How to use custom NSView to mimic NSAlert notification modal behavior?

Question

In Xcode for macOS I need to have a view pop up modally and synchronously into action on top of (within but smaller than) the frame of the app's main window to handle a situation requiring the user provide data and press one of several buttons and then retire, giving the app's main window's view control again. (Similar to the behavior of an alert.) I've got the interface designed, but the NSView documentation has floored me. I'm hoping there's a simple procedure for causing the behavior I seek. I can't be the only person to have done this kind of thing. I'm grateful for any help you can offer, and thank you.