Can I use AVAudioEngine to read from a file, process with an audio unit and write to a file, faster than real-time?

1. Configure your engine and player node.
2. Call play method for your player node.
3. Pause your engine.
4. Get an audio unit from your AVAudioOutputNode (audioEngine.outputNode)with this method.
5. Render from audio unit with AudioUnitRender in cycle and write audio buffer list to file with Extended Audio File Services.

Example:

Audio engine configuration

- (void)configureAudioEngine {    self.engine = [[AVAudioEngine alloc] init];    self.playerNode = [[AVAudioPlayerNode alloc] init];    [self.engine attachNode:self.playerNode];    AVAudioUnitDistortion *distortionEffect = [[AVAudioUnitDistortion alloc] init];    [self.engine attachNode:distortionEffect];    [self.engine connect:self.playerNode to:distortionEffect format:[distortionEffect outputFormatForBus:0]];    self.mixer = [self.engine mainMixerNode];    [self.engine connect:distortionEffect to:self.mixer format:[self.mixer outputFormatForBus:0]];    [distortionEffect loadFactoryPreset:AVAudioUnitDistortionPresetDrumsBitBrush];    NSError* error;    if (![self.engine startAndReturnError:&error])        NSLog(@"Can't start engine: %@", error);    else        [self scheduleFileToPlay];}- (void)scheduleFileToPlay {    NSError* error;    NSURL *fileURL = [[NSBundle mainBundle] URLForResource:@"filename" withExtension:@"m4a"];    self.file = [[AVAudioFile alloc] initForReading:fileURL error:&error];    if (self.file)        [self.playerNode scheduleFile:self.file atTime:nil completionHandler:nil];    else        NSLog(@"Can't read file: %@", error);}

Rendering methods

- (void)renderAudioAndWriteToFile {    [self.playerNode play];    [self.engine pause];    AVAudioOutputNode *outputNode = self.engine.outputNode;    AudioStreamBasicDescription const *audioDescription = [outputNode outputFormatForBus:0].streamDescription;    NSString *path = [self filePath];    ExtAudioFileRef audioFile = [self createAndSetupExtAudioFileWithASBD:audioDescription andFilePath:path];    if (!audioFile)        return;    AVURLAsset *asset = [AVURLAsset assetWithURL:self.file.url];    NSTimeInterval duration = CMTimeGetSeconds(asset.duration);    NSUInteger lengthInFrames = duration * audioDescription->mSampleRate;    const NSUInteger kBufferLength = 4096;    AudioBufferList *bufferList = AEAllocateAndInitAudioBufferList(*audioDescription, kBufferLength);    AudioTimeStamp timeStamp;    memset (&timeStamp, 0, sizeof(timeStamp));    timeStamp.mFlags = kAudioTimeStampSampleTimeValid;    OSStatus status = noErr;    for (NSUInteger i = kBufferLength; i < lengthInFrames; i += kBufferLength) {        status = [self renderToBufferList:bufferList writeToFile:audioFile bufferLength:kBufferLength timeStamp:&timeStamp];        if (status != noErr)            break;    }    if (status == noErr && timeStamp.mSampleTime < lengthInFrames) {        NSUInteger restBufferLength = (NSUInteger) (lengthInFrames - timeStamp.mSampleTime);        AudioBufferList *restBufferList = AEAllocateAndInitAudioBufferList(*audioDescription, restBufferLength);        status = [self renderToBufferList:restBufferList writeToFile:audioFile bufferLength:restBufferLength timeStamp:&timeStamp];        AEFreeAudioBufferList(restBufferList);    }    AEFreeAudioBufferList(bufferList);    ExtAudioFileDispose(audioFile);    if (status != noErr)        NSLog(@"An error has occurred");    else        NSLog(@"Finished writing to file at path: %@", path);}- (NSString *)filePath {    NSArray *documentsFolders =            NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);    NSString *fileName = [NSString stringWithFormat:@"%@.m4a", [[NSUUID UUID] UUIDString]];    NSString *path = [documentsFolders[0] stringByAppendingPathComponent:fileName];    return path;}- (ExtAudioFileRef)createAndSetupExtAudioFileWithASBD:(AudioStreamBasicDescription const *)audioDescription                                          andFilePath:(NSString *)path {    AudioStreamBasicDescription destinationFormat;    memset(&destinationFormat, 0, sizeof(destinationFormat));    destinationFormat.mChannelsPerFrame = audioDescription->mChannelsPerFrame;    destinationFormat.mSampleRate = audioDescription->mSampleRate;    destinationFormat.mFormatID = kAudioFormatMPEG4AAC;    ExtAudioFileRef audioFile;    OSStatus status = ExtAudioFileCreateWithURL(            (__bridge CFURLRef) [NSURL fileURLWithPath:path],            kAudioFileM4AType,            &destinationFormat,            NULL,            kAudioFileFlags_EraseFile,            &audioFile    );    if (status != noErr) {        NSLog(@"Can not create ext audio file");        return nil;    }    UInt32 codecManufacturer = kAppleSoftwareAudioCodecManufacturer;    status = ExtAudioFileSetProperty(            audioFile, kExtAudioFileProperty_CodecManufacturer, sizeof(UInt32), &codecManufacturer    );    status = ExtAudioFileSetProperty(            audioFile, kExtAudioFileProperty_ClientDataFormat, sizeof(AudioStreamBasicDescription), audioDescription    );    status = ExtAudioFileWriteAsync(audioFile, 0, NULL);    if (status != noErr) {        NSLog(@"Can not setup ext audio file");        return nil;    }    return audioFile;}- (OSStatus)renderToBufferList:(AudioBufferList *)bufferList                   writeToFile:(ExtAudioFileRef)audioFile                  bufferLength:(NSUInteger)bufferLength                     timeStamp:(AudioTimeStamp *)timeStamp {    [self clearBufferList:bufferList];    AudioUnit outputUnit = self.engine.outputNode.audioUnit;    OSStatus status = AudioUnitRender(outputUnit, 0, timeStamp, 0, bufferLength, bufferList);    if (status != noErr) {        NSLog(@"Can not render audio unit");        return status;    }    timeStamp->mSampleTime += bufferLength;    status = ExtAudioFileWrite(audioFile, bufferLength, bufferList);    if (status != noErr)        NSLog(@"Can not write audio to file");    return status;}- (void)clearBufferList:(AudioBufferList *)bufferList {    for (int bufferIndex = 0; bufferIndex < bufferList->mNumberBuffers; bufferIndex++) {        memset(bufferList->mBuffers[bufferIndex].mData, 0, bufferList->mBuffers[bufferIndex].mDataByteSize);    }}

