Today I discovered how to use Xcode to debug itself, and therefore also, my plugins!
However, I discovered today that you can edit the Run phase of your plugin’s scheme to start a new copy of Xcode that has your plugin in it. To do this, edit your scheme:
and in the Run phase, set the Executable to Xcode.app.
Now, when you press
⌘-R (or click on the play icon for your plugin), a new copy of Xcode will be launched and will stop on breakpoints within your plugin.