Implement WebVR presentation pausing for VR Shell Menu Mode

device/vr/android/gvr/gvr_device_provider.cc

Index: device/vr/android/gvr/gvr_device_provider.cc
diff --git a/device/vr/android/gvr/gvr_device_provider.cc b/device/vr/android/gvr/gvr_device_provider.cc
index 252aa117667e09cce9ca7041c791706dc8c2a220..a24a68c1629b9946ec36f5f2db20701d17b95ab2 100644
--- a/device/vr/android/gvr/gvr_device_provider.cc
+++ b/device/vr/android/gvr/gvr_device_provider.cc
@@ -107,6 +107,28 @@ void GvrDeviceProvider::OnGvrDelegateRemoved() {
   ExitPresent();
 }
 
+void GvrDeviceProvider::OnDisplayBlur() {
+  if (!main_thread_task_runner_->BelongsToCurrentThread()) {
+    main_thread_task_runner_->PostTask(
+        FROM_HERE,
+        base::Bind(&GvrDeviceProvider::OnDisplayBlur, base::Unretained(this)));

[dcheng, 2016/11/04 05:24:46]

Can you explain why this used of Unretained() is safe?

Also, why do we need to bounce threads? There's no comments on this class about
what threads it runs on or the threading requirements.

[mthiesse, 2016/11/04 17:20:36]

As Brandon wrote this class and I was really just following the pattern of
OnGvrDelegateReady, perhaps he'll give a better answer.

I'm not actually sure base::Unretained() is safe here. What's the best
alternative? Making this class refcounted and using base::RetainedRef()?

I'll add some comments regarding threading in the next patchset, but briefly,
OnDisplayFocus, OnDisplayBlur, and OnGvrDelegateReady are called from an
arbitrary GL thread, and everything else is called from the main thread. We need
to post to the main thread because the VRClientDispatcher can only be used from
the main thread. Whether or not the thread bounding should happen here or at the
callsite is a style question I don't know the correct answer to (maybe you have
strong opinions?).

+    return;
+  }
+  if (client_)
+    client_->OnDisplayBlur(vr_device_.get());
+}
+
+void GvrDeviceProvider::OnDisplayFocus() {
+  if (!main_thread_task_runner_->BelongsToCurrentThread()) {
+    main_thread_task_runner_->PostTask(
+        FROM_HERE,
+        base::Bind(&GvrDeviceProvider::OnDisplayFocus, base::Unretained(this)));
+    return;
+  }
+  if (client_)
+    client_->OnDisplayFocus(vr_device_.get());
+}
+
 void GvrDeviceProvider::GvrDelegateReady(GvrDelegate* delegate) {
   DCHECK(main_thread_task_runner_->BelongsToCurrentThread());