你如何解决“不能在不可变值上使用变异成员：'self'是不可变的”？

Question

如何解决错误Cannot use mutating member on immutable value: 'self' is immutable 在self.prepareConfetti(screenWidth: CGFloat(geo.size.width)) 行上？

关于这个主题有很多 StackOverflow 问题，但没有一个对我有帮助。

我的代码：

struct ContentView: View {
    @State private var confettiTimer = Timer.publish(every: 0.0166, on: .main, in: .common).autoconnect()
    private var confettiX = [CGFloat]()
    private var confettiY = [CGFloat]()
    private var confettiWidth = [CGFloat]()
    private var confettiHeight = [CGFloat]()
    private let numOfConfetti: Int = 80
 
    var body: some View {
        GeometryReader { geo in
            ForEach(0..<self.confettiY.count, id: \.self) { i in
                Path { path in
                    path.addRect(
                        CGRect(
                            x: self.confettiX[i],
                            y: self.confettiY[i],
                            width: self.confettiWidth[i],
                            height: self.confettiHeight[i]
                        )
                    )
                }
                .fill(Color.black)
                .onAppear {
                    self.prepareConfetti(screenWidth: CGFloat(geo.size.width)) //This gives the error
                }
                .onReceive(self.confettiTimer) { time in
                    
                }
            }
        }
    }
    
    mutating func prepareConfetti(screenWidth: CGFloat) {
        for _ in 0..<self.numOfConfetti {
            let x: CGFloat = 0.0
            let y = screenWidth * 0.5
                
            self.confettiX.append(x)
            self.confettiY.append(y)
            self.confettiWidth.append(20.0)
            self.confettiHeight.append(20.0)
        }
    }
    
}

Answer 1

你不能以这种方式改变 SwiftUI 视图，你应该使用这些属性作为状态

struct ContentView: View {
    @State private var confettiTimer = Timer.publish(every: 0.0166, on: .main, in: .common).autoconnect()
    
    @State private var confettiX = [CGFloat]()
    @State private var confettiY = [CGFloat]()
    @State private var confettiWidth = [CGFloat]()
    @State private var confettiHeight = [CGFloat]()
    private let numOfConfetti: Int = 80
 
    var body: some View {
        GeometryReader { geo in
            ForEach(0..<self.confettiY.count, id: \.self) { i in
                Path { path in
                    path.addRect(
                        CGRect(
                            x: self.confettiX[i],
                            y: self.confettiY[i],
                            width: self.confettiWidth[i],
                            height: self.confettiHeight[i]
                        )
                    )
                }
                .fill(Color.black)
                .onAppear {
                    self.prepareConfetti(screenWidth: CGFloat(geo.size.width)) //This gives the error
                }
                .onReceive(self.confettiTimer) { time in
                    
                }
            }
        }
    }
    
    private func prepareConfetti(screenWidth: CGFloat) {
        for _ in 0..<self.numOfConfetti {
            let x: CGFloat = 0.0
            let y = screenWidth * 0.5
                
            self.confettiX.append(x)
            self.confettiY.append(y)
            self.confettiWidth.append(20.0)
            self.confettiHeight.append(20.0)
        }
    }
}