Swift 2错误，在协议扩展中使用了变异函数“不能在不可变值上使用变异成员:'self'是不可变的 [英] Swift 2 Error using mutating function in Protocol extension "Cannot use mutating member on immutable value: 'self' is immutable

问题描述

不知道这是怎么回事，这似乎应该很简单.我有一个可变var的协议，具有可变功能的扩展.当我尝试使用扩展中声明的 mtkAnimQueAppend 时，事情在 testClass.testFunc 中崩溃了，我收到以下错误消息:不能在不可变的成员上使用变异成员值:自我"是一成不变的.

Not sure what's going on here, this seems like it should be pretty straight forward. I have a protocol that mutable var, an extension with a mutating function. Things are crapping out in the testClass.testFunc, when I try and use mtkAnimQueAppend declared in the extension, I get this error: "Cannot use mutating member on immutable value: 'self' is immutable.

protocol MTKAnimateValueDelegate {
    var mtkAnimQue:[MTKAnimateValue]? {get set}
}

extension MTKAnimateValueDelegate {
    ///Adds element to que
    mutating func mtkAnimQueAppend(element:MTKAnimateValue) {

        if mtkAnimQue != nil {
          mtkAnimQue?.append(element)
        } else {
          mtkAnimQue = [element]
        }
    }
}

class testClass: MTKAnimateValueDelegate {

  var mtkAnimQue:[MTKAnimateValue]?

  func testFunc() {
    var animValue = MTKAnimateValue(fromValue: 10, toValue: 20, inSeconds: 2)
    animValue.isAnimating = true
    mtkAnimQueAppend(animValue) //ERROR: "Cannot use mutating member on immutable value: 'self' is immutable
  }

}

推荐答案

问题是，在协议中，您将该函数标记为mutating，如果要在结构上使用该协议，则需要这样做.但是，传递给testFunc的self是不可变的(它是对该类实例的引用)，这会使编译器崩溃.如果testClass实际上是一个结构，并且可以对函数进行更改以解决该问题，那么这将是有道理的.

The problem is that, in the protocol you mark the function as mutating, which you need to do if you want to use the protocol on a struct. However, the self that is passed to testFunc is immutable (it's a reference to a instance of the class) and that is tripping up the compiler. This would make sense if testClass was actually a struct and you could make the function mutating to resolve the issue.

我可以看到两种解决方法:

I can see two work arounds:

1. 仅使协议类为

1. make the protocol class only

protocol MTKAnimateValueDelegate: class { ...

使testClass为一个结构，并将testFunc标记为变异.

Make testClass a struct and mark testFunc as mutating.

无论哪种方式，我认为这都是需要向Apple报告的错误.

Either way, I think this is a bug that needs to be reported to Apple.

修改

1. 另一种解决方法是制作self
的可变副本

1. Another way around it is to make a mutable copy of self

func testFunc() {
    var animValue = MTKAnimateValue(fromValue: 10, toValue: 20, inSeconds: 2)
    animValue.isAnimating = true
    var mutableSelf = self
    mutableSelf.mtkAnimQueAppend(animValue) 
  }

由于mutableSelf是参考，所以变异函数所做的任何更改仍将反映在self的状态中.

Since mutableSelf is a reference, any changes the mutating function makes will still be reflected in self's state.

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