get value of property returned by MIDIObjectPropertyChangeNotification

Question

I'm using the MIDI libraries of AudioKit with an app. Whilst that framework is proprietary, the code in question is returning a native struct, namely MIDIObjectPropertyChangeNotification.

Here is the code, where MIDIListener is a protocol defined by AudioKit and MIDIConnectionManager is my class for managing MIDI. In this callback, I am listening for connections and disconnections via RTP-MIDI

extension MIDIConnectionManager: MIDIListener {
func receivedMIDIPropertyChange(propertyChangeInfo: MIDIObjectPropertyChangeNotification) {
let propertyName = Unmanaged.fromOpaque(propertyChangeInfo.propertyName.toOpaque()).takeUnretainedValue() as CFString
        if propertyChangeInfo.objectType == .entity && propertyName as String == "apple.midirtp.session" {
// do something in response
}
}

As I connect and disconnect my app, the callback fires. But all it seems to know about is the property that's changed, not its value. I assumed that would also be in the MIDIObjectPropertyChangeNotification struct. But it isn't (https://developer.apple.com/documentation/coremidi/midiobjectpropertychangenotification).

I see that it does expose MIDIObjectRef ("The object whose property changed.", https://developer.apple.com/documentation/coremidi/midiobjectpropertychangenotification/1495180-object) but that refers only to "var propertyName: Unmanaged < CFString > The name of the modified property."

So I figure there must be a way of getting all the properties of that object and sure enough func MIDIObjectGetProperties (https://developer.apple.com/documentation/coremidi/1495206-midiobjectgetproperties)

But I'm fairly new to Swift and here is where my skills grind to a halt! Where next? I don't understand how to use that function and I'm a bit spooked by UnsafeMutablePointer!

score 0 · Answer 1 · answered Aug 12 '21 at 18:33

Ok partial answer here Need help converting (CFPropertyListRef *)nsdictionary to swift

From which I derive

var unmanagedProperties: Unmanaged<CFPropertyList>?
            MIDIObjectGetProperties(propertyChangeInfo.object, &unmanagedProperties, false)
            if let midiProperties: CFPropertyList = unmanagedProperties?.takeUnretainedValue() {
                let midiDictionary = midiProperties as! NSDictionary
                print("Midi properties: \n \(midiDictionary)");
            }

Hmm. All that funky close-to-the-metal pointer jazz makes me nervous :-|

But the object has a property "peers" that seems like a hook to detect a connection or otherwise.

get value of property returned by MIDIObjectPropertyChangeNotification

1 Answers1