Move the WebView instance count to native

This
- reduces unnecessary calls from native to java.
- makes it possible to add observer in native side.

Bug: 1042048
Change-Id: I0e5c32b98adfb56ac408556f9a0fcbd209b5813c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2001747Reviewed-by: Bo <boliu@chromium.org>
Commit-Queue: Tao Bai <michaelbai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#732264}

android_webview/browser/aw_contents_lifecycle_notifier.cc

@@ -5,19 +5,37 @@
 #include "android_webview/browser/aw_contents_lifecycle_notifier.h"
 
 #include "android_webview/browser_jni_headers/AwContentsLifecycleNotifier_jni.h"
+#include "content/public/browser/browser_thread.h"
 
 using base::android::AttachCurrentThread;
+using content::BrowserThread;
 
 namespace android_webview {
 
 // static
 void AwContentsLifecycleNotifier::OnWebViewCreated() {
-  Java_AwContentsLifecycleNotifier_onWebViewCreated(AttachCurrentThread());
+  DCHECK_CURRENTLY_ON(BrowserThread::UI);
+  if (getInstance().mNumWebViews++ == 0) {
+    Java_AwContentsLifecycleNotifier_onFirstWebViewCreated(
+        AttachCurrentThread());
+  }
 }
 
 // static
 void AwContentsLifecycleNotifier::OnWebViewDestroyed() {
-  Java_AwContentsLifecycleNotifier_onWebViewDestroyed(AttachCurrentThread());
+  DCHECK_CURRENTLY_ON(BrowserThread::UI);
+  if (--getInstance().mNumWebViews == 0) {
+    Java_AwContentsLifecycleNotifier_onLastWebViewDestroyed(
+        AttachCurrentThread());
+  }
 }
 
+// static
+AwContentsLifecycleNotifier& AwContentsLifecycleNotifier::getInstance() {
+  static base::NoDestructor<AwContentsLifecycleNotifier> instance;
+  return *instance;
+}
+
+AwContentsLifecycleNotifier::AwContentsLifecycleNotifier() = default;
+
 }  // namespace android_webview

android_webview/browser/aw_contents_lifecycle_notifier.h

@@ -7,6 +7,7 @@
 
 #include "base/android/jni_android.h"
 #include "base/macros.h"
+#include "base/no_destructor.h"
 
 namespace android_webview {
 
@@ -16,7 +17,16 @@ class AwContentsLifecycleNotifier {
   static void OnWebViewDestroyed();
 
  private:
-  DISALLOW_IMPLICIT_CONSTRUCTORS(AwContentsLifecycleNotifier);
+  friend base::NoDestructor<AwContentsLifecycleNotifier>;
+
+  static AwContentsLifecycleNotifier& getInstance();
+
+  AwContentsLifecycleNotifier();
+  ~AwContentsLifecycleNotifier() = delete;
+
+  int mNumWebViews = 0;
+
+  DISALLOW_COPY_AND_ASSIGN(AwContentsLifecycleNotifier);
 };
 
 }  // namespace android_webview

android_webview/java/src/org/chromium/android_webview/AwContentsLifecycleNotifier.java

@@ -28,7 +28,7 @@ public class AwContentsLifecycleNotifier {
 
     private static final ObserverList<Observer> sLifecycleObservers =
             new ObserverList<Observer>();
-    private static int sNumWebViews;
+    private static boolean sHasWebViewInstances;
 
     private AwContentsLifecycleNotifier() {}
 
@@ -41,34 +41,28 @@ public class AwContentsLifecycleNotifier {
     }
 
     public static boolean hasWebViewInstances() {
-        return sNumWebViews > 0;
+        return sHasWebViewInstances;
     }
 
     // Called on UI thread.
     @CalledByNative
-    private static void onWebViewCreated() {
+    private static void onFirstWebViewCreated() {
         ThreadUtils.assertOnUiThread();
-        assert sNumWebViews >= 0;
-        sNumWebViews++;
-        if (sNumWebViews == 1) {
-            // first webview created, notify observers.
-            for (Observer observer : sLifecycleObservers) {
-                observer.onFirstWebViewCreated();
-            }
+        sHasWebViewInstances = true;
+        // first webview created, notify observers.
+        for (Observer observer : sLifecycleObservers) {
+            observer.onFirstWebViewCreated();
         }
     }
 
     // Called on UI thread.
     @CalledByNative
-    private static void onWebViewDestroyed() {
+    private static void onLastWebViewDestroyed() {
         ThreadUtils.assertOnUiThread();
-        assert sNumWebViews > 0;
-        sNumWebViews--;
-        if (sNumWebViews == 0) {
-            // last webview destroyed, notify observers.
-            for (Observer observer : sLifecycleObservers) {
-                observer.onLastWebViewDestroyed();
-            }
+        sHasWebViewInstances = false;
+        // last webview destroyed, notify observers.
+        for (Observer observer : sLifecycleObservers) {
+            observer.onLastWebViewDestroyed();
         }
     }
 }