diff --git a/Packages/vChewing_MainAssembly/Sources/MainAssembly/PrefMgr_Extension.swift b/Packages/vChewing_MainAssembly/Sources/MainAssembly/PrefMgr_Extension.swift
index 731929d6..40ff9b29 100644
--- a/Packages/vChewing_MainAssembly/Sources/MainAssembly/PrefMgr_Extension.swift
+++ b/Packages/vChewing_MainAssembly/Sources/MainAssembly/PrefMgr_Extension.swift
@@ -8,6 +8,20 @@
 
 import InputMethodKit
 import Shared
+import SwiftUI
+
+// MARK: ObservableProject.
+
+@available(macOS 10.15, *)
+extension PrefMgr: ObservableObject {}
+
+extension PrefMgr {
+  func sendObjWillChange() {
+    if #available(macOS 10.15, *) {
+      objectWillChange.send()
+    }
+  }
+}
 
 // MARK: Auto parameter fix procedures, executed everytime on SessionCtl.activateServer().
 
diff --git a/Packages/vChewing_MainAssembly/Sources/MainAssembly/PrefMgr_Observable.swift b/Packages/vChewing_MainAssembly/Sources/MainAssembly/PrefMgr_Observable.swift
new file mode 100644
index 00000000..57812222
--- /dev/null
+++ b/Packages/vChewing_MainAssembly/Sources/MainAssembly/PrefMgr_Observable.swift
@@ -0,0 +1,357 @@
+// (c) 2021 and onwards The vChewing Project (MIT-NTL License).
+// ====================
+// This code is released under the MIT license (SPDX-License-Identifier: MIT)
+// ... with NTL restriction stating that:
+// No trademark license is granted to use the trade names, trademarks, service
+// marks, or product names of Contributor, except as required to fulfill notice
+// requirements defined in MIT License.
+
+import Shared
+import SwiftUI
+
+@available(macOS 13, *)
+public class PrefMgrObservable: NSObject, PrefMgrProtocol, ObservableObject {
+  public static let sharedInstance = PrefMgrObservable()
+  public static let kDefaultCandidateKeys = PrefMgr.kDefaultCandidateKeys
+  public static let kDefaultBasicKeyboardLayout = PrefMgr.kDefaultBasicKeyboardLayout
+  public static let kDefaultAlphanumericalKeyboardLayout = PrefMgr.kDefaultAlphanumericalKeyboardLayout
+  public static let kDefaultClientsIMKTextInputIncapable: [String: Bool] = PrefMgr.kDefaultClientsIMKTextInputIncapable
+
+  // MARK: - Settings (Tier 1)
+
+  @AppStorage(wrappedValue: false, UserDef.kIsDebugModeEnabled.rawValue)
+  public var isDebugModeEnabled: Bool
+
+  @AppStorage(wrappedValue: false, UserDef.kFailureFlagForUOMObservation.rawValue)
+  public var failureFlagForUOMObservation: Bool
+
+  @AppStorage(wrappedValue: false, UserDef.kSecurityHardenedCompositionBuffer.rawValue)
+  public var securityHardenedCompositionBuffer: Bool
+
+  @AppStorage(wrappedValue: -2000, UserDef.kDeltaOfCalendarYears.rawValue)
+  public var deltaOfCalendarYears: Int
+
+  @AppStorage(wrappedValue: "", UserDef.kMostRecentInputMode.rawValue)
+  public var mostRecentInputMode: String
+
+  @AppStorage(wrappedValue: false, UserDef.kCheckUpdateAutomatically.rawValue)
+  public var checkUpdateAutomatically: Bool
+
+  @AppStorage(wrappedValue: false, UserDef.kUseExternalFactoryDict.rawValue)
+  public var useExternalFactoryDict: Bool
+
+  @AppStorage(wrappedValue: "", UserDef.kCassettePath.rawValue)