I used some functions from this cool framework:

AudioBufferList *AEAllocateAndInitAudioBufferList(AudioStreamBasicDescription audioFormat, int frameCount) {    int numberOfBuffers = audioFormat.mFormatFlags & kAudioFormatFlagIsNonInterleaved ? audioFormat.mChannelsPerFrame : 1;    int channelsPerBuffer = audioFormat.mFormatFlags & kAudioFormatFlagIsNonInterleaved ? 1 : audioFormat.mChannelsPerFrame;    int bytesPerBuffer = audioFormat.mBytesPerFrame * frameCount;    AudioBufferList *audio = malloc(sizeof(AudioBufferList) + (numberOfBuffers-1)*sizeof(AudioBuffer));    if ( !audio ) {        return NULL;    }    audio->mNumberBuffers = numberOfBuffers;    for ( int i=0; i<numberOfBuffers; i++ ) {        if ( bytesPerBuffer > 0 ) {            audio->mBuffers[i].mData = calloc(bytesPerBuffer, 1);            if ( !audio->mBuffers[i].mData ) {                for ( int j=0; j<i; j++ ) free(audio->mBuffers[j].mData);                free(audio);                return NULL;            }        } else {            audio->mBuffers[i].mData = NULL;        }        audio->mBuffers[i].mDataByteSize = bytesPerBuffer;        audio->mBuffers[i].mNumberChannels = channelsPerBuffer;    }    return audio;}void AEFreeAudioBufferList(AudioBufferList *bufferList ) {    for ( int i=0; i<bufferList->mNumberBuffers; i++ ) {        if ( bufferList->mBuffers[i].mData ) free(bufferList->mBuffers[i].mData);    }    free(bufferList);}

For anyone looking for an answer to this question these days, it seems Apple has added offline processing capabilities just for this purpose: see the docs.