将泛型子类型约束传递给方法时Swift编译时错误 [英] Swift compile time error when passing object with generics subclass type constraint to method

问题描述

我有一个泛型类型的类，泛型类型有一个子类型约束。然后我尝试将一个类的实例传递给一个方法，但最终出现以下编译时错误：

 无法转换值类型'B< SomeType>'到期望的参数类型'B< NSObject>'
  

简单的例子：

  class A：NSObject {
 
 func testB（b：B< NSObject>） {
 $ b} 
} 
 
 class B< SomeType：NSObject> ;: NSObject {
 
 func test（）{
让a = A（）
 a.testB（b：self）//< -----上面提到的错误在这里显示
} 
} 
  

由于 SomeType 保证是 NSObject ，我预计编译器可以将 B > 转换为 B< NSObject> 并传递它。

类A只关心B的 SomeType 是 NSObject的而不是关于 SomeType 的实际类型。这就好像一个函数需要一个 UIView 参数，并且你传递了一个 UILabel 。

解决方案

更改：

  func testB（b： B< NSObject>）{
 
} 
  

收件人：

  func testB< SomeType：NSObject>（b：B< SomeType>）{
 
} 
  

这将起作用，因为 SomeType 约束为 NSObject ，不等于 NSObject 。

所以你的函数的 SomeType 也应该约束为 NSObject ：

func testB< SomeType：NSObject>（b：B< SomeType>）{...}

I have a class with a generic type and the generic type has a subclass type constraint. I then try and pass an instance of the class to a method but end up with the following compile time error:

 Cannot convert value of type 'B<SomeType>' to expected argument type 'B<NSObject>'

Below is a simple example:

class A: NSObject {

    func testB(b: B<NSObject>) {

    }
}

class B<SomeType: NSObject>: NSObject {

    func test() {
        let a = A()
        a.testB(b: self) // <----- The error noted above is shown here
    }
}

Because SomeType is guaranteed to be a subclass of NSObject, I would have expected the compiler would be ok with converting B<SomeType> to B<NSObject> and passing it along.

Class A only cares that B's SomeType is a subclass of NSObject and not about the actual type of SomeType. It's like if a function were to take a UIView parameter and you pass in a UILabel.

解决方案

Change:

func testB(b: B<NSObject>) {

}

To:

func testB<SomeType: NSObject>(b: B<SomeType>) {

}

This will work, because SomeType is constrain to NSObject, not equal to NSObject.

So your function's SomeType should be constrain to NSObject too:

func testB<SomeType: NSObject>(b: B<SomeType>) { ... }

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