Decode ProRes with AVFoundation and VideoToolbox

// Constructing an AVAssetReader

// Create an AVAsset with an URL pointing at a local asset
AVAsset *sourceMovieAsset = [AVAsset assetWithURL:sourceMovieURL];

// Create an AVAssetReader for the asset
AVAssetReader *assetReader = [AVAssetReader assetReaderWithAsset:sourceMovieAsset 
                                                           error:&error];

// Configuring AVAssetReaderTrackOutput

// Copy the array of video tracks from the source movie
NSArray<AVAssetTrack*>  *tracks = [sourceMovieAsset tracksWithMediaType:AVMediaTypeVideo];
    
// Get the first video track
AVAssetTrack *track = [sourceMovieVideoTracks objectAtIndex:0];

// Create the asset reader track output for this video track, requesting ‘y416’ output
NSDictionary *outputSettings = @{ (id)kCVPixelBufferPixelFormatTypeKey :
                                  @(kCVPixelFormatType_4444AYpCbCr16) };

AVAssetReaderTrackOutput* assetReaderTrackOutput
= [AVAssetReaderTrackOutput assetReaderTrackOutputWithTrack:track
                                             outputSettings:outputSettings];

// Set the property to instruct the track output to return the samples 
// without copying them
assetReaderTrackOutput.alwaysCopiesSampleData = NO;
   
// Connect the the AVAssetReaderTrackOutput to the AVAssetReader
[assetReader addOutput:assetReaderTrackOutput];

// Running AVAssetReader

BOOL success = [assetReader startReading];

if (success) {
   CMSampleBufferRef sampleBuffer = NULL;
        
   // output is a AVAssetReaderOutput
   while ((sampleBuffer = [output copyNextSampleBuffer]))
   {
       CVImageBufferRef imageBuffer = CMSampleBufferGetImageBuffer(sampleBuffer);
            
       if (imageBuffer)
       {
          // Use the image buffer here
          // if imageBuffer is NULL, this is likely a marker sampleBuffer
       }
    }
}

AVAssetReaderTrackOutput* assetReaderTrackOutput
= [AVAssetReaderTrackOutput assetReaderTrackOutputWithTrack:track
                                             outputSettings:nil];

AVSampleCursor* cursor = [assetTrack makeSampleCursorAtFirstSampleInDecodeOrder];
        
AVSampleBufferRequest* request = [[AVSampleBufferRequest alloc] initWithStartCursor:cursor];
        
request.direction = AVSampleBufferRequestDirectionForward;
request.preferredMinSampleCount = 1;
request.maxSampleCount = 1;
       
AVSampleBufferGenerator* generator
= [[AVSampleBufferGenerator alloc] initWithAsset:srcAsset timebase:nil];

BOOL notDone = YES;
    
while(notDone)
{
   CMSampleBufferRef sampleBuffer = [generator createSampleBufferForRequest:request];

   // do your thing with the sampleBuffer

   [cursor stepInDecodeOrderByCount:1];
}

CMBlockBufferCreateWithMemoryBlock(kCFAllocatorDefault, sampleData, sizeof(sampleData), 
                                   kCFAllocatorMalloc, NULL, 0, sizeof(sampleData), 0, 
                                   &blockBuffer);

CMVideoFormatDescriptionCreate(kCFAllocatorDefault, kCMVideoCodecType_AppleProRes4444, 1920, 
                               1080, extensionsDictionary, &formatDescription);

CMSampleTimingInfo timingInfo;

timingInfo.duration = CMTimeMake(10, 600);
timingInfo.presentationTimeStamp = CMTimeMake(frameNumber * 10, 600);

CMSampleBufferCreateReady(kCFAllocatorDefault, blockBuffer, formatDescription, 1, 1, 
                          &timingInfo, 1, &sampleSize, &sampleBuffer);

// VTDecompressionSession Creation

CMFormatDescriptionRef formatDesc = CMSampleBufferGetFormatDescription(sampleBuffer);

CFDictionaryRef pixelBufferAttributes = (__bridge CFDictionaryRef)@{
    (id)kCVPixelBufferPixelFormatTypeKey :
    @(kCVPixelFormatType_4444AYpCbCr16) };

VTDecompressionSessionRef decompressionSession;
    
OSStatus err = VTDecompressionSessionCreate(kCFAllocatorDefault, 
                                            formatDesc, 
                                            NULL,
                                            pixelBufferAttributes, 
                                            NULL, 
                                            &decompressionSession);

// Running a VTDecompressionSession

uint32_t inFlags = kVTDecodeFrame_EnableAsynchronousDecompression;

VTDecompressionOutputHandler  outputHandler
 = ^(OSStatus status,
     VTDecodeInfoFlags infoFlags,
     CVImageBufferRef imageBuffer,
     CMTime presentationTimeStamp,
     CMTime presentationDurationVTDecodeInfoFlags)
 {
     // Handle decoder output in this block
     // Status reports any decoder errors
     // imageBuffer contains the decoded frame if there were no errors
 };

VTDecodeInfoFlags outFlags;

OSStatus err = VTDecompressionSessionDecodeFrameWithOutputHandler(decompressionSession,
                                                   sampleBuffer, inFlags, 
                                                   &outFlags, outputHandler);

// CVPixelBuffer to Metal texture: IOSurface

IOSurfaceRef surface = CVPixelBufferGetIOSurface(imageBuffer);

id <MTLTexture> metalTexture = [metalDevice newTextureWithDescriptor:descriptor
                                                           iosurface:surface 
                                                               plane:0];

// Mark the IOSurface as in-use so that it won’t be recycled by the CVPixelBufferPool
IOSurfaceIncrementUseCount(surface);

// Set up command buffer completion handler to decrement IOSurface use count again
[cmdBuffer addCompletedHandler:^(id<MTLCommandBuffer> buffer) {
     IOSurfaceDecrementUseCount(surface);
 }];

// Create a CVMetalTextureCacheRef

CVMetalTextureCacheRef metalTextureCache = NULL;

id <MTLDevice> metalDevice = MTLCreateSystemDefaultDevice();
    
CVMetalTextureCacheCreate(kCFAllocatorDefault, NULL, metalDevice, NULL, &metalTextureCache);

// Create a CVMetalTextureRef using metalTextureCache and our pixelBuffer
CVMetalTextureCacheCreateTextureFromImage(kCFAllocatorDefault,
                                          metalTextureCache,
                                          pixelBuffer,
                                          NULL,
                                          pixelFormat,
                                          CVPixelBufferGetWidth(pixelBuffer),
                                          CVPixelBufferGetHeight(pixelBuffer),
                                          0,
                                          &cvTexture);

id <MTLTexture>  texture = CVMetalTextureGetTexture(cvTexture);
// Be sure to release the cvTexture object when the Metal command buffer completes!