重置 SwiftUI 中的绑定变量

Question

When if we pass in a binding variable (like $saveDialog as true) to alert or sheet, the variable will reset to false after that.

.alert(isPresented: $saveDialog) {}

But what if we want to do the same thing: says clear the screen when we tap on the button, by setting the $clear to true:

 Button("Clear") { self.clear = true }

Then

 DrawView(clear:$clear)

And in DrawView

struct DrawView: UIViewRepresentable {
    @Binding var clear:Bool

    func updateUIView(_ canvas: PKCanvasView, context: Context) {
        if clear {
            canvas.drawing = PKDrawing()
            self.clear = false // Issue
        }
    }

The issue is: Modifying state during view update, this will cause undefined behavior. Where do I set the clear variable to false?

Answer 1

Here is possible solution

func updateUIView(_ canvas: PKCanvasView, context: Context) {
    if clear {
        canvas.drawing = PKDrawing()

        // make on next event loop, so do not affect current update
        // which already uses `clear` state, thus avoid cycling
        DispatchQueue.main.async {
            self.clear = false
        }
    }
}