What exactly does the __block
keyword in Objective-C mean? I know it allows you to modify variables within blocks, but I'd like to know…
- What exactly does it tell the compiler?
- Does it do anything else?
- If that's all it does then why is it needed in the first place?
- Is it in the docs anywhere? (I can't find it).
Best Answer
It tells the compiler that any variable marked by it must be treated in a special way when it is used inside a block. Normally, variables and their contents that are also used in blocks are copied, thus any modification done to these variables don't show outside the block. When they are marked with
__block
, the modifications done inside the block are also visible outside of it.For an example and more info, see The __block Storage Type in Apple's Blocks Programming Topics.
The important example is this one:
In this example, both
localCounter
andlocalCharacter
are modified before the block is called. However, inside the block, only the modification tolocalCharacter
would be visible, thanks to the__block
keyword. Conversely, the block can modifylocalCharacter
and this modification is visible outside of the block.