Question

I need to paint an image using some data and show it on my iphone application. Since the painting takes significant time (2-3 seconds on device), I want to perform painting on a different thread. Also, I want to be able to cancel painting, change something in data and start it again. So it’s best for me to use NSOperation.

Now, when I do the drawing on the main thread, everything looks fine.
When I do exactly the same thing using NSOperation subclass, everything looks fine, but only 95% of the time. Sometimes it doesnt draw the full picture. Sometimes it doesnt draw text. Sometimes it uses different colors, there might be red/green/blue dots scattered all over the image etc etc etc.

---

I made a very short example to illustrate this:
First, we do all the painting on a main thread in a regular method:

//setting up bitmap context
 size_t width = 400;
 size_t height = 400;
 size_t bitsPerComponent = 8;
 size_t bytesPerRow = 4 * width;
 void* imageData = malloc(bytesPerRow * height);
 CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB(); 
 CGContextRef context = CGBitmapContextCreate(imageData, width, height, bitsPerComponent, bytesPerRow, colorSpace, kCGImageAlphaPremultipliedLast);
 CFRelease(colorSpace);

 //transforming it to usual coordinate system
 CGRect mapRect = CGRectMake(0, 0, width, height);
 UIGraphicsPushContext(context);
 CGContextTranslateCTM(context, 0, mapRect.size.height);
 CGContextScaleCTM(context, 1, -1);

 //actull drawing - nothing complicated here, 2 lines and 3 text strings on white background
 CGContextSetFillColorWithColor(context, [UIColor whiteColor].CGColor);
 CGContextFillRect(context, mapRect);

 CGContextSetLineWidth(context, 3);
 CGContextSetStrokeColorWithColor(context, [UIColor redColor].CGColor);

 CGContextMoveToPoint(context, 10, 10);
 CGContextAddLineToPoint(context, 20, 20);
 CGContextStrokePath(context);

 CGContextMoveToPoint(context, 20, 20);
 CGContextAddLineToPoint(context, 100, 100);
 CGContextStrokePath(context);

 [UIColor blackColor].set;
 [[NSString stringWithString:@"tag1"] drawInRect:CGRectMake(10, 10, 40, 15) withFont:[UIFont systemFontOfSize:15]];
 [[NSString stringWithString:@"tag2"] drawInRect:CGRectMake(20, 20, 40, 15) withFont:[UIFont systemFontOfSize:15]];
 [[NSString stringWithString:@"tag3"] drawInRect:CGRectMake(100, 100, 40, 15) withFont:[UIFont systemFontOfSize:15]];

 //getting UIImage from bitmap context
 CGImageRef _trueMap = CGBitmapContextCreateImage(context);
 if (_trueMap) {
  UIImage* _map = [UIImage imageWithCGImage:_trueMap];
  CFRelease(_trueMap);
  //displaying what we got
  //self.map leads to UIImageView
  self.map = _map;
 }

 //releasing context and memmory
 UIGraphicsPopContext();
 CFRelease(context);
 free(imageData);

No errors here. Always works.

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Now, I’ll subclass NSOperation and copy-paste this code there: Interface:

@interface Painter : NSOperation {
 //The controller which contains UIImageView we will use to display image
 MapViewController* mapViewController;

 CGContextRef context;
 void* imageData;
}

@property (nonatomic, assign) MapViewController* mapViewController;

- (id) initWithRootController:(MapViewController*)mvc__;

@end

Now the methods:

- (id) initWithRootController:(MapViewController*)mvc__ {
 if (self = [super init]) { 
  self.mapViewController = mvc__;
  size_t width = 400;
  size_t height = 400;
  size_t bitsPerComponent = 8;
  size_t bytesPerRow = 4 * width;
  imageData = malloc(bytesPerRow * height);
  CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB(); 
  context = CGBitmapContextCreate(imageData, width, height, bitsPerComponent, bytesPerRow, colorSpace, kCGImageAlphaPremultipliedLast);
  CFRelease(colorSpace);
 }
 return self;
}
- (void) main {
 size_t width = 400;
 size_t height = 400;

 //transforming it to usual coordinate system
 CGRect mapRect = CGRectMake(0, 0, width, height);
 UIGraphicsPushContext(context);
 CGContextTranslateCTM(context, 0, mapRect.size.height);
 CGContextScaleCTM(context, 1, -1);

 //actull drawing - nothing complicated here, 2 lines and 3 text strings on white background
 CGContextSetFillColorWithColor(context, [UIColor whiteColor].CGColor);
 CGContextFillRect(context, mapRect);

 CGContextSetLineWidth(context, 3);
 CGContextSetStrokeColorWithColor(context, [UIColor redColor].CGColor);

 CGContextMoveToPoint(context, 10, 10);
 CGContextAddLineToPoint(context, 20, 20);
 CGContextStrokePath(context);

 CGContextMoveToPoint(context, 20, 20);
 CGContextAddLineToPoint(context, 100, 100);
 CGContextStrokePath(context);

 [UIColor blackColor].set;
 [[NSString stringWithString:@"tag1"] drawInRect:CGRectMake(10, 10, 40, 15) withFont:[UIFont systemFontOfSize:15]];
 [[NSString stringWithString:@"tag2"] drawInRect:CGRectMake(20, 20, 40, 15) withFont:[UIFont systemFontOfSize:15]];
 [[NSString stringWithString:@"tag3"] drawInRect:CGRectMake(100, 100, 40, 15) withFont:[UIFont systemFontOfSize:15]];

 //getting UIImage from bitmap context
 CGImageRef _trueMap = CGBitmapContextCreateImage(context);
 if (_trueMap) {
  UIImage* _map = [UIImage imageWithCGImage:_trueMap];
  CFRelease(_trueMap);
  //displaying what we got
  [mapViewController performSelectorOnMainThread:@selector(setMap:) withObject:_map waitUntilDone:YES];
 }

 //releasing context and memmory
 UIGraphicsPopContext();
 CFRelease(context);
 free(imageData);
}

Again, no significant code changes between this 2 pieces of code. And when I start this operation like this:

NSOperationQueue* repaintQueue = [[NSOperationQueue alloc] init];
repaintQueue.maxConcurrentOperationCount = 1;
[repaintQueue addOperation:[[[Painter alloc] initWithRootController:self] autorelease]];

It will work. But not always, sometimes image will contain artifacts.

---

I’ve also made few screenshots to illustrate the issue, but couldn’t post them =(
Anyways, there is a screenshot which shows red line and 3 text lines (which is fine) and a screenshot which shows red line, no text lines and “tag2” written upside down on tab bar controller.

So what’s the problem?
I cant use Quartz with NSOperation? Is there some kind of restriction on drawing on separate threads? Is there a way to bypass those restrictions if so?
If anyone ever seen this problem, please reply.

Answer 1

A few calls in your code are form UIKit (the ones prefixed UI), and UIKit is not threadsafe. Any UI operations should be called on the main thread, or you risk weird things happening, such as artefacting.

I can’t speak for Quartz2d (or Core Grahics) itself, as I haven’t used a lot of it directly. But I do know that UIKit is not threadsafe.

The method drawInRect:withFont: is from a category added to NSString from UIKit (UIStringDrawing). These methods are not threadsafe, and that is why you are seeing strange behaviour.