一块在编译时会发生什么，我可以在运行时创建一个吗? [英] What happens to a block at compile time, and can I create one at runtime?
问题描述
这是关于 Objective-C 中 blocks (^ {})的两部分问题.我已经搜索了一些答案，但是Google或SO中什么都没有显示.这个问题源于创建针对iOS的自定义XML布局引擎的愿望，该引擎具有对块的支持-这意味着我想在运行时解析NSStrings并创建一个块.
This is a two-part question about blocks (^{}) in Objective-C. I have searched for some answers, but nothing has showed up in Google or SO. This question stems from a desire to create an custom XML Layout Engine for iOS, with support for blocks - meaning that I want to parse NSStrings and create a block during runtime.
1)这甚至可能吗?如果是这样，怎么办?
1) Is this even possible? If so, how can it be done?
在NSString to Block上找不到更多内容，我认为原因可能是编译器如何处理块-因此我再次搜索答案，但空手而归.所以:
Not being able to find much on NSString to Block, I thought that the reason may be how a block is handled by the compiler - so again I searched for an answer but came up empty-handed. So:
2)在Objective-C中编译时一个块会发生什么?
2) What happens to a block at compile time in Objective-C?
推荐答案
块是两件事情.在执行通过块的那一刻捕获的大量可执行代码和状态.
A block is two things; a hunk of executable code and state captured at the moment when execution passes over the block.
即给出:
myBlock = ^{ return someVariable + someOtherVariable; };
编译后，这将创建一段代码，其功能非常类似于将两个变量加在一起的函数，返回结果.没有创建块实例.
When compiled, this creates a chunk of code that acts very much like a function that adds two variables together returns the result. No block instance is created.
在执行时，当对表达式myBlock = ^{...};求值时，将创建一个块实例.在该块实例内部是对编译器创建的代码的引用，以及对myBlock进行赋值时两个变量中包含的值的副本(当然，除非
When executing, when the expression myBlock = ^{...}; is evaluated, then a block instance is created. Inside that block instance is a reference to the code the compiler created and a copy of the values contained in the two variables at the time the assignment to myBlock was made (unless, of course, __block is in play, etc...).
答案(2)，但与答案(1)有关.
That answers (2), but is relevant to answering (1).
在运行时，您可以整天创建现有的，已编译的，阻止的实例.但是，您不能创建新的种类块.这样做需要编译器，并且仅限于运行时环境，在该环境中可以编译新的可执行代码并实际执行它.
At runtime, you can create instances of existing, compiled, blocks all day long. You cannot, however, create new kinds of blocks. Doing so would require a compiler and would be limited to runtime environments where compiling new executable code and actually executing it is possible.
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