Objective-C的变量...指着自己？ [英] Objective-C variable... pointing to itself?

问题描述

我发现这个结构在​​一些苹果公司的例子code的处理键 - 值观察。当添加一个观察者，你可以添加上下文（以一个void *变量的形式），可以唯一地标识志愿通话 - 如果你想特别有用多个志愿调用触发同样的动作，作为单一的情况下，可避免使用一堆链或报表做好一切准备。这是公司用来声明用于环境变量的行：

I spotted this construct in some of Apple's example code for dealing with key-value observing. When adding an observer, you can add a context (in the form of a void* variable) that can uniquely identify the KVO call - particularly useful if you want multiple KVO calls to trigger the same action, as the single context can avoid using a bunch of chained or statements to check all the possibilities. This is the line that's used to declare the variable used for the context:

static void *aContext = &aContext;

这基本上宣布aContext引用本身，分配本身它自己的内存地址 - 一个辉煌的技巧，创造了韩国国际志愿者组织范围内的唯一标识符。抛开细节，我很好奇，究竟这就是所谓的（自赋值？圆形指针？别的东西吗？），它可能有志愿之外还有什么其他用途。我试着谷歌搜索不同的东西，但我不能拿出任何东西完全一样，缺乏适当的术语。 ：）

It's basically declaring aContext to reference itself, assigning itself its own memory location - a brilliant trick that creates a unique identifier for the KVO context. Specifics aside, I'm curious what exactly this is called (self-assignment? circular pointer? something else?) and what other uses it may have besides KVO. I tried Googling different things but I couldn't come up with anything exactly like this, lacking the proper terminology. :)

我肯定会经常使用这一招，因为它减少的数量，如果必要志愿处理，这使得它更优雅。

I'm certainly going to be using this trick regularly, as it reduces the number of if statements necessary for KVO handling, which makes it that much more elegant.

推荐答案

我认为这是过于复杂和混乱。当你想为KVO一个独特的环境只需要声明并使用一个指向它：

I think this is overly complicated and confusing. When you want to have a unique context for KVO just declare it and use a pointer to it:

static int kMyObjectPropertyObservationContext;

...

[object addObserver:self
         forKeyPath:@"myProperty"
            options:0
            context:&kMyObjectPropertyObservationContext];

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