Oct 2022 12:03:14 +0800
Subject: [PATCH] InputHandler // Remove stateCallback().

---
 Packages/vChewing_Tekkon/README.md            |   7 +-
 Source/Modules/InputHandler_Core.swift        |   2 +-
 .../InputHandler_HandleCandidate.swift        |  26 +--
 .../InputHandler_HandleComposition.swift      |  24 +-
 Source/Modules/InputHandler_HandleEvent.swift |  18 +-
 Source/Modules/InputHandler_HandleInput.swift | 109 +++++----
 Source/Modules/InputHandler_States.swift      | 208 ++++++++----------
 Source/Modules/SessionCtl_Core.swift          |  12 +-
 Source/Modules/SessionCtl_Delegates.swift     |  24 +-
 Source/Modules/SessionCtl_HandleEvent.swift   |   2 +-
 Source/Modules/SessionCtl_HandleStates.swift  |  22 +-
 .../SessionCtl_IMKCandidatesData.swift        |   2 +-
 12 files changed, 223 insertions(+), 233 deletions(-)

diff --git a/Packages/vChewing_Tekkon/README.md b/Packages/vChewing_Tekkon/README.md
index e7a073e0..248a8a79 100644
--- a/Packages/vChewing_Tekkon/README.md
+++ b/Packages/vChewing_Tekkon/README.md
@@ -174,7 +174,6 @@ extension InputHandler {
   func handle(
     input: InputHandler,
     state: InputState,
-    stateCallback: @escaping (InputState) -> Void,
     errorCallback: @escaping (String) -> Void
   ) -> Bool {
     let charCode: UniChar = input.charCode
@@ -217,7 +216,7 @@ if !skipPhoneticHandling && _composer.inputValidityCheck(key: charCode) {
   // 有調號的話，則不需要這樣處理，轉而繼續在此之後的處理。
   let composeReading = _composer.hasToneMarker()
   if !composeReading {
-    stateCallback(generateStateOfInputting())
+    delegate.switchState(generateStateOfInputting())
     return true
   }
 }
@@ -246,7 +245,7 @@ if composeReading {  // 符合按鍵組合條件
     errorCallback("114514")  // 向狀態管理引擎回呼一個錯誤狀態
     _composer.clear()  // 清空注拼槽的內容
     // 根據「天權星引擎 (威注音) 或 Gramambular (小麥) 的組字器是否為空」來判定回呼哪一種狀態
-    stateCallback(
+    delegate.switchState(
       (getCompositorLength() == 0) ? InputState.EmptyIgnoringPreviousState() : generateStateOfInputting())
     return true  // 向 IMK 報告說這個按鍵訊號已經被輸入法攔截處理了
   }
@@ -269,7 +268,7 @@ if composeReading {  // 符合按鍵組合條件
   // 再以回呼組字狀態的方式來執行updateClientComposingBuffer()
   let inputting = generateStateOfInputting()
   inputting.poppedText = poppedText
-  stateCallback(inputting)
+  delegate.switchState(inputting)
 
   return true  // 向 IMK 報告說這個按鍵訊號已經被輸入法攔截處理了
 }
diff --git a/Source/Modules/InputHandler_Core.swift b/Source/Modules/InputHandler_Core.swift
index 2176af29..a3532e41 100644
--- a/Source/Modules/InputHandler_Core.swift
+++ b/Source/Modules/InputHandler_Core.swift
@@ -37,7 +37,7 @@ public protocol InputHandlerDelegate {
   var selectionKeys: String { get }
   var state: IMEStateProtocol { get set }
   var clientBundleIdentifier: String { get }
-  func handle(state newState: IMEStateProtocol, replaceCurrent: Bool)
+  func switchState(_ newState: IMEStateProtocol)
   func candidateController() -> CtlCandidateProtocol
   func candidateSelectionCalledByInputHandler(at index: Int)
   func performUserPhraseOperation(with state: IMEStateProtocol, addToFilter: Bool)
diff --git a/Source/Modules/InputHandler_HandleCandidate.swift b/Source/Modules/InputHandler_HandleCandidate.swift
index 6ca272a0..d23183c5 100644
--- a/Source/Modules/InputHandler_HandleCandidate.swift
+++ b/Source/Modules/InputHandler_HandleCandidate.swift
@@ -18,20 +18,20 @@ extension InputHandler {
   /// - Parameters:
   ///   - input: 輸入訊號。
   ///   - state: 給定狀態（通常為當前狀態）。
-  ///   - stateCallback: 狀態回呼，交給對應的型別內的專有函式來處理。
   ///   - errorCallback: 錯誤回呼。
   /// - Returns: 告知 IMK「該按鍵是否已經被輸入法攔截處理」。
   func handleCandidate(
     state: IMEStateProtocol,
     input: InputSignalProtocol,
-    stateCallback: @escaping (IMEStateProtocol) -> Void,
     errorCallback: @escaping (String) -> Void
   ) -> Bool {
-    guard var ctlCandidate = delegate?.candidateController() else {
+    guard let delegate = delegate else {
       errorCallback("06661F6E")
       return true
     }
 
+    var ctlCandidate = delegate.candidateController()
+
     // MARK: 取消選字 (Cancel Candidate)
 
     let cancelCandidateKey =
@@ -47,12 +47,12 @@ extension InputHandler {
         // 就將當前的組字緩衝區析構處理、強制重設輸入狀態。
         // 否則，一個本不該出現的真空組字緩衝區會使前後方向鍵與 BackSpace 鍵失靈。
         // 所以這裡需要對 compositor.isEmpty 做判定。
-        stateCallback(IMEState.ofAbortion())
+        delegate.switchState(IMEState.ofAbortion())
       } else {
-        stateCallback(generateStateOfInputting())
+        delegate.switchState(generateStateOfInputting())
       }
       if state.type == .ofSymbolTable, let nodePrevious = state.node.previous, !nodePrevious.members.isEmpty {
-        stateCallback(IMEState.ofSymbolTable(node: nodePrevious))
+        delegate.switchState(IMEState.ofSymbolTable(node: nodePrevious))
       }
       return true
     }
@@ -61,10 +61,10 @@ extension InputHandler {
 
     if input.isEnter {
       if state.type == .ofAssociates, !prefs.alsoConfirmAssociatedCandidatesByEnter {
-        stateCallback(IMEState.ofAbortion())
+        delegate.switchState(IMEState.ofAbortion())
         return true
       }
-      delegate?.candidateSelectionCalledByInputHandler(at: ctlCandidate.selectedCandidateIndex)
+      delegate.candidateSelectionCalledByInputHandler(at: ctlCandidate.selectedCandidateIndex)
       return true
     }
 
@@ -232,7 +232,7 @@ extension InputHandler {
     let match: String =
       (state.type == .ofAssociates) ? input.inputTextIgnoringModifiers ?? "" : input.text
 
-    let selectionKeys = delegate?.selectionKeys ?? PrefMgr.shared.candidateKeys
+    let selectionKeys = delegate.selectionKeys
 
