protocols - 如何将协议(protocol)定义为 @ObservedObject 属性的类型？

Question

我有一个依赖于 View 模型的 swiftui View ， View 模型有一些已发布的属性。我想为 View 模型层次结构定义一个协议(protocol)和默认实现，并使 View 依赖于协议(protocol)而不是具体类？

我希望能够编写以下内容:

protocol ItemViewModel: ObservableObject {
    @Published var title: String

    func save()
    func delete()
}

extension ItemViewModel {
    @Published var title = "Some default Title"

    func save() {
        // some default behaviour
    }

    func delete() {
        // some default behaviour
    }
}


struct ItemView: View {
    @ObservedObject var viewModel: ItemViewModel

    var body: some View {
        TextField($viewModel.title, text: "Item Title")
        Button("Save") { self.viewModel.save() }  
    }
}

// What I have now is this:

class AbstractItemViewModel: ObservableObject {
    @Published var title = "Some default Title"

    func save() {
        // some default behaviour
    }

    func delete() {
        // some default behaviour
    }
}

class TestItemViewModel: AbstractItemViewModel {
    func delete() {
        // some custom behaviour
    }
}

struct ItemView: View {
    @ObservedObject var viewModel: AbstractItemViewModel

    var body: some View {
        TextField($viewModel.title, text: "Item Title")
        Button("Save") { self.viewModel.save() } 
    }
}

Answer 1

swift 协议(protocol)和扩展中不允许使用包装器和存储属性，至少目前是这样。所以我会采用以下混合协议(protocol)、泛型和类的方法......(所有这些都可以用 Xcode 11.2/iOS 13.2 编译和测试)

// base model protocol
protocol ItemViewModel: ObservableObject {
    var title: String { get set }

    func save()
    func delete()
}

// generic view based on protocol
struct ItemView<Model>: View where Model: ItemViewModel {
    @ObservedObject var viewModel: Model

    var body: some View {
        VStack {
            TextField("Item Title", text: $viewModel.title)
            Button("Save") { self.viewModel.save() }
        }
    }
}

// extension with default implementations
extension ItemViewModel {
    
    var title: String {
        get { "Some default Title" }
        set { }
    }
    
    func save() {
        // some default behaviour
    }

    func delete() {
        // some default behaviour
    }
}

// concrete implementor
class SomeItemModel: ItemViewModel {
    @Published var title: String
    
    init(_ title: String) {
        self.title = title
    }
}

// testing view
struct TestItemView: View {
    var body: some View {
        ItemView(viewModel: SomeItemModel("test"))
    }
}

backup