The UIImagePickerController class has been the developer's best friend and worst
enemy since its introduction into the iPhone SDK in iOS 2.0. For those who have not
used it, this class provides a way for an application to have the user select and
return an image, whether it be existing from the device's library or fresh from the
camera; this image may then be included into the application in some manner. Today
we are going to discuss a problem that is almost as common as the use of the class
itself! I'm talking about memory warnings and crashes.
Images on Display
I think one of the most common issues stems from developers trying to do something akin to the following code:
- (void)someActionMethod {
UIImagePickerController *controller = [[UIImagePickerController alloc] init];
[controller setDelegate:self];
[self presentModalViewController:controller animated:YES];
[controller release];
}
- (void)imagePickerController:(UIImagePickerController *)picker
didFinishPickingMediaWithInfo:(NSDictionary *)info {
[[picker parentViewController] dismissModalViewControllerAnimated:YES];
UIImage *image = [info objectForKey:UIImagePickerControllerOriginalImage];
[myImageView setImage:image];
}
Translated into plain text, "as soon as the user selects an image and we get a callback, display that image in some view and move on". Eek!
Most often (even on the high-powered iPhone 4), this will result in one of the two following behaviors:
- Your app will crash...hard.
- After the user dismisses the picker, you get a memory warning, and the view controller that presented the picker looks as though it has been reloaded from scratch (...because it has)
This will happen most often with images from the camera, but as images stored in the library get larger and larger you will see it happen there as well. The issue is a combination of memory recovery behavior and processing power.
My Brain Hurts
The first thing many developers notice is the memory warning that arises in the console circa the time of the failure...and so they tend to focus on it. Because it's the only visual clue they have to start diagnostics from. The memory warning is usually marked with a "Level=1", which indicates it's a warning and not critical or some other level. Even when your application is running perfectly, you will likely still get these warnings every time you use an image picker...and that's okay! However, what the warning does signal to you is that the wrong choice of your next move will result in one of the two behaviors above.
On an iPhone 3GS, for example, the camera is 3 megapixels. UIImagePickerController
gives you the full image back when you retrieve it in the delegate callback
(that's 2048 x 1536 pixels)! Storing this much image data in memory is one thing,
but forcing your UI thread to try and do something with it will only spell disaster.
iOS will try to first free up as much memory as it can by releasing view controller
objects (symptom #2). If it is unsuccessful, dire circumstances await (symptom #1).
What's the Right Move?
What you do with the image (display it, save it to disk, etc.) depends on your application. But there are two universal truths about the image data that comes back:
- The image SHOULD be scaled down
- The processing MUST not be done on the main/UI thread
Scaling
There is a really great piece of useful code in the community that I cannot take credit for, but I will post here for convenience. This code uses UIKit methods to resize and scale down an image.
- (void)useImage:(UIImage *)image {
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
// Create a graphics image context
CGSize newSize = CGSizeMake(320, 480);
UIGraphicsBeginImageContext(newSize);
// Tell the old image to draw in this new context, with the desired
// new size
[image drawInRect:CGRectMake(0,0,newSize.width,newSize.height)];
// Get the new image from the context
UIImage* newImage = UIGraphicsGetImageFromCurrentImageContext();
// End the context
UIGraphicsEndImageContext();
[userPhotoView setImage:newImage];
[pool release];
}
Even if you intend to save the full image to disk and you want to display a preview,
that preview should not be larger than the viewport of the device (320x480 or 640x960).
Forcing UIKit to scale the image for you at runtime with a call to setImage: is
a bad plan.
Backround Threading
Many of you may have looked at the previous method and asked why an NSAutoreleasePool
was created; that is because of the next step. Even if you have the presence of mind
to scale down your image before displaying it, if you try to do so on the main thread
you will end up with the same results as if you had left that job up to iOS.
Because of this, the scaling method we have written should be dispatched into a
background thread, and our delegate method gets modified like so:
- (void)imagePickerController:(UIImagePickerController *)picker
didFinishPickingMediaWithInfo:(NSDictionary *)info {
[[picker parentViewController] dismissModalViewControllerAnimated:YES];
UIImage *image = [info objectForKey:UIImagePickerControllerOriginalImage];
[NSThread detachNewThreadSelector:@selector(useImage:) toTarget:self withObject:image];
}
You may also want to consider popping up a progress dialog or something during the operation (Apple does this as well, try taking a picture for your contacts), as it can take a few seconds.
Conclusion
There, now we have a working example of how to properly handle picked images that we want to display in our applications. Hopefully this will keep some of you from going bald too soon.