Objective-C Blocks Cheat Sheet

Blocks are an incredibly powerful addition to Objective-C, introduced in iOS 4. However, their syntax can be maddeningly difficult to remember. Matt Gallagher has an excellent post that breaks down the syntax to help you understand it. If you haven’t read this article, go do it now.

Even after working with blocks for a while, I still get tripped up. So I created the cheat sheet below, which I frequently refer to when declaring blocks.

// http://cocoawithlove.com/2009/10/ugly-side-of-blocks-explicit.html has a nice breakdown of the syntax--it helps to think of the ^ as similar to a pointer dereference symbol *

// block typedef:

    typedef void(^Block)();
    typedef void(^ConditionalBlock)(BOOL);
    typedef NSString*(^BlockThatReturnsString)();
    typedef NSString*(^ConditionalBlockThatReturnsString)(BOOL);

// block property with typedef:

    @property(copy)Block theBlock;

// block property without typedef:

    @property(copy)void(^block)();
    @property(copy)void(^conditionalBlock)(BOOL);
    @property(copy)NSString*(^blockThatReturnsString)();
    @property(copy)NSString*(^conditionalBlockThatReturnsString)(BOOL);


// block definition inline:

    ReturnType(^block_name)(parmeter, types, here) = ^(parameter, types, here) {};
    
    void(^block)(void) = ^{};    
    void(^conditionalBlock)(BOOL shouldWork) = ^(BOOL shouldWork){};
    NSString*(^blockThatReturnsString)(void) = ^ NSString* { return @"foo"; };    
    NSString*(^conditionalBlockThatReturnsString)(BOOL shouldWork) = ^ NSString* (BOOL shouldWork){ 
        return shouldWork ? @"foo" : @"bar"; 
    };

    -(void)runBlocks {
        block();
        conditionalBlock(NO);
        NSString *someString = blockThatReturnsString();
        NSString *conditionalString = conditionalBlockThatReturnsString(NO);
    }

// blocks as return values:

    -(void (^)(void))doSomething;
    -(void (^)(BOOL))doSomethingConditionally;
    -(NSString* (^)(void))returnString;
    -(NSString* (^)(BOOL))returnStringConditionally;

    -(NSString* (^)(BOOL))returnStringConditionally {
        return ^(BOOL shouldReturn) {
            if (shouldReturn) return @"foo";
            return @"bar";
        }
    }
    
// blocks as arguments:

    -(void)doSomethingWithBlock:(void (^)(void))block;
    -(void)doSomethingWithBlock:(void (^)(BOOL))conditionalBlock;
    -(void)doSomethingWithBlock:(NSString* (^)(void))blockThatReturnsString;
    -(void)doSomethingWithBlock:(NSString* (^)(BOOL))conditionalBlockThatReturnsString;