如何在ObservableObject上触发onChange？为何当前代码未生效？

假设我们有一个实现了Equatable协议的ObservableObject视图模型，包含多个@Published属性：

class ViewModel: ObservableObject, Equatable {
    static func == (lhs: ViewModel, rhs: ViewModel) -> Bool {
        lhs.value == rhs.value &&
        lhs.anotherProperty1 == rhs.anotherProperty1 &&
        lhs.anotherProperty2 == rhs.anotherProperty2 &&
        lhs.anotherProperty3 == rhs.anotherProperty3
    }
    
    @Published var value = 0
    @Published var anotherProperty1 = "foo"
    @Published var anotherProperty2 = "bar"
    @Published var anotherProperty3 = "baz"
}

我们期望ViewModel的任意属性修改时都能触发onChange回调，但在如下DemoView中，点击按钮修改value属性时，绑定viewModel的onChange并未触发：

struct DemoView: View {
    @StateObject var viewModel = ViewModel()
    
    var body: some View {
        Button {
            viewModel.value += 1
        } label: {
            Text("Increment value \(viewModel.value.formatted())")
        }
        .onChange(of: viewModel) { _ in debugPrint("viewModel did change") } // 未触发回调
    }
}

原因分析 #

1. onChange(of: viewModel) 监听的是视图模型实例的引用变化，而非实例内部属性的变化。修改viewModel.value时，viewModel本身的内存地址（引用）没有改变，SwiftUI就认为它没有“变化”，不会触发回调。
2. 虽然你实现了Equatable协议定义了内容相等的规则，但SwiftUI对引用类型的onChange判断，默认是比较引用地址，不会调用==方法来判断内容是否变化。

解决方法 #

方法一：合并所有属性的变化信号 #

在ViewModel中创建一个合并所有@Published属性变化的Publisher，然后在视图中用onReceive监听：

修改ViewModel：

class ViewModel: ObservableObject, Equatable {
    static func == (lhs: ViewModel, rhs: ViewModel) -> Bool {
        lhs.value == rhs.value &&
        lhs.anotherProperty1 == rhs.anotherProperty1 &&
        lhs.anotherProperty2 == rhs.anotherProperty2 &&
        lhs.anotherProperty3 == rhs.anotherProperty3
    }
    
    @Published var value = 0
    @Published var anotherProperty1 = "foo"
    @Published var anotherProperty2 = "bar"
    @Published var anotherProperty3 = "baz"
    
    // 合并所有属性的变化信号，任意属性修改都会发送通知
    var anyPropertyChanged: AnyPublisher<Void, Never> {
        Publishers.Merge4(
            $value.map { _ in },
            $anotherProperty1.map { _ in },
            $anotherProperty2.map { _ in },
            $anotherProperty3.map { _ in }
        )
        .eraseToAnyPublisher()
    }
}

修改DemoView：

struct DemoView: View {
    @StateObject var viewModel = ViewModel()
    
    var body: some View {
        Button {
            viewModel.value += 1
        } label: {
            Text("Increment value \(viewModel.value.formatted())")
        }
        .onReceive(viewModel.anyPropertyChanged) {
            debugPrint("viewModel did change")
        }
    }
}

方法二：监听所有属性的组合值 #

直接在onChange中监听ViewModel所有属性的组合元组，只要任意属性变化，组合值就会更新，从而触发回调：

struct DemoView: View {
    @StateObject var viewModel = ViewModel()
    
    var body: some View {
        Button {
            viewModel.value += 1
        } label: {
            Text("Increment value \(viewModel.value.formatted())")
        }
        .onChange(of: (viewModel.value, viewModel.anotherProperty1, viewModel.anotherProperty2, viewModel.anotherProperty3)) { _ in
            debugPrint("viewModel did change")
        }
    }
}

这种方法无需修改ViewModel，但属性较多时代码会比较繁琐。

方法三：手动触发对象变化通知 #

利用ObservableObject的objectWillChange发送器，给每个@Published属性添加didSet钩子，手动发送对象变化通知：

修改ViewModel：

class ViewModel: ObservableObject, Equatable {
    static func == (lhs: ViewModel, rhs: ViewModel) -> Bool {
        lhs.value == rhs.value &&
        lhs.anotherProperty1 == rhs.anotherProperty1 &&
        lhs.anotherProperty2 == rhs.anotherProperty2 &&
        lhs.anotherProperty3 == rhs.anotherProperty3
    }
    
    @Published var value = 0 {
        didSet { objectWillChange.send() }
    }
    @Published var anotherProperty1 = "foo" {
        didSet { objectWillChange.send() }
    }
    @Published var anotherProperty2 = "bar" {
        didSet { objectWillChange.send() }
    }
    @Published var anotherProperty3 = "baz" {
        didSet { objectWillChange.send() }
    }
}

这样修改任何属性时，都会额外发送对象变化通知，SwiftUI会认为整个ViewModel发生了变化，从而触发onChange(of: viewModel)的回调。

内容的提问来源于stack exchange，提问作者parapote