customtabs: Send time to first contentful paint to the caller.

chrome/android/java/src/org/chromium/chrome/browser/metrics/PageLoadMetrics.java

Index: chrome/android/java/src/org/chromium/chrome/browser/metrics/PageLoadMetrics.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/metrics/PageLoadMetrics.java b/chrome/android/java/src/org/chromium/chrome/browser/metrics/PageLoadMetrics.java
new file mode 100644
index 0000000000000000000000000000000000000000..baedce9c764513ee2558c2ca5daef27325d58180
--- /dev/null
+++ b/chrome/android/java/src/org/chromium/chrome/browser/metrics/PageLoadMetrics.java
@@ -0,0 +1,60 @@
+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+package org.chromium.chrome.browser.metrics;
+
+import org.chromium.base.ThreadUtils;
+import org.chromium.base.annotations.CalledByNative;
+import org.chromium.content_public.browser.WebContents;
+
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * Receives the page load metrics updates from AndroidPageLoadMetricsObserver, and notifies the
+ * observers.
+ *
+ * Threading: everything here must happen on the UI thread.
+ */
+public class PageLoadMetrics {
+    public static final String FIRST_CONTENTFUL_PAINT = "firstContentfulPaint";
+
+    /** Observer for page load metrics. */
+    public interface Observer {
+        /**
+         * Called when the first contentful paint page load metric is available.
+         *
+         * @param webContents the WebContents this metrics is related to.
+         * @param firstContentfulPaintMs Time to first contentful paint from navigation start.
+         */
+        public void onFirstContentfulPaint(WebContents webContents, long firstContentfulPaintMs);
+    }
+
+    private static List<Observer> sObservers = null;
+
+    /** Adds an observer. */
+    public static void addObserver(Observer observer) {
+        ThreadUtils.assertOnUiThread();
+        if (sObservers == null) sObservers = new ArrayList<Observer>();
+        sObservers.add(observer);
+    }
+
+    /** Removes an observer. */
+    public static void removeObserver(Observer observer) {
+        ThreadUtils.assertOnUiThread();
+        sObservers.remove(observer);

[Bryan McQuade, 2016/07/26 14:29:33]

is this safe if a listener removes themselves during a callback dispatch? if
not, is there a java class that handles removal during iteration?

[Benoit L, 2016/07/27 16:23:45]

Yes, there is ObserverList in Java as well. I deliberately wanted to avoid it
here for now, to keep things simpler.

But coming back to it you're absolutely right, it's better to make this more
robust right away.
Done.

+        if (sObservers.size() == 0) sObservers = null;
+    }
+
+    @CalledByNative
+    static void onFirstContentfulPaint(WebContents webContents, long firstContentfulPaintMs) {
+        ThreadUtils.assertOnUiThread();
+        if (sObservers == null) return;
+        for (Observer observer : sObservers) {
+            observer.onFirstContentfulPaint(webContents, firstContentfulPaintMs);
+        }
+    }
+
+    private PageLoadMetrics() {}
+}