     for j in 0..<selectionKeys.count {
       let label = selectionKeys.charComponents[j]
@@ -245,7 +245,7 @@ extension InputHandler {
     if index != NSNotFound {
       let candidateIndex = ctlCandidate.candidateIndexAtKeyLabelIndex(index)
       if candidateIndex != -114_514 {
-        delegate?.candidateSelectionCalledByInputHandler(at: candidateIndex)
+        delegate.candidateSelectionCalledByInputHandler(at: candidateIndex)
         return true
       }
     }
@@ -285,10 +285,10 @@ extension InputHandler {
       if shouldAutoSelectCandidate {
         let candidateIndex = ctlCandidate.candidateIndexAtKeyLabelIndex(0)
         if candidateIndex != -114_514 {
-          delegate?.candidateSelectionCalledByInputHandler(at: candidateIndex)
-          stateCallback(IMEState.ofAbortion())
+          delegate.candidateSelectionCalledByInputHandler(at: candidateIndex)
+          delegate.switchState(IMEState.ofAbortion())
           return handleInput(
-            event: input, state: IMEState.ofEmpty(), stateCallback: stateCallback, errorCallback: errorCallback
+            event: input, state: IMEState.ofEmpty(), errorCallback: errorCallback
           )
         }
         return true
diff --git a/Source/Modules/InputHandler_HandleComposition.swift b/Source/Modules/InputHandler_HandleComposition.swift
index 4c601733..63dd1bb7 100644
--- a/Source/Modules/InputHandler_HandleComposition.swift
+++ b/Source/Modules/InputHandler_HandleComposition.swift
@@ -15,14 +15,14 @@ extension InputHandler {
   /// 用來處理 InputHandler.HandleInput() 當中的與組字有關的行為。
   /// - Parameters:
   ///   - input: 輸入訊號。
-  ///   - stateCallback: 狀態回呼，交給對應的型別內的專有函式來處理。
   ///   - errorCallback: 錯誤回呼。
   /// - Returns: 告知 IMK「該按鍵是否已經被輸入法攔截處理」。
   func handleComposition(
     input: InputSignalProtocol,
-    stateCallback: @escaping (IMEStateProtocol) -> Void,
     errorCallback: @escaping (String) -> Void
   ) -> Bool? {
+    guard let delegate = delegate else { return nil }
+
     // MARK: 注音按鍵輸入處理 (Handle BPMF Keys)
 
     var keyConsumedByReading = false
@@ -65,7 +65,7 @@ extension InputHandler {
       // 沒有調號的話，只需要 setInlineDisplayWithCursor() 且終止處理（return true）即可。
       // 有調號的話，則不需要這樣，而是轉而繼續在此之後的處理。
       if !composer.hasToneMarker() {
-        stateCallback(generateStateOfInputting())
+        delegate.switchState(generateStateOfInputting())
         return true
       }
     }
@@ -90,16 +90,16 @@ extension InputHandler {
 
         if prefs.keepReadingUponCompositionError {
           composer.intonation.clear()  // 砍掉聲調。
-          stateCallback(generateStateOfInputting())
+          delegate.switchState(generateStateOfInputting())
           return true
         }
 
         composer.clear()
         // 根據「組字器是否為空」來判定回呼哪一種狀態。
         switch compositor.isEmpty {
-          case false: stateCallback(generateStateOfInputting())
+          case false: delegate.switchState(generateStateOfInputting())
           case true:
-            stateCallback(IMEState.ofAbortion())
+            delegate.switchState(IMEState.ofAbortion())
         }
         return true  // 向 IMK 報告說這個按鍵訊號已經被輸入法攔截處理了。
       }
@@ -122,27 +122,27 @@ extension InputHandler {
       // 再以回呼組字狀態的方式來執行 setInlineDisplayWithCursor()。
       var inputting = generateStateOfInputting()
       inputting.textToCommit = textToCommit
-      stateCallback(inputting)
+      delegate.switchState(inputting)
 
       /// 逐字選字模式的處理。
       if prefs.useSCPCTypingMode {
         let candidateState: IMEStateProtocol = generateStateOfCandidates(state: inputting)
         switch candidateState.candidates.count {
-          case 2...: stateCallback(candidateState)
+          case 2...: delegate.switchState(candidateState)
           case 1:
             let firstCandidate = candidateState.candidates.first!  // 一定會有，所以強制拆包也無妨。
             let reading: String = firstCandidate.0
             let text: String = firstCandidate.1
-            stateCallback(IMEState.ofCommitting(textToCommit: text))
+            delegate.switchState(IMEState.ofCommitting(textToCommit: text))
 
             if !prefs.associatedPhrasesEnabled {
-              stateCallback(IMEState.ofEmpty())
+              delegate.switchState(IMEState.ofEmpty())
             } else {
               let associatedPhrases =
                 generateStateOfAssociates(
                   withPair: .init(key: reading, value: text)
                 )
-              stateCallback(associatedPhrases.candidates.isEmpty ? IMEState.ofEmpty() : associatedPhrases)
+              delegate.switchState(associatedPhrases.candidates.isEmpty ? IMEState.ofEmpty() : associatedPhrases)
             }
           default: break
         }
@@ -154,7 +154,7 @@ extension InputHandler {
     /// 是說此時注拼槽並非為空、卻還沒組音。這種情況下只可能是「注拼槽內只有聲調」。
     if keyConsumedByReading {
       // 以回呼組字狀態的方式來執行 setInlineDisplayWithCursor()。
-      stateCallback(generateStateOfInputting())
+      delegate.switchState(generateStateOfInputting())
       return true
     }
     return nil
diff --git a/Source/Modules/InputHandler_HandleEvent.swift b/Source/Modules/InputHandler_HandleEvent.swift
index 96aabbd6..f06df3c0 100644
--- a/Source/Modules/InputHandler_HandleEvent.swift
+++ b/Source/Modules/InputHandler_HandleEvent.swift
@@ -18,22 +18,18 @@ extension InputHandler {
   /// - Parameter event: 由 IMK 選字窗接收的裝置操作輸入事件。
   /// - Returns: 回「`true`」以將該案件已攔截處理的訊息傳遞給 IMK；回「`false`」則放行、不作處理。
   public func handleEvent(_ event: NSEvent) -> Bool {
-    imkCandidatesEventPreHandler(event: event) ?? commonEventHandler(event)
+    imkCandidatesEventPreHandler(event: event) ?? doHandleInput(event)
   }
 
   /// 將按鍵行為與當前輸入法狀態結合起來、交給按鍵調度模組來處理。
   /// 再根據返回的 result bool 數值來告知 IMK「這個按鍵事件是被處理了還是被放行了」。
   /// 這裡不用 handleCandidate() 是因為需要針對聯想詞輸入狀態做額外處理。
-  private func commonEventHandler(_ event: NSEvent) -> Bool {
+  private func doHandleInput(_ event: NSEvent) -> Bool {
     guard let delegate = delegate else { return false }
-
-    let result = handleInput(event: event, state: delegate.state) { newState in
-      delegate.handle(state: newState, replaceCurrent: true)
-    } errorCallback: { errorString in
+    return handleInput(event: event, state: delegate.state) { errorString in
       vCLog(errorString)
       IMEApp.buzz()
     }
-    return result
   }
 
   /// 專門處理與 IMK 選字窗有關的判斷語句。
@@ -84,7 +80,7 @@ extension InputHandler {
     let eventArray = [event]
     guard let imkC = delegate.candidateController() as? CtlCandidateIMK else { return false }
     if event.isEsc || event.isBackSpace || event.isDelete || (event.isShiftHold && !event.isSpace) {
-      return commonEventHandler(event)
+      return doHandleInput(event)
     } else if event.isSymbolMenuPhysicalKey {
       // 符號鍵的行為是固定的，不受偏好設定影響。
       switch imkC.currentLayout {
@@ -113,7 +109,7 @@ extension InputHandler {
         if let newEvent = newEvent {
           if prefs.useSCPCTypingMode, delegate.state.type == .ofAssociates {
             // 註：input.isShiftHold 已經在 Self.handle() 內處理，因為在那邊處理才有效。
-            return event.isShiftHold ? true : commonEventHandler(event)
+            return event.isShiftHold ? true : doHandleInput(event)
           } else {
             if #available(macOS 10.14, *) {
               imkC.handleKeyboardEvent(newEvent)
@@ -126,7 +122,7 @@ extension InputHandler {
       }
 
       if prefs.useSCPCTypingMode, !event.isReservedKey {
-        return commonEventHandler(event)
+        return doHandleInput(event)
       }
 
