返回在 swift 协议 function 中查看相关类型

Question

public protocol NodeBuilder {
  associatedtype OutputView: View
  func build() -> OutputView
}

public struct Builder {
  private let builder: NodeBuilder // Protocol 'NodeBuilder' can only be used as a generic constraint because it has Self or associated type requirements
  
  public init(builder: NodeBuilder) { // Protocol 'NodeBuilder' can only be used as a generic constraint because it has Self or associated type requirements
    self.builder = builder
  }
  
  public func build<OutputView: View>() -> OutputView {
    builder.build() // Member 'build' cannot be used on value of protocol type 'NodeBuilder'; use a generic constraint instead
  }
}

struct Component: NodeBuilder {
  func build() -> some View {
    Text("Some View")
  }
}

我正在尝试在这里创建一个可重用的协议。 我收到错误，这些错误在此处添加为评论。 在网上找不到任何解决方案来解决这个问题。 我怎样才能使这段代码工作？ 或者有什么建议可以在哪里查找有关它的更多信息？ 谢谢！

Answer 1

具有关联类型的协议不能用作类型（但是在 Swift 5.7 中可以）

由于错误状态，您必须使用受协议约束的 generics

例如

public protocol NodeBuilder {
    associatedtype OutputView: View
    func build() -> OutputView
}

public struct Builder<N: NodeBuilder>  {
    private let builder: N
    
    public init(builder: N) {
        self.builder = builder
    }
    
    public func build<N>() -> N {
        builder.build() as! N
    }
}

struct Component: NodeBuilder {
    func build() -> some View {
        Text("Some View")
    }
}