Bug 1753131 - use MediaManager device list change coalescing for MediaDevices r=jib

There is a chance that this may result in some associated events no longer
being coalesced into a single devicechange event, but there is little
disadvantage to this while there is an advantage to getting devicechange
events promptly.

Future changes in https://phabricator.services.mozilla.com/D132908 will
track exactly which devices have changed and so filter out no-op
notifications.

Depends on D132892

Differential Revision: https://phabricator.services.mozilla.com/D132893

dom/media/MediaDevices.cpp

@@ -22,8 +22,6 @@
 #include "nsPIDOMWindow.h"
 #include "nsQueryObject.h"
 
-#define DEVICECHANGE_HOLD_TIME_IN_MS 1000
-
 namespace mozilla::dom {
 
 using EnumerationFlag = MediaManager::EnumerationFlag;
@@ -34,9 +32,6 @@ MediaDevices::MediaDevices(nsPIDOMWindowInner* aWindow)
 
 MediaDevices::~MediaDevices() {
   MOZ_ASSERT(NS_IsMainThread());
-  if (mFuzzTimer) {
-    mFuzzTimer->Cancel();
-  }
   mDeviceChangeListener.DisconnectIfExists();
 }
 
@@ -534,26 +529,10 @@ void MediaDevices::OnDeviceChange() {
     return;
   }
 
-  if (mFuzzTimer) {
-    // An event is already in flight.
-    return;
-  }
-
-  mFuzzTimer = NS_NewTimer();
-
-  if (!mFuzzTimer) {
-    MOZ_ASSERT(false);
-    return;
-  }
-
-  mFuzzTimer->InitWithNamedFuncCallback(
-      [](nsITimer*, void* aClosure) {
-        MediaDevices* md = static_cast<MediaDevices*>(aClosure);
-        md->DispatchTrustedEvent(u"devicechange"_ns);
-        md->mFuzzTimer = nullptr;
-      },
-      this, DEVICECHANGE_HOLD_TIME_IN_MS, nsITimer::TYPE_ONE_SHOT,
-      "MediaDevices::mFuzzTimer Callback");
+  NS_DispatchToCurrentThread(
+      NS_NewRunnableFunction("devicechange", [self = RefPtr(this), this] {
+        DispatchTrustedEvent(u"devicechange"_ns);
+      }));
 }
 
 mozilla::dom::EventHandlerNonNull* MediaDevices::GetOndevicechange() {

dom/media/MediaDevices.h

@@ -14,7 +14,6 @@
 #include "nsCOMPtr.h"
 #include "nsID.h"
 #include "nsISupports.h"
-#include "nsITimer.h"
 #include "nsTHashSet.h"
 
 class AudioDeviceInfo;
@@ -95,7 +94,6 @@ class MediaDevices final : public DOMEventTargetHelper {
 
   nsTHashSet<nsString> mExplicitlyGrantedAudioOutputIds;
   nsTArray<RefPtr<Promise>> mPendingEnumerateDevicesPromises;
-  nsCOMPtr<nsITimer> mFuzzTimer;
 
   // Connect/Disconnect on main thread only
   MediaEventListener mDeviceChangeListener;

dom/media/MediaManager.cpp

@@ -2187,8 +2187,6 @@ void MediaManager::DeviceListChanged() {
   if (sHasShutdown) {
     return;
   }
-  mDeviceListChangeEvent.Notify();
-
   // Wait 200 ms, because
   // A) on some Windows machines, if we call EnumerateRawDevices immediately
   //    after receiving devicechange event, we'd get an outdated devices list.
@@ -2227,6 +2225,8 @@ void MediaManager::DeviceListChanged() {
 }
 
 void MediaManager::HandleDeviceListChanged() {
+  mDeviceListChangeEvent.Notify();
+
   EnumerateRawDevices(MediaSourceEnum::Camera, MediaSourceEnum::Microphone,
                       EnumerationFlag::EnumerateAudioOutputs)
       ->Then(

dom/media/systemservices/MediaTaskUtils.h

@@ -17,7 +17,11 @@ namespace media {
 
 /* media::NewTaskFrom() - Create a Task from a lambda.
  *
- * Similar to media::NewRunnableFrom() - Create an nsRunnable from a lambda.
+ * Similar to media::NewRunnableFrom() - Create an nsRunnable from a lambda,
+ * but ignore the return value from the lambda.
+ *
+ * Prefer NS_NewRunnableFunction(), which provides a specific name, unless the
+ * lambda really must have a non-void return value that is to be ignored.
  */
 
 template <typename OnRunType>