Thread safe UIImage resizing?

Question

I made this UIImage extension to get a rescaled copy:

-(UIImage*)scaleByRatio:(float) scaleRatio
{ 
    CGSize scaledSize = CGSizeMake(self.size.width * scaleRatio, self.size.height * scaleRatio);

    //The output context.   
    UIGraphicsBeginImageContext(scaledSize);
    CGContextRef context = UIGraphicsGetCurrentContext();

//Percent (101%)    
#define SCALE_OVER_A_BIT 1.01

    //Scale.
    CGContextScaleCTM(context, scaleRatio * SCALE_OVER_A_BIT, scaleRatio * SCALE_OVER_A_BIT);
    [self drawAtPoint:CGPointZero];

    //End?
    UIImage *scaledImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return scaledImage;
}

It works in the most cases, but in my recent project it outputs the original image (I save to disk right after scaling using UIImageJPEGRepresentation and imageWithData:).

I invoke the method within a background thread. Could this be the problem? How can I rewrite this to be thread safe (supposing the problem is caused by threading).

Answer 1

-(UIImage*)scaleByRatio:(float) scaleRatio
{ 
   CGSize scaledSize = CGSizeMake(self.size.width * scaleRatio, self.size.height * scaleRatio);
   CGColorSpaceRef colorSpace;
   int   bitmapBytesPerRow;
   bitmapBytesPerRow   = (size.width * 4);

   //The output context.   
   UIGraphicsBeginImageContext(scaledSize);
   CGContextRef context = context = CGBitmapContextCreate (NULL,
                                 scaledSize .width,
                                 scaledSize .height,
                                 8,      // bits per component
                                 bitmapBytesPerRow,
                                 colorSpace,
                                 kCGImageAlphaPremultipliedLast);

 //Percent (101%)    
 #define SCALE_OVER_A_BIT 1.01

    //Scale.
   CGContextScaleCTM(context, scaleRatio * SCALE_OVER_A_BIT, scaleRatio * SCALE_OVER_A_BIT);
[self drawAtPoint:CGPointZero];

   //End?
   UIImage *scaledImage = UIGraphicsGetImageFromCurrentImageContext();
   UIGraphicsEndImageContext();
   return scaledImage;
 }

In-short you have to Create CGContextRef using CGBitmapContextCreate not using UIGraphicsGetCurrentContext(); becuase UIGraphicsGetCurrentContext(); is not thrad safe.

Hope, this will help you...enjoy