       if delegate.state.type == .ofAssociates,
@@ -134,7 +130,7 @@ extension InputHandler {
         !event.isCursorClockLeft, !event.isCursorClockRight, !event.isSpace,
         !event.isEnter || !prefs.alsoConfirmAssociatedCandidatesByEnter
       {
-        return commonEventHandler(event)
+        return doHandleInput(event)
       }
       imkC.interpretKeyEvents(eventArray)
       return true
diff --git a/Source/Modules/InputHandler_HandleInput.swift b/Source/Modules/InputHandler_HandleInput.swift
index 620ebe95..48c78f5e 100644
--- a/Source/Modules/InputHandler_HandleInput.swift
+++ b/Source/Modules/InputHandler_HandleInput.swift
@@ -21,17 +21,17 @@ extension InputHandler {
   /// - Parameters:
   ///   - input: 輸入訊號。
   ///   - state: 給定狀態（通常為當前狀態）。
-  ///   - stateCallback: 狀態回呼，交給對應的型別內的專有函式來處理。
   ///   - errorCallback: 錯誤回呼。
   /// - Returns: 告知 IMK「該按鍵是否已經被輸入法攔截處理」。
   func handleInput(
     event input: InputSignalProtocol,
     state: IMEStateProtocol,
-    stateCallback: @escaping (IMEStateProtocol) -> Void,
     errorCallback: @escaping (String) -> Void
   ) -> Bool {
     // 如果按鍵訊號內的 inputTest 是空的話，則忽略該按鍵輸入，因為很可能是功能修飾鍵。
-    guard !input.text.isEmpty, input.charCode.isPrintable else { return false }
+    // 不處理任何包含不可列印字元的訊號。
+    // delegate 必須存在，否則不處理。
+    guard !input.text.isEmpty, input.charCode.isPrintable, let delegate = delegate else { return false }
 
     let inputText: String = input.text
     var state = state  // 常數轉變數。
@@ -44,7 +44,7 @@ extension InputHandler {
         return false
       }
       errorCallback("550BCF7B: InputHandler just refused an invalid input.")
-      stateCallback(state)
+      delegate.switchState(state)
       return true
     }
 
@@ -66,7 +66,7 @@ extension InputHandler {
       // 略過對 BackSpace 的處理。
     } else if input.isCapsLockOn || state.isASCIIMode {
       // 但願能夠處理這種情況下所有可能的按鍵組合。
-      stateCallback(IMEState.ofEmpty())
+      delegate.switchState(IMEState.ofEmpty())
 
       // 字母鍵摁 Shift 的話，無須額外處理，因為直接就會敲出大寫字母。
       if (input.isUpperCaseASCIILetterKey && state.isASCIIMode)
@@ -82,8 +82,8 @@ extension InputHandler {
       }
 
       // 將整個組字區的內容遞交給客體應用。
-      stateCallback(IMEState.ofCommitting(textToCommit: inputText.lowercased()))
-      stateCallback(IMEState.ofEmpty())
+      delegate.switchState(IMEState.ofCommitting(textToCommit: inputText.lowercased()))
+      delegate.switchState(IMEState.ofEmpty())
 
       return true
     }
@@ -97,9 +97,9 @@ extension InputHandler {
       if !(state.type == .ofCandidates || state.type == .ofAssociates
         || state.type == .ofSymbolTable)
       {
-        stateCallback(IMEState.ofEmpty())
-        stateCallback(IMEState.ofCommitting(textToCommit: inputText.lowercased()))
-        stateCallback(IMEState.ofEmpty())
+        delegate.switchState(IMEState.ofEmpty())
+        delegate.switchState(IMEState.ofCommitting(textToCommit: inputText.lowercased()))
+        delegate.switchState(IMEState.ofEmpty())
         return true
       }
     }
@@ -108,7 +108,7 @@ extension InputHandler {
 
     if [.ofCandidates, .ofSymbolTable].contains(state.type) {
       return handleCandidate(
-        state: state, input: input, stateCallback: stateCallback, errorCallback: errorCallback
+        state: state, input: input, errorCallback: errorCallback
       )
     }
 
@@ -116,11 +116,11 @@ extension InputHandler {
 
     if state.type == .ofAssociates {
       if handleCandidate(
-        state: state, input: input, stateCallback: stateCallback, errorCallback: errorCallback
+        state: state, input: input, errorCallback: errorCallback
       ) {
         return true
       } else {
-        stateCallback(IMEState.ofEmpty())
+        delegate.switchState(IMEState.ofEmpty())
       }
     }
 
@@ -128,19 +128,19 @@ extension InputHandler {
 
     if state.type == .ofMarking {
       if handleMarkingState(
-        state, input: input, stateCallback: stateCallback,
+        state, input: input,
         errorCallback: errorCallback
       ) {
         return true
       }
       state = state.convertedToInputting
-      stateCallback(state)
+      delegate.switchState(state)
     }
 
     // MARK: 注音按鍵輸入處理 (Handle BPMF Keys)
 
     if let compositionHandled = handleComposition(
-      input: input, stateCallback: stateCallback, errorCallback: errorCallback
+      input: input, errorCallback: errorCallback
     ) {
       return compositionHandled
     }
@@ -157,10 +157,10 @@ extension InputHandler {
           if compositor.cursor >= compositor.length {
             let displayedText = state.displayedText
             if !displayedText.isEmpty {
-              stateCallback(IMEState.ofCommitting(textToCommit: displayedText))
+              delegate.switchState(IMEState.ofCommitting(textToCommit: displayedText))
             }
-            stateCallback(IMEState.ofCommitting(textToCommit: " "))
-            stateCallback(IMEState.ofEmpty())
+            delegate.switchState(IMEState.ofCommitting(textToCommit: " "))
+            delegate.switchState(IMEState.ofEmpty())
           } else if currentLM.hasUnigramsFor(key: " ") {
             compositor.insertKey(" ")
             walk()
@@ -168,12 +168,12 @@ extension InputHandler {
             let textToCommit = commitOverflownComposition
             var inputting = generateStateOfInputting()
             inputting.textToCommit = textToCommit
-            stateCallback(inputting)
+            delegate.switchState(inputting)
           }
           return true
         } else if input.isShiftHold {  // 臉書等網站會攔截 Tab 鍵，所以用 Shift+Command+Space 對候選字詞做正向/反向輪替。
           return handleInlineCandidateRotation(
-            state: state, reverseModifier: input.isCommandHold, stateCallback: stateCallback,
+            state: state, reverseModifier: input.isCommandHold,
             errorCallback: errorCallback
           )
         }
@@ -182,7 +182,7 @@ extension InputHandler {
       if candidateState.candidates.isEmpty {
         errorCallback("3572F238")
       } else {
-        stateCallback(candidateState)
+        delegate.switchState(candidateState)
       }
       return true
     }
@@ -191,56 +191,56 @@ extension InputHandler {
 
