在SwiftUI中为TextField设置初始值--比较新旧值。

Question

我看到了很多关于如何使用空TextField来收集新的值的例子和教程，但是没有一个例子显示如何使用TextField来编辑一个值。

在我的使用案例中，我想让TextField从我的viewmodel中预先填充数据，然后当用户编辑数据时，应该启用一个保存按钮。在我的表单中，我也有一个导航链接，导致一个子页面，用户可以从列表中选择一些东西，然后被路由回表单。

只要我使用一个空字段，它的表现就像描述的那样；用户可以在字段中输入一些临时的东西，然后导航到子页面，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")
    }
}

Answer 1

要用模型中的值初始化文本字段，你需要定义你自己的初始化器并使用 State(wrappedValue:) 的初始化器 @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")
    }
}

Answer 2

怎么样像这样的测试代码。关键是使用 "ObservableObject"。

import SwiftUI


class MyModel: ObservableObject {
@Published var model = "model1"
}

struct ContentView: View {

@ObservedObject var myModel = MyModel()
@State var editedValue = ""

var body: some View {
    NavigationView {
        VStack(alignment: .leading, spacing: 20) {
            Group{
                Text("Some label")
                TextField("Placeholder text", text: Binding<String>(
                    get: { self.editedValue },
                    set: {
                        self.editedValue = $0
                        self.myModel.model = self.editedValue
                })).onAppear(perform: loadData)
            }
            Divider()

            NavigationLink(destination: Text("the nex page")) {
                Text("Click Me To Display The next View")
            }

            // 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
                self.myModel.model = self.editedValue
            },label: {
                Text("SAVE")
            })
        }
    }.navigationViewStyle(StackNavigationViewStyle())
}

func loadData() {
    self.editedValue = myModel.model
}

}