How to write an Objective-C Completion Block

You define the block as a custom type:

typedef void (^ButtonCompletionBlock)(int buttonIndex);

Then use it as an argument to a method:

+ (SomeButtonView*)buttonViewWithTitle:(NSString *)title 
                      cancelAction:(ButtonCompletionBlock)cancelBlock
                  completionAction:(ButtonCompletionBlock)completionBlock

When calling this in code it is just like any other block:

[SomeButtonView buttonViewWithTitle:@"Title"
                   cancelAction:^(int buttonIndex) {
                         NSLog(@"User cancelled");
               } 
                 completionAction:^(int buttonIndex) {
                         NSLog(@"User tapped index %i", buttonIndex);
               }];

If it comes time to trigger the block, simply call completionBlock() (where completionBlock is the name of your local copy of the block

I always use this when I want to write a block:

http://fuckingblocksyntax.com

EDIT

If you are writing Swift then use this:

http://fuckingswiftblocksyntax.com

If the above links are not opening because of the obscene language, use this one.

http://goshdarnblocksyntax.com/

I wrote a completionBlock for a class which will return the values of a dice after they have been shaked:

1. Define typedef with returnType (.h above @interface declaration)

   typedef void (^CompleteDiceRolling)(NSInteger diceValue);
   

2. Define a @property for the block (.h)

   @property (copy, nonatomic) CompleteDiceRolling completeDiceRolling;
   

3. Define a method with finishBlock (.h)

   - (void)getDiceValueAfterSpin:(void (^)(NSInteger diceValue))finishBlock;
   

4. Insert previous defined method in .m file and commit finishBlock to @property defined before

   - (void)getDiceValueAfterSpin:(void (^)(NSInteger diceValue))finishBlock{
       self.completeDiceRolling = finishBlock;
   }
   

5. To trigger completionBlock pass predefined variableType to it (Don't forget to check whether the completionBlock exists)

   if( self.completeDiceRolling ){
       self.completeDiceRolling(self.dieValue);
   }
   

Regarding to http://goshdarnblocksyntax.com/

As a local variable:

returnType (^blockName)(parameterTypes) = ^returnType(parameters) {...};

As a property:

@property (nonatomic, copy) returnType (^blockName)(parameterTypes);

As a method parameter:

- (void)someMethodThatTakesABlock:(returnType (^)(parameterTypes))blockName;

As an argument to a method call:

[someObject someMethodThatTakesABlock:^returnType (parameters) {...}];

As a typedef:

typedef returnType (^TypeName)(parameterTypes);
TypeName blockName = ^returnType(parameters) {...};