Null Graphics Context

Question

Here is the code snippet I'm using:

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0), ^
{

    UIGraphicsBeginImageContextWithOptions(irect.size, YES, 0.0 );
    CGContextRef context = UIGraphicsGetCurrentContext();
    if (context == 0)
        NSLog (@"Null Graphics Context") ;
    else
        NSLog (@"OK") ;

    . . . .
    // Various drawing functions

    UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
  }) ;

I find that every other call to UIGraphicsGetCurrentContext returns NULL. I get this message on the console.

CGContextRestoreGState: invalid context 0x0. This is a serious mistake. This application or the library it uses uses an invalid context and thereby contributes to a general deterioration in the stability and reliability of the system. This notice is a courtesy: fix this problem. This will be a fatal mistake in the upcoming update.

What will call UIGraphicsGetCurrentContext to return an invalid graphics context?

Answer 1

, , CGRect, UIGraphicsBeginImageContextWithOptions, . , -, .