Invoke non void functions with performSelectorOnMainThread

Question

As the title of my question said .. I'm try to call a non-void function with performSelectorOnMainThread .. my function return UIImage , when I call it like this:

UImage *img = [self performSelectorOnMainThread:@selector(captureScreen:) withObject:webView waitUntilDone:YES];

it gives me an error that I have incompatible assigning type , also I try to cast it like this

UImage *img = (UIImage*)[self performSelectorOnMainThread:@selector(captureScreen:) withObject:webView waitUntilDone:YES];

and I got Semantic Issue: Operand of type 'void' where arithmetic or pointer type is required

I know that I can call it as normal , but Im trying some multithreading stuff and need it to invoke like this .. so how I can prevent this error please?

Edit :

I try to use GCD inside this function (captureScreen) its keep making Exc_bad_access on the "viewToCapture" .. so I decide to call the parent function inside the GCD block

dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0ul);

dispatch_async(queue, ^{
    [self captureScreenDelayed:pageView];

    dispatch_sync(dispatch_get_main_queue(), ^{
    });
});

and "captureScreenDelayed:" is as follow

- (void) captureScreenDelayed:(EpubPageViewController*)pageView
{
    if(!pageView)
        pageView = [self currentPageView];

    if(pageView.pageImageView)
        pageView.pageImageView.image = (UIImage*)[self performSelectorOnMainThread:@selector(captureScreen:) withObject:webView waitUntilDone:YES];
}

and the captureScreen :

-(UIImage*)captureScreen:(UIView*) viewToCapture
{

    UIGraphicsBeginImageContextWithOptions(viewToCapture.bounds.size, viewToCapture.opaque, 0.0);
    [viewToCapture.layer renderInContext:UIGraphicsGetCurrentContext()];
    UIImage *viewImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return viewImage;
}

to be honest Im not sure if this the correct way to get the image in separated thread... I appreciate any advice.

Answer 1

The thing you have to remember here is that the method that you're actually calling is -performSelectorOnMainThread:withObject:waitUntilDone: which has a (void) return type. Thus, you can't get the result of the performed selector returned through this method. If you need to do this type of multithreading, you should check out the concurrency programming guide and, in particular, grand central dispatch.