objective-c - 在 Objective C block 中隐藏 `self` 的成语？

Question

当我有一个 Objective C 实例创建一个需要引用该实例的 block 时，我经常通过一个弱指针这样做，该指针不会使该实例保持事件状态并产生一个保留周期，如下所示:

__weak MyType *const weakSelf = self;
void (^aBlock)() = ^(){
  // Do things with weakSelf instead of self.
}

我想要一个成语来阻止我在 block 中使用强大的 self 。理想情况下，在使用惯用语时，如果我尝试在 block 中使用 self 而不是 weakSelf，则会出现编译错误。运行时错误也可以。

Answer 1

我有一个我不是特别喜欢的解决方案，但它可能会引发更好的答案。我将不回答这个问题，希望有更好的解决方案到来。

这是一种方法:

// Here's a method definition…
-(void) aMethod
{
  // Want to create a block in which its impossible to refer to strong "self".
  // Begin a new scope to do this in.
  {
    // Within this scope, cover the existing "self" with a weak variant.
    __weak STWeatherTableViewController const *weakSelf = self;
    __weak STWeatherTableViewController const *self = weakSelf;

    // Sadly it's _not_ possible to simply do:
    //   __weak STWeatherTableViewController const *self = self;
    // … it gives a warning about initialisation of a variable form its own
    // uninitialised value, which makes sense, though you might hope the
    // compiler would work out what's going on.

    // Make a block that captures the covered self and does something with it.
    void (^exampleBlock)() = ^(){ [self lineHeight]; };
    exampleBlock();
  }

  // Now, back in the scope of the original method, "self" is non weak
  // again.
  [self doSomething];
}

我想，如果你真的很关心这个，你可以使用宏。它至少会抽象出这个想法并使代码中的用法易于查找和注意:

#define WEAKEN_SELF(classType) \
__weak classType const *weakSelf = self; \
__weak classType const *self = weakSelf

甚至:

#define WEAKEN_SELF(classType) \
__weak classType const *weakSelfTemporary##__LINE__ = self; __weak classType const *self = weakSelfTemporary##__LINE__;

你会像这样使用它:

-(void) testMethod
{
  // You still need that scope or you cover the original "self".
  {
    WEAKEN_SELF(STWeatherTableViewController)
    void (^exampleBlock)() = ^(){ [self someMethodOrOther]; };
    exampleBlock();
  }
}

我不相信这是值得的。有编译器警告可能就足够了，它们可能会变成错误？