Confusion over CoreMIDI Destinations - swift

Question

Given the following code if I use the first method in the if branch to obtain a MIDIDestination the code works correctly, and MIDI data is sent. If I use the second method from the else branch, no data is sent.
var client = MIDIClientRef()
var port = MIDIPortRef()
var dest = MIDIEndpointRef()
MIDIClientCreate("jveditor" as CFString, nil, nil, &client)
MIDIOutputPortCreate(client, "output" as CFString, &port)
if false {
dest = MIDIGetDestination(1)
} else {
var device = MIDIGetExternalDevice(0)
var entity = MIDIDeviceGetEntity(device, 0)
dest = MIDIEntityGetDestination(entity, 0)
}
var name: Unmanaged<CFString>?
MIDIObjectGetStringProperty(dest, kMIDIPropertyDisplayName, &name)
print(name?.takeUnretainedValue() as! String)
var gmOn : [UInt8] = [ 0xf0, 0x7e, 0x7f, 0x09, 0x01, 0xf7 ]
var pktlist = MIDIPacketList()
var current = MIDIPacketListInit(&pktlist)
current = MIDIPacketListAdd(&pktlist, MemoryLayout<MIDIPacketList>.stride, current, 0, gmOn.count, &gmOn)
MIDISend(port, dest, &pktlist)
In both cases the printed device name is correct, and the status of every call is noErr.
I have noticed that if I ask for the kMIDIManufacturerName property that I get different results - specifically using the first method I get Generic, from the USB MIDI interface to which the MIDI device is connected, and with the second method I get the value of Roland configured via the Audio MIDI Setup app.
The reason I want to use the second method is specifically so that I can filter out devices that don't have the desired manufacturer name, but as above I can't then get working output.
Can anyone explain the difference between these two methods, and why the latter doesn't work, and ideally offer a suggestion as to how I can work around that?

Answer 1

It sounds like you want to find only the MIDI destination endpoints to talk to a certain manufacturer's devices. Unfortunately that isn't really possible, since there is no protocol for discovering what MIDI devices exist, what their attributes are, and how they are connected to the computer.
(Remember that MIDI is primitive 1980s technology. It doesn't even require bidirectional communication. There are perfectly valid MIDI setups with MIDI devices that you can send data to, but can never receive data from, and vice versa.)
The computer knows what MIDI interfaces are connected to it (for instance, a USB-MIDI interface). CoreMIDI calls these "Devices". You can find out how many there are, how many ports each has, etc. But there is no way to find out anything about the physical MIDI devices like keyboards and synthesizers that are connected to them.
"External devices" are an attempt to get around the discovery problem. They are the things that appear in Audio MIDI Setup when you press the "Add Device" button. That's all!
Ideally your users would create an external device for each physical MIDI device in their setup, enter all the attributes of each one, and set up all the connections in a way that perfectly mirrors their physical MIDI cables.
Unfortunately, in reality:
There may not be any external devices. There is not much benefit to creating them in Audio MIDI Setup, and it's a lot of boring data entry, so most people don't bother.
If there are external devices, you can't trust any of the information that the users added. The manufacturer might not be right, or might be spelled wrong, for instance.
It's pretty unfriendly to force your users to set things up in Audio MIDI Setup before they can use your software. Therefore, no apps do that... and therefore nobody sets anything up in Audio MIDI Setup. It's a chicken-and-egg problem.
Even if there are external devices, your users might want to send MIDI to other endpoints (like virtual endpoints created by other apps) that are not apparently connected to external devices. You should let them do what they want.
The documentation for MIDIGetDevice() makes a good suggestion:
If a client iterates through the devices and entities in the system, it will not ever visit any virtual sources and destinations created by other clients. Also, a device iteration will return devices which are "offline" (were present in the past but are not currently present), while iterations through the system's sources and destinations will not include the endpoints of offline devices.
Thus clients should usually use MIDIGetNumberOfSources, MIDIGetSource, MIDIGetNumberOfDestinations and MIDIGetDestination, rather iterating through devices and entities to locate endpoints.
In other words: use MIDIGetNumberOfDestinations and MIDIGetDestination to get the possible destinations, then let your users pick one of them. That's all.
If you really want to do more:
Given a destination endpoint, you can use MIDIEndpointGetEntity and MIDIEndpointGetDevice to get to the MIDI interface.
Given any MIDI object, you can find its connections to other objects. Use MIDIObjectGetDataProperty to get the value of property kMIDIPropertyConnectionUniqueID, which is an array of the unique IDs of connected objects. Then use MIDIObjectFindByUniqueID to get to the object. The outObjectType will tell you what kind of object it is.
But that's pretty awkward, and you're not guaranteed to find any useful information.

Answer 2

Based on a hint from Kurt Revis's answer, I've found the solution.
The destination that I needed to find is associated with the source of the external device, with the connection between them found using the kMIDIPropertyConnectionUniqueID property of that source.
Replacing the code in the if / else branch in the question with the code below works:
var external = MIDIGetExternalDevice(0)
var entity = MIDIDeviceGetEntity(external, 0)
var src = MIDIEntityGetSource(entity, 0)
var connID : Int32 = 0
var dest = MIDIObjectRef()
var type = MIDIObjectType.other
MIDIObjectGetIntegerProperty(src, kMIDIPropertyConnectionUniqueID, &connID)
MIDIObjectFindByUniqueID(connID, &dest, &type)
A property dump suggests that the connection Unique ID property is really a data property (perhaps containing multiple IDs) but the resulting CFData appears to be in big-endian format so reading it as an integer property instead seems to work fine.