WiFiMIDI is a remote keyboard, drum pad and controller for MIDI. You can send MIDI signals to your MIDI compatible applications on your mac : GarageBand, Reason…

Your iPhone or iPod need to be on the same WiFi network than the Mac running MIDI software. On the MacOS computer, the application "Audio MIDI Setup" is in the Utilities folder inside the Applications folder. This application routes the MIDI signal to all MIDI applications running on the Mac.

works with an iPod Touch 4G ou an iPhone 3GS or later.
Needs iOS 12 or later.
Needs MacOS 10.4 or later.
You need specific software such as rtpMIDI to run on Windows XP/Vista/7.
rtpMIDI : http://www.tobias-erichsen.de/rtpMIDI.html