Show shadow when entity header starts scrolling.

- Add a controller to manage add/remove onScrollChangedListener to
  recyclerviews.
- When recyclerview on each screen is scrolled to top, set actionbar
  elevation to 0, otherwise set it to non-zero.
- When screen is moved to background, detach the listener.
- Use the controller in entity header.

Change-Id: Iecf194d885098c98c392810f62893ae9189f3936
Fix: 37670670
Test: make RunSettingsRoboTests

src/com/android/settings/widget/ActionBarShadowController.java

@@ -0,0 +1,97 @@
+/*
+ * Copyright (C) 2017 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.android.settings.widget;
+
+import android.app.ActionBar;
+import android.app.Activity;
+import android.support.v7.widget.RecyclerView;
+import android.view.View;
+
+import com.android.settingslib.core.lifecycle.Lifecycle;
+import com.android.settingslib.core.lifecycle.LifecycleObserver;
+import com.android.settingslib.core.lifecycle.events.OnStart;
+import com.android.settingslib.core.lifecycle.events.OnStop;
+
+public class ActionBarShadowController implements LifecycleObserver, OnStart, OnStop {
+
+    private ScrollChangeWatcher mScrollChangeWatcher;
+    private RecyclerView mRecyclerView;
+    private boolean isScrollWatcherAttached;
+
+    public static ActionBarShadowController attachToRecyclerView(Activity activity,
+            Lifecycle lifecycle, RecyclerView recyclerView) {
+        return new ActionBarShadowController(activity, lifecycle, recyclerView);
+    }
+
+    private ActionBarShadowController(Activity activity, Lifecycle lifecycle,
+            RecyclerView recyclerView) {
+        mScrollChangeWatcher = new ScrollChangeWatcher(activity);
+        mRecyclerView = recyclerView;
+        attachScrollWatcher();
+        lifecycle.addObserver(this);
+    }
+
+    @Override
+    public void onStop() {
+        detachScrollWatcher();
+    }
+
+    private void detachScrollWatcher() {
+        mRecyclerView.removeOnScrollListener(mScrollChangeWatcher);
+        isScrollWatcherAttached = false;
+    }
+
+    @Override
+    public void onStart() {
+        attachScrollWatcher();
+    }
+
+    private void attachScrollWatcher() {
+        if (!isScrollWatcherAttached) {
+            isScrollWatcherAttached = true;
+            mRecyclerView.addOnScrollListener(mScrollChangeWatcher);
+            mScrollChangeWatcher.updateDropShadow(mRecyclerView);
+        }
+    }
+
+    /**
+     * Update the drop shadow as the scrollable entity is scrolled.
+     */
+    private final class ScrollChangeWatcher extends RecyclerView.OnScrollListener {
+
+        private Activity mActivity;
+
+        public ScrollChangeWatcher(Activity activity) {
+            mActivity = activity;
+        }
+
+        // RecyclerView scrolled.
+        @Override
+        public void onScrolled(RecyclerView view, int dx, int dy) {
+            updateDropShadow(view);
+        }
+
+        public void updateDropShadow(View view) {
+            final boolean shouldShowShadow = view.canScrollVertically(-1);
+            final ActionBar actionBar = mActivity.getActionBar();
+            if (actionBar != null) {
+                actionBar.setElevation(shouldShowShadow ? 8 : 0);
+            }
+        }
+    }
+
+}