How to trigger onAppear in SwiftUI for macOS

Issue #626

SwiftUI does not trigger onAppear and onDisappear like we expect. We can use NSView to trigger

import SwiftUI

struct AppearAware: NSViewRepresentable {
    var onAppear: () -> Void

    func makeNSView(context: NSViewRepresentableContext<AppearAware>) -> AwareView {
        let view = AwareView()
        view.onAppear = onAppear
        return view
    }

    func updateNSView(_ nsView: AwareView, context: NSViewRepresentableContext<AppearAware>) {

    }
}

final class AwareView: NSView {
    private var trigged: Bool = false
    var onAppear: () -> Void = {}

    override func viewDidMoveToSuperview() {
        super.viewDidMoveToSuperview()

        guard !trigged else { return }
        trigged = true
        onAppear()
    }
}

Then we can use it as an hidden view, like in a ZStack

ZStack {
    AppearAware(onAppear: {
        LocalImageCache.shared.load(url: url) { image in
            self.image = image
        }
    })
    Image(image)
}