AudioEngine.m

- (void)startRecordingMixerOutput
{
    // install a tap on the main mixer output bus and write output buffers to file
    
    /*  The method installTapOnBus:bufferSize:format:block: will create a "tap" to record/monitor/observe the output of the node.
	
        @param bus
            the node output bus to which to attach the tap
        @param bufferSize
            the requested size of the incoming buffers. The implementation may choose another size.
        @param format
            If non-nil, attempts to apply this as the format of the specified output bus. This should
            only be done when attaching to an output bus which is not connected to another node; an
            error will result otherwise.
            The tap and connection formats (if non-nil) on the specified bus should be identical. 
            Otherwise, the latter operation will override any previously set format.
            Note that for AVAudioOutputNode, tap format must be specified as nil.
        @param tapBlock
            a block to be called with audio buffers

		Only one tap may be installed on any bus. Taps may be safely installed and removed while
		the engine is running. */
    
    NSError *error;
    if (!_mixerOutputFileURL) _mixerOutputFileURL = [NSURL URLWithString:[NSTemporaryDirectory() stringByAppendingString:@"mixerOutput.caf"]];
    
    AVAudioMixerNode *mainMixer = [_engine mainMixerNode];
    AVAudioFile *mixerOutputFile = [[AVAudioFile alloc] initForWriting:_mixerOutputFileURL settings:[[mainMixer outputFormatForBus:0] settings] error:&error];
    NSAssert(mixerOutputFile != nil, @"mixerOutputFile is nil, %@", [error localizedDescription]);
    
    [self startEngine];
    [mainMixer installTapOnBus:0 bufferSize:4096 format:[mainMixer outputFormatForBus:0] block:^(AVAudioPCMBuffer *buffer, AVAudioTime *when) {
        NSError *error;
        BOOL success = NO;
        
        // as AVAudioPCMBuffer's are delivered this will write sequentially. The buffer's frameLength signifies how much of the buffer is to be written
        // IMPORTANT: The buffer format MUST match the file's processing format which is why outputFormatForBus: was used when creating the AVAudioFile object above
        success = [mixerOutputFile writeFromBuffer:buffer error:&error];
        NSAssert(success, @"error writing buffer data to file, %@", [error localizedDescription]);
    }];
    _isRecording = YES;
}

- (void)stopRecordingMixerOutput
{
    if (_isRecording) {
        [[_engine mainMixerNode] removeTapOnBus:0];
        _isRecording = NO;
        
        if (self.recordingIsAvailable) {            
            //Post a notificaiton that the record is complete
            //Other nodes/objects can listen to this update accordingly
            [[NSNotificationCenter defaultCenter] postNotificationName:kRecordingCompletedNotification object:nil];
        }
    }
}