自定义鼠标上下事件View Modifier导致SwiftUI视图层级无法更新

问题根源在于你自定义的View Modifier如果是基于AppKit/UIKit底层视图实现鼠标事件监听，事件回调的闭包执行上下文没有和SwiftUI的视图更新机制绑定，导致状态变更无法触发视图body的重建。哪怕用@State或ObservableObject，只要回调不在SwiftUI的更新周期内，或者闭包捕获的是状态的旧副本，视图就不会更新。

方案一：用SwiftUI原生Gesture实现（推荐） #

直接利用SwiftUI的DragGesture（最小距离设为0）来监听鼠标按下和抬起，原生Gesture的回调会自动纳入SwiftUI的更新周期，状态变更能直接触发视图刷新：

struct MousePressModifier: ViewModifier {
    let onPress: () -> Void
    let onRelease: () -> Void
    
    func body(content: Content) -> some View {
        content
            .gesture(
                DragGesture(minimumDistance: 0)
                    .onChanged { _ in
                        onPress()
                    }
                    .onEnded { _ in
                        onRelease()
                    }
            )
    }
}

// 使用示例
struct PlayerView: View {
    @State private var isPlaying = false
    
    var body: some View {
        Circle()
            .frame(width: 60, height: 60)
            .foregroundColor(.blue)
            .overlay {
                Image(systemName: isPlaying ? "pause.fill" : "play.fill")
                    .foregroundColor(.white)
            }
            .modifier(MousePressModifier(
                onPress: { isPlaying.toggle() },
                onRelease: { /* 处理抬起逻辑 */ }
            ))
    }
}

方案二：修复底层视图实现的Modifier #

如果必须用AppKit/UIKit底层视图实现，需要通过Coordinator持有状态绑定，并确保在主线程更新状态：

struct MouseEventModifier: ViewModifier {
    let onMouseDown: () -> Void
    let onMouseUp: () -> Void
    
    func body(content: Content) -> some View {
        content
            .background(MouseEventCaptureView(onDown: onMouseDown, onUp: onMouseUp))
    }
}

fileprivate struct MouseEventCaptureView: NSViewRepresentable {
    let onDown: () -> Void
    let onUp: () -> Void
    
    func makeNSView(context: Context) -> NSView {
        let view = NSView()
        view.wantsLayer = true
        // 设置跟踪区域，监听鼠标按下/抬起
        let trackingArea = NSTrackingArea(
            rect: .zero,
            options: [.activeAlways, .mouseDownAndUp, .inVisibleRect],
            owner: context.coordinator,
            userInfo: nil
        )
        view.addTrackingArea(trackingArea)
        return view
    }
    
    func makeCoordinator() -> Coordinator {
        Coordinator(onDown: onDown, onUp: onUp)
    }
    
    func updateNSView(_ nsView: NSView, context: Context) {}
    
    class Coordinator: NSObject {
        private let onDown: () -> Void
        private let onUp: () -> Void
        
        init(onDown: @escaping () -> Void, onUp: @escaping () -> Void) {
            self.onDown = onDown
            self.onUp = onUp
        }
        
        override func mouseDown(with event: NSEvent) {
            // 确保在主线程执行状态更新
            DispatchQueue.main.async {
                self.onDown()
            }
        }
        
        override func mouseUp(with event: NSEvent) {
            DispatchQueue.main.async {
                self.onUp()
            }
        }
    }
}

// 使用示例
struct PlayerView: View {
    @State private var isPlaying = false
    
    var body: some View {
        Circle()
            .frame(width: 60, height: 60)
            .foregroundColor(.blue)
            .overlay {
                Image(systemName: isPlaying ? "pause.fill" : "play.fill")
                    .foregroundColor(.white)
            }
            .modifier(MouseEventModifier(
                onMouseDown: { isPlaying.toggle() },
                onMouseUp: { /* 处理抬起逻辑 */ }
            ))
    }
}

内容的提问来源于stack exchange，提问作者Marcel Tesch