How to create a CGBitmapContext which works for Retina display and not wasting space for regular display?

Question

Is it true that if it is in UIKit, including drawRect, the HD aspect of Retina display is automatically handled? So does that mean in drawRect, th

Answer 1

After beginning the new image context, you can retrieve it using UIGraphicsGetCurrentContext. Then, if you want to hang onto it and reuse it thereafter, just retain it like you would any CF object (and remember to release it when you're done with it, in accordance with the rules). You still have to call UIGraphicsEndImageContext to pop it off of UIKit's context stack, but if you've retained the context, then the context will live on afterward and you should be able to continue using it until you release it.

Later, if you want to use the context again (and haven't released it yet), one way would be to call UIGraphicsPushContext, which will push the context back onto the context stack.

The other way to use the context would be to assume it's a CGBitmapContext (the UIKit docs call it a “bitmap-based context” but don't say CGBitmapContext by name) and use CGBitmapContextCreateImage to capture a new image from the context after drawing.

The main difference is that, if you've created the context with UIGraphicsCreateImageContextWithOptions, UIGraphicsGetImageFromCurrentImageContext returns a UIImage whose scale should match the value you created the context with. (I don't know whether that scale value gets preserved if you pop the context and then push it back later.) CGBitmapContextCreateImage returns a CGImage, and CGImage only knows pixels.

The other difference is that UIKit drawing APIs, such as UIBezierPath, work on the current context in UIKit's context stack. Thus, if you don't push the context, you can only use Quartz APIs to draw into the context.

I haven't tested any of the above, so you should test it thoroughly yourself before doing this in code that you will deliver to users.