How can I retrieve a return value from a completion block?

问题

Is it possible to run a completion block on the main thread?

For example, I have one method which returns a value:

- (int)test
{

    /* here one method is called with completion block with return type void */

    [obj somemethodwithcompeltionblock:
      {
         /* here I am getting my Int which I want to return */
      }
      ];
}

but I can\'t see how to return the integer value from within the completion block as the result of this method, because the completion block runs on a background thread.

How can I do this?

回答1:

You're missing some basics about asynchronous development with blocks. You can't have a dispatched block return from anywhere but its own scope. Think of each block as its own method, instead of inline code.

I think what you're looking for is something similar to this...

- (void)testWithHandler:(void(^)(int result))handler
{
    [obj somemethodwithcompeltionblock:^{
            int someInt = 10;
            dispatch_async(dispatch_get_main_queue(), ^{
                handler(10);
            });
      }
      ];
}


- (void)callSite
{
    [self testWithHandler:^(int testResult){
        NSLog(@"Result was %d", testResult);
    }];
}

来源：https://stackoverflow.com/questions/8432134/how-can-i-retrieve-a-return-value-from-a-completion-block