+  public var cassettePath: String
+
+  @AppStorage(wrappedValue: "", UserDef.kUserDataFolderSpecified.rawValue)
+  public var userDataFolderSpecified: String
+
+  public var appleLanguages: [String] {
+    get { PrefMgr.shared.appleLanguages }
+    set { PrefMgr.shared.appleLanguages = newValue }
+  }
+
+  @AppStorage(wrappedValue: 0, UserDef.kKeyboardParser.rawValue)
+  public var keyboardParser: Int
+
+  @AppStorage(wrappedValue: kDefaultBasicKeyboardLayout, UserDef.kBasicKeyboardLayout.rawValue)
+  public var basicKeyboardLayout: String
+
+  @AppStorage(wrappedValue: kDefaultAlphanumericalKeyboardLayout, UserDef.kAlphanumericalKeyboardLayout.rawValue)
+  public var alphanumericalKeyboardLayout: String
+
+  @AppStorage(wrappedValue: true, UserDef.kShowNotificationsWhenTogglingCapsLock.rawValue)
+  public var showNotificationsWhenTogglingCapsLock: Bool
+
+  @AppStorage(wrappedValue: 16, UserDef.kCandidateListTextSize.rawValue)
+  public var candidateListTextSize: Double {
+    didSet {
+      // 必須確立條件，否則就會是無限迴圈。
+      if !(12 ... 196).contains(candidateListTextSize) {
+        candidateListTextSize = max(12, min(candidateListTextSize, 196))
+      }
+    }
+  }
+
+  @AppStorage(wrappedValue: false, UserDef.kAlwaysExpandCandidateWindow.rawValue)
+  public var alwaysExpandCandidateWindow: Bool
+
+  @AppStorage(wrappedValue: false, UserDef.kCandidateWindowShowOnlyOneLine.rawValue)
+  public var candidateWindowShowOnlyOneLine: Bool
+
+  @AppStorage(wrappedValue: true, UserDef.kShouldAutoReloadUserDataFiles.rawValue)
+  public var shouldAutoReloadUserDataFiles: Bool
+
+  @AppStorage(wrappedValue: false, UserDef.kUseRearCursorMode.rawValue)
+  public var useRearCursorMode: Bool
+
+  @AppStorage(wrappedValue: true, UserDef.kMoveCursorAfterSelectingCandidate.rawValue)
+  public var moveCursorAfterSelectingCandidate: Bool
+
+  @AppStorage(wrappedValue: true, UserDef.kUseDynamicCandidateWindowOrigin.rawValue)
+  public var useDynamicCandidateWindowOrigin: Bool
+
+  @AppStorage(wrappedValue: true, UserDef.kUseHorizontalCandidateList.rawValue)
+  public var useHorizontalCandidateList: Bool
+
+  @AppStorage(wrappedValue: true, UserDef.kChooseCandidateUsingSpace.rawValue)
+  public var chooseCandidateUsingSpace: Bool
+
+  @AppStorage(wrappedValue: false, UserDef.kAllowBoostingSingleKanjiAsUserPhrase.rawValue)
+  public var allowBoostingSingleKanjiAsUserPhrase: Bool
+
+  @AppStorage(wrappedValue: true, UserDef.kFetchSuggestionsFromUserOverrideModel.rawValue)
+  public var fetchSuggestionsFromUserOverrideModel: Bool
+
+  @AppStorage(wrappedValue: false, UserDef.kUseFixedCandidateOrderOnSelection.rawValue)
+  public var useFixedCandidateOrderOnSelection: Bool
+
+  @AppStorage(wrappedValue: true, UserDef.kAutoCorrectReadingCombination.rawValue)
+  public var autoCorrectReadingCombination: Bool
+
+  @AppStorage(wrappedValue: false, UserDef.kAlsoConfirmAssociatedCandidatesByEnter.rawValue)
+  public var alsoConfirmAssociatedCandidatesByEnter: Bool
+
+  @AppStorage(wrappedValue: false, UserDef.kKeepReadingUponCompositionError.rawValue)
