UIImage setImage is very, very slow

Question

I'm trying to load an image from a local URL using the following:

UIImage *image = [UIImage imageWithData:[NSData dataWithContentsOfURL:fileURL]];
[self.imageView setImage:image];
NSLog(@"imageView set");

So I see in the console "imageView set" almost immediately, but it takes a very long time for it to be reflected in the UI (sometimes a few minutes!).

Any idea why this is happening?

Answer 1

This happened to me when I was setting the image in a background thread (in which I was downloading the image file). Just make sure your code is run in the main thread. The image will change immediately.

// In a background thread...

UIImage *image = ... // Build or download the image

dispatch_async(dispatch_get_main_queue(), ^{ 
    [self.imageView setImage:image]; // Run in main thread (UI thread)
});

Answer 2

You should load up instruments and see what exactly it is doing.

For starters, you should do what you can to avoid I/O on the drawing thread. Also, are there other I/O requests at the same time?

A UIImage does not necessarily need to be a singular bitmap representation -- it may be backed by a cache and/or loaded lazily. So just because an 'image' is 'set', does not mean that the optimal bitmap has been loaded into memory and prepared for rendering -- it may be deferred until render (draw) is requested.

Profiling OTOH will tell you (generally) why it is taking longer than expected.