     if let keyCodeType = KeyCode(rawValue: input.keyCode) {
       switch keyCodeType {
-        case .kEscape: return handleEsc(state: state, stateCallback: stateCallback)
+        case .kEscape: return handleEsc(state: state)
         case .kTab:
           return handleInlineCandidateRotation(
-            state: state, reverseModifier: input.isShiftHold, stateCallback: stateCallback, errorCallback: errorCallback
+            state: state, reverseModifier: input.isShiftHold, errorCallback: errorCallback
           )
         case .kUpArrow, .kDownArrow, .kLeftArrow, .kRightArrow:
           if (input.isControlHold || input.isShiftHold) && (input.isOptionHold) {
             if input.isLeft {  // Ctrl+PgLf / Shift+PgLf
-              return handleHome(state: state, stateCallback: stateCallback, errorCallback: errorCallback)
+              return handleHome(state: state, errorCallback: errorCallback)
             } else if input.isRight {  // Ctrl+PgRt or Shift+PgRt
-              return handleEnd(state: state, stateCallback: stateCallback, errorCallback: errorCallback)
+              return handleEnd(state: state, errorCallback: errorCallback)
             }
           }
           if input.isCursorBackward {  // Forward
             return handleBackward(
-              state: state, input: input, stateCallback: stateCallback, errorCallback: errorCallback
+              state: state, input: input, errorCallback: errorCallback
             )
           }
           if input.isCursorForward {  // Backward
             return handleForward(
-              state: state, input: input, stateCallback: stateCallback, errorCallback: errorCallback
+              state: state, input: input, errorCallback: errorCallback
             )
           }
           if input.isCursorClockLeft || input.isCursorClockRight {  // Clock keys
             if input.isOptionHold, state.type == .ofInputting {
               if input.isCursorClockRight {
                 return handleInlineCandidateRotation(
-                  state: state, reverseModifier: false, stateCallback: stateCallback, errorCallback: errorCallback
+                  state: state, reverseModifier: false, errorCallback: errorCallback
                 )
               }
               if input.isCursorClockLeft {
                 return handleInlineCandidateRotation(
-                  state: state, reverseModifier: true, stateCallback: stateCallback, errorCallback: errorCallback
+                  state: state, reverseModifier: true, errorCallback: errorCallback
                 )
               }
             }
-            return handleClockKey(state: state, stateCallback: stateCallback, errorCallback: errorCallback)
+            return handleClockKey(state: state, errorCallback: errorCallback)
           }
-        case .kHome: return handleHome(state: state, stateCallback: stateCallback, errorCallback: errorCallback)
-        case .kEnd: return handleEnd(state: state, stateCallback: stateCallback, errorCallback: errorCallback)
+        case .kHome: return handleHome(state: state, errorCallback: errorCallback)
+        case .kEnd: return handleEnd(state: state, errorCallback: errorCallback)
         case .kBackSpace:
-          return handleBackSpace(state: state, input: input, stateCallback: stateCallback, errorCallback: errorCallback)
+          return handleBackSpace(state: state, input: input, errorCallback: errorCallback)
         case .kWindowsDelete:
-          return handleDelete(state: state, input: input, stateCallback: stateCallback, errorCallback: errorCallback)
+          return handleDelete(state: state, input: input, errorCallback: errorCallback)
         case .kCarriageReturn, .kLineFeed:
           return (input.isCommandHold && input.isControlHold)
             ? (input.isOptionHold
-              ? handleCtrlOptionCommandEnter(state: state, stateCallback: stateCallback)
-              : handleCtrlCommandEnter(state: state, stateCallback: stateCallback))
-            : handleEnter(state: state, stateCallback: stateCallback)
+              ? handleCtrlOptionCommandEnter(state: state)
+              : handleCtrlCommandEnter(state: state))
+            : handleEnter(state: state)
         default: break
       }
     }
@@ -257,12 +257,12 @@ extension InputHandler {
             let textToCommit = commitOverflownComposition
             var inputting = generateStateOfInputting()
             inputting.textToCommit = textToCommit
-            stateCallback(inputting)
+            delegate.switchState(inputting)
             let candidateState = generateStateOfCandidates(state: inputting)
             if candidateState.candidates.isEmpty {
               errorCallback("B5127D8A")
             } else {
-              stateCallback(candidateState)
+              delegate.switchState(candidateState)
             }
           } else {  // 不要在注音沒敲完整的情況下叫出統合符號選單。
             errorCallback("17446655")
@@ -273,8 +273,8 @@ extension InputHandler {
         // 得在這裡先 commit buffer，不然會導致「在摁 ESC 離開符號選單時會重複輸入上一次的組字區的內容」的不當行為。
         // 於是這裡用「模擬一次 Enter 鍵的操作」使其代為執行這個 commit buffer 的動作。
         // 這裡不需要該函式所傳回的 bool 結果，所以用「_ =」解消掉。
-        _ = handleEnter(state: state, stateCallback: stateCallback)
-        stateCallback(IMEState.ofSymbolTable(node: CandidateNode.root))
+        _ = handleEnter(state: state)
+        delegate.switchState(IMEState.ofSymbolTable(node: CandidateNode.root))
         return true
       }
     }
@@ -286,10 +286,10 @@ extension InputHandler {
         guard let stringRAW = input.mainAreaNumKeyChar else { return false }
         let newStringFW = stringRAW.applyingTransform(.fullwidthToHalfwidth, reverse: true) ?? stringRAW
         let newStringHW = stringRAW.applyingTransform(.fullwidthToHalfwidth, reverse: false) ?? stringRAW
-        stateCallback(
+        delegate.switchState(
           IMEState.ofCommitting(textToCommit: prefs.halfWidthPunctuationEnabled ? newStringHW : newStringFW)
         )
-        stateCallback(IMEState.ofEmpty())
+        delegate.switchState(IMEState.ofEmpty())
         return true
       }
     }
@@ -307,7 +307,6 @@ extension InputHandler {
     if handlePunctuation(
       customPunctuation,
       state: state,
-      stateCallback: stateCallback,
       errorCallback: errorCallback
     ) {
       return true
@@ -321,7 +320,6 @@ extension InputHandler {
     if handlePunctuation(
       punctuation,
       state: state,
-      stateCallback: stateCallback,
       errorCallback: errorCallback
     ) {
       return true
@@ -332,8 +330,8 @@ extension InputHandler {
     /// 該功能僅可在當前組字區沒有任何內容的時候使用。
     if state.type == .ofEmpty {
       if input.isSpace, !input.isOptionHold, !input.isControlHold, !input.isCommandHold {
-        stateCallback(IMEState.ofCommitting(textToCommit: input.isShiftHold ? "　" : " "))
-        stateCallback(IMEState.ofEmpty())
+        delegate.switchState(IMEState.ofCommitting(textToCommit: input.isShiftHold ? "　" : " "))
+        delegate.switchState(IMEState.ofEmpty())
         return true
       }
     }
@@ -344,22 +342,21 @@ extension InputHandler {
       if input.isShiftHold {  // 這裡先不要判斷 isOptionHold。
         switch prefs.upperCaseLetterKeyBehavior {
           case 1:
-            stateCallback(IMEState.ofEmpty())
-            stateCallback(IMEState.ofCommitting(textToCommit: inputText.lowercased()))
-            stateCallback(IMEState.ofEmpty())
+            delegate.switchState(IMEState.ofEmpty())
+            delegate.switchState(IMEState.ofCommitting(textToCommit: inputText.lowercased()))
+            delegate.switchState(IMEState.ofEmpty())
             return true
 
           case 2:
-            stateCallback(IMEState.ofEmpty())
-            stateCallback(IMEState.ofCommitting(textToCommit: inputText.uppercased()))
-            stateCallback(IMEState.ofEmpty())
+            delegate.switchState(IMEState.ofEmpty())
+            delegate.switchState(IMEState.ofCommitting(textToCommit: inputText.uppercased()))
+            delegate.switchState(IMEState.ofEmpty())
             return true
           default:  // 包括 case 0，直接塞給組字區。
             let letter = "_letter_\(inputText)"
             if handlePunctuation(
               letter,
               state: state,
-              stateCallback: stateCallback,
               errorCallback: errorCallback
             ) {
               return true
@@ -378,7 +375,7 @@ extension InputHandler {
       errorCallback(
         "Blocked data: charCode: \(input.charCode), keyCode: \(input.keyCode)")
       errorCallback("A9BFF20E")
-      stateCallback(state)
+      delegate.switchState(state)
       return true
     }
 
diff --git a/Source/Modules/InputHandler_States.swift b/Source/Modules/InputHandler_States.swift
index d1030132..c86ea1d6 100644
--- a/Source/Modules/InputHandler_States.swift
+++ b/Source/Modules/InputHandler_States.swift
@@ -123,17 +123,17 @@ extension InputHandler {
   /// - Parameters:
   ///   - state: 當前狀態。
   ///   - input: 輸入按鍵訊號。
-  ///   - stateCallback: 狀態回呼。
   ///   - errorCallback: 錯誤回呼。
   /// - Returns: 將按鍵行為「是否有處理掉」藉由 SessionCtl 回報給 IMK。
   func handleMarkingState(
     _ state: IMEStateProtocol,
     input: InputSignalProtocol,
-    stateCallback: @escaping (IMEStateProtocol) -> Void,
     errorCallback: @escaping (String) -> Void
   ) -> Bool {
+    guard let delegate = delegate else { return false }
+
     if input.isEsc {
-      stateCallback(generateStateOfInputting())
+      delegate.switchState(generateStateOfInputting())
       return true
     }
 