+  public var keepReadingUponCompositionError: Bool
+
+  @AppStorage(wrappedValue: 0, UserDef.kUpperCaseLetterKeyBehavior.rawValue)
+  public var upperCaseLetterKeyBehavior: Int
+
+  @AppStorage(wrappedValue: true, UserDef.kTogglingAlphanumericalModeWithLShift.rawValue)
+  public var togglingAlphanumericalModeWithLShift: Bool {
+    didSet {
+      SessionCtl.theShiftKeyDetector.toggleWithLShift = togglingAlphanumericalModeWithLShift
+    }
+  }
+
+  @AppStorage(wrappedValue: true, UserDef.kTogglingAlphanumericalModeWithRShift.rawValue)
+  public var togglingAlphanumericalModeWithRShift: Bool {
+    didSet {
+      SessionCtl.theShiftKeyDetector.toggleWithRShift = togglingAlphanumericalModeWithRShift
+    }
+  }
+
+  @AppStorage(wrappedValue: true, UserDef.kConsolidateContextOnCandidateSelection.rawValue)
+  public var consolidateContextOnCandidateSelection: Bool
+
+  @AppStorage(wrappedValue: false, UserDef.kHardenVerticalPunctuations.rawValue)
+  public var hardenVerticalPunctuations: Bool
+
+  @AppStorage(wrappedValue: true, UserDef.kTrimUnfinishedReadingsOnCommit.rawValue)
+  public var trimUnfinishedReadingsOnCommit: Bool
+
+  @AppStorage(wrappedValue: false, UserDef.kAlwaysShowTooltipTextsHorizontally.rawValue)
+  public var alwaysShowTooltipTextsHorizontally: Bool
+
+  public var clientsIMKTextInputIncapable: [String: Bool] {
+    get { PrefMgr.shared.clientsIMKTextInputIncapable }
+    set { PrefMgr.shared.clientsIMKTextInputIncapable = newValue }
+  }
+
+  @AppStorage(wrappedValue: true, UserDef.kOnlyLoadFactoryLangModelsIfNeeded.rawValue)
+  public var onlyLoadFactoryLangModelsIfNeeded: Bool {
+    didSet {
+      if !onlyLoadFactoryLangModelsIfNeeded { LMMgr.loadDataModelsOnAppDelegate() }
+    }
+  }
+
+  @AppStorage(wrappedValue: true, UserDef.kShowTranslatedStrokesInCompositionBuffer.rawValue)
+  public var showTranslatedStrokesInCompositionBuffer: Bool
+
+  @AppStorage(wrappedValue: 0, UserDef.kForceCassetteChineseConversion.rawValue)
+  public var forceCassetteChineseConversion: Int
+
+  @AppStorage(wrappedValue: true, UserDef.kShowReverseLookupInCandidateUI.rawValue)
+  public var showReverseLookupInCandidateUI: Bool
+
+  @AppStorage(wrappedValue: true, UserDef.kAutoCompositeWithLongestPossibleCassetteKey.rawValue)
+  public var autoCompositeWithLongestPossibleCassetteKey: Bool
+
+  @AppStorage(wrappedValue: false, UserDef.kShareAlphanumericalModeStatusAcrossClients.rawValue)
+  public var shareAlphanumericalModeStatusAcrossClients: Bool
+
+  @AppStorage(wrappedValue: true, UserDef.kPhraseEditorAutoReloadExternalModifications.rawValue)
+  public var phraseEditorAutoReloadExternalModifications: Bool
+
+  @AppStorage(wrappedValue: false, UserDef.kClassicHaninKeyboardSymbolModeShortcutEnabled.rawValue)
+  public var classicHaninKeyboardSymbolModeShortcutEnabled: Bool
+
+  // MARK: - Settings (Tier 2)
+
+  @AppStorage(wrappedValue: true, UserDef.kUseSpaceToCommitHighlightedSCPCCandidate.rawValue)
+  public var useSpaceToCommitHighlightedSCPCCandidate: Bool
+
+  @AppStorage(wrappedValue: false, UserDef.kEnableMouseScrollingForTDKCandidatesCocoa.rawValue)
