iOS how to play midi notes?

Question

I have searched and already have done an OS X app that can play MIDI notes, but when i tried in iOS, nothing happened. Here is the core code:

AUGraph graph;
AudioUnit synthUnit;
AUNode synthNode, outNode;

NewAUGraph(&graph);

AudioComponentDescription cd;
cd.componentManufacturer = kAudioUnitManufacturer_Apple;

cd.componentType = kAudioUnitType_MusicDevice;
cd.componentSubType = kAudioUnitSubType_MIDISynth;
AUGraphAddNode(graph, &cd, &synthNode);

cd.componentType = kAudioUnitType_Output;
cd.componentSubType = kAudioUnitSubType_GenericOutput;
AUGraphAddNode(graph, &cd, &outNode), "AUGraphAddNode");

CheckError(AUGraphOpen(graph), "AUGraphOpen");

AUGraphConnectNodeInput(graph, synthNode, 0, outNode, 0);

AUGraphNodeInfo(graph, synthNode, 0, &synthUnit);

AUGraphInitialize(graph);
CAShow(graph);

AUGraphStart(graph);

CFURLRef bankURL = ... //gs_instruments.dls
AudioUnitSetProperty(synthUnit,
                    kMusicDeviceProperty_SoundBankURL,
                    kAudioUnitScope_Global,
                    0,
                    &bankURL,
                    sizeof(bankURL));

static UInt32 kChannelMessage_NoteOn = 0x90;
UInt8 channel = 0;
UInt8 note = 60;
UInt32 velocity = 127;

MusicDeviceMIDIEvent(synthUnit,
                    kChannelMessage_NoteOn | channel,
                    note,
                    velocity,
                    0);

AUGraphStop(graph);
DisposeAUGraph(graph);

I know the cd.componentType and cd.componentSubType settings may be not correct, because the difference between iOS app and OS X app is just it. In OS X:

AudioComponentDescription cd;
cd.componentManufacturer = kAudioUnitManufacturer_Apple;

cd.componentType = kAudioUnitType_MusicDevice;
cd.componentSubType = kAudioUnitSubType_DLSSynth;
AUGraphAddNode(graph, &cd, &synthNode);

cd.componentType = kAudioUnitType_Output;
cd.componentSubType = kAudioUnitSubType_DefaultOutput;
AUGraphAddNode(graph, &cd, &outNode);

How to set cd and play MIDI note correctly in iOS? I don't have clearly or deeply understanding on Audio Unit / AUGraph...

UPDATED: Using cd.componentSubType = kAudioUnitSubType_RemoteIO; when adding outNode will be right and play soft MIDI notes successfully.

Answer 1

kAudioUnitSubType_GenericOutput should be kAudioUnitSubType_RemoteIO for iOS.

You need to let the graph run for audio to play, so don't immediately call AUGraphStop() and DisposeAUGraph().

For testing the graph, I would use kAudioUnitSubType_Sampler instead of kAudioUnitSubType_MIDISynth because it has a built in wave generator if you don't set it's preset. Then when you get the graph working, try out your synth.