SwiftUI：如何知道 ObservableObject 的已发布变量的变化

Question

我有一个基于模型数据显示切换列表的视图。这个模型有一个@Published 变量，它会根据切换选择进行更改。

class Model: ObservableObject, Hashable {
      
    var id: String
    @Published var isSelected: Bool
    
    init(id: String, isSelected: Bool) {
        self.id = id
        self.isSelected = isSelected
    }
    ...
}

class ViewModel: ObservableObject {

    var list: [Model]
    
    init() { ...}
    ...
    
    func save() {
        
    }
    
    func clear() {
        
    }
}


struct MyView: View {

    @ObservedObject var viewModel: ViewModel

    var body: some View {
        VStack {
            ForEach(viewModel.list, id: \.self) { model in
                Toggle(model.id, isOn: $model.isSelected)
            }
            Button("Done") {
                viewModel.save()
            }
            Button("Clear") {
                viewModel.clear()
            }
            ...
        }
    }
}

问题是，因为我有一系列带有@Publsihed 的模型，

• 如何知道用户是否更改了任何切换，所以我可以启用/禁用保存按钮
• 如何知道列出所有已更改的切换（即模型 isSelected 已更改），例如当我按保存时

Answer 1

尝试类似这种方法，以更改模型列表 isSelected 属性。

struct Model: Identifiable, Hashable {
    var id: String
    var isSelected: Bool
    var hasChanged: Bool = false // <--- here
}

class ViewModel: ObservableObject {
    
    // for testing
    @Published var list: [Model] = [Model(id: "1", isSelected: false),
                                    Model(id: "2", isSelected: false),
                                    Model(id: "3", isSelected: true)]
    
     func save() {
         for mdl in list {
             if mdl.hasChanged {
                print("---> (mdl.id) has changed")  // <-- here
             }
         }
     }
    
    func clear() {
        list = []
        print("---> clear list: (list)")
    }
    
    func reset() {
        for i in list.indices {
            list[i].isSelected = false
        }
        print("---> reset list: (list)")
    }
    
}

struct ContentView: View {
    @StateObject var viewModel = ViewModel()
    
    var body: some View {
        MyView(viewModel: viewModel)
    }
}

struct MyView: View {
    @ObservedObject var viewModel: ViewModel
    @State var hasChanged = false
    
    var body: some View {
        VStack {
            ForEach($viewModel.list) { $model in
                Toggle(model.id, isOn: $model.isSelected)
                    .onChange(of: model.isSelected) { val in  // <-- here
                        model.hasChanged.toggle()
                    }
            }
            Button("Save") {
                viewModel.save()
            }
            
            Button("Clear") {
                viewModel.clear()
            }
            Button("Reset") {
                viewModel.reset()
            }
        }
    }
}

编辑-1：

如果要计算特定Model 更改的次数，则 只需为其添加一个计数器，例如在此示例代码中：

struct MyView: View {
    @ObservedObject var viewModel: ViewModel
    
    var body: some View {
        VStack {
            ForEach($viewModel.list) { $model in
                Toggle(model.id, isOn: $model.isSelected)
                    .onChange(of: model.isSelected) { val in  
                        model.hasChanged.toggle()
                        model.changeCount += 1    // <-- here
                    }
            }
            Button("Save") {
                viewModel.save()
            }
            
            Button("Clear") {
                viewModel.clear()
            }
            Button("Reset") {
                viewModel.reset()
            }
        }
    }
}

struct Model: Identifiable, Hashable {
    var id: String
    var isSelected: Bool
    var hasChanged: Bool = false 
    var changeCount: Int = 0  // <--- here
}

class ViewModel: ObservableObject {
    
    // for testing
    @Published var list: [Model] = [Model(id: "1", isSelected: false),
                                    Model(id: "2", isSelected: false),
                                    Model(id: "3", isSelected: true)]
    
    func save() {
        for var mdl in list {
            print("---> (mdl.id) has changed = (mdl.hasChanged)  count: (mdl.changeCount)")  // <-- here
        }
    }
    
    func clear() {
        list = []
        print("---> clear list: (list)")
    }
    
    func reset() {
        for i in list.indices {
            list[i].isSelected = false
        }
        print("---> reset list: (list)")
    }
    
}