Question

I’ve read up on blocks as much as I can, however I can’t seem to wrap my head around this concept. I’ve used blocks regularly where they are utilized to call success and failure blocks in an object’s owner’s class.

So, say I have ClassA with a variable of ClassB. I call [ClassB method1]. What exactly must be put into method1 in order to add the ability to run a success block assuming all goes well within method1? How does it know to invoke the success block that is written into ClassA? Any examples would be great.

Answer 1

Here is a rough and quick example.

In ClassB.h define the function with a success / failure block:

- (void)doSomethingOnSuccess:(void (^)(NSString *resultString))success
                     failure:(void (^)(NSError *error))failure;

In ClassB.m:

- (void)doSomethingOnSuccess:(void (^)(NSString *resultString))success
                     failure:(void (^)(NSError *error))failure
{
  // Do some task
  BOOL taskSucceed = [self doSomeTask];
  if (taskSucceed) {
    NSString *result = @"done";
    // Check for success block
    if (success) 
      success(result);
  } else {
    // Generate error
    NSError *error = ...;
    if (failure)
      failure(error);
  }

}

Finally, in ClassA:

[ClassB doSomethingOnSuccess:^(NSString *resultString) {
  // method succeeded
} failure:^(NSError *error) {
  // something failed
}