+  public var enableMouseScrollingForTDKCandidatesCocoa: Bool
+
+  @AppStorage(wrappedValue: false, UserDef.kDisableSegmentedThickUnderlineInMarkingModeForManagedClients.rawValue)
+  public var disableSegmentedThickUnderlineInMarkingModeForManagedClients: Bool
+
+  // MARK: - Settings (Tier 3)
+
+  @AppStorage(wrappedValue: 10, UserDef.kMaxCandidateLength.rawValue)
+  public var maxCandidateLength: Int
+
+  @AppStorage(wrappedValue: true, UserDef.kShouldNotFartInLieuOfBeep.rawValue)
+  public var shouldNotFartInLieuOfBeep: Bool
+
+  @AppStorage(wrappedValue: false, UserDef.kShowHanyuPinyinInCompositionBuffer.rawValue)
+  public var showHanyuPinyinInCompositionBuffer: Bool
+
+  @AppStorage(wrappedValue: false, UserDef.kInlineDumpPinyinInLieuOfZhuyin.rawValue)
+  public var inlineDumpPinyinInLieuOfZhuyin: Bool
+
+  @AppStorage(wrappedValue: false, UserDef.kCNS11643Enabled.rawValue)
+  public var cns11643Enabled: Bool {
+    didSet {
+      LMMgr.setCNSEnabled(cns11643Enabled) // 很重要
+    }
+  }
+
+  @AppStorage(wrappedValue: true, UserDef.kSymbolInputEnabled.rawValue)
+  public var symbolInputEnabled: Bool {
+    didSet {
+      LMMgr.setSymbolEnabled(symbolInputEnabled) // 很重要
+    }
+  }
+
+  @AppStorage(wrappedValue: false, UserDef.kCassetteEnabled.rawValue)
+  public var cassetteEnabled: Bool {
+    didSet {
+      LMMgr.setCassetteEnabled(cassetteEnabled) // 很重要
+    }
+  }
+
+  @AppStorage(wrappedValue: false, UserDef.kChineseConversionEnabled.rawValue)
+  public var chineseConversionEnabled: Bool {
+    didSet {
+      // 康熙轉換與 JIS 轉換不能同時開啟，否則會出現某些奇奇怪怪的情況
+      if chineseConversionEnabled, shiftJISShinjitaiOutputEnabled {
+        shiftJISShinjitaiOutputEnabled.toggle()
+        UserDefaults.current.set(
+          shiftJISShinjitaiOutputEnabled, forKey: UserDef.kShiftJISShinjitaiOutputEnabled.rawValue
+        )
+      }
+      UserDefaults.current.set(
+        chineseConversionEnabled, forKey: UserDef.kChineseConversionEnabled.rawValue
+      )
+    }
+  }
+
+  @AppStorage(wrappedValue: false, UserDef.kShiftJISShinjitaiOutputEnabled.rawValue)
+  public var shiftJISShinjitaiOutputEnabled: Bool {
+    didSet {
+      // 康熙轉換與 JIS 轉換不能同時開啟，否則會出現某些奇奇怪怪的情況
+      if shiftJISShinjitaiOutputEnabled, chineseConversionEnabled {
+        chineseConversionEnabled.toggle()
+        UserDefaults.current.set(
+          chineseConversionEnabled, forKey: UserDef.kChineseConversionEnabled.rawValue
+        )
+      }
+      UserDefaults.current.set(
+        shiftJISShinjitaiOutputEnabled, forKey: UserDef.kShiftJISShinjitaiOutputEnabled.rawValue
+      )
+    }
+  }
+
+  @AppStorage(wrappedValue: false, UserDef.kCurrencyNumeralsEnabled.rawValue)
+  public var currencyNumeralsEnabled: Bool
+
+  @AppStorage(wrappedValue: false, UserDef.kHalfWidthPunctuationEnabled.rawValue)
+  public var halfWidthPunctuationEnabled: Bool
+
+  @AppStorage(wrappedValue: true, UserDef.kEscToCleanInputBuffer.rawValue)
+  public var escToCleanInputBuffer: Bool
+
+  @AppStorage(wrappedValue: true, UserDef.kAcceptLeadingIntonations.rawValue)
