Question

I am pretty sure this is a straightforward problem I must be confused about (I am a total newbie on the iPhone): I am trying to continuously draw shapes using the CG APIs into a bitmap object, and to always show an up-to-date version of that bitmap on the screen, inside a UIScrollView object so the user can pan, zoom, etc.

Using UIView with drawRect is no good because this is an ongoing thing — I have a separate thread where all the drawing commands are issued, and I need them to be applied and accumulated on that same framebuffer (kind of like a web page rendering engine). What I keep seeing is that all GCContext APIs related to images seem to CREATE a brand new image whenever you want it rendered to the screen. That sounds bad because it forces the creation of another copy of the image.

I guess I am looking for a way to create an off-screen bitmap, render to it as much as I want using Core Graphics, and whenever I want to, blt that image to the screen, but still retaining the ability to keep drawing to it and blt it to the screen again, later on.

Thanks in advance!

Answer 1

You could use a CGLayer. Create it once with CGLayerCreateWithContext() and later retrieve the layer’s context with CGLayerGetContext() whenever you need to draw into it.

To draw the layer into another graphics context, call CGContextDrawLayerAtPoint() or CGContextDrawLayerInRect().