AudioUnit Render callback issue (callback not invoked) - ios

Question

I have a simple requirement at this point, an iOS app that reads from an audio file and outputs to a speaker using AudioUnits. The reason behind not using high-level APIs is, at some point, I need to process the samples coming out of the audio file and eventually send it across network.
I have a code that works, reads the audio file and plays back to the speaker. The only issue here is, the render callback isn't working. The callback never gets called, neither do I receive any error while registering the same. Help is much appreciated (I am a beginner on Core Audio and this is my first question on stackoverflow, so please pardon any basic mistakes/overlooks). The piece of code I use for initializing the graph is attached.
void createMyAUGraph (MyAUGraphPlayerST *player) {
// Create a new AUGraph
CheckError(NewAUGraph(&player->graph), "New AUGraph failed");
// Generate description for output
AudioComponentDescription outputcd = {0};
outputcd.componentType = kAudioUnitType_Output;
outputcd.componentSubType = kAudioUnitSubType_RemoteIO;
outputcd.componentManufacturer = kAudioUnitManufacturer_Apple;
outputcd.componentFlags = 0;
outputcd.componentFlagsMask = 0;
// Add new node
AUNode outputNode;
CheckError(AUGraphAddNode(player->graph, &outputcd, &outputNode), "Add output node failed");
// Node for file player
AudioComponentDescription fileplayercd = {0};
fileplayercd.componentType = kAudioUnitType_Generator;
fileplayercd.componentSubType = kAudioUnitSubType_AudioFilePlayer;
fileplayercd.componentManufacturer = kAudioUnitManufacturer_Apple;
// Add new node
AUNode fileNode;
CheckError(AUGraphAddNode(player->graph, &fileplayercd, &fileNode), "Add file node failed");
// Open graph
CheckError(AUGraphOpen(player->graph), "Graph open failed");
// Retrive AudioUnit
CheckError(AUGraphNodeInfo(player->graph, outputNode, NULL, &player->outputAU), "file unit retrive failed");
CheckError(AUGraphNodeInfo(player->graph, fileNode, NULL, &player->fileAU), "file unit retrive failed");
// connect nodes
CheckError(AUGraphConnectNodeInput(player->graph, fileNode, 0, outputNode, 0), "failed to connect nodes");
// some other setup
UInt32 flag = 1;
CheckError(AudioUnitSetProperty(player->outputAU,
kAudioOutputUnitProperty_EnableIO,
kAudioUnitScope_Output,
0,
&flag,
sizeof (flag)), "Set io property failed");
// Register render callback
AURenderCallbackStruct output_cb;
output_cb.inputProc = recording_cb;
output_cb.inputProcRefCon = player;
CheckError(AudioUnitSetProperty(player->outputAU, kAudioUnitProperty_SetRenderCallback, kAudioUnitScope_Global, 0, &output_cb, sizeof (output_cb)), "callback register failed");
// initialize graph
CheckError(AUGraphInitialize(player->graph), "graph initialization failed");
}

Answer 1

You told the graph to connect your RemoteIO's input to the file player node, not to your render callback. Then you initialized the graph, which overrode your render property.
If you want to pull samples from a file to process, your processing routine or render callback will have to do so, not the connection from the player output to the RemoteIO input. So don't let the graph make that connection.
Updated answer:
On any recent iOS version, you need to first use the Audio Session API to request microphone privacy permission before starting the Audio Units, otherwise you will only get silence from the microphone.
To use Audio Units to record and play thru as well as record, try putting callbacks on both the output and input of RemoteIO, then pass the sample data between the two callbacks using a circular buffer. Inside one or both of the callbacks, you can record or modify the samples as needed. Make sure to heed real-time restrictions inside the audio context (no locks or memory management, etc.)