Concurrency programming

Blocks

1. Create a simple block and invoke it,
2. Create a block with params and invoke it
3. Show how block (__block) variables work
4. show how to avoid self strong referencing
5. show how to pass blocks to methods

- (void) runBlock:(void (^)(NSNumber* number, NSString* text))completionBlock;

6. show typedefing

typedef void (^ImageCompletionBlock)(UIImage *);  
typedef void (^ActionBlock)();

Grand Central dispatch

1. Use gcd to create a background task
2. Create a dead lock
3. Understand:

• serial queues
• global concurrent queues
• main dispatch queue

4. Singleton with GCD

+ (MyClass *)sharedInstance
{
    //  Static local predicate must be initialized to 0
    static MyClass *sharedInstance = nil;
    static dispatch_once_t onceToken = 0;
    dispatch_once(&onceToken, ^{
        sharedInstance = [[MyClass alloc] init];
        // Do any other initialisation stuff here
    });
    return sharedInstance;
}

5. Dispatch groups

dispatch_queue_t queue = dispatch_get_global_qeueue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_group_t group = dispatch_group_create();
for(id obj in array)
    dispatch_group_async(group, queue, ^{
        [self doSomethingIntensiveWith:obj];
    });
dispatch_group_notify(group, queue, ^{
    [self doSomethingWith:array];
});
dispatch_release(group);

NSOperation queues

NSInvocationOperation

NSOperationQueue *queue = [NSOperationQueue new];
 
NSInvocationOperation *operation = [[NSInvocationOperation alloc] initWithTarget:self
    selector:@selector(methodToCall)
    object:objectToPassToMethod];
 
[queue addOperation:operation];
[operation release];

NSBlockOperation

NSBlockOperation* theOp = [NSBlockOperation blockOperationWithBlock: ^{
    NSLog(@"Beginning operation.\n");
    // Do some work.
}];

NSOperation subclassing