@@ -145,38 +145,34 @@ extension InputHandler {
 
     // Enter
     if input.isEnter {
-      if let sessionCtl = delegate {
-        // 先判斷是否是在摁了降權組合鍵的時候目標不在庫。
-        if input.isShiftHold, input.isCommandHold, !state.isFilterable {
-          errorCallback("2EAC1F7A")
-          return true
-        }
-        if !state.isMarkedLengthValid {
-          errorCallback("9AAFAC00")
-          return true
-        }
-        if !sessionCtl.performUserPhraseOperation(with: state, addToFilter: false) {
-          errorCallback("5B69CC8D")
-          return true
-        }
+      // 先判斷是否是在摁了降權組合鍵的時候目標不在庫。
+      if input.isShiftHold, input.isCommandHold, !state.isFilterable {
+        errorCallback("2EAC1F7A")
+        return true
       }
-      stateCallback(generateStateOfInputting())
+      if !state.isMarkedLengthValid {
+        errorCallback("9AAFAC00")
+        return true
+      }
+      if !delegate.performUserPhraseOperation(with: state, addToFilter: false) {
+        errorCallback("5B69CC8D")
+        return true
+      }
+      delegate.switchState(generateStateOfInputting())
       return true
     }
 
     // BackSpace & Delete
     if input.isBackSpace || input.isDelete {
-      if let inputHandlerDelegate = delegate {
-        if !state.isFilterable {
-          errorCallback("1F88B191")
-          return true
-        }
-        if !inputHandlerDelegate.performUserPhraseOperation(with: state, addToFilter: true) {
-          errorCallback("68D3C6C8")
-          return true
-        }
+      if !state.isFilterable {
+        errorCallback("1F88B191")
+        return true
       }
-      stateCallback(generateStateOfInputting())
+      if !delegate.performUserPhraseOperation(with: state, addToFilter: true) {
+        errorCallback("68D3C6C8")
+        return true
+      }
+      delegate.switchState(generateStateOfInputting())
       return true
     }
 
@@ -194,10 +190,10 @@ extension InputHandler {
           marker: convertCursorForDisplay(compositor.marker)
         )
         marking.tooltipBackupForInputting = state.tooltipBackupForInputting
-        stateCallback(marking.markedRange.isEmpty ? marking.convertedToInputting : marking)
+        delegate.switchState(marking.markedRange.isEmpty ? marking.convertedToInputting : marking)
       } else {
         errorCallback("1149908D")
-        stateCallback(state)
+        delegate.switchState(state)
       }
       return true
     }
@@ -216,10 +212,10 @@ extension InputHandler {
           marker: convertCursorForDisplay(compositor.marker)
         )
         marking.tooltipBackupForInputting = state.tooltipBackupForInputting
-        stateCallback(marking.markedRange.isEmpty ? marking.convertedToInputting : marking)
+        delegate.switchState(marking.markedRange.isEmpty ? marking.convertedToInputting : marking)
       } else {
         errorCallback("9B51408D")
-        stateCallback(state)
+        delegate.switchState(state)
       }
       return true
     }
@@ -231,15 +227,15 @@ extension InputHandler {
   /// 標點輸入的處理。
   /// - Parameters:
   ///   - customPunctuation: 自訂標點索引鍵頭。
-  ///   - stateCallback: 狀態回呼。
   ///   - errorCallback: 錯誤回呼。
   /// - Returns: 將按鍵行為「是否有處理掉」藉由 SessionCtl 回報給 IMK。
   func handlePunctuation(
     _ customPunctuation: String,
     state: IMEStateProtocol,
-    stateCallback: @escaping (IMEStateProtocol) -> Void,
     errorCallback: @escaping (String) -> Void
   ) -> Bool {
+    guard let delegate = delegate else { return false }
+
     if !currentLM.hasUnigramsFor(key: customPunctuation) {
       return false
     }
@@ -247,7 +243,7 @@ extension InputHandler {
     guard composer.isEmpty else {
       // 注音沒敲完的情況下，無視標點輸入。
       errorCallback("A9B69908D")
-      stateCallback(state)
+      delegate.switchState(state)
       return true
     }
 
@@ -257,21 +253,21 @@ extension InputHandler {
     let textToCommit = commitOverflownComposition
     var inputting = generateStateOfInputting()
     inputting.textToCommit = textToCommit
-    stateCallback(inputting)
+    delegate.switchState(inputting)
 
     // 從這一行之後開始，就是針對逐字選字模式的單獨處理。
     guard prefs.useSCPCTypingMode, composer.isEmpty else { return true }
 
     let candidateState = generateStateOfCandidates(state: inputting)
     switch candidateState.candidates.count {
-      case 2...: stateCallback(candidateState)
+      case 2...: delegate.switchState(candidateState)
       case 1:
         clear()  // 這句不要砍，因為下文可能會回呼 candidateState。
         if let candidateToCommit: (String, String) = candidateState.candidates.first, !candidateToCommit.1.isEmpty {
-          stateCallback(IMEState.ofCommitting(textToCommit: candidateToCommit.1))
-          stateCallback(IMEState.ofEmpty())
+          delegate.switchState(IMEState.ofCommitting(textToCommit: candidateToCommit.1))
+          delegate.switchState(IMEState.ofEmpty())
         } else {
-          stateCallback(candidateState)
+          delegate.switchState(candidateState)
         }
       default: errorCallback("8DA4096E")
     }
@@ -283,16 +279,15 @@ extension InputHandler {
   /// Enter 鍵的處理。
   /// - Parameters:
   ///   - state: 當前狀態。
-  ///   - stateCallback: 狀態回呼。
   /// - Returns: 將按鍵行為「是否有處理掉」藉由 SessionCtl 回報給 IMK。
   func handleEnter(
-    state: IMEStateProtocol,
-    stateCallback: @escaping (IMEStateProtocol) -> Void
+    state: IMEStateProtocol
   ) -> Bool {
+    guard let delegate = delegate else { return false }
     guard state.type == .ofInputting else { return false }
 
-    stateCallback(IMEState.ofCommitting(textToCommit: state.displayedText))
-    stateCallback(IMEState.ofEmpty())
+    delegate.switchState(IMEState.ofCommitting(textToCommit: state.displayedText))
+    delegate.switchState(IMEState.ofEmpty())
     return true
   }
 
@@ -301,12 +296,11 @@ extension InputHandler {
   /// Command+Enter 鍵的處理（注音文）。
   /// - Parameters:
   ///   - state: 當前狀態。
-  ///   - stateCallback: 狀態回呼。
   /// - Returns: 將按鍵行為「是否有處理掉」藉由 SessionCtl 回報給 IMK。
   func handleCtrlCommandEnter(
-    state: IMEStateProtocol,
-    stateCallback: @escaping (IMEStateProtocol) -> Void
+    state: IMEStateProtocol
   ) -> Bool {
+    guard let delegate = delegate else { return false }
     guard state.type == .ofInputting else { return false }
 
     var displayedText = compositor.keys.joined(separator: "-")
@@ -315,12 +309,12 @@ extension InputHandler {
       displayedText = Tekkon.cnvPhonaToHanyuPinyin(target: displayedText)  // 注音轉拼音
     }
 
-    if let delegate = delegate, !delegate.clientBundleIdentifier.contains("vChewingPhraseEditor") {
+    if !delegate.clientBundleIdentifier.contains("vChewingPhraseEditor") {
       displayedText = displayedText.replacingOccurrences(of: "-", with: " ")
     }
 
-    stateCallback(IMEState.ofCommitting(textToCommit: displayedText))
-    stateCallback(IMEState.ofEmpty())
+    delegate.switchState(IMEState.ofCommitting(textToCommit: displayedText))
+    delegate.switchState(IMEState.ofEmpty())
     return true
   }
 
@@ -329,12 +323,11 @@ extension InputHandler {
   /// Command+Option+Enter 鍵的處理（網頁 Ruby 注音文標記）。
   /// - Parameters:
   ///   - state: 當前狀態。
-  ///   - stateCallback: 狀態回呼。
   /// - Returns: 將按鍵行為「是否有處理掉」藉由 SessionCtl 回報給 IMK。
   func handleCtrlOptionCommandEnter(
-    state: IMEStateProtocol,
-    stateCallback: @escaping (IMEStateProtocol) -> Void
+    state: IMEStateProtocol
   ) -> Bool {
+    guard let delegate = delegate else { return false }
     guard state.type == .ofInputting else { return false }
 
     var composed = ""
@@ -355,8 +348,8 @@ extension InputHandler {
       composed += key.contains("_") ? value : "<ruby>\(value)<rp>(</rp><rt>\(key)</rt><rp>)</rp></ruby>"
     }
 
