Question

There is a great wiki about image loading from the camera picker. Which made me aware of costs of taking an image at full resolution.

At the moment, when a photo is picked, I push a new view controller and display the image at full resolution. Pushing the view is a really slow and choppy experience (about 1 fps!) that I want to smooth out. Comparing to picking a photo on Instagram, I notice that they use a low resolution image and later swap in the full image. (I need the full res image because the user should be able to zoom and pan)

The idea I want is somthing like this:

- (void)imagePickerController:(UIImagePickerController *)picker
        didFinishPickingMediaWithInfo:(NSDictionary *)info 
{

    UIImage* fullImage = [info objectForKey:UIImagePickerControllerOriginalImage];

    // Push a view controller and give it the image.....
}

- (void) viewDidLoad {

    CGSize smallerImageSize = _imageView.bounds;
    UIImage* smallerImage = [MyHelper quickAndDirtyImageResize:_fullImage     
                                                        toSize:smallerImageSize];

    // Set the low res image for now... then later swap in the high res
    _imageView.image = smallerImage;

    // Swap in high res image async
    // This is the part im unsure about... Im sure UIKit isn't thread-safe!
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, NULL), ^{
        _imageView.image = _fullImage;
    });
}

I think that UIImage isn’t memory mapped in until it is used. Therefore it dons’t slow things down until its given to the imageView. Is this correct?

I think image decoding is already done asynchronously by the system, however, it is sill slowing the phone down considerably while its loading.

Is there a way do perform some of the work required to display an image in a very low priority background queue?

Answer 1

You’re trying to do things the most complicated way 🙂
Why not just prepare the small image before pushing the view controller and pass it to them? Look at this code:

- (void)imagePickerController:(UIImagePickerController *)picker
        didFinishPickingMediaWithInfo:(NSDictionary *)info 
{

    UIImage *fullImage = [info objectForKey:UIImagePickerControllerOriginalImage];
    UIImage *smallImage = [fullImage imageScaledToSize:self.view.bounds];

    // Push a view controller and give it BOTH images
}

// And in your pushed view controller

- (void)viewDidLoad
{
    _imageView.image = self.smallImage;
}

- (void)viewDidAppear:(BOOL)animated
{
    [super viewDidAppear:animated];

    _imageView.image = self.fullImage;
}

The main thing is that viewDidAppear: will be called right after the animation is done so you can switch images here without any worries.