Add RTCFileVideoCapturer class.

webrtc/sdk/objc/Framework/Classes/RTCFileVideoCapturer.m

+/**
+ *  Copyright 2017 The WebRTC Project Authors. All rights reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#import "RTCFileVideoCapturer.h"
+
+#import "WebRTC/RTCLogging.h"
+
+@implementation RTCFileVideoCapturer {
+  AVAssetReader *_reader;
+  AVAssetReaderTrackOutput *_outTrack;
+  BOOL _capturerStopped;
+  CMTime _lastPresentationTime;
+  dispatch_queue_t _frameQueue;
+}
+
+- (void)startCapturingFromFileNamed:(NSString *)nameOfFile {
+  dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
+    if (_reader && _reader.status == AVAssetReaderStatusReading) {
+      RTCLog("Capturer exists and reads anoth6er file. Start capture request failed.");

[magjed_webrtc, 2017/06/01 13:44:37]

nit: still anoth6er

[daniela-webrtc, 2017/06/01 14:13:27]

The patch didn't upload properly :D Should be fixed now.

+      return;
+    }
+    NSString *pathForFile = [self pathForFileName:nameOfFile];
+    if (!pathForFile) {
+      RTCLog("File %@ not found in bundle", nameOfFile);
+      return;
+    }
+
+    _lastPresentationTime = CMTimeMake(0, 0);
+
+    NSURL *URLForFile = [NSURL fileURLWithPath:pathForFile];
+    AVURLAsset *asset = [AVURLAsset URLAssetWithURL:URLForFile options:nil];
+
+    NSArray *allTracks = [asset tracksWithMediaType:AVMediaTypeVideo];
+    NSError *error = nil;
+    _reader = [[AVAssetReader alloc] initWithAsset:asset error:&error];
+    if (error) {
+      RTCLog("File reader failed with error: %@", error);
+      return;
+    }
+
+    NSDictionary *options = @{
+      (NSString *)
+      kCVPixelBufferPixelFormatTypeKey : @(kCVPixelFormatType_420YpCbCr8BiPlanarFullRange)
+    };
+    _outTrack = [[AVAssetReaderTrackOutput alloc] initWithTrack:allTracks.firstObject
+                                                 outputSettings:options];
+    [_reader addOutput:_outTrack];
+
+    [_reader startReading];
+    RTCLog(@"File capturer started reading");
+    [self readNextBuffer];
+  });
+}
+
+- (void)stopCapture {
+  _capturerStopped = YES;
+  RTCLog(@"File capturer stopped.");
+}
+
+#pragma mark - Private
+
+- (nullable NSString *)pathForFileName:(NSString *)fileName {
+  NSArray *nameComponents = [fileName componentsSeparatedByString:@"."];
+  if (nameComponents.count != 2) {
+    return nil;
+  }
+
+  NSString *path =
+      [[NSBundle mainBundle] pathForResource:nameComponents[0] ofType:nameComponents[1]];
+  return path;
+}
+
+- (dispatch_queue_t)frameQueue {
+  if (!_frameQueue) {
+    _frameQueue = dispatch_queue_create("org.webrtc.filecapturer.video", DISPATCH_QUEUE_SERIAL);
+    dispatch_set_target_queue(_frameQueue,
+                              dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0));
+  }
+  return _frameQueue;
+}
+
+- (void)readNextBuffer {
+  if (_reader.status != AVAssetReaderStatusReading || _capturerStopped) {
+    [_reader cancelReading];
+    _reader = nil;
+    return;
+  }
+
+  CMSampleBufferRef sampleBuffer = [_outTrack copyNextSampleBuffer];
+  if (!sampleBuffer) {
+    [self readNextBuffer];
+    return;
+  }
+  if (CMSampleBufferGetNumSamples(sampleBuffer) != 1 || !CMSampleBufferIsValid(sampleBuffer) ||
+      !CMSampleBufferDataIsReady(sampleBuffer)) {
+    [self readNextBuffer];
+    return;
+  }
+
+  [self publishSampleBuffer:sampleBuffer];
+}
+
+- (void)publishSampleBuffer:(CMSampleBufferRef)sampleBuffer {
+  CMTime presentationTime = CMSampleBufferGetPresentationTimeStamp(sampleBuffer);
+  Float64 presentationDifference =
+      CMTimeGetSeconds(CMTimeSubtract(presentationTime, _lastPresentationTime));
+  _lastPresentationTime = presentationTime;
+  int64_t presentationDifferenceRound = lroundf(presentationDifference * NSEC_PER_SEC);
+
+  __block dispatch_source_t timer = [self createStrictTimer];
+  // Strict timer that will fire |presentationDifferenceRound| ns from now and never again.
+  dispatch_source_set_timer(timer,
+                            dispatch_time(DISPATCH_TIME_NOW, presentationDifferenceRound),
+                            DISPATCH_TIME_FOREVER,
+                            0);
+  dispatch_source_set_event_handler(timer, ^{
+    dispatch_source_cancel(timer);
+    timer = nil;
+
+    CVPixelBufferRef pixelBuffer = CMSampleBufferGetImageBuffer(sampleBuffer);
+    if (!pixelBuffer) {