-    stateCallback(IMEState.ofCommitting(textToCommit: composed))
-    stateCallback(IMEState.ofEmpty())
+    delegate.switchState(IMEState.ofCommitting(textToCommit: composed))
+    delegate.switchState(IMEState.ofEmpty())
     return true
   }
 
@@ -366,15 +359,14 @@ extension InputHandler {
   /// - Parameters:
   ///   - state: 當前狀態。
   ///   - input: 輸入按鍵訊號。
-  ///   - stateCallback: 狀態回呼。
   ///   - errorCallback: 錯誤回呼。
   /// - Returns: 將按鍵行為「是否有處理掉」藉由 SessionCtl 回報給 IMK。
   func handleBackSpace(
     state: IMEStateProtocol,
     input: InputSignalProtocol,
-    stateCallback: @escaping (IMEStateProtocol) -> Void,
     errorCallback: @escaping (String) -> Void
   ) -> Bool {
+    guard let delegate = delegate else { return false }
     guard state.type == .ofInputting else { return false }
 
     // 引入 macOS 內建注音輸入法的行為，允許用 Shift+BackSpace 解構前一個漢字的讀音。
@@ -386,16 +378,16 @@ extension InputHandler {
         compositor.dropKey(direction: .rear)
         walk()  // 這裡必須 Walk 一次、來更新目前被 walk 的內容。
         prevReading.1.charComponents.forEach { composer.receiveKey(fromPhonabet: $0) }
-        stateCallback(generateStateOfInputting())
+        delegate.switchState(generateStateOfInputting())
         return true
       case 1:
-        stateCallback(IMEState.ofAbortion())
+        delegate.switchState(IMEState.ofAbortion())
         return true
       default: break
     }
 
     if input.isShiftHold, input.isOptionHold {
-      stateCallback(IMEState.ofAbortion())
+      delegate.switchState(IMEState.ofAbortion())
       return true
     }
 
@@ -407,7 +399,7 @@ extension InputHandler {
         walk()
       } else {
         errorCallback("9D69908D")
-        stateCallback(state)
+        delegate.switchState(state)
         return true
       }
     } else {
@@ -415,9 +407,9 @@ extension InputHandler {
     }
 
     switch composer.isEmpty && compositor.isEmpty {
-      case false: stateCallback(generateStateOfInputting())
+      case false: delegate.switchState(generateStateOfInputting())
       case true:
-        stateCallback(IMEState.ofAbortion())
+        delegate.switchState(IMEState.ofAbortion())
     }
     return true
   }
@@ -428,25 +420,24 @@ extension InputHandler {
   /// - Parameters:
   ///   - state: 當前狀態。
   ///   - input: 輸入按鍵訊號。
-  ///   - stateCallback: 狀態回呼。
   ///   - errorCallback: 錯誤回呼。
   /// - Returns: 將按鍵行為「是否有處理掉」藉由 SessionCtl 回報給 IMK。
   func handleDelete(
     state: IMEStateProtocol,
     input: InputSignalProtocol,
-    stateCallback: @escaping (IMEStateProtocol) -> Void,
     errorCallback: @escaping (String) -> Void
   ) -> Bool {
+    guard let delegate = delegate else { return false }
     guard state.type == .ofInputting else { return false }
 
     if input.isShiftHold {
-      stateCallback(IMEState.ofAbortion())
+      delegate.switchState(IMEState.ofAbortion())
       return true
     }
 
     if compositor.cursor == compositor.length, composer.isEmpty {
       errorCallback("9B69938D")
-      stateCallback(state)
+      delegate.switchState(state)
       return true
     }
 
@@ -460,9 +451,9 @@ extension InputHandler {
     let inputting = generateStateOfInputting()
     // 這裡不用「count > 0」，因為該整數變數只要「!isEmpty」那就必定滿足這個條件。
     switch inputting.displayedText.isEmpty {
-      case false: stateCallback(inputting)
+      case false: delegate.switchState(inputting)
       case true:
-        stateCallback(IMEState.ofAbortion())
+        delegate.switchState(IMEState.ofAbortion())
     }
     return true
   }
@@ -472,19 +463,18 @@ extension InputHandler {
   /// 處理與當前文字輸入排版前後方向呈 90 度的那兩個方向鍵的按鍵行為。
   /// - Parameters:
   ///   - state: 當前狀態。
-  ///   - stateCallback: 狀態回呼。
   ///   - errorCallback: 錯誤回呼。
   /// - Returns: 將按鍵行為「是否有處理掉」藉由 SessionCtl 回報給 IMK。
   func handleClockKey(
     state: IMEStateProtocol,
-    stateCallback: @escaping (IMEStateProtocol) -> Void,
     errorCallback: @escaping (String) -> Void
   ) -> Bool {
+    guard let delegate = delegate else { return false }
     guard state.type == .ofInputting else { return false }
     if !composer.isEmpty {
       errorCallback("9B6F908D")
     }
-    stateCallback(state)
+    delegate.switchState(state)
     return true
   }
 
@@ -493,28 +483,27 @@ extension InputHandler {
   /// 處理 Home 鍵的行為。
   /// - Parameters:
   ///   - state: 當前狀態。
-  ///   - stateCallback: 狀態回呼。
   ///   - errorCallback: 錯誤回呼。
   /// - Returns: 將按鍵行為「是否有處理掉」藉由 SessionCtl 回報給 IMK。
   func handleHome(
     state: IMEStateProtocol,
-    stateCallback: @escaping (IMEStateProtocol) -> Void,
     errorCallback: @escaping (String) -> Void
   ) -> Bool {
+    guard let delegate = delegate else { return false }
     guard state.type == .ofInputting else { return false }
 
     if !composer.isEmpty {
       errorCallback("ABC44080")
-      stateCallback(state)
+      delegate.switchState(state)
       return true
     }
 
     if compositor.cursor != 0 {
       compositor.cursor = 0
-      stateCallback(generateStateOfInputting())
+      delegate.switchState(generateStateOfInputting())
     } else {
       errorCallback("66D97F90")
-      stateCallback(state)
+      delegate.switchState(state)
     }
 
     return true
@@ -525,28 +514,27 @@ extension InputHandler {
   /// 處理 End 鍵的行為。
   /// - Parameters:
   ///   - state: 當前狀態。
-  ///   - stateCallback: 狀態回呼。
   ///   - errorCallback: 錯誤回呼。
   /// - Returns: 將按鍵行為「是否有處理掉」藉由 SessionCtl 回報給 IMK。
   func handleEnd(
     state: IMEStateProtocol,
-    stateCallback: @escaping (IMEStateProtocol) -> Void,
     errorCallback: @escaping (String) -> Void
   ) -> Bool {
+    guard let delegate = delegate else { return false }
     guard state.type == .ofInputting else { return false }
 
     if !composer.isEmpty {
       errorCallback("9B69908D")
-      stateCallback(state)
+      delegate.switchState(state)
       return true
     }
 
     if compositor.cursor != compositor.length {
       compositor.cursor = compositor.length
-      stateCallback(generateStateOfInputting())
+      delegate.switchState(generateStateOfInputting())
     } else {
       errorCallback("9B69908E")
-      stateCallback(state)
+      delegate.switchState(state)
     }
 
     return true
@@ -557,26 +545,25 @@ extension InputHandler {
   /// 處理 Esc 鍵的行為。
   /// - Parameters:
   ///   - state: 當前狀態。
-  ///   - stateCallback: 狀態回呼。
   /// - Returns: 將按鍵行為「是否有處理掉」藉由 SessionCtl 回報給 IMK。
   func handleEsc(
-    state: IMEStateProtocol,
-    stateCallback: @escaping (IMEStateProtocol) -> Void
+    state: IMEStateProtocol
   ) -> Bool {
+    guard let delegate = delegate else { return false }
     guard state.type == .ofInputting else { return false }
 
