How to tell NSTextView which side to extend selection on?

Question

If I have an NSTextView which is in this state:

How can I tell the textview that if a user presses shift+right, that rather than extending right towards the 'o', it instead de-selects the 'e'? I though this had to do with the affinity, but I have tried setting it to both NSSelectionAffinityUpstream and NSSelectionAffinityDownstream via the following code:

[self setSelectionRange: NSMakeRange(9,6)
               affinity: x
         stillSelecting: NO];

But that made no different. Hitting shift+right still selected the 'o'.

NSTextView knows how to do this SOMEHOW, because if you cursor position between 'w' and 'o', then hit shift+left until it matches the screenshot, then hit shift+right, it matches the behaviour I mentioned.

I'm ok to override the shift+arrow code and roll my own, but I would rather allow NSTextView to do its own thing. Anyone know if I am missing anything?

Answer 1

I'm not sure what you're trying to do, since this is the default text movement/selection modification behavior. Are you trying to override this to always do this one thing or are you trying to add another keyboard shortcut that overrides this behavior? In either case, some background:

Selection affinity doesn't quite work the way it sounds (this is a surprise to me after researching it just now). In fact there seems to be a disconnect between the affinity and the inherited NSResponder actions corresponding to movement and selection modification. I'll get to that in a moment.

What you want to look at are the responder actions like -moveBackwardAndModifySelection:, -moveWordBackwardAndModifySelection:, and so on. Per the documentation, the first call to such selection-modifying movement actions determines the "end" of the selection that will be modified with subsequent calls to modify selection in either direction. This means that if your first action was to select forward (-moveForwardAndModifySelection:), the "forward end" (right end in left-to-right languages; left end in right-to-left, automagically) is what will be modified from that point forward, whether you call -moveForward... or -moveBackward....

I discovered the disconnect between affinity and -move...AndModifySelection: by looking at the open source Cocoatron version of NSTextView. It appears the internal _affinity property isn't consulted in any of the move-and-select methods. It determines whether the selection range change should be "upstream" or "downstream" based on _selectionOrigin, a private property that is set when the selection ranges are modified (via the -setSelectedRange(s)... method or a mouse down / drag). I think the affinity property is just there for you to consult. Overriding it to always return one value or another doesn't change any behavior, it just misreports to outsiders. The _selectionOrigin seems only to be modified via -setSelectedRanges... method if the selection is zero-length (ie, just cursor placement).

So, with all this in mind, you might have to add one step to your manual selection modification: First set an empty selection with a location where you'd like the selection origin to be (forward end if you want backward affinity; backward end if you want forward affinity), then set a non-zero-length selection with the desired affinity.

Roundabout and ridiculous, I know, but I think that's how it has to be, given the CocoaTron source code.