How do I create an Inter App MIDI In port - objective-c

Question

I will program an inter App MIDI In Port in my Arranger App, that can be accessed by other MIDI App's. I would appreciate very much to get some sample code. I built a virtual MIDI In port like this, but how to make it visible for other App's:
MIDIClientRef virtualMidi;
result = MIDIClientCreate(CFSTR("Virtual Client"), MyMIDINotifyProc, NULL, &virtualMidi);

Answer 1

You need to use MIDIDestinationCreate, which will be visible to other MIDI Clients. You need to provide a MIDIReadProc callback that will be notified when a MIDI event arrives to your MIDI Destination. You may create another MIDI Input Port as well, with the same callback, that you can connect yourself from within your own program to an external MIDI Source.
Here is an example (in C++):
void internalCreate(CFStringRef name)
{
OSStatus result = noErr;
result = MIDIClientCreate( name , nullptr, nullptr, &m_client );
if (result != noErr) {
qDebug() << "MIDIClientCreate() err:" << result;
return;
}
result = MIDIDestinationCreate ( m_client, name, MacMIDIReadProc, (void*) this, &m_endpoint );
if (result != noErr) {
qDebug() << "MIDIDestinationCreate() err:" << result;
return;
}
result = MIDIInputPortCreate( m_client, name, MacMIDIReadProc, (void *) this, &m_port );
if (result != noErr) {
qDebug() << "MIDIInputPortCreate() error:" << result;
return;
}
}
Another example, in ObjectiveC from symplesynth
- (id)initWithName:(NSString*)newName
{
PYMIDIManager* manager = [PYMIDIManager sharedInstance];
MIDIEndpointRef newEndpoint;
OSStatus error;
SInt32 newUniqueID;
// This makes sure that we don't get notified about this endpoint until after
// we're done creating it.
[manager disableNotifications];
MIDIDestinationCreate ([manager midiClientRef], (CFStringRef)newName, midiReadProc, self, &newEndpoint);
// This code works around a bug in OS X 10.1 that causes
// new sources/destinations to be created without unique IDs.
error = MIDIObjectGetIntegerProperty (newEndpoint, kMIDIPropertyUniqueID, &newUniqueID);
if (error == kMIDIUnknownProperty) {
newUniqueID = PYMIDIAllocateUniqueID();
MIDIObjectSetIntegerProperty (newEndpoint, kMIDIPropertyUniqueID, newUniqueID);
}
MIDIObjectSetIntegerProperty (newEndpoint, CFSTR("PYMIDIOwnerPID"), [[NSProcessInfo processInfo] processIdentifier]);
[manager enableNotifications];
self = [super initWithMIDIEndpointRef:newEndpoint];
ioIsRunning = NO;
return self;
}

Answer 2

Ports can't be discovered from the API, but sources and destinations can. You want to create a MIDISource or MIDIDestination so that MIDI clients can call MIDIGetNumberOfDestinations/MIDIGetDestination or MIDIGetNumberOfSources/MIDIGetSource and discover it.
FYI, there is no need to do what you are planning to do on macOS because the IAC driver already does it. If this is for iOS, these are the steps to follow:
Create at least one MIDI Client.
Create a MIDIInputPort with a read block for I/O.
Use MIDIPortConnectSource to attach the input port to every MIDI Source of interest.
[From now, every MIDI message received by the source will come to your read block.]
If you want to resend this data to a different destination, you'll need to have created a MIDIOutputPort as well. Use MIDISend with that port to the desired MIDI Destination.