DeviceMonitorMac: move CrAVFoundationDeviceObserver and most of SuspendObserverDelegate to UI Thread

content/browser/device_monitor_mac.mm

Index: content/browser/device_monitor_mac.mm
diff --git a/content/browser/device_monitor_mac.mm b/content/browser/device_monitor_mac.mm
index eb429afe6402bb07bfc3f100ec31e9c05e2665e9..deeb3986f47d78344146636ffe3bbd3e49e13705 100644
--- a/content/browser/device_monitor_mac.mm
+++ b/content/browser/device_monitor_mac.mm
@@ -10,9 +10,11 @@
 
 #include "base/bind_helpers.h"
 #include "base/logging.h"
+#include "base/mac/bind_objc_block.h"
 #include "base/mac/scoped_nsobject.h"
 #include "base/threading/thread_checker.h"
 #include "content/public/browser/browser_thread.h"
+#include "media/base/bind_to_current_loop.h"
 #import "media/video/capture/mac/avfoundation_glue.h"
 
 namespace {
@@ -216,15 +218,17 @@ class SuspendObserverDelegate;
 
 // This class is a Key-Value Observer (KVO) shim. It is needed because C++
 // classes cannot observe Key-Values directly. Created, manipulated, and
-// destroyed on the Device Thread by SuspendedObserverDelegate.
+// destroyed on the UI Thread by SuspendObserverDelegate.
 @interface CrAVFoundationDeviceObserver : NSObject {
  @private
-  SuspendObserverDelegate* receiver_;  // weak
+  base::Closure onDeviceChangedCallback_;
+  SuspendObserverDelegate* receiver_;  // Weak.

[tommi (sloooow) - chröme, 2014/07/07 07:41:07]

you don't need this variable anymore

[mcasas, 2014/07/08 15:11:45]

Done.

   // Member to keep track of the devices we are already monitoring.
   std::set<CrAVCaptureDevice*> monitoredDevices_;
 }
 
-- (id)initWithChangeReceiver:(SuspendObserverDelegate*)receiver;
+- (id)initWithChangeReceiver:(SuspendObserverDelegate*)receiver

[tommi (sloooow) - chröme, 2014/07/07 07:41:07]

Since this method doesn't need the receiver pointer anymore, its name should
probably be changed (e.g. to simply "init" or better yet, "initialize")

[mcasas, 2014/07/08 15:11:45]

Changed, but still kept the initWithX as is part of Cocoa coding guidelines,
"Add new keywords to the end of an existing method when you create a method that
is more specific than the inherited one."...

+     onDeviceChangedCallback:(const base::Closure&)callback;
 - (void)startObserving:(CrAVCaptureDevice*)device;
 - (void)stopObserving:(CrAVCaptureDevice*)device;
 
@@ -233,14 +237,15 @@ class SuspendObserverDelegate;
 namespace {
 
 // This class owns and manages the lifetime of a CrAVFoundationDeviceObserver.
-// Provides a callback for this device observer to indicate that there has been
-// a device change of some kind. Created by AVFoundationMonitorImpl in UI thread
-// but living in Device Thread.
+// AVFoundationMonitorImpl creates and destroys it in UI thread. Runs the
+// expensive device enumerations in OnDeviceChanged() and StartObserver() on
+// Device Thread.
 class SuspendObserverDelegate :
     public base::RefCountedThreadSafe<SuspendObserverDelegate> {
  public:
   explicit SuspendObserverDelegate(DeviceMonitorMacImpl* monitor)
       : avfoundation_monitor_impl_(monitor) {
+    DCHECK(content::BrowserThread::CurrentlyOn(content::BrowserThread::UI));
     device_thread_checker_.DetachFromThread();
   }
 
@@ -251,12 +256,15 @@ class SuspendObserverDelegate :
  private:
   friend class base::RefCountedThreadSafe<SuspendObserverDelegate>;
 
-  virtual ~SuspendObserverDelegate() {}
+  virtual ~SuspendObserverDelegate() {
+    DCHECK(content::BrowserThread::CurrentlyOn(content::BrowserThread::UI));
+  }
 
   void OnDeviceChangedOnUIThread(
       const std::vector<DeviceInfo>& snapshot_devices);
 
   base::ThreadChecker device_thread_checker_;
+  // Created, used and released in UI thread.
   base::scoped_nsobject<CrAVFoundationDeviceObserver> suspend_observer_;
   DeviceMonitorMacImpl* avfoundation_monitor_impl_;
 };
@@ -266,7 +274,8 @@ void SuspendObserverDelegate::OnDeviceChanged() {
   NSArray* devices = [AVCaptureDeviceGlue devices];
   std::vector<DeviceInfo> snapshot_devices;
   for (CrAVCaptureDevice* device in devices) {
-    [suspend_observer_ startObserving:device];
+    content::BrowserThread::PostTask(content::BrowserThread::UI, FROM_HERE,
+        base::BindBlock(^{ [suspend_observer_ startObserving:device]; }));
     BOOL suspended = [device respondsToSelector:@selector(isSuspended)] &&
         [device isSuspended];
     DeviceInfo::DeviceType device_type = DeviceInfo::kUnknown;
@@ -290,10 +299,19 @@ void SuspendObserverDelegate::OnDeviceChanged() {
 
 void SuspendObserverDelegate::StartObserver() {
   DCHECK(device_thread_checker_.CalledOnValidThread());
-  suspend_observer_.reset([[CrAVFoundationDeviceObserver alloc]
-                               initWithChangeReceiver:this]);
-  for (CrAVCaptureDevice* device in [AVCaptureDeviceGlue devices])
-    [suspend_observer_ startObserving:device];
+
+  base::Closure on_device_changed_callback = media::BindToCurrentLoop(

