You simply select a musical scale and white keys of your MIDI keyboard instantly conforms to that scale. However, you also can fiddle with a scale and come up with your own version.
If you feel experimental, you can instantly change the scale after recording your performance and hear your melody from different scale perspectives. Sometimes it works, sometimes not.
Admittedly getting some influence from iOS Garageband's scale adjustable software keyboard, I wanted to have the same musical freedom (freedom of limits) with better haptic feedback. WhiteKeys minimises the risk of playing discord notes in a song, given that you know your scale.
Currently I cannot provide any download links, but please do not hesitate to contact with me if you want to give it a try. I will be happy to send you a copy in Application form (Mac Only) and get some feedback. I can give you instructions to make it work with Pro Tools but I am afraid I can not provide help with other DAWs.