What are blocks in iOS?

Ans : 

Blocks are first-class functions, which is a fancy way of saying that Blocks are regular Objective-C objects. Since they’re objects, they can be passed as parameters, returned from methods and functions, and assigned to variables. A block creates a const copy of any local variable that is referenced inside of its scope.

Block is a chunk of code that can be executed at some future time.

Blocks can greatly simplify code. They can help you reduce code, reduce dependency on delegates, and write cleaner, more readable code.

Block is alternative of delegate or NSNotificationCenter. In delegate and NSNotificationCenter, callback methods and main method are written different places. So its more difficult to read. In block, call back method written in main method as parameter.

Declaration : return_type (^block_name)(param_type, param_type, ...)

int (^add)(int,int)

Definition : ^return_type(param_type param_name, param_type param_name, ...) { ... return return_type; }

^(int number1, int number2){ return number1+number2 }

Declaration + Definition : 

int (^add)(int,int) = ^(int number1, int number2){ return number1+number2; }

We can call block also like following : 

int resultFromBlock = add(2,2);

If we take example of NSArray using block :

[theArray enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop){

    NSLog(@"The object at index %d is %@",idx,obj);

}];

If we take example of UIView animation using block :

[UIView animateWithDuration:5.0 

                     animations:^{

                        [animatingView setAlpha:0];

                        [animatingView setCenter:CGPointMake(animatingView.center.x+50.0, 

                                                             animatingView.center.y+50.0)];

                     } 

                     completion:^(BOOL finished) {

                         [animatingView removeFromSuperview];

                     }];

Apple also suggest to use block instead of call back methods.

Difference between blocks and completion handler : 

We have understand what is block. Completion handler is a way (technique) for implementing callback functionality using blocks. As a completion handler parameter, we have to pass block.

Example of Completion handler is seen above as last example of blocks.

Completion handler always comes as last parameter.

Read more : Difference between blocks and completion handler