Completion Block with Data Return

CompletionBlocksWithBOOL.m

.h = - (void)signInAccountWithUserName:(NSString *)userName
                         password:(NSString *)password
                       completion:(void (^)(BOOL success))completionBlock;
                       
.m = - (void)signInAccountWithUserName:(NSString *)userName
                         password:(NSString *)password
                       completion:(void (^)(BOOL success))completionBlock
{
    // Notice that we are passing a BOOL back to the completion block.
    if (completionBlock != nil) completionBlock(loginSuccessful);
}

//Call the method:
[self signInAccountWithUserName:@"Bob"
                       password:@"BobsPassword"
                     completion:^{
                         [self doMethod]; //do something like call a method
                     }];
                     
OR

[self signInAccountWithUserName:@"Bob"
                       password:@"BobsPassword"
                     completion:^(BOOL success) {
                         if (success) {
                             [self doMethod]; //do something like call a method
                         } else {
                             // do something like display an alert
                         }
                     }];

CompletionBlocksWithReturn.m

//Block that passes in an MKMapView and returns NSData called mapData
.h = - (void)ImageFromMapView:(MKMapView*)mapView completion:(void (^)(NSData* mapData))completionBlock;

.m = - (void)ImageFromMapView:(MKMapView*)mapView completion:(void (^)(NSData* mapData))completionBlock
{
  //method does what it needs to do
  //self.data holds the NSData that was created inside this method
  completionBlock(self.data);
}

Calling it = 
  [classWithMethod ImageFromMapView:mapView completion:^(NSData *mapData) {
    //do something with mapData
  }];