iOS/MacOS:Refactor metal rendering by extracting common implementation

webrtc/sdk/objc/Framework/Classes/Metal/RTCMTLNSVideoView.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 "WebRTC/RTCMTLNSVideoView.h"
 
 #import <Metal/Metal.h>
 #import <MetalKit/MetalKit.h>
 
 #import "WebRTC/RTCVideoFrame.h"
 
 #import "RTCMTLI420Renderer.h"
 
-@interface RTCMTLNSVideoView () <MTKViewDelegate>
+@interface RTCMTLNSVideoView ()<MTKViewDelegate>
 @property(nonatomic) id<RTCMTLRenderer> renderer;
 @property(nonatomic, strong) MTKView *metalView;
 @property(atomic, strong) RTCVideoFrame *videoFrame;
 @end
 
 @implementation RTCMTLNSVideoView {
   id<RTCMTLRenderer> _renderer;
 }
 
 @synthesize renderer = _renderer;
@@ skipping 52 matching lines @@
       [NSLayoutConstraint constraintsWithVisualFormat:@"V:|-0-[_metalView]-0-|"
                                               options:0
                                               metrics:nil
                                                 views:views];
   [self addConstraints:constraintsVertical];
   [super updateConstraints];
 }
 
 #pragma mark - MTKViewDelegate methods
 - (void)drawInMTKView:(nonnull MTKView *)view {
+  if (self.videoFrame == nil) {
+    return;
+  }
   if (view == self.metalView) {
     [_renderer drawFrame:self.videoFrame];
   }
 }
 
 - (void)mtkView:(MTKView *)view drawableSizeWillChange:(CGSize)size {
 }
 
 #pragma mark - RTCVideoRenderer
 
 - (void)setSize:(CGSize)size {
   _metalView.drawableSize = size;
   [_metalView draw];
 }
 
 - (void)renderFrame:(nullable RTCVideoFrame *)frame {
   if (frame == nil) {
     return;
   }
   self.videoFrame = [frame newI420VideoFrame];
 }
 
 @end