ios - @Published 属性包装器不适用于 ObservableObject 的子类

Question

我有一个符合@ObservableObject 协议(protocol)的类，并使用它自己的变量从它创建了一个子类，并使用@Published 属性包装器来管理状态。

似乎在使用子类时忽略了@published 属性包装器。有谁知道这是否是预期的行为，是否有解决方法？

我正在运行 iOS 13 Beta 8 和 xCode Beta 6。

这是我所看到的示例。在 MyTestObject 上更新 TextField 时，Text View 会使用 aString 值正确更新。如果我更新 MyInheritedObjectTextField，则 TextView 中的 anotherString 值不会更新。

import SwiftUI

class MyTestObject: ObservableObject {
    @Published var aString: String = ""

}

class MyInheritedObject: MyTestObject {
    @Published var anotherString: String = ""
}

struct TestObserverWithSheet: View {
    @ObservedObject var myTestObject = MyInheritedObject()
    @ObservedObject var myInheritedObject = MyInheritedObject()

    var body: some View {
        NavigationView {
            VStack(alignment: .leading) {
                TextField("Update aString", text: self.$myTestObject.aString)
                Text("Value of aString is: \(self.myTestObject.aString)")

                TextField("Update anotherString", text: self.$myInheritedObject.anotherString)
                Text("Value of anotherString is: \(self.myInheritedObject.anotherString)")
            }
        }
    }
}

Answer 1

终于找到了这个问题的解决方案/解决方法。如果您从子类中删除属性包装器，并在变量上调用基类 objectWillChange.send()，则状态会正确更新。

注意:不要重新声明 let objectWillChange = PassthroughSubject<Void, Never>()在子类上，因为这将再次导致状态无法正确更新。

我希望这会在未来的版本中作为 objectWillChange.send() 得到修复。有很多样板需要维护。

这是一个完整的示例:

    import SwiftUI

    class MyTestObject: ObservableObject {
        @Published var aString: String = ""

    }

    class MyInheritedObject: MyTestObject {
        // Using @Published doesn't work on a subclass
        // @Published var anotherString: String = ""

        // If you add the following to the subclass updating the state also doesn't work properly
        // let objectWillChange = PassthroughSubject<Void, Never>()

        // But if you update the value you want to maintain state 
        // of using the objectWillChange.send() method provided by the 
        // baseclass the state gets updated properly... Jaayy!
        var anotherString: String = "" {
            willSet { self.objectWillChange.send() }
        }
    }

    struct MyTestView: View {
        @ObservedObject var myTestObject = MyTestObject()
        @ObservedObject var myInheritedObject = MyInheritedObject()

        var body: some View {
            NavigationView {
                VStack(alignment: .leading) {
                    TextField("Update aString", text: self.$myTestObject.aString)
                    Text("Value of aString is: \(self.myTestObject.aString)")

                    TextField("Update anotherString", text: self.$myInheritedObject.anotherString)
                    Text("Value of anotherString is: \(self.myInheritedObject.anotherString)")
                }
            }
        }
    }