使用CGBitmapContextCreate（）和基础位图内存管理创建的CGContextRef

Question

I'm trying to get used to iOS's CGContextRef graphic contexts and can't find an answer to the following questions.

When you create a bitmap context with CGBitmapContextCreate() , you have an option to pass either a pre-allocated buffer or NULL in its data parameter. In the second case:

Pass NULL if you want this function to allocate memory for the bitmap. This frees you from managing your own memory, which reduces memory leak issues.

Ok, that seems to be a good approach to adopt. But what Apple's documentation lacks is a clear explanations of the following moments:

• When does the data buffer get freed in the case you passed NULL to CGBitmapContextCreate() ? Is it correct that it happens when you release the context with CGContextRelease() ?
• And as opposed, when you passed a pointer to a pre-allocated buffer to CGBitmapContextCreate() , is it safe to suppose that after calling CGContextRelease() , you still have valid (not corrupted in some way) bitmap in your buffer? Or you must not use the generated bitmap after the CGContextRelease() call?

Could someone experienced share their knowledge on this matter?

Answer 1

If you didn't create the buffer (passed NULL), it will get released by it's creator when you release the context..

If you create and pass a buffer, you're responsible for freeing it after you release the context. The context is only an abstraction for writing and reading that buffer, and releasing it doesn't affect the data in your buffer, your bitmap data is still in the buffer. You should be able to verify this easily by making a second bitmap context out of the buffer and saving it to a file for example.