onReceive String.publisher 导致无限循环

Question

我在 View 中使用了两个发布者：

A：String.publisher

B：ObservableObject 包含一个@Published String 类型

如果我监控发布者 A，我会得到一个无限循环。 但是监控发布者B是可以的！

    import SwiftUI
    import Combine

    class Model: ObservableObject{
        @Published var someBool = false
        @Published var name:String = ""
    }

    struct ContentView: View {
        // Publisher A
        @State var name = ""
        // Publisher B
        @ObservedObject var model = Model()

        var body: some View {
            VStack {
                // Plan A: lead to infinite loop!!!
                TextField("Input Name", text: $name)
                // Plan B: It's OK
                //TextField("Input Name", text: $model.name)

                .onReceive(name.publisher.reduce("", {t,c in
                    t + String(c)
                })) {text in
                    print("change to \(text)")
                    self.model.someBool.toggle()    //Plan A: infinite loop!!!
                }
                /*
                .onReceive(model.$name){name in
                    print("change to \(name)")
                    self.model.someBool.toggle()    //Plan B: It's OK!!!
                }
                */
            }
        }
    }

虽然我在onReceive()中改变了model.someBool的值，但是Plan B没问题，Plan A导致死循环。 这是为什么？？？谢谢:)

Answer 1

希望，你需要一个真实的来源。如果您不喜欢使用您的模型，具有 State / Binding 对的等效代码可能如下所示

struct ContentView: View {
    @State var name: String = ""
    @State var flag = false
    var body: some View {
        let subject = CurrentValueSubject<String, Never>(name)
        return VStack {
            TextField("Input Name", text: $name).textFieldStyle(RoundedBorderTextFieldStyle()).padding()
            .onReceive(subject) { name in
                print("change to \(name)")
                self.flag.toggle() // toggle every char typing
            }
        }
    }
}

在您的示例中，我禁用（参见注释行）模型中的默认“请求”

import SwiftUI
import Combine

class Model: ObservableObject{
    var someBool = false {
        willSet {
            print("will change to", newValue)
            print("ask SwiftUI to update from model")
            //self.objectWillChange.send()
        }
        didSet {
            print(oldValue, "changed")
        }
    }
}

struct ContentView: View {
    @State var name = ""
    @StateObject var model = Model()

    var body: some View {
        VStack {
            TextField("Input Name", text: $name).textFieldStyle(RoundedBorderTextFieldStyle())

            .onReceive(name.publisher.reduce("", {t,c in
                t + String(c)
            })) {text in
                print("change to \(text)")
                self.model.someBool.toggle()
            }
        }
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

打字时会打印出来

true changed
change to Qw
will change to true
ask SwiftUI to update from model
false changed
change to Qwe
will change to false
ask SwiftUI to update from model
true changed
change to Qwer
will change to true
ask SwiftUI to update from model
false changed
change to Qwert
will change to false
ask SwiftUI to update from model
true changed
...

现在取消注释模型中的行

class Model: ObservableObject{
    var someBool = false {
        willSet {
            print("will change to", newValue)
            print("ask SwiftUI to update from model")
            self.objectWillChange.send()
        }
        didSet {
            print(oldValue, "changed")
        }
    }
}

然后再次运行它... 它将无限循环打印

...
change to 
will change to true
ask SwiftUI to update from model
false changed
change to 
will change to false
ask SwiftUI to update from model
true changed
change to 
will change to true
ask SwiftUI to update from model
false changed
change to 
will change to false
ask SwiftUI to update from model
true changed
change to 
...

您的模型发生了变化，SwiftUI 正在重新评估它的主体，因此模型再次发生变化......在一个循环中。

最小循环示例

import SwiftUI
import Combine

class Model: ObservableObject {
    @Published var flag = false
}
struct ContentView: View {
    @StateObject var model = Model()
    var body: some View {
            Color.yellow
                .onReceive(model.$flag) {_ in
                    print(".")
                self.model.flag.toggle()
            }
    }
}