更新PreviewProvider中的@State变量似乎不会直接更新只读计算属性previews。解决方案是将@State变量 Package 在测试支架视图中。然后在previews属性中使用此测试视图，以便实时预览正确刷新。已在Xcode 11.2.1中测试并运行。

struct ChildView: View {
    @Binding var itemName: String

    var body: some View {
        VStack {
            Text("Name: \(itemName)")
            Button(action: {
                self.itemName = "different value"
            }) {
                Text("Change")
            }
        }
    }
}

struct ChildView_Previews: PreviewProvider {

    struct BindingTestHolder: View {
        @State var testItem: String = "Initial"
        var body: some View {
            ChildView(itemName: $testItem)
        }
    }

    static var previews: some View {
        BindingTestHolder()
    }
}

如果您需要一个可以在实时预览中更改的值，我喜欢使用这个helper类：

struct BindingProvider<StateT, Content: View>: View {
    
    @State private var state: StateT
    private var content: (_ binding: Binding<StateT>) -> Content
    
    init(_ initialState: StateT, @ViewBuilder content: @escaping (_ binding: Binding<StateT>) -> Content) {
        self.content = content
        self._state = State(initialValue: initialState)
    }
    
    var body: some View {
        self.content($state)
    }
}

像这样使用它：

struct YourView_Previews: PreviewProvider {
    
    static var previews: some View {
        var yourVar = "example"
        BindingProvider(yourVar) { binding in
            YourView(initVar: binding)
        }
    }

}

这允许您在实时预览中测试绑定更改。