SwiftUI GeometryReader未能更新其父项'；s州

昨天我看到了这篇文章：，我对

GeometryReader

的行为很好奇，所以我进行了一些实验，下面是我读完这篇文章后所做的：

import SwiftUI
import PlaygroundSupport

/* ------- Approach 1 ------- */

struct ViewGettingSize: View {
    
    @Binding var size: CGSize
    
    func makeView(with geometry: GeometryProxy) -> some View {
        
        // ⭐️ Try to update @Binding var `size`,
        //    but SwiftUI ignores this assignment, why?
        //    @Binding var `size` is NOT updated.
        self.size = geometry.size
        
        print(geometry.size)       // (158.5, 45.5)
        print(self.size)           // (50, 50)
        
        return Color.pink
    }
    
    var body: some View {
        GeometryReader { geo in
            self.makeView(with: geo)  // Color.pink
        }
    }
}

/* ------- Approach 2 ------- */

struct SizePreferenceKey: PreferenceKey {
    static var defaultValue: CGSize = .zero
    static func reduce(value: inout CGSize, nextValue: () -> CGSize) {
        value = nextValue()
    }
}

struct ViewSettingSizePreference: View {
    func makeView(with geometry: GeometryProxy) -> some View {
        print(geometry.size)       // (158.5, 45.5)
        return Color.orange
            .preference(key: SizePreferenceKey.self, value: geometry.size)
    }
    var body: some View {
        GeometryReader { geo in
            self.makeView(with: geo)   // Color.orange
        }
    }
}

/* ------- Test These Approaches ------- */

let text = Text("some text").font(.largeTitle)

// live view
struct ContentView: View {
    
    @State private var size = CGSize(50, 50)
    @State private var size2 = CGSize(50, 50)
    
    var body: some View {
        VStack {
            
            Group {
                
                /* ------- Approach 1 ------- */
                text
                    // ⭐️ this one doesn't work.
                    .background(ViewGettingSize(size: $size))
                Color.blue
                    // ⭐️ `size` is still (50,50)
                    .frame(width: self.size.width, height: self.size.height)
                
                /* ------- Approach 2 ------- */
                text
                    // ⭐️ this one works.
                    .background(ViewSettingSizePreference())
                    .onPreferenceChange(SizePreferenceKey.self) { (size) in
                        print(size)        // (158.5, 45.5)
                        self.size2 = size  // ⭐️ `size2` updated successfully.
                        print(self.size2)  // (158.5, 45.5)
                }
                Color.purple
                    .frame(width: self.size2.width, height: self.size2.height)
                
            }// Group
                .border(Color.black)
            
        }// VStack (container)
            .padding()
            .background(Color.gray)
    }
}

PlaygroundPage.current.setLiveView(ContentView())

结果:

从上面开始，我使用了两种方法，试图通过更新

@State

变量来更新

ContentView

，尽管第二种方法成功，但第一种方法失败了，有人知道它失败的原因吗？谢谢

因为它会产生渲染周期（更改状态会启动刷新，从而更改状态，等等），而SwiftUI渲染引擎足够聪明，可以删除此类更新

您需要为下一个事件循环延迟此类更新，如

func makeView(with geometry: GeometryProxy) -> some View {
    DispatchQueue.main.async {        
       self.size = geometry.size
    }
    
    print(geometry.size)       // (158.5, 45.5)
    print(self.size)           // (50, 50)
    
    return Color.pink
}

查看此方法的正确用法，例如在

中，但@Peter Warbo认为此方法有时会崩溃，不是吗？在我看来，这种方法对于SwiftUI来说非常“不自然”，这是一种黑客行为吗？

func makeView(with geometry: GeometryProxy) -> some View {
    DispatchQueue.main.async {        
       self.size = geometry.size
    }
    
    print(geometry.size)       // (158.5, 45.5)
    print(self.size)           // (50, 50)
    
    return Color.pink
}