Question

I’m trying to load views asynchronously. The problem is that the frame of the views to be loaded depends on the data that’s loaded asynchronously. In other words there are some long calculations that decide where to actually display the UIViews.

I know that there are problems when trying to actually display a UIView in a thread and that you should always load them back in the main thread, so this is the code I’ve been trying out:

asyncQueue = [[NSOperationQueue alloc] init];
[asyncQueue addOperationWithBlock:^{
    // Do work to load the UIViews and figure out where they should be
    UIButton *test = [[UIButton alloc] initWithFrame:[self doWorkToGetFrame]];

    [[NSOperationQueue mainQueue] addOperationWithBlock:^{
            [self addSubview:test];
        }
    }];
}];

This all resides in a UIView container.

Answer 1

Try something like this:

UIButton *test = [[UIButton alloc] initWithFrame:CGRectZero];
test.hidden = YES;
test.alpha = 0.0f;
[self addSubview:test];

dispatch_queue_t downloadQueue = dispatch_queue_create("myDownloadQueue",NULL);
dispatch_async(downloadQueue, ^
{
  // do work to load UIViews and calculate frame
  CGRect frameButton = ...;

  dispatch_async(dispatch_get_main_queue(), ^
  {
    test.hidden = NO; 
    [UIView animateWithDuration:0.4f animations:^
    {
     test.frame = frameButton;
     test.alpha = 1.0f;
    } 
    completion:^(BOOL finished){}];


  });
});

dispatch_release(downloadQueue);

This adds your button in the main thread, but makes it invisible initially. After your background work is done, you’ll make it visible and set the frame using an animation.