🔰update gesture states

SwiftUI ⟩ Gestures ⟩ update gesture states

Gesture state is "transient" which means it will reset back to its initial state when user ends or cancels the gesture.

⭐️ 重點

• To update a view as a gesture changes, add a GestureState property to your view and update it with updating(_:body:).

• SwiftUI invokes the updating callback as soon as it recognizes the gesture and whenever the value of the gesture changes.

• SwiftUI doesn’t invoke the updating callback when the user ends or cancels a gesture. Instead, the gesture state property automatically resets its state back to its initial value.

🔸 定義

// ⭐️ updates @GestureState var as the gesture’s value changes.
//    `updating($state){ value, state, transaction in ... }`
func updating<State>(
    _ state: GestureState<State>, 
    body: @escaping (
        Self.Value,         // gesture's current value
        inout State,        // @GestureState var to be updated
        inout Transaction   // animation "context" (❓)
    ) -> Void
) -> GestureStateGesture<Self, State>

💾 程式

⬆️ 需要： TimerView

struct ContentView: View {
    
    // ⭐️ gesture state:
    //    reset to initial value when ends or cancels.
    @GestureState var isPressing = false
    
    var body: some View {
        
        // long press (for 3 sec)
        let press = LongPressGesture(minimumDuration: 3)
            // ⭐️ update gesture state
            .updating($isPressing) { value, state, _ in
                state = value
            }
        
        return Circle()
            .fill(isPressing ? .pink : .green)
            .frame(width: 100, height: 100, alignment: .center)
            .gesture(press)    // ⭐️ apply gesture
            .overlay {
                if isPressing {
                    // 👔 TimeView: count-down timer
                    TimerView(seconds: 3) { sec in
                        Text("\(sec)").font(.system(size: 50)).bold()
                    }   
                }
            }
    }
}

📗 參考

• SwiftUI ⟩ Gestures ⟩ Adding Interactivity with Gestures ⭐️

❓

問：「 .updating() 的 body closure 的第三個參數 transaction 是幹什麼的❓」

問：「 .updating() 似乎只能更新 GestureState，不能更新 ＠State 變數❓」

答：「 更新 view states 要用 .onChanged() 」

👉 Adding Interactivity with Gestures ⭐️