+  public var acceptLeadingIntonations: Bool
+
+  @AppStorage(wrappedValue: 0, UserDef.kSpecifyIntonationKeyBehavior.rawValue)
+  public var specifyIntonationKeyBehavior: Int
+
+  @AppStorage(wrappedValue: 0, UserDef.kSpecifyShiftBackSpaceKeyBehavior.rawValue)
+  public var specifyShiftBackSpaceKeyBehavior: Int
+
+  @AppStorage(wrappedValue: false, UserDef.kSpecifyShiftTabKeyBehavior.rawValue)
+  public var specifyShiftTabKeyBehavior: Bool
+
+  @AppStorage(wrappedValue: false, UserDef.kSpecifyShiftSpaceKeyBehavior.rawValue)
+  public var specifyShiftSpaceKeyBehavior: Bool
+
+  // MARK: - Optional settings
+
+  @AppStorage(wrappedValue: "", UserDef.kCandidateTextFontName.rawValue)
+  public var candidateTextFontName: String
+
+  @AppStorage(wrappedValue: kDefaultCandidateKeys, UserDef.kCandidateKeys.rawValue)
+  public var candidateKeys: String {
+    didSet {
+      let optimized = candidateKeys.lowercased().deduplicated
+      if candidateKeys != optimized { candidateKeys = optimized }
+      if CandidateKey.validate(keys: candidateKeys) != nil {
+        candidateKeys = Self.kDefaultCandidateKeys
+      }
+    }
+  }
+
+  @AppStorage(wrappedValue: false, UserDef.kUseSCPCTypingMode.rawValue)
+  public var useSCPCTypingMode: Bool {
+    willSet {
+      if newValue {
+        LMMgr.loadUserSCPCSequencesData()
+      }
+    }
+  }
+
+  @AppStorage(wrappedValue: false, UserDef.kPhraseReplacementEnabled.rawValue)
+  public var phraseReplacementEnabled: Bool {
+    willSet {
+      LMMgr.setPhraseReplacementEnabled(newValue)
+      if newValue {
+        LMMgr.loadUserPhraseReplacement()
+      }
+    }
+  }
+
+  @AppStorage(wrappedValue: false, UserDef.kAssociatedPhrasesEnabled.rawValue)
+  public var associatedPhrasesEnabled: Bool {
+    willSet {
+      if newValue {
+        LMMgr.loadUserAssociatesData()
+      }
+    }
+  }
+
+  // MARK: - Keyboard HotKey Enable / Disable
+
+  @AppStorage(wrappedValue: true, UserDef.kUsingHotKeySCPC.rawValue)
+  public var usingHotKeySCPC: Bool
+
+  @AppStorage(wrappedValue: true, UserDef.kUsingHotKeyAssociates.rawValue)
+  public var usingHotKeyAssociates: Bool
+
+  @AppStorage(wrappedValue: true, UserDef.kUsingHotKeyCNS.rawValue)
+  public var usingHotKeyCNS: Bool
+
+  @AppStorage(wrappedValue: true, UserDef.kUsingHotKeyKangXi.rawValue)
+  public var usingHotKeyKangXi: Bool
+
+  @AppStorage(wrappedValue: true, UserDef.kUsingHotKeyJIS.rawValue)
+  public var usingHotKeyJIS: Bool
+
+  @AppStorage(wrappedValue: true, UserDef.kUsingHotKeyHalfWidthASCII.rawValue)
+  public var usingHotKeyHalfWidthASCII: Bool
+
+  @AppStorage(wrappedValue: true, UserDef.kUsingHotKeyCurrencyNumerals.rawValue)
+  public var usingHotKeyCurrencyNumerals: Bool
+
+  @AppStorage(wrappedValue: true, UserDef.kUsingHotKeyCassette.rawValue)
+  public var usingHotKeyCassette: Bool
+
+  @AppStorage(wrappedValue: true, UserDef.kUsingHotKeyRevLookup.rawValue)
+  public var usingHotKeyRevLookup: Bool
+
+  @AppStorage(wrappedValue: true, UserDef.kUsingHotKeyInputMode.rawValue)
+  public var usingHotKeyInputMode: Bool
+}