diff --git a/Packages/vChewing_SwiftExtension/Sources/SwiftExtension/SwiftExtension.swift b/Packages/vChewing_SwiftExtension/Sources/SwiftExtension/SwiftExtension.swift
index f32b21a6..fabe4640 100644
--- a/Packages/vChewing_SwiftExtension/Sources/SwiftExtension/SwiftExtension.swift
+++ b/Packages/vChewing_SwiftExtension/Sources/SwiftExtension/SwiftExtension.swift
@@ -105,6 +105,8 @@ extension Bool {
 
 // MARK: - Property wrapper
 
+// Ref: https://www.avanderlee.com/swift/property-wrappers/
+
 @propertyWrapper
 public struct AppProperty<Value> {
   public let key: String