在使用SwiftUI 开发 MacOS应用时，使用View的.OnHover来更新View样式时发现，当Window失去焦点，再次在该View处点击Window获得焦点，该View不会触发onHover。

struct DemoView: View {
  @State var isHovering: Bool = false
  var body: some View {
    ZStack(alignment: .topLeading) {
      VStack(alignment: .center, spacing: 8){
          ...
      }
      .frame(width:400)
      .onHover { isHovering in
         self.isHovering = isHovering
      }

      if isHovering {
          // 添加视图
      }
    }
  }
}

优化方案

加入鼠标监听视图层

import AppKit
import SwiftUI

public class MouseMonitorNSView : NSView {
    
    var trackingArea : NSTrackingArea?
    public var onHover: ((Bool) -> Void)?
    
    public override func draw(_ dirtyRect: NSRect) {
        super.draw(dirtyRect)
        self.wantsLayer = true
        self.layer?.backgroundColor = NSColor.clear.cgColor
        
    }
        
    override public func updateTrackingAreas() {
        if let ta = trackingArea {
            self.removeTrackingArea(ta)
        }
        
        if !self.isHidden {
            let opt = (
                NSTrackingArea.Options.mouseEnteredAndExited.rawValue
//                | NSTrackingArea.Options.mouseMoved.rawValue
                | NSTrackingArea.Options.activeAlways.rawValue
            )
            trackingArea = NSTrackingArea(rect: self.bounds, options: NSTrackingArea.Options(rawValue: opt), owner: self, userInfo: nil)
            self.addTrackingArea(trackingArea!)
        }
    }
    
    public override func mouseEntered(with event: NSEvent) {
//        print("mouseEntered")
        if let hoverBlock = onHover {
            hoverBlock(true)
        }
    }
    
    public override func mouseExited(with event: NSEvent) {
//        print("mouseExited")
        if let hoverBlock = onHover {
            hoverBlock(false)
        }
    }
    
//    public override func mouseMoved(with event: NSEvent) {
//        print("mouseMoved")
//    }
//
}

public struct MouseMonitorView : NSViewRepresentable {
    
    private let onHover: (Bool) -> Void
    
    public init(onHover: @escaping (Bool) -> Void){
        self.onHover = onHover
    }
    
    public func makeNSView(context: Context) -> MouseMonitorNSView {
        let view = MouseMonitorNSView()
        view.onHover = onHover
        return view
    }
    
    public func updateNSView(_ nsView: MouseMonitorNSView, context: Context) {
        nsView.onHover = onHover
    }
}

DemoView 改造后

struct DemoView: View {
  @State var isHovering: Bool = false
  var body: some View {
    ZStack(alignment: .topLeading) {
      MouseMonitorView { isHovering in
        self.isHovering = isHovering
      }//此视图在window失去焦点亦能监听到鼠标事件，保持与VStack位置大小一致
      VStack(alignment: .center, spacing: 8){
          ...
      }
      .frame(width:400)
//      .onHover { isHovering in
//         self.isHovering = isHovering
//      }

      if isHovering {
          // 添加视图
      }
    }
  }
}

此方案有个小小注意点：在window失去焦点时，视图亦能监听到鼠标进出事件

后记

是否有更优雅的方案，评论区留言