I would like to know how to switch the code view between Release and Debug modes.

You can see: in the first image, the default project have DEBUG macro. So in perview, release code segment is grey.

But!!!

In edit scheme, I change all of item to release mode! (See second image)

At this point, the DEBUG code block is still highlighted.

If I compile and run it, the output is actually from the Release code block.

In other words, I have changed all the modes in the scheme, but the preview mode is still in DEBUG.