用Avfoundation 录像(续)

除了直接用AVCaptureMovieFileOutput，还可以把AVCaptureVideoDataOutput，AVCaptureAudioDataOutput 当作output。这两个class的delegate AVCaptureVideoDataOutputSampleBufferDelegate, AVCaptureAudioDataOutputSampleBufferDelegate 有一个- (void)captureOutput:(AVCaptureOutput )captureOutput didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer fromConnection:(AVCaptureConnection )connection 的method, 开始录像之后你可以从这个method 里面拿到每一个录下的frame，然后可以用AVAssetWriter来写入local的文件。

1. 加output

   _videoDataOutput = [AVCaptureVideoDataOutput new];
   _audioDataOutput = [AVCaptureAudioDataOutput new];
   if ([_captureSession canAddOutput:_videoDataOutput]) {
       DLog(@"added video out");
       [_captureSession addOutput:_videoDataOutput];
   }
   _videoConnection = [_videoDataOutput connectionWithMediaType:AVMediaTypeVideo];
   _videoConnection.videoOrientation = AVCaptureVideoOrientationPortrait;
   
   [_videoDataOutput setSampleBufferDelegate:self queue:_videoDataOutputQueue];
   
   if ([_captureSession canAddOutput:_audioDataOutput]) {
       DLog(@"added audio out");
       [_captureSession addOutput:_audioDataOutput];
   }
   _audioConnection = [_audioDataOutput connectionWithMediaType:AVMediaTypeAudio];
   [_audioDataOutput setSampleBufferDelegate:self queue:_audioDataOutputQueue];

2. 准备AVAssetWriter, 需要两个writerInput 分别写入video 和audio

   _assetWriter = [AVAssetWriter assetWriterWithURL:[self outputURL] fileType:AVFileTypeMPEG4 error:nil];
   _videoAssetWriterInput = [[AVAssetWriterInput alloc] initWithMediaType:AVMediaTypeVideo
                                                                   outputSettings:[_videoDataOutput recommendedVideoSettingsForAssetWriterWithOutputFileType:AVFileTypeMPEG4]];
   _videoAssetWriterInput.expectsMediaDataInRealTime = YES;
   _audioAssetWriterInput = [[AVAssetWriterInput alloc] initWithMediaType:AVMediaTypeAudio
                                                                   outputSettings:[_audioDataOutput recommendedAudioSettingsForAssetWriterWithOutputFileType:AVFileTypeMPEG4]];
   _audioAssetWriterInput.expectsMediaDataInRealTime = YES;
   
   if ([_assetWriter canAddInput:_videoAssetWriterInput]) {
       DLog(@"added video out writer");
       [_assetWriter addInput:_videoAssetWriterInput];
   }
   if ([_assetWriter canAddInput:_audioAssetWriterInput]) {
       DLog(@"added audio out writer");
       [_assetWriter addInput:_audioAssetWriterInput];
   }

3. 填写 - (void)captureOutput:(AVCaptureOutput )captureOutput didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer fromConnection:(AVCaptureConnection )connection

   - (void)captureOutput:(AVCaptureOutput *)captureOutput didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer fromConnection:(AVCaptureConnection *)connection
   {
       CFRetain(sampleBuffer);
       CMFormatDescriptionRef formatDesc = CMSampleBufferGetFormatDescription(sampleBuffer);
       CMMediaType mediaType = CMFormatDescriptionGetMediaType(formatDesc);
       CMTime currentSampleTime = CMSampleBufferGetPresentationTimeStamp(sampleBuffer);
   
       if (mediaType == kCMMediaType_Video) {
           if(_firstSample) {
               if ([_assetWriter startWriting]) {
                   [_assetWriter startSessionAtSourceTime:currentSampleTime];
                   _firstSample = NO;
               } else {
                   DLog(@"Error when starting avseet writer, %ld %@, %@", (long)_assetWriter.status, [_assetWriter.error localizedDescription], [_assetWriter.error userInfo]);
               }
               return;
           }
   
           if (_videoAssetWriterInput.readyForMoreMediaData) {
               [_videoAssetWriterInput appendSampleBuffer:bufferToWrite];
           }
       } else if (mediaType == kCMMediaType_Audio && !_firstSample) {
           if (_audioAssetWriterInput.readyForMoreMediaData) {
               [_audioAssetWriterInput appendSampleBuffer:bufferToWrite];
           }
       }
       
       CFRelease(sampleBuffer);
   }

4. 在didOutputSampleBuffer里面拿到sample buffer 之后还可以对其进行各种处理－－将其转化为CIImage ，然后就可以apply 各种filter。

   CFRetain(pixelBuffer);
   CVPixelBufferLockBaseAddress(pixelBuffer, 0 );
   
   CIImage *img = [CIImage imageWithCVPixelBuffer:pixelBuffer];
   CIFilter *filter = [CIFilter filterWithName:<filterName>]; //具体 filter可以参考[Core Image Filter Reference](https://developer.apple.com/library/prerelease/mac/documentation/GraphicsImaging/Reference/CoreImageFilterReference/index.html)
   
   ... apply filters
   
   CIImage *outputImage = [filter outputImage];
   [_ciContext render:outputImage toCVPixelBuffer:pixelBuffer]; //把处理好的ciimage 再写回buffer
   
   CVPixelBufferUnlockBaseAddress(pixelBuffer, 0);
   CFRelease(pixelBuffer);

5. 那么live preview 呢，其实如果没有别的要求的话可以直接用之前用的AVCaptureVideoPreviewLayer，但是如果要在live preview 里面显示filter 的效果的话。那么就要将frame 画到一个GLKView 上面，然后把这个view 展示给user.

   create GLKView。这里要用 EAGLContext来create CIContext 这样具体draw的时候就会用GPU 来draw 了，否则将使用CPU 来画

   _eaglContext = [[EAGLContext alloc] initWithAPI:kEAGLRenderingAPIOpenGLES2];
   _ciContext = [CIContext contextWithEAGLContext:_eaglContext options:@{kCIContextWorkingColorSpace : [NSNull null]} ];
       
   GLKView *glkView = [[GLKView alloc] initWithFrame:<frame> context:_eaglContext];
   [glkView bindDrawable];
   _livePreviewView = glkView;

   画frame

   CFRetain(pixelBuffer);
   CVPixelBufferLockBaseAddress(pixelBuffer, 0 );
   
   CIImage *img = [CIImage imageWithCVPixelBuffer:pixelBuffer];
   
   [_ciContext drawImage:outputImage inRect:previewRect fromRect:cropRect];
   [((GLKView *)_livePreviewView) display];
   
   CVPixelBufferUnlockBaseAddress(pixelBuffer, 0);
   CFRelease(pixelBuffer);

6. 最后保存到Library

   [_captureSession stopRunning];
   [_videoAssetWriterInput markAsFinished];
   [_audioAssetWriterInput markAsFinished];
   
   [_assetWriter finishWritingWithCompletionHandler:^{
       ALAssetsLibrary *library = [[ALAssetsLibrary alloc] init];
       if ([library videoAtPathIsCompatibleWithSavedPhotosAlbum:videoURL]) {
           [library writeVideoAtPathToSavedPhotosAlbum:videoURL completionBlock:^(NSURL *assetURL, NSError *error){
               if (error) {
                   ALog(@"Error when saving captured video to library, error = %@, url = %@", error, assetURL);
               }
           }];
       }
   }];