diff --git a/Packages/vChewing_SwiftExtension/Sources/SwiftExtension/SwiftExtension.swift b/Packages/vChewing_SwiftExtension/Sources/SwiftExtension/SwiftExtension.swift
index f32b21a6..998e4224 100644
--- a/Packages/vChewing_SwiftExtension/Sources/SwiftExtension/SwiftExtension.swift
+++ b/Packages/vChewing_SwiftExtension/Sources/SwiftExtension/SwiftExtension.swift
@@ -104,6 +104,7 @@ extension Bool {
 }
 
 // MARK: - Property wrapper
+// Ref: https://www.avanderlee.com/swift/property-wrappers/
 
 @propertyWrapper
 public struct AppProperty<Value> {