iphone - Must drawInRect: for a separate context be executed on the main thread?

Question

[update: this problem has been resolved; the issue was not in drawInRect: but in UIGraphicsBeginImageContext()]

In my app, I'm grabbing a bunch of large images, cropping them down to thumbnail size and storing the thumbnails for previewing.

Note that I'm doing this in a separate image context -- this is not about redrawing a UIView that is on the screen.

This code is rather intensive so I'm running it in a separate thread. The actual scaling looks like this, and is a category implementation on top of UIImage:

- (UIImage *) scaledImageWithWidth:(CGFloat)width andHeight:(CGFloat)height
{
    CGRect rect = CGRectMake(0.0, 0.0, width, height);
    UIGraphicsBeginImageContext(rect.size);
    [self drawInRect:rect]; // <-- crashing on this line
    UIImage *scaledImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return scaledImage;
}

This is called from a separate method, which loops through the images in turn and does the processing. The actual call to the above method looks like this:

UIImage *small = [bigger scaledImageWithWidth:24.f andHeight:32.f];

This all works most of the time, but occasionally I get an EXC_BAD_ACCESS.

Backtrace:

#0  0x330d678c in ripc_RenderImage ()
#1  0x330dd5aa in ripc_DrawImage ()
#2  0x300e3276 in CGContextDelegateDrawImage ()
#3  0x300e321a in CGContextDrawImage ()
#4  0x315164c8 in -[UIImage drawInRect:blendMode:alpha:] ()
#5  0x31516098 in -[UIImage drawInRect:] ()
#6  0x0000d6e4 in -[UIImage(Scaling) scaledImageWithWidth:andHeight:] (self=0x169320, _cmd=0x30e6e, width=48, height=64) at /Users/me/Documents/svn/app/trunk/Classes/UIImage+Scaling.m:20
#7  0x00027df0 in -[mgMinimap loadThumbnails] (self=0x13df00, _cmd=0x30d05) at /Users/me/Documents/svn/app/trunk/Classes/mgMinimap.m:167
#8  0x32b15bd0 in -[NSThread main] ()
#9  0x32b81cfe in __NSThread__main__ ()
#10 0x30c8f78c in _pthread_start ()
#11 0x30c85078 in thread_start ()

[update 4] When I run this in the Simulator, and this problem happens, the console additionally shows the following:

// the below is before loading the first thumbnail
<Error>: CGContextSaveGState: invalid context
<Error>: CGContextSetBlendMode: invalid context
<Error>: CGContextSetAlpha: invalid context
<Error>: CGContextTranslateCTM: invalid context
<Error>: CGContextScaleCTM: invalid context
<Error>: CGContextDrawImage: invalid context
<Error>: CGContextRestoreGState: invalid context
<Error>: CGBitmapContextCreateImage: invalid context
// here, the first thumbnail has finished loading and the second one
// is about to be generated
<Error>: CGContextSetStrokeColorWithColor: invalid context
<Error>: CGContextSetFillColorWithColor: invalid context

My gut feeling is that I occasionally end up trying to drawInRect: while the OS is also trying to draw something, which results, occasionally, in a crash. I always presumed that as long as you don't draw on the actual screen, this is acceptable -- is this not the case? Or if it is the case, any idea what might be causing this?

Update (r2): I forgot to mention that this app is running under rather severe memory constraints (I've got a lot of images loaded at any given time and these are swapped in/out), so this may be a case of running out of memory (read on -- it's not). I'm not sure how to verify that, though, so thoughts on this would be welcome too. I did verify this by severely cutting down on the number of images being loaded and adding a check to make sure they're properly deallocated (they are, and the crash still occurs).

Update 3: I thought I found the problem. Below is the answer I wrote, before the crash happened again:

The code would (after I posted this question) occasionally start exiting with exit code 0, and sometimes with exit code 10 (SIGBUS). 0 means "no error", so that was extremely odd. 10 seems to mean a bit of everything so that was unhelpful too. The drawInRect: call was a big hint, though, when the crash happened there.

The looping through to get the thumbnails was generating a lot of autoreleased images. I had an autorelease pool but it was wrapping the entire for loop. I added a second autorelease pool within the for loop:

- (void)loadThumbnails
{
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
    for (...) {
        NSAutoreleasePool *cyclePool = 
           [[NSAutoreleasePool alloc] init]; // <-- here
        UIImage *bigger = ...;
        UIImage *small = [bigger scaledImageWithWidth:24.f andHeight:32.f];
        UIImage *bloated = [i scaledImageWithWidth:48.f andHeight:64.f];
        [cyclePool release]; // <-- ending here
    }
    [pool release];
}

I thought the above fixed the issue, until I ran the app and it crashed on me with "exit code 0" again just earlier. Back to the drawing board...

Answer 1

Have you looked at Matt Gemmell's latest release, MGImageUtilities? I extracted this from his source on github:

// Create appropriately modified image.
UIImage *image;
UIGraphicsBeginImageContextWithOptions(destRect.size, NO, 0.0); // 0.0 for scale means "correct scale for device's main screen".
CGImageRef sourceImg = CGImageCreateWithImageInRect([self CGImage], sourceRect); // cropping happens here.
image = [UIImage imageWithCGImage:sourceImg scale:0.0 orientation:self.imageOrientation]; // create cropped UIImage.
[image drawInRect:destRect]; // the actual scaling happens here, and orientation is taken care of automatically.
CGImageRelease(sourceImg);
image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();

Not sure if thread safety is the issue, but it may be worth trying Matt's code before going too far down that path.