Ensure UIView.layer is accessed on main thread.

webrtc/sdk/objc/Framework/Classes/UI/RTCCameraPreviewView.m

 /*
  *  Copyright 2015 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/RTCCameraPreviewView.h"
 
 #import <AVFoundation/AVFoundation.h>
 #import <UIKit/UIKit.h>
 
 #import "RTCDispatcher+Private.h"
 
 @implementation RTCCameraPreviewView
 
 @synthesize captureSession = _captureSession;
 
 + (Class)layerClass {
   return [AVCaptureVideoPreviewLayer class];
 }
 
 - (void)setCaptureSession:(AVCaptureSession *)captureSession {
   if (_captureSession == captureSession) {
     return;
   }
-  _captureSession = captureSession;
-  AVCaptureVideoPreviewLayer *previewLayer = [self previewLayer];
-  [RTCDispatcher dispatchAsyncOnType:RTCDispatcherTypeCaptureSession
+  [RTCDispatcher dispatchAsyncOnType:RTCDispatcherTypeMain
                                block:^{
-    previewLayer.session = captureSession;
+    _captureSession = captureSession;
+    AVCaptureVideoPreviewLayer *previewLayer = [self previewLayer];
+    [RTCDispatcher dispatchAsyncOnType:RTCDispatcherTypeCaptureSession
+                                 block:^{
+      previewLayer.session = captureSession;
+    }];
   }];
 }
 
 - (void)layoutSubviews {
   [super layoutSubviews];
 
   // Update the video orientation based on the device orientation.
   [self setCorrectVideoOrientation];
 }
 
@@ skipping 21 matching lines @@
   }
 }
 
 #pragma mark - Private
 
 - (AVCaptureVideoPreviewLayer *)previewLayer {
   return (AVCaptureVideoPreviewLayer *)self.layer;
 }
 
 @end