Can I use api `UIApplicationVolumeUpButtonDownNotification` for getting volume up/down button?

I cannot see this notification in the documentation. Does it mean it is private ?

some discussion here: UIApplicationVolumeUpButtonDownNotification

Review guidelines state that you are not authorised to change the behaviour of hardware buttons:

2.5.9 Apps that alter or disable the functions of standard switches, such as the Volume Up/Down and Ring/Silent switches, or other native user interface elements or behaviors will be rejected.

Depending on how you want to use the Volume Up/Down button, you may infringe this rule and get your app rejected. If you do so in your app however, I would recommend you, on submitting app for review, to explain carefully in the comments to reviewer why you consider you don't infringe guideline 2.5.9.

AVAudioSession has a property "outputVolume".
You could look for changes to this, using Key-value observing...
...and use that to trigger your photo.

That might go against rule 2.5.9, but since Apple's Camera app uses the volume buttons, I think it should be okay.

(Note: this technique does not work in the Simulator, it needs a real device)

Perhaps something like this:

import AVFAudio

class VolumeButtonObserver {
    
    let audioSession = AVAudioSession.sharedInstance()
    var outputVolumeObservation: NSKeyValueObservation?
    
    init() {
        observeVolumeButtons()
    }

    func observeVolumeButtons() {
        do {
            try audioSession.setActive(true)
        } catch {}
        
        outputVolumeObservation = audioSession.observe(\.outputVolume) { (audioSession, changes) in
            print("A volume button was pressed")
            /// Take action here...
        }
    }
}