What are blocks and how are they used?

Refence

Blocks are a way of defining a single task or unit of behavior without having to write an entire Objective-C class. Under the covers Blocks are still Objective C objects. They are a language level feature that allow programming techniques like lambdas and closures to be supported in Objective-C. Creating a block is done using the ^ { } syntax:

myBlock = ^{
   NSLog(@"This is a block");
}

It can be invoked like so:

myBlock();

It is essentially a function pointer which also has a signature that can be used to enforce type safety at compile and runtime. For example you can pass a block with a specific signature to a method like so:

- (void)callMyBlock:(void (^)(void))callbackBlock;

If you wanted the block to be given some data you can change the signature to include them:

- (void)callMyBlock:(void (^)(double, double))block 
{
    ...
    block(3.0, 2.0);
}

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