Question

Right now I am dispatching a block of code inside a method to go download stuff for me using following format:

dispatch_queue_t downloader("downloader", NULL);
dispatch_async (downloader, ^{

//do stuff

});

what I am trying to do now is have that block return a UIImage for me, which I can then use as the return for the method:

-(UIImage *) myMethod:

dispatch_queue_t downloader("downloader", NULL);
dispatch_async (downloader, ^{

//do stuff to get UIImage

     dispatch_async (dispatch_get_main_queue, ^{

     return UIImage;  //this is the image that image I want myMethod to return

     });    
});

how ever I am getting passing parameter to incompatible type errors. I realize that is because I am not declaring that the block has a return value.

How can I declare that the block returns a UIImage right inside dispatch_async? Is this even possible or is would the method already be done by the time the block finishes execution?

Answer 1

Well, using the return would require your method to be synchronous, so there’s no solution that way. You will need to use a block as part of the function, like this:

- (void)myMethodOnFinish:(void(^)(UIImage *))finishBlock {

    dispatch_queue_t downloader("downloader", NULL);
    dispatch_async (downloader, ^{

        //do stuff to get UIImage
        UIImage *result = ...;

         dispatch_async (dispatch_get_main_queue, ^{
         //this is the image that image I want myMethod to return
             finishBlock(result);
        });    
    });
}

and call like this:

[self myMethodOnFinish:^(UIImage *image) {

    self.imageView.image = image; // or whatever you need to do            

}];