     if prefs.escToCleanInputBuffer {
       /// 若啟用了該選項，則清空組字器的內容與注拼槽的內容。
       /// 此乃 macOS 內建注音輸入法預設之行為，但不太受 Windows 使用者群體之待見。
-      stateCallback(IMEState.ofAbortion())
+      delegate.switchState(IMEState.ofAbortion())
     } else {
       if composer.isEmpty { return true }
       /// 如果注拼槽不是空的話，則清空之。
       composer.clear()
       switch compositor.isEmpty {
-        case false: stateCallback(generateStateOfInputting())
+        case false: delegate.switchState(generateStateOfInputting())
         case true:
-          stateCallback(IMEState.ofAbortion())
+          delegate.switchState(IMEState.ofAbortion())
       }
     }
     return true
@@ -588,20 +575,19 @@ extension InputHandler {
   /// - Parameters:
   ///   - state: 當前狀態。
   ///   - input: 輸入按鍵訊號。
-  ///   - stateCallback: 狀態回呼。
   ///   - errorCallback: 錯誤回呼。
   /// - Returns: 將按鍵行為「是否有處理掉」藉由 SessionCtl 回報給 IMK。
   func handleForward(
     state: IMEStateProtocol,
     input: InputSignalProtocol,
-    stateCallback: @escaping (IMEStateProtocol) -> Void,
     errorCallback: @escaping (String) -> Void
   ) -> Bool {
+    guard let delegate = delegate else { return false }
     guard state.type == .ofInputting else { return false }
 
     if !composer.isEmpty {
       errorCallback("B3BA5257")
-      stateCallback(state)
+      delegate.switchState(state)
       return true
     }
 
@@ -619,32 +605,32 @@ extension InputHandler {
           marker: convertCursorForDisplay(compositor.marker)
         )
         marking.tooltipBackupForInputting = state.tooltip
-        stateCallback(marking)
+        delegate.switchState(marking)
       } else {
         errorCallback("BB7F6DB9")
-        stateCallback(state)
+        delegate.switchState(state)
       }
     } else if input.isOptionHold {
       if input.isControlHold {
-        return handleEnd(state: state, stateCallback: stateCallback, errorCallback: errorCallback)
+        return handleEnd(state: state, errorCallback: errorCallback)
       }
       // 游標跳轉動作無論怎樣都會執行，但如果出了執行失敗的結果的話則觸發報錯流程。
       if !compositor.jumpCursorBySpan(to: .front) {
         errorCallback("33C3B580")
-        stateCallback(state)
+        delegate.switchState(state)
         return true
       }
-      stateCallback(generateStateOfInputting())
+      delegate.switchState(generateStateOfInputting())
     } else {
       if compositor.cursor < compositor.length {
         compositor.cursor += 1
         if isCursorCuttingChar() {
           compositor.jumpCursorBySpan(to: .front)
         }
-        stateCallback(generateStateOfInputting())
+        delegate.switchState(generateStateOfInputting())
       } else {
         errorCallback("A96AAD58")
-        stateCallback(state)
+        delegate.switchState(state)
       }
     }
 
@@ -657,20 +643,19 @@ extension InputHandler {
   /// - Parameters:
   ///   - state: 當前狀態。
   ///   - input: 輸入按鍵訊號。
-  ///   - stateCallback: 狀態回呼。
   ///   - errorCallback: 錯誤回呼。
   /// - Returns: 將按鍵行為「是否有處理掉」藉由 SessionCtl 回報給 IMK。
   func handleBackward(
     state: IMEStateProtocol,
     input: InputSignalProtocol,
-    stateCallback: @escaping (IMEStateProtocol) -> Void,
     errorCallback: @escaping (String) -> Void
   ) -> Bool {
+    guard let delegate = delegate else { return false }
     guard state.type == .ofInputting else { return false }
 
     if !composer.isEmpty {
       errorCallback("6ED95318")
-      stateCallback(state)
+      delegate.switchState(state)
       return true
     }
 
@@ -688,32 +673,32 @@ extension InputHandler {
           marker: convertCursorForDisplay(compositor.marker)
         )
         marking.tooltipBackupForInputting = state.tooltip
-        stateCallback(marking)
+        delegate.switchState(marking)
       } else {
         errorCallback("D326DEA3")
-        stateCallback(state)
+        delegate.switchState(state)
       }
     } else if input.isOptionHold {
       if input.isControlHold {
-        return handleHome(state: state, stateCallback: stateCallback, errorCallback: errorCallback)
+        return handleHome(state: state, errorCallback: errorCallback)
       }
       // 游標跳轉動作無論怎樣都會執行，但如果出了執行失敗的結果的話則觸發報錯流程。
       if !compositor.jumpCursorBySpan(to: .rear) {
         errorCallback("8D50DD9E")
-        stateCallback(state)
+        delegate.switchState(state)
         return true
       }
-      stateCallback(generateStateOfInputting())
+      delegate.switchState(generateStateOfInputting())
     } else {
       if compositor.cursor > 0 {
         compositor.cursor -= 1
         if isCursorCuttingChar() {
           compositor.jumpCursorBySpan(to: .rear)
         }
-        stateCallback(generateStateOfInputting())
+        delegate.switchState(generateStateOfInputting())
       } else {
         errorCallback("7045E6F3")
-        stateCallback(state)
+        delegate.switchState(state)
       }
     }
 
@@ -726,15 +711,14 @@ extension InputHandler {
   /// - Parameters:
   ///   - state: 當前狀態。
   ///   - reverseModifier: 是否有控制輪替方向的修飾鍵輸入。
-  ///   - stateCallback: 狀態回呼。
   ///   - errorCallback: 錯誤回呼。
   /// - Returns: 將按鍵行為「是否有處理掉」藉由 SessionCtl 回報給 IMK。
   func handleInlineCandidateRotation(
     state: IMEStateProtocol,
     reverseModifier: Bool,
-    stateCallback: @escaping (IMEStateProtocol) -> Void,
     errorCallback: @escaping (String) -> Void
   ) -> Bool {
+    guard let delegate = delegate else { return false }
     if composer.isEmpty, compositor.isEmpty || compositor.walkedNodes.isEmpty { return false }
     guard state.type == .ofInputting else {
       guard state.type == .ofEmpty else {
@@ -814,7 +798,7 @@ extension InputHandler {
 
     consolidateNode(candidate: candidates[currentIndex], respectCursorPushing: false, preConsolidate: false)
 
-    stateCallback(generateStateOfInputting())
+    delegate.switchState(generateStateOfInputting())
     return true
   }
 }
diff --git a/Source/Modules/SessionCtl_Core.swift b/Source/Modules/SessionCtl_Core.swift
index d7214f0a..e58a1688 100644
--- a/Source/Modules/SessionCtl_Core.swift
+++ b/Source/Modules/SessionCtl_Core.swift
@@ -119,7 +119,7 @@ public class SessionCtl: IMKInputController {
         // ----------------------------
         Self.isVerticalTyping = isVerticalTyping
         // 強制重設當前鍵盤佈局、使其與偏好設定同步。這裡的這一步也不能省略。
-        handle(state: IMEState.ofEmpty())
+        switchState(IMEState.ofEmpty())
       }
     }
   }
@@ -170,14 +170,14 @@ extension SessionCtl {
     // 過濾掉尚未完成拼寫的注音。
     if state.type == .ofInputting, PrefMgr.shared.trimUnfinishedReadingsOnCommit {
       inputHandler.composer.clear()
-      handle(state: inputHandler.generateStateOfInputting())
+      switchState(inputHandler.generateStateOfInputting())
     }
     let isSecureMode = PrefMgr.shared.clientsIMKTextInputIncapable.contains(clientBundleIdentifier)
     if state.hasComposition, !isSecureMode {
       /// 將傳回的新狀態交給調度函式。
-      handle(state: IMEState.ofCommitting(textToCommit: state.displayedText))
+      switchState(IMEState.ofCommitting(textToCommit: state.displayedText))
     }
-    handle(state: isSecureMode ? IMEState.ofAbortion() : IMEState.ofEmpty())
+    switchState(isSecureMode ? IMEState.ofAbortion() : IMEState.ofEmpty())
   }
 }
 
@@ -208,7 +208,7 @@ extension SessionCtl {
       UpdateSputnik.shared.checkForUpdate(forced: false, url: kUpdateInfoSourceURL)
     }
 
