Question

I want to move some code that takes a couple seconds to generate a UIImage on another thread, but im getting a context error when using

    UIGraphicsBeginImageContextWithOptions(size,false,0);

before calling the dispatch to generate the image saying “invalid context 0x0” for each operation i try to do. Is this at all possible?

Answer 1

What’s New in iOS: iOS 4.0 says this:

• Drawing to a graphics context in UIKit is now thread-safe. Specifically:
  • The routines used to access and manipulate the graphics context can now correctly handle contexts residing on different threads.
  • String and image drawing is now thread-safe.
  • Using color and font objects in multiple threads is now safe to do.

It sounds like you tried something like this:

UIGraphicsBeginImageContextWithOptions(size,false,0);
dispatch_async(someQueue, ^{
    [UIColor.whiteColor setFill];
    UIRectFill(0, 0, 20, 20);
    dispatch_async(dispatch_get_main_queue(), ^{
        UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
        UIGraphicsEndImageContext();
        self.imageView.image = image;
    });
};

That won’t work because each thread has its own stack of graphics contexts (starting in iOS 4.0). You need to do it like this:

dispatch_async(someQueue, ^{
    UIGraphicsBeginImageContextWithOptions(size,false,0);
    [UIColor.whiteColor setFill];
    UIRectFill(0, 0, 20, 20);
    UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    dispatch_async(dispatch_get_main_queue(), ^{
        self.imageView.image = image;
    });
};

UPDATE

The documentation for UIGraphicsBeginImageContextWithOptions and other UIKit graphics functions now says

In iOS 4 and later, you may call this function from any thread of your app.

The documentation for UIColor says

Color objects are immutable and so it is safe to use them from multiple threads in your app.

The documentation for UIFont says

Font objects are immutable and so it is safe to use them from multiple threads in your app.

However, the documentation for the UIKit NSString-drawing additions says

The methods described in this class extension must be used from your app’s main thread.

So you must not try something like [@"hello" drawAtPoint:CGPointZero withAttributes:attrs] from a background thread.