Block example

gistfile1.m

// In the project far, far away...

// .h
typedef void(^SuccessBlock)();
typedef void(^FailureBlock)(NSString *errorMessage);

- (void)executeCommandWithSuccessBlock:(SuccessBlock)successBlock failureBlock:(FailureBlock)failureBlock;

// usage
[someCommand executeCommandWithSuccessBlock:^{
    NSLog(@"Well, well, splendid.");
} failureBlock:^(NSString *errorMessage){
    NSLog(@"Uh oh!");
    NSLog(@"By the way, the error message is %@", errorMessage);
}];