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swift - getting unresolved identifier 'self' in SwiftUI code trying to use Timer.scheduledTimer

问题描述

In SwiftUI I'm noting to use a Timer that:

Try 1 - This doesn't work as get "Use of unresolved identifier 'self'"

var timer2: Timer = Timer.scheduledTimer(withTimeInterval: 1, repeats: true) {
    self.angle = self.angle + .degrees(1)
}

Try 2 - Works, but have to put in an "_ = self.timer" to start it later

var timer: Timer {
    Timer.scheduledTimer(withTimeInterval: 0.1, repeats: true) {_ in
        self.angle = self.angle + .degrees(1)
    }
}
// then after need to use " .onAppear(perform: {_ = self.timer}) "

Is there a way to get my Try1 working? That is where in a SwiftUI file I can create the timer up front? Or actually where in SwiftUI would one normally start and stop the timer? i.e. where are lifecycle methods

Whole file:

import SwiftUI

struct ContentView : View {

    @State var angle: Angle = .degrees(55)

//    var timer2: Timer = Timer.scheduledTimer(withTimeInterval: 1, repeats: true) {
//        self.angle = self.angle + .degrees(1)
//    }

    var timer: Timer {
        Timer.scheduledTimer(withTimeInterval: 0.1, repeats: true) {_ in
            self.angle = self.angle + .degrees(1)
        }
    }

    private func buttonAction() {
        print("test")
        self.angle = self.angle + .degrees(5)
    }

    var body: some View {
        VStack{
            Text("Start")
            ZStack {
                Circle()
                    .fill(Color.blue)
                    .frame(
                        width: .init(integerLiteral: 100),
                        height: .init(integerLiteral: 100)
                    )
                Rectangle()
                    .fill(Color.green)
                    .frame(width: 20, height: 100)
                    // .rotationEffect(Angle(degrees: 25.0))
                    .rotationEffect(self.angle)
            }
            Button(action: self.buttonAction) {
                Text("CLICK HERE")
            }
            Text("End")
        }
         .onAppear(perform: {_ = self.timer})

    }
}

标签： swiftswiftuiswift5

解决方案

我不清楚您的示例是否需要一个计时器，但由于存在大量关于如何在 SwiftUI 应用程序中包含计时器的错误信息，我将进行演示。

关键是将计时器放在其他地方并在每次触发时发布。我们可以通过添加一个将计时器作为可绑定对象的类添加到我们的环境中来轻松做到这一点（请注意，您需要导入 Combine）：

class TimerHolder : BindableObject {
    var timer : Timer!
    let didChange = PassthroughSubject<TimerHolder,Never>()
    var count = 0 {
        didSet {
            self.didChange.send(self)
        }
    }
    func start() {
        self.timer?.invalidate()
        self.count = 0
        self.timer = Timer.scheduledTimer(withTimeInterval: 1, repeats: true) { 
            [weak self] _ in
            guard let self = self else { return }
            self.count += 1
        }
    }
}

我们需要通过环境向下传递此类的一个实例，因此我们修改场景委托：

window.rootViewController = 
    UIHostingController(rootView: ContentView())

变成

window.rootViewController = 
    UIHostingController(rootView: ContentView()
        .environmentObject(TimerHolder()))

最后，让我们建立一些 UI 来启动计时器并显示计数，以证明它正在工作：

struct ContentView : View {
    @EnvironmentObject var timerHolder : TimerHolder
    var body: some View {
        VStack {
            Button("Start Timer") { self.timerHolder.start() }
            Text(String(self.timerHolder.count))
        }
    }
}

为那些没有关注剧情的人编辑更新：BindableObject 已迁移到 ObservableObject，不再需要手动信号。所以：

class TimerHolder : ObservableObject {
    var timer : Timer!
    @Published var count = 0
    // ... and the rest is as before ...

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