如何在 CADisplayLink 動畫期間更新視圖

Question

我正在嘗試使用CADisplayLink實現一個簡單的動畫。 黑色方塊應該在屏幕上緩慢移動。

但是正方形看起來是靜止的，因為當step()增加 testX 和 testY 時Squares的body沒有得到更新。

據我所知（我是 Swift 新手），我正確使用了@ObservedObject范例，所以我不明白為什么視圖不會刷新。

如何讓Squares的body與動畫一起更新？

我的代碼如下，感謝您的幫助！

class MyAnimations: NSObject, ObservableObject {
    @Published var testX: Double = 0
    @Published var testY: Double = 0
    
    func createDisplayLink() {
        let displaylink = CADisplayLink(target: self, selector: #selector(step))
        
        displaylink.add(to: .current, forMode: RunLoop.Mode.default)
    }
         
    @objc func step(link: CADisplayLink) {
        testX += 0.5
        testY += 0.5
    }
    
}



struct Squares: View {
    @ObservedObject var animation = MyAnimations()
    
    var body: some View {
        Path { path in
            path.addRect(CGRect(x: self.animation.testX, y: self.animation.testY, width: 50, height: 50))
        }
            .fill(Color.black)
    }
}


struct SomeOtherView: View {
    var body: some View {
        Squares()
    }
}

Answer 1

這是因為你displaylink是在棧創建的，就這么被毀了從函數退出之后。 你需要讓它成為財產。

這是修改后的測試代碼。 使用 Xcode 12 / iOS 14 測試。

class MyAnimations: NSObject, ObservableObject {
    @Published var testX: Double = 0
    @Published var testY: Double = 0

    private var displaylink: CADisplayLink!       // << here !!
    func createDisplayLink() {
        if nil == displaylink {
            displaylink = CADisplayLink(target: self, selector: #selector(step))
            displaylink.add(to: .current, forMode: RunLoop.Mode.default)
        }
    }

    @objc func step(link: CADisplayLink) {
        testX += 0.5
        testY += 0.5
    }

}



struct Squares: View {
    @ObservedObject var animation = MyAnimations()

    var body: some View {
        Path { path in
            path.addRect(CGRect(x: self.animation.testX, y: self.animation.testY, width: 50, height: 50))
        }
        .fill(Color.black)
        .onAppear {
            animation.createDisplayLink()
        }
    }
}

Answer 2

從我看來，您只需要一個ObservableObject的共享實例

class MyAnimations: NSObject, ObservableObject {
    @Published var testX: Double = 0
    @Published var testY: Double = 0
    
    static let sharedInstance: MyAnimations = MyAnimations()//Singleton Pattern
    func createDisplayLink() {
        let displaylink = CADisplayLink(target: self, selector: #selector(step))
        
        displaylink.add(to: .current, forMode: RunLoop.Mode.default)
    }
    
    @objc func step(link: CADisplayLink) {
        testX += 0.5
        testY += 0.5
    }
    
}
struct Squares: View {
    //@ObservedObject var animation = MyAnimations.sharedInstance//Singleton Pattern
    @EnvironmentObject var animation: MyAnimations
    var body: some View {
        VStack{
            Button("Move", action: {
                animation.createDisplayLink()
            })
            Path { path in
                path.addRect(CGRect(x: self.animation.testX, y: self.animation.testY, width: 50, height: 50))
            }
            .fill(Color.black)
        }
    }
}


struct SomeOtherView: View {
    @ObservedObject var animation = MyAnimations()
    //@ObservedObject var animation = MyAnimations.sharedInstance//Singleton Pattern
    var body: some View {
        VStack{
            Button("Trigger to Create", action: {
                animation.createDisplayLink()
            })//Sample Trigger
            Squares()
                .environmentObject(animation)//Omit if using Singleton
        }
    }
}