为什么 didSet 在 SwiftUI 中的 TextField 绑定上调用了两次？

Question

I have a very basic view that only shows a TextField :

View

struct ContentView: View {

    @StateObject var viewModel = ViewModel()
    
    var body: some View {
        TextField("Enter a string...", text: $viewModel.string)
    }
    
}

The TextField 's text is bound to a string property on the view model:

ViewModel

class ViewModel: ObservableObject {
    
    @Published var string: String = "" {
        didSet {
            print("didSet string:", string)
        }
    }
    
}

I added a didSet property observer to perform a custom action whenever the string changes. For this simple example, I only print a string on the console.

Observation

When I run this code and enter the string "123" into the text field, this is the output I get:

didSet string: 1
didSet string: 1
didSet string: 12
didSet string: 12
didSet string: 123
didSet string: 123

Question:

Why?
Why is the didSet closure called twice for each character I type? (I would expect it to be called once for each character.)

Is there anything wrong with the code or is this expected behavior somehow?

Answer 1

I'm seeing this issue on Xcode 14.2 RC and iOS 16.2 RC, but weirdly what fixes it is adding a .textFieldStyle(.plain) or .textFieldStyle(.roundedBorder) .

I'm really not sure why having no textFieldStyle would affect this, but the binding calls set:{} twice when I have no textFieldStyle set, and as soon as I add one of those, it behaves normally and only calls set:{} once at a time.

I hope this helps someone!

Answer 2

 let binding = Binding<String>(get: {
                textvariable
            }, set: {
                viewModel.setText(query: $0) //add event inside setText
                // do whatever you want here
            })