[magjed_webrtc, 2017/06/01 13:44:37]

Maybe we can extract the CVPixelBufferRef and do this check in readNextBuffer
instead, and publish the final pixelBuffer?

[daniela-webrtc, 2017/06/01 14:13:27]

We need to release the sampleBuffer after it was used to create the frame so
passing the CVPixelBuffer would cause us to leak the sampleBuffer. And we need
to create the frame in the block because we need to use the correct timestamp. 

[magjed_webrtc, 2017/06/01 15:43:03]

Can we use CVBufferRetain(pixelBuffer) in readNextBuffer and then
CVBufferRelease(pixelBuffer) after we have created the frame to overcome this
problem?

+      CFRelease(sampleBuffer);
+      dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
+        [self readNextBuffer];
+      });

[magjed_webrtc, 2017/06/01 13:44:38]

We should return here I guess?

[daniela-webrtc, 2017/06/01 14:13:27]

Done.

+    }
+
+    NSTimeInterval timeStampSeconds = CACurrentMediaTime();
+    int64_t timeStampNs = timeStampSeconds * NSEC_PER_SEC;

[magjed_webrtc, 2017/06/01 13:44:37]

Should we use lroundf here?

[daniela-webrtc, 2017/06/01 14:13:27]

Yes. NSTimeInteval is double.

+    RTCVideoFrame *videoFrame =
+        [[RTCVideoFrame alloc] initWithPixelBuffer:pixelBuffer rotation:0 timeStampNs:timeStampNs];
+    CFRelease(sampleBuffer);

[magjed_webrtc, 2017/06/01 13:44:37]

I noticed we don't do 'CFRelease(sampleBuffer);' in RTCCameraVideoCapturer.m. Is
that something we missed?

[daniela-webrtc, 2017/06/01 14:13:27]

We need to explicitly release the sampleBuffer here because of the way it is
obtained in line 96.
From the documentation of copyNextSampleBuffer
"The client is responsible for calling CFRelease on the returned CMSampleBuffer
object when finished with it"

That's not the case in the RTCCameraVideoCapturer where we don't own the buffer
passed to us by the delegate callback.

+
+    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
+      [self readNextBuffer];
+    });
+
+    [self.delegate capturer:self didCaptureVideoFrame:videoFrame];
+  });
+  dispatch_activate(timer);
+}
+
+- (dispatch_source_t)createStrictTimer {
+  dispatch_source_t timer = dispatch_source_create(
+      DISPATCH_SOURCE_TYPE_TIMER, 0, DISPATCH_TIMER_STRICT, [self frameQueue]);
+  return timer;
+}
+
+- (void)dealloc {
+  [self stopCapture];
+}
+
+@end