Add a handler for enabled/disabling assist.

chrome/android/java/src/org/chromium/chrome/browser/AssistStatusHandler.java

Index: chrome/android/java/src/org/chromium/chrome/browser/AssistStatusHandler.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/AssistStatusHandler.java b/chrome/android/java/src/org/chromium/chrome/browser/AssistStatusHandler.java
new file mode 100644
index 0000000000000000000000000000000000000000..64477a876eb6a50dafa9c934a92db4951b2019f7
--- /dev/null
+++ b/chrome/android/java/src/org/chromium/chrome/browser/AssistStatusHandler.java
@@ -0,0 +1,105 @@
+// 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;
+
+import android.app.Activity;
+import android.view.View;
+
+import org.chromium.base.BuildInfo;
+import org.chromium.chrome.browser.tabmodel.EmptyTabModelSelectorObserver;
+import org.chromium.chrome.browser.tabmodel.TabModelSelector;
+import org.chromium.chrome.browser.tabmodel.TabModelSelectorObserver;
+
+import java.lang.reflect.InvocationTargetException;
+import java.lang.reflect.Method;
+
+/**
+ * Handler for tracking and updating the assist status for a given activity.
+ */
+public class AssistStatusHandler {
+    private final Activity mActivity;
+    private final TabModelSelectorObserver mSelectorObserver;
+
+    private Boolean mAssistSupported;
+    private TabModelSelector mTabModelSelector;
+    private Method mSetAssistBlockedMethod;
+
+    /**
+     * Builds an assist status handler for the specified activity.
+     * @param activity The activity whose assist status should be updated.
+     */
+    public AssistStatusHandler(Activity activity) {
+        mActivity = activity;
+
+        mSelectorObserver = new EmptyTabModelSelectorObserver() {
+            @Override
+            public void onChange() {
+                updateAssistState();
+            }
+        };
+    }
+
+    /**
+     * Set the selector that the assist handler should track for tab updates.
+     * @param selector The selector broadcasting tab updates for the activity associated with
+     *                 this handler.
+     */
+    public void setTabModelSelector(TabModelSelector selector) {
+        if (mTabModelSelector != null) {
+            mTabModelSelector.removeObserver(mSelectorObserver);
+        }
+
+        mTabModelSelector = selector;
+        if (mTabModelSelector != null) {
+            mTabModelSelector.addObserver(mSelectorObserver);
+        }
+
+        updateAssistState();
+    }
+
+    /**
+     * Destroy the handler and removes any remaining dependencies.
+     */
+    public void destroy() {
+        if (mTabModelSelector != null) {
+            mTabModelSelector.removeObserver(mSelectorObserver);
+            mTabModelSelector = null;
+        }
+    }
+
+    /**
+     * @return Whether assist is currently supported based on the Activity state.
+     */
+    public boolean isAssistSupported() {
+        return mTabModelSelector == null || !mTabModelSelector.isIncognitoSelected();
+    }
+
+    /**
+     * Trigger an update of the assist state.
+     */
+    protected final void updateAssistState() {
+        if (!BuildInfo.isMncOrLater()) return;
+
+        boolean isAssistSupported = isAssistSupported();
+        if (mAssistSupported == null || mAssistSupported != isAssistSupported) {
+            if (mSetAssistBlockedMethod == null) {
+                try {
+                    mSetAssistBlockedMethod =
+                            View.class.getMethod("setAssistBlocked", boolean.class);
+                } catch (NoSuchMethodException e) {
+                    return;
+                }
+            }
+            View rootContent = mActivity.findViewById(android.R.id.content);
+            try {
+                mSetAssistBlockedMethod.invoke(rootContent, !isAssistSupported);
+            } catch (IllegalAccessException | IllegalArgumentException
+                    | InvocationTargetException e) {
+                return;
+            }
+        }
+        mAssistSupported = isAssistSupported;
+    }
+}