Swift 有什么方法可以快速为泛型参数分配默认值

发布于07月26日

我定义了OptionClosureView的 struct .

struct OptionClosureView<Content: View>: View {
    var content: (()->Content)? = nil
    
    var body: some View {
        if content == nil{
            Text("Hello, W或ld!")
        }else{
            content!()
        }
    }
}

如果我不想给"content"的属性赋值，我指的是OptionClosureView的默认文本视图.在这种情况下，我只想声明如下:

OptionClosureView()

而不是:

OptionClosureView<Text>()

或

OptionClosureView<Image>()

或

....

IMO, I don't care what is "Content" standing f或 in this case. Is there any way to assign a default value to the generic parameter "Content" in 或der to allow me to invoke OptionClosureView() directly? Something like:

struct OptionClosureView<Content: View = Text>: View {
....
}

推荐答案

您可以添加一个方便的init，它调用具有nil个内容的默认init，并让编译器通过使用泛型where子句仅将此init用于单个Content类型来推断类型.类型可以是任何视图类型，我刚刚使用EmptyView作为示例.

extension OptionClosureView where Content == EmptyView {
  init() {
    self.init(content: nil)
  }
}

let noContentView = OptionClosureView() // the type of this is OptionClosureView<EmptyView>
let contentView = OptionClosureView { Text("Content") } // OptionClosureView<Text>