Question

I have the following code:

CGContextRef ctx = UIGraphicsGetCurrentContext();
UIGraphicsBeginImageContext(size);
screenImageContext = UIGraphicsGetCurrentContext();
ctx = screenImageContext;

UIGraphicsPushContext(UIGraphicsGetCurrentContext());
ctx  = UIGraphicsGetCurrentContext();

NSLog(@" %@",screenImageContext);
UIImage * result = UIGraphicsGetImageFromCurrentImageContext(); // Returns nil

UIImageWriteToSavedPhotosAlbum(result, nil, nil, nil);

UIGraphicsPopContext();
result = UIGraphicsGetImageFromCurrentImageContext(); // returns valid result

My problem is that UIGraphicsGetImageFromCurrentImageContext returns nil, while the second one after UIGraphicsPopContext returns the correct result.

The docs clearly states that UIGraphicsGetImageFromCurrentImageContext will return nil when either the context is nil or the current context isn’t a graphic context, but both these problems aren’t happening here.

If anyone could shed some light over this i’d be very grateful
Shai.

Answer 1

From my understanding of things your issue is stemming from the following line

UIGraphicsPushContext(UIGraphicsGetCurrentContext());

With this call you’re trying to make the current context the current context. This really doesn’t make any sense.

Also with your code it’s a little scrambled, you call UIGraphicsGetCurrentContext() which will return the current context, then you call UIGraphisBeginImageContext(CGSize size) which as stated in the doc’s

Creates a bitmap-based graphics context and makes it the current context

Then you get the current graphics context again which is now a bitmap-based graphics context thanks to the previous call, then you overwrite the original CGContextRef (“ctx”) that you just retrieved.

I’m not 100% certain what you were aiming to achieve with your code but if you were just trying to capture the contents of a bitmap-based context in an image and save it to the photo album then the following code will do that.

CGSize size = CGSizeMake(320, 480); //Screen Size on iPhone device
UIGraphicsBeginImageContext(size);  //Create a new Bitmap-based graphics context (also makes this the current context)
CGContextRef screenImageContext = UIGraphicsGetCurrentContext(); //get a reference to the context we just made above
NSLog(@" %@",screenImageContext);
//NOTE: without any drawring code in here this will just be a blank image (white/alpha)
// or an image set to whatever the current UIColor is set to
//So you may want to add some drawing code in here. Although TBH I'm not sure what you were originally
// trying to achieve.
UIImage * result = UIGraphicsGetImageFromCurrentImageContext(); // Returns nil
NSLog(@" %@",result); //just output this to demonstrate that it's non null/nil
UIImageWriteToSavedPhotosAlbum(result, nil, nil, nil);
UIGraphicsEndImageContext(); //Removes the current bitmap-based graphics context from the top of the stack

Hope that helps.