What does the “__block” keyword mean?

by

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:

extern NSInteger CounterGlobal;
static NSInteger CounterStatic;

{
    NSInteger localCounter = 42;
    __block char localCharacter;

    void (^aBlock)(void) = ^(void) {
        ++CounterGlobal;
        ++CounterStatic;
        CounterGlobal = localCounter; // localCounter fixed at block creation
        localCharacter="a"; // sets localCharacter in enclosing scope
    };

    ++localCounter; // unseen by the block
    localCharacter="b";

    aBlock(); // execute the block
    // localCharacter now 'a'
}

In this example, both localCounter and localCharacter are modified before the block is called. However, inside the block, only the modification to localCharacter would be visible, thanks to the __block keyword. Conversely, the block can modify localCharacter and this modification is visible outside of the block.

More Related Contents:

  1. How do I avoid capturing self in blocks when implementing an API?
  2. Implementing a method taking a block to use as callback
  3. How to dispatch on main queue synchronously without a deadlock?
  4. How change the size of sprite in cocos2d? [closed]
  5. UIRefreshControl without UITableViewController
  6. How do you trigger a block after a delay, like -performSelector:withObject:afterDelay:?
  7. File write with [NSBundle mainBundle] fails
  8. What’s the best way to develop a sideswipe menu like the one in Facebook’s new iOS app?
  9. Getting row of UITableView cell on button press
  10. Best practice? – Array/Dictionary as a Core Data Entity Attribute [closed]
  11. Display animated GIF in iOS
  12. Xcode intellisense meaning of letters in colored boxes like f,T,C,M,P,C,K,# etc
  13. Can I use Objective-C blocks as properties?
  14. Phone number formatting on iOS
  15. ViewController respondsToSelector: message sent to deallocated instance (CRASH)
  16. Supporting Open In… menu item in my app for iOS Mail And Safari
  17. iOS – Merging two images of different size
  18. What does @synchronized() do as a singleton method in objective C?
  19. UIScrollView contentSize not working
  20. What makes a keychain item unique (in iOS)?
  21. Move UIView up when the keyboard appears in iOS
  22. List of class properties in Objective-C
  23. Access iOS dictionary programmatically
  24. RootViewController Switch Transition Animation
  25. Creating delegates on the spot with blocks
  26. Custom nav bar styling – iOS [duplicate]
  27. Unable to set frame correctly before viewDidAppear
  28. How to enable the new Objective-C object literals on iOS?
  29. Detecting if UIView is intersecting other UIViews
  30. IBOutletCollection set ordering in Interface Builder

Leave a Comment