Upstream oodles of Chrome for Android code into Chromium.

chrome/android/java_staging/src/org/chromium/chrome/browser/ntp/NativePageAssassin.java

+// Copyright 2015 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.ntp;
+
+import org.chromium.chrome.browser.FrozenNativePage;
+import org.chromium.chrome.browser.NativePage;
+import org.chromium.chrome.browser.Tab;
+
+import java.lang.ref.WeakReference;
+import java.util.ArrayList;
+
+/**
+ * NativePageAssassin tracks recent tabs and freezes each native page when it hasn't been visible
+ * for a while. This keeps hidden NativePages from using up precious memory.
+ *
+ * The NativePageAssassin is a singleton since having full knowledge of the user's hidden tabs --
+ * across all local instances of Chrome and all TabModels -- allows the NativePageAssassin to better
+ * estimate which hidden tabs the user is likely to return to.
+ *
+ * Thread safety: this should only be accessed on the UI thread.
+ */
+public class NativePageAssassin {
+
+    private static final NativePageAssassin sInstance = new NativePageAssassin();
+
+    /**
+     * The number of hidden tabs to consider "recent". Any non-recent native page will be frozen.
+     */
+    private static final int MAX_RECENT_TABS = 3;
+
+    /**
+     * The most recently hidden tabs, limited to MAX_RECENT_TABS elements, ordered from oldest to
+     * newest. Visible tabs are not included in this list.
+     */
+    private ArrayList<WeakReference<Tab>> mRecentTabs = new ArrayList<WeakReference<Tab>>(
+            MAX_RECENT_TABS + 1);
+
+    private NativePageAssassin() {}
+
+    /**
+     * @return The one and only NativePageAssassin.
+     */
+    public static NativePageAssassin getInstance() {
+        return sInstance;
+    }
+
+    /**
+     * Call this whenever a tab is shown.
+     *
+     * @param tab The tab being shown.
+     */
+    public void tabShown(Tab tab) {
+        // Remove the tab from the list of recently hidden tabs.
+        for (int i = 0; i < mRecentTabs.size(); i++) {
+            Tab t = mRecentTabs.get(i).get();
+            if (t == tab) {
+                mRecentTabs.remove(i);
+            }
+        }
+    }
+
+    /**
+     * Call this whenever a tab is hidden.
+     *
+     * @param tab The tab being hidden.
+     */
+    public void tabHidden(Tab tab) {
+        mRecentTabs.add(new WeakReference<Tab>(tab));
+
+        // If a tab has just passed the threshold from "recent" to "not recent" and it's displaying
+        // a native page, freeze the native page.
+        if (mRecentTabs.size() <= MAX_RECENT_TABS) return;
+        freeze(mRecentTabs.remove(0).get());
+    }
+
+    /**
+     * Freezes all hidden NativePages that aren't already frozen.
+     */
+    public void freezeAllHiddenPages() {
+        for (int i = 0; i < mRecentTabs.size(); i++) {
+            freeze(mRecentTabs.get(i).get());
+        }
+        mRecentTabs.clear();
+    }
+
+    private void freeze(Tab tab) {
+        if (tab == null) return;
+        NativePage pageToFreeze = tab.getNativePage();
+        if (pageToFreeze == null || pageToFreeze instanceof FrozenNativePage
+                || pageToFreeze.getView().getParent() != null) {
+            return;
+        }
+        tab.freezeNativePage();
+    }
+}