-    handle(state: IMEState.ofEmpty())
+    switchState(IMEState.ofEmpty())
     Self.allInstances.insert(self)
   }
 
@@ -217,7 +217,7 @@ extension SessionCtl {
   public override func deactivateServer(_ sender: Any!) {
     _ = sender  // 防止格式整理工具毀掉與此對應的參數。
     resetInputHandler()  // 這條會自動搞定 Empty 狀態。
-    handle(state: IMEState.ofDeactivated())
+    switchState(IMEState.ofDeactivated())
     Self.allInstances.remove(self)
   }
 
diff --git a/Source/Modules/SessionCtl_Delegates.swift b/Source/Modules/SessionCtl_Delegates.swift
index f912950b..dbd06c11 100644
--- a/Source/Modules/SessionCtl_Delegates.swift
+++ b/Source/Modules/SessionCtl_Delegates.swift
@@ -75,11 +75,11 @@ extension SessionCtl: CtlCandidateDelegate {
     if state.type == .ofSymbolTable, (0..<state.node.members.count).contains(index) {
       let node = state.node.members[index]
       if !node.members.isEmpty {
-        handle(state: IMEState.ofEmpty())  // 防止縱橫排選字窗同時出現
-        handle(state: IMEState.ofSymbolTable(node: node))
+        switchState(IMEState.ofEmpty())  // 防止縱橫排選字窗同時出現
+        switchState(IMEState.ofSymbolTable(node: node))
       } else {
-        handle(state: IMEState.ofCommitting(textToCommit: node.name))
-        handle(state: IMEState.ofEmpty())
+        switchState(IMEState.ofCommitting(textToCommit: node.name))
+        switchState(IMEState.ofEmpty())
       }
       return
     }
@@ -94,29 +94,29 @@ extension SessionCtl: CtlCandidateDelegate {
       let inputting = inputHandler.generateStateOfInputting()
 
       if PrefMgr.shared.useSCPCTypingMode {
-        handle(state: IMEState.ofCommitting(textToCommit: inputting.displayedText))
+        switchState(IMEState.ofCommitting(textToCommit: inputting.displayedText))
         // 此時是逐字選字模式，所以「selectedValue.1」是單個字、不用追加處理。
         if PrefMgr.shared.associatedPhrasesEnabled {
           let associates = inputHandler.generateStateOfAssociates(
             withPair: .init(key: selectedValue.0, value: selectedValue.1)
           )
-          handle(state: associates.candidates.isEmpty ? IMEState.ofEmpty() : associates)
+          switchState(associates.candidates.isEmpty ? IMEState.ofEmpty() : associates)
         } else {
-          handle(state: IMEState.ofEmpty())
+          switchState(IMEState.ofEmpty())
         }
       } else {
-        handle(state: inputting)
+        switchState(inputting)
       }
       return
     }
 
     if state.type == .ofAssociates {
       let selectedValue = state.candidates[index]
-      handle(state: IMEState.ofCommitting(textToCommit: selectedValue.1))
+      switchState(IMEState.ofCommitting(textToCommit: selectedValue.1))
       // 此時是聯想詞選字模式，所以「selectedValue.1」必須只保留最後一個字。
       // 不然的話，一旦你選中了由多個字組成的聯想候選詞，則連續聯想會被打斷。
       guard let valueKept = selectedValue.1.last else {
-        handle(state: IMEState.ofEmpty())
+        switchState(IMEState.ofEmpty())
         return
       }
       if PrefMgr.shared.associatedPhrasesEnabled {
@@ -124,11 +124,11 @@ extension SessionCtl: CtlCandidateDelegate {
           withPair: .init(key: selectedValue.0, value: String(valueKept))
         )
         if !associates.candidates.isEmpty {
-          handle(state: associates)
+          switchState(associates)
           return
         }
       }
-      handle(state: IMEState.ofEmpty())
+      switchState(IMEState.ofEmpty())
     }
   }
 }
diff --git a/Source/Modules/SessionCtl_HandleEvent.swift b/Source/Modules/SessionCtl_HandleEvent.swift
index 35cd8d94..eb80d131 100644
--- a/Source/Modules/SessionCtl_HandleEvent.swift
+++ b/Source/Modules/SessionCtl_HandleEvent.swift
@@ -27,7 +27,7 @@ extension SessionCtl {
 
     // 如果是 deactivated 狀態的話，強制糾正其為 empty()。
     if let client = client(), state.type == .ofDeactivated {
-      handle(state: IMEState.ofEmpty())
+      switchState(IMEState.ofEmpty())
       return handle(event, client: client)
     }
 
diff --git a/Source/Modules/SessionCtl_HandleStates.swift b/Source/Modules/SessionCtl_HandleStates.swift
index a8ee5e15..012feebe 100644
--- a/Source/Modules/SessionCtl_HandleStates.swift
+++ b/Source/Modules/SessionCtl_HandleStates.swift
@@ -12,6 +12,20 @@ import Shared
 // MARK: - 狀態調度 (State Handling)
 
 extension SessionCtl {
+  /// 針對傳入的新狀態進行調度、且將當前會話控制器的狀態切換至新狀態。
+  ///
+  /// 先將舊狀態單獨記錄起來，再將新舊狀態作為參數，
+  /// 根據新狀態本身的狀態種類來判斷交給哪一個專門的函式來處理。
+  /// - Remark: ⚠️ 任何在這個函式當中被改變的變數均不得是靜態 (Static) 變數。
+  /// 針對某一個客體的 deactivateServer() 可能會在使用者切換到另一個客體應用
+  /// 且開始敲字之後才會執行。這個過程會使得不同的 SessionCtl 副本之間出現
+  /// 不必要的互相干涉、打斷彼此的工作。
+  /// - Note: 本來不用這麼複雜的，奈何 Swift Protocol 不允許給參數指定預設值。
+  /// - Parameter newState: 新狀態。
+  public func switchState(_ newState: IMEStateProtocol) {
+    handle(state: newState, replace: true)
+  }
+
   /// 針對傳入的新狀態進行調度。
   ///
   /// 先將舊狀態單獨記錄起來，再將新舊狀態作為參數，
@@ -21,9 +35,9 @@ extension SessionCtl {
   /// 且開始敲字之後才會執行。這個過程會使得不同的 SessionCtl 副本之間出現
   /// 不必要的互相干涉、打斷彼此的工作。
   /// - Parameter newState: 新狀態。
-  public func handle(state newState: IMEStateProtocol, replaceCurrent: Bool = true) {
+  public func handle(state newState: IMEStateProtocol, replace: Bool) {
     var previous = state
-    if replaceCurrent { state = newState }
+    if replace { state = newState }
     switch newState.type {
       case .ofDeactivated:
         ctlCandidateCurrent.visible = false
@@ -43,14 +57,14 @@ extension SessionCtl {
           for instance in Self.allInstances {
             guard let imkC = instance.ctlCandidateCurrent as? CtlCandidateIMK else { continue }
             if instance.state.isCandidateContainer, !imkC.visible {
-              instance.handle(state: instance.state, replaceCurrent: false)
+              instance.handle(state: instance.state, replace: false)
             }
           }
         }
       case .ofEmpty, .ofAbortion:
         if newState.type == .ofAbortion {
           previous = IMEState.ofEmpty()
-          if replaceCurrent { state = previous }
+          if replace { state = previous }
         }
         ctlCandidateCurrent.visible = false
         tooltipInstance.hide()
diff --git a/Source/Modules/SessionCtl_IMKCandidatesData.swift b/Source/Modules/SessionCtl_IMKCandidatesData.swift
index 37bb68df..6f8e76bd 100644
--- a/Source/Modules/SessionCtl_IMKCandidatesData.swift
+++ b/Source/Modules/SessionCtl_IMKCandidatesData.swift
@@ -75,7 +75,7 @@ extension SessionCtl {
     let candidateString: String = candidateString?.string ?? ""
     if state.type == .ofAssociates {
       if !PrefMgr.shared.alsoConfirmAssociatedCandidatesByEnter {
-        handle(state: IMEState.ofAbortion())
+        switchState(IMEState.ofAbortion())
         return
       }
     }