SwiftUI view with window resizing.

ContentView.swift

//
//  ContentView.swift
//  window-demo
//
//  Created by Asia Fu on 2024/10/20.
//

import SwiftUI

struct WindowAccessor: NSViewRepresentable {
    var onWindowAvailable: (NSWindow) -> Void

    func makeNSView(context: Context) -> NSView {
        let view = NSView()
        DispatchQueue.main.async {
            guard let window = view.window else { return }
            onWindowAvailable(window)
        }
        return view
    }

    func updateNSView(_ nsView: NSView, context: Context) {}
}

struct ContentView: View {
    var titleHeight: CGFloat = 26
    
    @State private var texts: [String] = []
    @State private var textWidth: CGFloat = 240
    @State private var viewSize: NSSize = .zero
    @State private var window: NSWindow? = nil

    var body: some View {
        VStack {
            VStack {
                VStack {
                    ForEach(texts, id: \.self) { text in
                        HStack {
                            Text(text)
                        }
                        .padding(8)
                    }
                }
                .padding(8)
                .frame(width: textWidth)
                HStack {
                    Button(action: {
                        addText()
                    }) {
                        Text("Add")
                    }
                    
                    Button(action: {
                        clearTexts()
                    }) {
                        Text("Clear")
                    }
                    
                    Button(action: {
                        textWidth += (300 - textWidth) * 2
                    }) {
                        Text("Toggle width")
                    }
                }
                .padding(.vertical, 12)
            }
            .background(
                GeometryReader { geometry in
                    Color.clear
                        .onAppear {
                            viewSize = geometry.size
                            setInitialWindowFrame()
                        }
                        .onChange(of: geometry.size) {
                            viewSize = geometry.size
                        }
                }
            )
            WindowAccessor(onWindowAvailable: { self.window = $0 })
        }
        .frame(minHeight: 0, idealHeight: 100, alignment: .top)
        .background(.yellow.opacity(0.3))
        .onChange(of: viewSize.height, {
            guard let window = getWindow() else { return }
            let targetHeight = viewSize.height + titleHeight
            let step = 10
            let gap = (targetHeight - window.frame.size.height) / CGFloat(step)
            var i = 0
            var currentHeight = window.frame.size.height
            Timer.scheduledTimer(withTimeInterval: 0.016, repeats: true, block: { timer in
                var frameSize = window.frame.size
                if i >= 10 {
                    frameSize.height = targetHeight
                    timer.invalidate()
                } else {
                    i += 1
                    currentHeight += gap
                    frameSize.height = currentHeight
                }
                // 这里可能会有警告，但是其实 frameSize.height 正常
                window.setContentSize(frameSize)
            })
        })
        .onChange(of: viewSize.width) {
            guard let window = getWindow() else { return }
            var frameSize = window.frame.size
            frameSize.width = viewSize.width
            window.setContentSize(frameSize)
        }
    }
    
    private func setInitialWindowFrame() {
        guard let window = getWindow() else { return }
        var initialSize = viewSize
        initialSize.height += titleHeight
        window.setContentSize(initialSize)
        window.center()
    }
    
    // TODO: 这里 window 不一定对，如果有 menubar 的话
    private func getWindow() -> NSWindow? {
        if let window { return window }
        if NSApp.windows.count == 1 {
            return NSApp.windows.first
        }
        return NSApp.keyWindow ?? NSApp.windows.first
    }
    
    private func addText() {
        withAnimation(.easeOut, { texts.append("Row \(texts.count + 1)") })
    }
    
    private func clearTexts() {
        withAnimation(.easeOut, { texts.removeAll() })
    }
}

#Preview {
    ContentView()
}