• Blocks
  

What are blocks?

Why should we use blocks?

• Grand Central Dispatch
  

How does GCD help iOS  development?

packages of code that usually appear in the form of methods in Objective-C.

first-class objects

            void (^helloWorldBlock)(void) = ^ {

           printf ("Hello, World!\n");

            };

            void (^helloWorldBlock)(void) = ^ {

            printf ("Hello, World!\n");

            };

            helloWorldBlock();

Blocks return just like functions.

            int (^fortyTwo)(void) = ^{

                 return 42;

            }

            int a = fortyTwo();

Just like functions:

            BOOL (^isFortyTwo)(int) = ^(int a) {

            return (a == 42);

            }

            BOOL myBool = isFortyTwo(5); // NO

            int a = 42;

            void (^myBlock)(void) = ^{

            printf("a = %d\n", a);

            };

            myBlock();

            int a = 42;

            void (^myBlock)(void) = ^{

            a++;

            printf("a = %d\n", a);

            };

            myBlock();

            __block int a = 42;

            void (^myBlock)(void) = ^{

            a++;

            printf("a = %d\n", a);

            };

            myBlock();

Simple block that returns an int and  takes a float:

typedef int (^floatToIntBlock)(float);

Now initialize it:

floatToIntBlock foo = ^(float a) {      return (int)a; };

• Method Signature:
  
• - (void)doThisBlock:(void (^)(id obj))block
  
• returns void, takes id argument
  

First typedef the block, then use it as  an argument:

typedef int (^floatToIntBlock)(float);

void useMyBlock(floatToIntBlock foo);

• Open-source threading library  (dispatch)
  
• dispatch tasks to multiple cores
  
• Automatically optimizes threading
  

dispatch_queue_t queue =  dispatch_get_global_queue(DISPATCH_Q UEUE_PRIORITY_HIGH, 0ul);

dispatch_async(queue, ^{

[self  performLongTask];

}

dispatch_async(queue, block);
dispatch_async_f(queue, context, func);

Schedules block or function on queue, returns immediately

dispatch_sync(queue, block);
dispatch_sync_f(queue, context, func);

Schedules block or function on queue, blocks until completion

• dispatch_queue_t
  
• Main Queue
  
• Analogous to main thread (Do your  UI operations here)
  
• dispatch_get_main_queue()
  

dispatch_get_global_queue(priority, flags);

priority is one of three constants:

• DISPATCH_QUEUE_PRIORITY_LOW
  
• DISPATCH_QUEUE_PRIORITY_NORMAL
  
• DISPATCH_QUEUE_PRIORITY_HIGH
  

second arg should always be 0ul (for now)

dispatch_queue_create(label, attr)

Use reverse DNS for label

com.example.myQueue

pass NULL for attr

Be sure to use dispatch_release()

• Custom queues are serial
  
• First-in, first-out, one at a time
  
• Global queues are concurrent
  
• GCD automatically chooses how  many (usually # of CPU cores)