如果@EnvironmentObject，如何创建通用？

Question

我最近发现需要编写 Class 的 Mock，因为它会导致 SwiftUI preview 无法正常工作。不幸的是，我得到了错误：

Property type 'T' does not match that of the 'wrappedValue' property of its wrapper type 'EnvironmentObject'

在视图结构中：

struct ContentView<T>: View {
    @EnvironmentObject var mockFoobar: T
    ...
}

还有错误：

Type of expression is ambiguous without more context

对于预览结构：

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        let mockFoobar: MockFoobar = MockFoobar()
        return ContentView<MockFoobar>()
            .environmentObject(mockFoobar)
    }
}

MockFoobar class 是：

class MockFoobar: ObservableObject {
  ...
}

正如用户@Asperi 所提供的，按照建议测试了以下内容：

class Foobar: ObservableObject {
    @Published var param: Bool = false
    func start() {
        self.param = true
    }
}

struct MyFoobarView<T: ObservableObject>: View {
    @EnvironmentObject var foobar: T
    
    var body: some View {
        VStack {
            Text("Hello Foobar")
        }
        .onAppear {
            self.foobar.start()
        }
    }
}

struct MyFoobarView_Previews: PreviewProvider {
    static var previews: some View {
        let foobar: Foobar = Foobar()
        return MyFoobarView()
            .environmentObject(foobar)
    }
}

但我收到以下错误（第一个在 .onAppear 中，第二个在 PreviewProvider 中）：

Cannot call value of non-function type 'Binding<Subject>'

Generic parameter 'T' could not be inferred

Answer 1

EnvironmentObject必须是ObservableObject，所以这里修正

struct ContentView<T: ObservableObject>: View {
    @EnvironmentObject var mockFoobar: T
    
    // .. other code here

更新： 添加了引入模型协议的演示

protocol Foobaring {
    var param: Bool { get set }
    func start()
}

class Foobar: ObservableObject, Foobaring {
    @Published var param: Bool = false
    func start() {
        self.param = true
    }
}

struct MyFoobarView<T: ObservableObject & Foobaring>: View {
    @EnvironmentObject var foobar: T

    var body: some View {
        VStack {
            Text("Hello Foobar")
        }
        .onAppear {
            self.foobar.start()
        }
    }
}

struct MyFoobarView_Previews: PreviewProvider {
    static var previews: some View {
        let foobar: Foobar = Foobar()
        return MyFoobarView<Foobar>()
            .environmentObject(foobar)
    }
}

backup

如果@EnvironmentObject，如何创建通用？

How to create Generic if @EnvironmentObject?

xcode

swiftui

xcode11.4