Using AVAssetWriter to create QuickTime movie from raw H.264 / AAC files

Using Mac OSX Objective-C, I'm trying to create a command line tool that takes a single H.264 and a single AAC file as inputs - encoded from the same source material - and, using AVAssetWriter, create a single QuickTime-compatible mov (and/or m4v) file for further editing / distribution.

What I've done so far: 1) Using AVFoundation Framework components, i.e., AVAssetWriter, AVAssetWriterInput, etc. and Core Media Framework components, i.e., CMSampleBufferCreate, CMBlockBufferReplaceDataBytes, etc. - I've prototyped the CL tool. 2) I've hooked up the input/output file URLs, created the AVAssetWriter and the AVAssetWriterInput, the CMBlockBuffer, etc. 3) When I execute my runLoop, AVAssetWriter creates the m4v file but, although it is well-formed, it is only a 136 byte file representing the movie header atoms with no video track data. 4) I have searched StackOverflow - as well as the Apple forums and the Internet generally - to find an answer to my specific set of issues.

Using the error checking in my code as well as the Xcode debugger, I find that AVAssetWriter is set up properly - it starts to construct the movie file - but CMBlockBufferReplaceDataBytes DOES NOT write H.264 NAL data into the CMBlockBuffer (as I believe I should do). So what am I missing?

Here is the relevant portion of my runLoop code:

// Create the videoFile.m4v AVAssetWriter.
AVAssetWriter *videoFileWriter = [[AVAssetWriter alloc] initWithURL:destinationURL fileType:AVFileTypeQuickTimeMovie error:&error];
NSParameterAssert(videoFileWriter);
if (error) {
    NSLog(@"AVAssetWriter initWithURL failed with error= %@", [error localizedDescription]);
}

// Create the video file settings dictionary.
NSDictionary *videoSettings = [NSDictionary dictionaryWithObjectsAndKeys: AVVideoCodecH264, AVVideoCodecKey, [NSNumber numberWithInt:1280],
                               AVVideoWidthKey, [NSNumber numberWithInt:720], AVVideoHeightKey, nil];

// Perform video settings check.
if ([videoFileWriter canApplyOutputSettings:videoSettings forMediaType:AVMediaTypeVideo]) {
    NSLog(@"videoFileWriter can apply videoSettings...");    
}

// Create the input to the videoFileWriter AVAssetWriter.
AVAssetWriterInput *videoFileWriterInput = [[AVAssetWriterInput alloc] initWithMediaType:AVMediaTypeVideo outputSettings:videoSettings];
videoFileWriterInput.expectsMediaDataInRealTime = YES;
NSParameterAssert(videoFileWriterInput);
NSParameterAssert([videoFileWriter canAddInput:videoFileWriterInput]);

// Connect the videoFileWriterInput to the videoFileWriter.
if ([videoFileWriter canAddInput:videoFileWriterInput]) {
   [videoFileWriter addInput:videoFileWriterInput]; 
}

// Get the contents of videoFile.264 (using current Mac OSX methods).
NSData *sourceData = [NSData dataWithContentsOfURL:sourceURL];
const char *videoFileData = [sourceData bytes];
size_t sourceDataLength = [sourceData length];
NSLog(@"The value of 'sourceDataLength' is: %ld", sourceDataLength);

// Set up to create the videoSampleBuffer.
int32_t videoWidth = 1280;
int32_t videoHeight = 720;
CMBlockBufferRef videoBlockBuffer = NULL;
CMFormatDescriptionRef videoFormat = NULL;
CMSampleBufferRef videoSampleBuffer = NULL;
CMItemCount numberOfSampleTimeEntries = 1;
CMItemCount numberOfSamples = 1;

// More set up to create the videoSampleBuffer.
CMVideoFormatDescriptionCreate(kCFAllocatorDefault, kCMVideoCodecType_H264, videoWidth, videoHeight, NULL, &videoFormat);
result = CMBlockBufferCreateWithMemoryBlock(kCFAllocatorDefault, NULL, 150000, kCFAllocatorDefault, NULL, 0, 150000, kCMBlockBufferAssureMemoryNowFlag,
                                            &videoBlockBuffer);
NSLog(@"After 'CMBlockBufferCreateWithMemoryBlock', 'result' is: %d", result);

// The CMBlockBufferReplaceDataBytes method is supposed to write videoFile.264 data bytes into the videoSampleBuffer.
result = CMBlockBufferReplaceDataBytes(videoFileData, videoBlockBuffer, 0, 150000);
NSLog(@"After 'CMBlockBufferReplaceDataBytes', 'result' is: %d", result);

CMSampleTimingInfo videoSampleTimingInformation = {CMTimeMake(1, 30)};
result = CMSampleBufferCreate(kCFAllocatorDefault, videoBlockBuffer, TRUE, NULL, NULL, videoFormat, numberOfSamples, numberOfSampleTimeEntries,
                              &videoSampleTimingInformation, 0, NULL, &videoSampleBuffer);
NSLog(@"After 'CMSampleBufferCreate', 'result' is: %d", result);

// Set the videoSampleBuffer to ready (is this needed?).
result = CMSampleBufferMakeDataReady(videoSampleBuffer);
NSLog(@"After 'CMSampleBufferMakeDataReady', 'result' is: %d", result);

// Start writing...
if ([videoFileWriter startWriting]) {
    [videoFileWriter startSessionAtSourceTime:kCMTimeZero];
}

// Start the first while loop (DEBUG)...

All ideas, comments, and suggestions are welcome.

Thank you!