UIGraphicsGetImageFromCurrentImageContext memory leak with previews

Question

I'm trying to create previews images of pages in a PDF but I have some problems with the release of memory.

I wrote a simple test algorithm that cycles on the problem, the app crashes near the 40th iteration:

NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *documentsDirectory = [paths objectAtIndex:0];
NSString *pdfPath = [documentsDirectory stringByAppendingPathComponent:@"myPdf.pdf"];
CFURLRef url = CFURLCreateWithFileSystemPath( NULL, (CFStringRef)pdfPath, kCFURLPOSIXPathStyle, NO );
CGPDFDocumentRef myPdf = CGPDFDocumentCreateWithURL( url );
CFRelease (url);
CGPDFPageRef page = CGPDFDocumentGetPage( myPdf, 1 );

int i=0;
while(i < 1000){

    UIGraphicsBeginImageContext(CGSizeMake(768,1024));
    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextSetRGBFillColor(context, 1.0,1.0,1.0,1.0);
    CGContextFillRect(context,CGRectMake(0, 0, 768, 1024));
    CGContextSaveGState(context);
    CGContextTranslateCTM(context, 0.0, 1024);
    CGContextScaleCTM(context, 1.0, -1.0);
    CGContextDrawPDFPage(context, page);
    CGContextRestoreGState(context);

    // --------------------------
    // The problem is here (without this line the application doesn't crash)
    UIImageView *backgroundImageView1 = [[UIImageView alloc] initWithImage:UIGraphicsGetImageFromCurrentImageContext()];
    // --------------------------

    UIGraphicsEndImageContext();
    [backgroundImageView1 release];

    NSLog(@"Loop: %d", i++);
}

CGPDFDocumentRelease(myPdf);

The above-mentioned line seems to generate a memory leak, however, instruments doesn't show memory problems;

Can I escape from this kind of mistake?someone can explain me in which way? Are there other ways to show previews of a pdf?

UPDATE

I think the problem isn't the release of UIImage created by the method UIGraphicsGetImageFromCurrentImageContext() but the release of UIImageView created with this autorelease image.

I have divided the line of code in three steps:

UIImage *myImage = UIGraphicsGetImageFromCurrentImageContext();
UIImageView *myImageView = [[UIImageView alloc] init];
[myImageView setImage: myImage]; // Memory Leak

The first and second lines doesn't create memory leaks so I think that the method UIGraphicsGetImageFromCurrentImageContext is not the problem.

I also tried as follows but the problem persists:

UIImageView *myImageView = [[UIImageView alloc] initWithImage:myImage];

I think there is a memory leak in the release of a UIImageView that contains a UIImage with the autorelease property.

I tried to write my object UIImageView inheriting a UIView as explained in this thread.

This solution works but isn't very elegant, it's a workaround, I would prefer to use the object UIImageView solving the memory problem.

Answer 1

The problem is this:

UIGraphicsGetImageFromCurrentImageContext()

returns an autoreleased UIImage. The autorelease pool holds on to this image until your code returns control to the runloop, which you do not do for a long time. To solve this problem, you would have to create and drain a fresh autorelease pool on every iteration (or every few iterations) of your while loop.

Answer 2

I know it's an old question, but I've just been banging my head against the wall on this for a few hours. In my app repeatedly calling

UIImage *image = UIGraphicsGetImageFromCurrentImageContext()

in a loop does hold on to the memory despite me calling image = nil; Not sure how long the app would keep hold of the memory before freeing, but it's certainly long enough for my app to get a memory warning then crash.

I managed to solve it finally by wrapping the code that calls / uses the image from UIGraphicsGetImageFromCurrentImageContext() in @autoreleasepool. So I have:

@autoreleasepool {
    UIImage *image = [self imageWithView:_outputImageView]; //create the image
    [movie addImage:image frameNum:i fps:kFramesPerSec];    //use the image as a frame in movie
    image = nil;
}

Hope that might help someone.

Answer 3

For future reference here's what I did to solve this (tested in Swift 4).

I was calling the function below for every new image downloaded from the internet (on a utility queue). Before implementing the autorelease pool it would crash after processing about 100.

For simplicity, in the resizeImage function I've removed needed code except for the autoreleasepool and the part that was leaking.

private func resizeImage(image: UIImage, toHeight: CGFloat) -> UIImage {
    return autoreleasepool { () -> UIImage in
        [...]

        let newImage = UIGraphicsGetImageFromCurrentImageContext() //Leaked
        UIGraphicsEndImageContext()

        return newImage!
    }

}

I hope this helps!