SwiftUI how to change state on keyUp macOS event?

Question

I am trying to change a state value when keyUp is detected from window. However, it doesn't seem to work for me. Here is my code

/// Window
class Window: NSWindow {
  override func keyUp(with event: NSEvent) {
    (contentView as! NSHostingView<ContentView>).rootView.keyPressed(with: event)
  }
}

/// ContentView
struct ContentView {
  @State var index: Int = 0
  var body: some View { ... }

  func keyPressed(with event: NSEvent) {
    self.index = self.index + 1
  }
}

I used debugger to test it, apparently, the keyPressed is called successfully, but the index is not set correctly. Anyone knows why? Or what is the correct strategy to listen to keyboard in SwiftUI for macOS?

Check this [How to detect keyboard events in SwiftUI on macOS?](https://stackoverflow.com/a/61155272/12299030) — Asperi, Apr 17 '20 at 04:35
Hello, thanks for commenting. I actually went through this before, but it is not quite what I want. When I use his method, it blocks. What I am trying to achieve is more like this question here: https://stackoverflow.com/questions/59764338/swiftui-macos-scroll-a-list-with-arrow-keys-while-a-textfield-is-active — WatashiJ, Apr 17 '20 at 16:35

score 8 · Accepted Answer · edited Aug 23 '22 at 14:16

Here is a demo of possible approach. Tested with Xcode 11.4 / macOS 10.15.4

The idea is to join custom NSWindow, generating key events, with SwiftUI View via injected publisher to environment values. This gives possibility to listen/handle events at any level of view hierarchy.

Below is full module (AppDelegate.swift) code. Also see useful comments in code.

import Cocoa
import SwiftUI
import Combine

// Environment key to hold even publisher
struct WindowEventPublisherKey: EnvironmentKey {
    static let defaultValue: AnyPublisher<NSEvent, Never> = 
        Just(NSEvent()).eraseToAnyPublisher() // just default stub
}


// Environment value for keyPublisher access
extension EnvironmentValues {
    var keyPublisher: AnyPublisher<NSEvent, Never> {
        get { self[WindowEventPublisherKey.self] }
        set { self[WindowEventPublisherKey.self] = newValue }
    }
}

// Custom window holding publisher and sending events to it. In general 
// it can be any event, but for originated question we limit to keyUp events
class Window: NSWindow {
    private let publisher = PassthroughSubject<NSEvent, Never>() // private

    var keyEventPublisher: AnyPublisher<NSEvent, Never> { // public
        publisher.eraseToAnyPublisher()
    }

    override func keyUp(with event: NSEvent) {
        publisher.send(event)
    }
}

// Root demo view
struct DemoKeyPressedView: View {
    @Environment(\.keyPublisher) var keyPublisher // << access to publisher

    @State private var index: Int = 0
    var body: some View {
        Text("Demo \(index)")
            .onReceive(keyPublisher) { event in // << listen to events
                self.keyPressed(with: event)
            }
    }
    
    func keyPressed(with event: NSEvent) {
        self.index += 1
    }
}

@NSApplicationMain
class AppDelegate: NSObject, NSApplicationDelegate {

    var window: Window!


    func applicationDidFinishLaunching(_ aNotification: Notification) {

        // Create the custom window
        window = Window(
            contentRect: NSRect(x: 0, y: 0, width: 480, height: 300),
            styleMask: [.titled, .closable, .miniaturizable, .resizable, .fullSizeContentView],
            backing: .buffered, defer: false)
        window.center()
        window.setFrameAutosaveName("Main Window")

        // Create the SwiftUI view that provides the window contents.
        let contentView = DemoKeyPressedView()
            .frame(minWidth: 400, maxWidth: .infinity, maxHeight: .infinity)
            .environment(\.keyPublisher, window.keyEventPublisher) // inject publisher

        window.contentView = NSHostingView(rootView: contentView)
        window.makeKeyAndOrderFront(nil)
    }

    func applicationWillTerminate(_ aNotification: Notification) {
        // Insert code here to tear down your application
    }

    func applicationShouldTerminateAfterLastWindowClosed(_ sender: NSApplication) -> Bool {
        return true
    }
}

SwiftUI how to change state on keyUp macOS event?

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