Allow mixed state but not on click with NSButton (checkbox)

Question

I have a checkbox (NSButton) that I set programmatically. This checkbox will be set to other on, off or mixed, but when the user clicks it I want it to only cycle between on and off.

Currently, when button.allowsMixedState = true when the user clicks on the checkbox it will cycle between all three states.

Is there a way to have button.allowsMixedState = true but only have the checkbox cycle between on and off when it's clicked?

Answer 1

I was able to finally get this working, by subclassing the NSButtonCell rather than the NSButton and overriding its nextState property:

(This example was run with Swift 4)

// 0 is .off, 1 is .on and -1 is .mixed.
// We override nextState so that it can never be -1.
class MyButtonCell: NSButtonCell {
    override var nextState: Int {
        get {
            return self.state == .on ? 0 : 1
        }
    }
}

This means I can set the checkbox's state to mixed, but when the user clicks it the checkbox will only cycle between on or off.