Understanding @Binding in SwiftUI

Question

I watched some WWDC videos and Apple docs about data binding, and according to my current understanding, @State as a property delegate will provide a binding connection between the view and the annotated property, for example:

@State var myText: String

var body: some View {
  VStack {
    TextField($myText, placeholder: Text("input"))
    Text(myText)
  }
}

This will bind myText with the content of the text field I added (i.e. one changes the other will follows up to update)

However, though I know $myText refers to the binding type of Binding, I noticed that Binding is also a property delegate, and I noticed it appears in some code examples from Apple. I have no idea what this is used for as a property delegate. @State already does the binding work, then what do we need @Binding for? Apple docs suck for now about this.

Answer 1

According to this WWDC Talk (Data Flow through Swift UI):

https://developer.apple.com/wwdc19/226

@State should be used for local/private changes inside a View. Ideally, they would be private.

@Binding should be used in subviews/reusable components when the value lives outside the current view domain.

You can see it in presentation(:_) APIs.

There are probably bunch of states inside them, that tell SwiftUI how to display them - but the decision of whether it should appear or not it's up to the superview, hence the @Binding (isShowing) you need to provide.