在 SwiftUI 中设置 TextField 的初始值 - 比较新旧值

我看过很多有关如何使用空 TextField 来收集新值的示例和教程，但没有一个展示如何使用 TextField 来编辑值。

在我的用例中，我希望用视图模型中的数据预填充/预填充 TextField，然后当用户编辑数据时，应启用“保存”按钮。在我的表单中，我还有一个导航链接，该链接指向一个子页面，用户可以在其中从列表中选择某些内容，然后路由回表单。

只要我使用空字段，它的行为就如所描述的那样；用户可以在字段中输入临时内容，导航到子页面，临时值仍然像他离开时一样。

struct TextFieldDemo: View {

    var model:String    // Actual a more complex view model
    @State var editedValue:String = ""

    var body: some View {
        VStack(alignment: .leading, spacing: 20) {
            Group{
                Text("Some label")
                TextField("Placeholder text", text: $editedValue)
            }
            Divider()
            Text("Some navigation link to push in a page where " +
                "the user can select something from a list and click back...")

            // If the user starts to edit the textfield - follows a navigation link and comes back
            // he should be able to continue edit the field where he left of - the text field should
            // not have been reset to the original value.

            Button(action: {
                // Call some save function in the ViewModel
                },label: {
                    Text("SAVE")
                }
            ).disabled(model == editedValue)
        }.onAppear(){
            // I could have done something like:
            //   self.editedValue = model
            // but it seems like this will fire if the user navigates into the described page and reset
            // the TextField to the model value.
        }
    }
}

struct TextFieldDemo_Previews: PreviewProvider {
    static var previews: some View {
        TextFieldDemo(model: "The old value")
    }
}

要使用模型中的值初始化文本字段，您需要定义自己的初始值设定项并使用State(wrappedValue:)初始化程序@State vars:

struct TextFieldDemo: View {

    var model:String    // Actual a more complex view model
    @State var editedValue: String

    init(model: String) {
        self.model = model
        self._editedValue = State(wrappedValue: model) // _editedValue is State<String>
    }

    var body: some View {
        VStack(alignment: .leading, spacing: 20) {
            Group{
                Text("Some label")
                TextField("Placeholder text", text: $editedValue)
            }
            Divider()
            Text("Some navigation link to push in a page where " +
                "the user can select something from a list and click back...")

            // If the user starts to edit the textfield - follows a navigation link and comes back
            // he should be able to continue edit the field where he left of - the text field should
            // not have been reset to the original value.

            Button(action: {
                // Call some save function in the ViewModel
                },label: {
                    Text("SAVE")
                }
            ).disabled(model == editedValue)
        }.onAppear(){
            // I could have done something like:
            //   self.editedValue = model
            // but it seems like this will fire if the user navigates into the described page and reset
            // the TextField to the model value.
        }
    }
}

struct TextFieldDemo_Previews: PreviewProvider {
    static var previews: some View {
        TextFieldDemo(model: "The old value")
    }
}