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

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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

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