如何让fullSizeContentView样式的NSWindow匹配SwiftUI视图固有尺寸？

我希望带有fullSizeContentView样式的NSWindow能够完全匹配具有固有内容尺寸的SwiftUI视图大小。之前看过一些方案，但那些都需要在顶层设置固定frame，我不想这么做，希望窗口大小完全贴合视图。

以下是可在Xcode 14.1中运行的Playground代码片段：

import AppKit
import SwiftUI

class MyWindow: NSWindow {
    override func setFrame(_ frameRect: NSRect, display flag: Bool) {
        print("\(Date().timeIntervalSince1970) setFrame called \(frameRect)")
        super.setFrame(frameRect, display: flag)
    }
}

let window = MyWindow()

window.styleMask = [
    .titled,
    .closable,
    .resizable,
    .fullSizeContentView
]

window.toolbar = nil

window.titlebarAppearsTransparent = true
window.titleVisibility = .hidden
window.isMovable = true
window.isMovableByWindowBackground = true
window.standardWindowButton(.closeButton)?.isHidden = false
window.standardWindowButton(.miniaturizeButton)?.isHidden = true
window.standardWindowButton(.zoomButton)?.isHidden = true

print("\(Date().timeIntervalSince1970) Before content \(window.frame)")
window.contentView = NSHostingView(rootView: ContentView())
print("\(Date().timeIntervalSince1970) After setting content \(window.frame)")

window.makeKeyAndOrderFront(nil)

print("\(Date().timeIntervalSince1970) After makeKeyAndOrderFront \(window.frame)")

DispatchQueue.main.asyncAfter(deadline: .now() + 1) {
    print("\(Date().timeIntervalSince1970) After 1 second \(window.frame)")
}

struct ContentView: View {
    var body: some View {
        Text("Hello")
            .font(.system(size: 200))
            .background(.blue)
            .fixedSize()
            .ignoresSafeArea()
    }
}

当前运行后窗口底部会留有多余空间，窗口截图如下：

打印的frame变更信息：

1674086812.362426 setFrame called (100.0, 100.0, 100.0, 100.0)
1674086812.363435 Before content (100.0, 100.0, 100.0, 100.0)
1674086812.373186 setFrame called (100.0, -63.0, 431.0, 263.0)
1674086812.3741732 After setting content (100.0, -63.0, 431.0, 263.0)
1674086812.374618 setFrame called (100.0, 85.0, 431.0, 263.0)
1674086812.375651 After makeKeyAndOrderFront (100.0, 85.0, 431.0, 263.0)
1674086812.4359 setFrame called (100.0, 57.0, 431.0, 291.0)
1674086813.41998 After 1 second (198.0, 99.0, 431.0, 291.0)

问题原因 #

• fullSizeContentView的布局逻辑：启用fullSizeContentView后，内容视图会延伸到标题栏下方，但系统仍会默认保留标题栏的高度计算，导致窗口整体高度被额外加上了标题栏的默认高度（28pt），这就是底部多余空间的来源。
• SwiftUI视图尺寸的延迟计算：SwiftUI视图的固有尺寸不是初始化时立即确定的，而是在渲染流程中异步计算完成。窗口显示后，视图尺寸最终确定，会触发NSWindow重新调整frame适配，这就是多次调用setFrame的原因。
• ignoresSafeArea的局限性：ignoresSafeArea()只影响SwiftUI内部布局，不会告知NSWindow忽略标题栏的额外空间计算，窗口仍按传统结构计算整体尺寸。

解决办法 #

方法1：手动调整窗口frame匹配视图尺寸 #

设置contentView后，手动获取视图固有尺寸并修正窗口frame：

// 设置完window.contentView后添加这段代码
if let hostingView = window.contentView as? NSHostingView<ContentView> {
    hostingView.sizeToFit()
    let contentSize = hostingView.frame.size
    // 减去标题栏默认高度28pt，抵消fullSizeContentView的额外高度
    let windowSize = NSSize(width: contentSize.width, height: contentSize.height - 28)
    window.setFrame(window.frame.withSize(windowSize), display: true)
}

方法2：通过NSWindowDelegate控制窗口尺寸 #

实现代理方法强制窗口尺寸匹配内容视图：

class WindowDelegate: NSObject, NSWindowDelegate {
    func windowWillResize(_ sender: NSWindow, to frameSize: NSSize) -> NSSize {
        guard let hostingView = sender.contentView as? NSHostingView<ContentView> else {
            return frameSize
        }
        hostingView.sizeToFit()
        let contentSize = hostingView.frame.size
        return NSSize(width: contentSize.width, height: contentSize.height - 28)
    }
}

// 创建window后设置代理
let delegate = WindowDelegate()
window.delegate = delegate

方法3：在完整App中使用SwiftUI原生WindowGroup #

如果是开发完整macOS App，直接用SwiftUI的WindowGroup可自动适配：

@main
struct MyApp: App {
    var body: some Scene {
        WindowGroup {
            ContentView()
                .fixedSize()
                .background(.blue)
        }
        .windowStyle(.hiddenTitleBar)
        .windowResizability(.contentSize)
    }
}

内容的提问来源于stack exchange，提问作者narduk