[tommi (sloooow) - chröme, 2014/07/07 07:41:07]

is media::BindToCurrentLoop used elsewhere in content?
It feels like this is a utility method for media so using it in content might
not be a good thing... I wonder if something comparable is available in base?

[mcasas, 2014/07/08 15:11:45]

Yes, f.i. in browser/renderer_host/media/video_capture_manager.cc and in
other 29 locations not including unittests.

Is a very useful class, I wonder if it should be ported?

+      base::Bind(&SuspendObserverDelegate::OnDeviceChanged, this));
+  content::BrowserThread::PostTask(content::BrowserThread::UI, FROM_HERE,
+      base::BindBlock(^{ suspend_observer_.reset(
+          [[CrAVFoundationDeviceObserver alloc]
+              initWithChangeReceiver:this
+             onDeviceChangedCallback:on_device_changed_callback]);
+      }));
+  for (CrAVCaptureDevice* device in [AVCaptureDeviceGlue devices]) {
+    content::BrowserThread::PostTask(content::BrowserThread::UI, FROM_HERE,
+        base::BindBlock(^{ [suspend_observer_ startObserving:device]; }));
+  }
 }
 
 void SuspendObserverDelegate::ResetDeviceMonitorOnUIThread() {
@@ -384,22 +402,29 @@ void AVFoundationMonitorImpl::OnDeviceChanged() {
 
 @implementation CrAVFoundationDeviceObserver
 
-- (id)initWithChangeReceiver:(SuspendObserverDelegate*)receiver {
+- (id)initWithChangeReceiver:(SuspendObserverDelegate*)receiver
+     onDeviceChangedCallback:(const base::Closure&)callback {
+  DCHECK(content::BrowserThread::CurrentlyOn(content::BrowserThread::UI));
   if ((self = [super init])) {
     DCHECK(receiver != NULL);
     receiver_ = receiver;
+    onDeviceChangedCallback_ = callback;
   }
   return self;
 }
 
 - (void)dealloc {
+  DCHECK(content::BrowserThread::CurrentlyOn(content::BrowserThread::UI));
   std::set<CrAVCaptureDevice*>::iterator it = monitoredDevices_.begin();
-  while (it != monitoredDevices_.end())
-    [self stopObserving:*it++];
+  while (it != monitoredDevices_.end()) {
+    [self removeObservers:*it];
+    monitoredDevices_.erase(it++);
+  }
   [super dealloc];
 }
 
 - (void)startObserving:(CrAVCaptureDevice*)device {
+  DCHECK(content::BrowserThread::CurrentlyOn(content::BrowserThread::UI));
   DCHECK(device != nil);
   // Skip this device if there are already observers connected to it.
   if (std::find(monitoredDevices_.begin(), monitoredDevices_.end(), device) !=
@@ -418,28 +443,32 @@ void AVFoundationMonitorImpl::OnDeviceChanged() {
 }
 
 - (void)stopObserving:(CrAVCaptureDevice*)device {
+  DCHECK(content::BrowserThread::CurrentlyOn(content::BrowserThread::UI));
   DCHECK(device != nil);
   std::set<CrAVCaptureDevice*>::iterator found =
       std::find(monitoredDevices_.begin(), monitoredDevices_.end(), device);
   DCHECK(found != monitoredDevices_.end());
-  // Every so seldom, |device| might be gone when getting here, in that case
-  // removing the observer causes a crash. Try to avoid it by checking sanity of
-  // the |device| via its -observationInfo. http://crbug.com/371271.
+  [self removeObservers:*found];
+  monitoredDevices_.erase(found);
+}
+
+- (void)removeObservers:(CrAVCaptureDevice*)device {
+  // Check sanity of |device| via its -observationInfo. http://crbug.com/371271.
   if ([device observationInfo]) {
     [device removeObserver:self
                 forKeyPath:@"suspended"];
     [device removeObserver:self
                 forKeyPath:@"connected"];
   }
-  monitoredDevices_.erase(found);
 }
 
 - (void)observeValueForKeyPath:(NSString*)keyPath
                       ofObject:(id)object
                         change:(NSDictionary*)change
                        context:(void*)context {
+  DCHECK(content::BrowserThread::CurrentlyOn(content::BrowserThread::UI));
   if ([keyPath isEqual:@"suspended"])
-    receiver_->OnDeviceChanged();
+    onDeviceChangedCallback_.Run();
   if ([keyPath isEqual:@"connected"])
     [self stopObserving:static_cast<CrAVCaptureDevice*>(context)];
 }