Creating an aggregate device using Core Audio in Objective-C

main.m

#import <Foundation/Foundation.h>
#import <CoreAudio/CoreAudio.h>
#import "utilities.h"

/*
    This example demonstrates how to create an aggregate device using Core Audio in Objective-C.
    It creates an aggregate device with two sub-devices, which are the default input and output devices.
*/

int main(int argc, const char * argv[]) {
    OSStatus status = noErr;
    
    // Get the default input and output device IDs
    AudioDeviceID defaultInputDeviceID = getDefaultInputDeviceID();
    AudioDeviceID defaultOutputDeviceID = getDefaultOutputDeviceID();
    
    // Get the UIDs of the default input and output devices
    NSString* defaultInputDeviceUID = getDeviceUID(defaultInputDeviceID);
    NSString* defaultOutputDeviceUID = getDeviceUID(defaultOutputDeviceID);
    
    // Create a dictionary to hold the aggregate device description
    NSMutableDictionary *description = [NSMutableDictionary dictionary];
    
    // Set the name of the new aggregate device
    description[@(kAudioAggregateDeviceNameKey)] = @"YourNameHere";
    
    // Set the UID of the aggregate device
    description[@(kAudioAggregateDeviceUIDKey)] = @"YourUniqueId";
    
    // Determines if the device should be visible in the "Audio MIDI Setup"
    description[@(kAudioAggregateDeviceIsPrivateKey)] = @(NO);
    
    // Array to hold the sub devices
    NSMutableArray *subDevices = [NSMutableArray array];
    
    // Input device
    NSDictionary *inputSubDevice = @{
        @(kAudioSubDeviceUIDKey) : defaultInputDeviceUID,
        @(kAudioSubDeviceDriftCompensationKey) : @(YES),
    };
    
    [subDevices addObject:inputSubDevice];

    // Output device
    NSDictionary *outputSubDevice = @{
        @(kAudioSubDeviceUIDKey) : defaultOutputDeviceUID,
        @(kAudioSubDeviceDriftCompensationKey) : @(YES),
    };
    
    [subDevices addObject:outputSubDevice];

    // Add the devices to the description
    description[@(kAudioAggregateDeviceSubDeviceListKey)] = subDevices;
    
    // This will be the ID of the newly created aggregate device
    AudioDeviceID aggregateDeviceID = 0;
    
    // Create the aggregate device
    status = AudioHardwareCreateAggregateDevice((__bridge CFDictionaryRef)description, &aggregateDeviceID);
    
    if (status != noErr) {
        NSLog(@"An error occurred while creating the aggregate device, code: %d", status);
        return -1;
    }
    
    NSLog(@"A new aggregate device has been created with ID: %d", aggregateDeviceID);
    return 0;
}

utilities.h

#ifndef helpers_h
#define helpers_h

// Get the default input device ID
AudioDeviceID getDefaultInputDeviceID(void) {
    // Initialize the device ID variable
    AudioDeviceID deviceID = kAudioDeviceUnknown;
    UInt32 propertySize = sizeof(deviceID);
    
    // Set the property address for retrieving the default input device ID
    AudioObjectPropertyAddress propertyAddress = {
        kAudioHardwarePropertyDefaultInputDevice,
        kAudioObjectPropertyScopeGlobal,
        kAudioObjectPropertyElementMain
    };
    
    // Retrieve the default input device ID
    OSStatus status = AudioObjectGetPropertyData(kAudioObjectSystemObject, &propertyAddress, 0, NULL, &propertySize, &deviceID);
    
    if (status != noErr) {
        // Error handling
        // Handle the failure to retrieve the default input device ID
    }
    
    return deviceID;
}

// Get the default output device ID
AudioDeviceID getDefaultOutputDeviceID(void) {
    // Initialize the device ID variable
    AudioDeviceID deviceID = kAudioDeviceUnknown;
    UInt32 propertySize = sizeof(deviceID);
    
    // Set the property address for retrieving the default output device ID
    AudioObjectPropertyAddress propertyAddress = {
        kAudioHardwarePropertyDefaultOutputDevice,
        kAudioObjectPropertyScopeGlobal,
        kAudioObjectPropertyElementMain
    };
    
    // Retrieve the default output device ID
    OSStatus status = AudioObjectGetPropertyData(kAudioObjectSystemObject, &propertyAddress, 0, NULL, &propertySize, &deviceID);
    
    if (status != noErr) {
        // Error handling
        // Handle the failure to retrieve the default output device ID
    }
    
    return deviceID;
}

// Get the UID of an audio device
NSString *getDeviceUID(AudioDeviceID deviceID) {
    // Initialize the status variable
    OSStatus status = noErr;
    
    // Initialize the UID and its size variables
    CFStringRef uid = NULL;
    UInt32 uidSize = sizeof(uid);
    
    // Set the property address for retrieving the device UID
    AudioObjectPropertyAddress propertyAddress = {
        kAudioDevicePropertyDeviceUID,
        kAudioObjectPropertyScopeGlobal,
        kAudioObjectPropertyElementMain
    };
    
    // Retrieve the device UID
    status = AudioObjectGetPropertyData(deviceID, &propertyAddress, 0, NULL, &uidSize, &uid);
    
    if (status != noErr) {
        return nil;
    }
    
    return (__bridge NSString *)uid;
}

#endif /* helpers_h */