[Preferences] Use getListView() over R.id.list

The usage of R.id.list here causes problems when updating support
library version. Switch to the PreferenceFragmentCompat.getListView()
method instead.

Also add some clarification about when to use
getShowShadowOnScrollListener in preference fragments, and document the
lack of such listener on SearchEnginePreference with a TODO.

Bug: 987748
Change-Id: Ieb76455e6744ec1ed0f300c8fe0914ed630e3562
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1725099Reviewed-by: Theresa <twellington@chromium.org>
Commit-Queue: Natalie Chouinard <chouinard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#682333}

chrome/android/java/src/org/chromium/chrome/browser/preferences/Preferences.java

@@ -24,8 +24,6 @@ import android.util.Log;
 import android.view.Menu;
 import android.view.MenuItem;
 import android.view.View;
-import android.view.ViewGroup;
-import android.widget.FrameLayout;
 
 import org.chromium.base.ApiCompatibilityUtils;
 import org.chromium.base.VisibleForTesting;
@@ -40,12 +38,13 @@ import org.chromium.chrome.browser.profiles.ProfileManagerUtils;
 /**
  * The Chrome settings activity.
  *
- * This activity displays a single Fragment, typically a PreferenceFragment. As the user navigates
- * through settings, a separate Preferences activity is created for each screen. Thus each fragment
- * may freely modify its activity's action bar or title. This mimics the behavior of
- * android.preference.PreferenceActivity.
+ * This activity displays a single {@link Fragment}, typically a {@link PreferenceFragmentCompat}.
+ * As the user navigates through settings, a separate Preferences activity is created for each
+ * screen. Thus each fragment may freely modify its activity's action bar or title. This mimics the
+ * behavior of {@link android.preference.PreferenceActivity}.
  *
- * If the preference overrides the root layout (e.g. {@link HomepageEditor}), add the following:
+ * If the main fragment is not an instance of {@link PreferenceFragmentCompat} (e.g. {@link
+ * HomepageEditor}) or overrides {@link PreferenceFragmentCompat}'s layout, add the following:
  * 1) preferences_action_bar_shadow.xml to the custom XML hierarchy and
  * 2) an OnScrollChangedListener to the main content's view's view tree observer via
  *    PreferenceUtils.getShowShadowOnScrollListener(...).
@@ -162,18 +161,22 @@ public class Preferences extends ChromeBaseAppCompatActivity
     @Override
     public void onAttachedToWindow() {
         super.onAttachedToWindow();
+
         Fragment fragment = getMainFragment();
-        if (fragment == null || fragment.getView() == null
-                || fragment.getView().findViewById(R.id.list) == null) {
+        if (!(fragment instanceof PreferenceFragmentCompat)) {
             return;
         }
-        View contentView = fragment.getActivity().findViewById(android.R.id.content);
-        if (contentView == null || !(contentView instanceof FrameLayout)) {
+
+        RecyclerView recyclerView = ((PreferenceFragmentCompat) fragment).getListView();
+        if (recyclerView == null) {
             return;
         }
-        View inflatedView = View.inflate(getApplicationContext(),
-                R.layout.preferences_action_bar_shadow, (ViewGroup) contentView);
-        RecyclerView recyclerView = fragment.getView().findViewById(R.id.list);
+
+        // Append action bar shadow to layout.
+        View inflatedView = getLayoutInflater().inflate(
+                R.layout.preferences_action_bar_shadow, findViewById(android.R.id.content));
+
+        // Display shadow on scroll.
         recyclerView.getViewTreeObserver().addOnScrollChangedListener(
                 PreferenceUtils.getShowShadowOnScrollListener(
                         recyclerView, inflatedView.findViewById(R.id.shadow)));

chrome/android/java/src/org/chromium/chrome/browser/preferences/SearchEnginePreference.java

@@ -13,8 +13,10 @@ import org.chromium.base.VisibleForTesting;
 import org.chromium.chrome.R;
 
 /**
-* A preference fragment for selecting a default search engine.
-*/
+ * A preference fragment for selecting a default search engine.
+ *
+ * TODO(crbug.com/988877): Add on scroll shadow to action bar.
+ */
 public class SearchEnginePreference extends ListFragment {
     private SearchEngineAdapter mSearchEngineAdapter;