📰 Adjust the card display depending on the screen width.

chrome/android/java/src/org/chromium/chrome/browser/ntp/DisplayStyleObserver.java

+// 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.ntp;
+
+import android.view.View;
+
+/**
+ * Gets notified of changes in the display style.
+ * @see UiConfig.DisplayStyle
+ * @see UiConfig#getDisplayStyle()
+ */
+public interface DisplayStyleObserver {
+    void onDisplayStyleChanged(@UiConfig.DisplayStyle int newDisplayStyle);
+
+    /**
+     * Implementation of {@link DisplayStyleObserver} designed to play nicely with
+     * {@link android.support.v7.widget.RecyclerView}. It will not notify of changes when the
+     * associated view is not attached to the window.
+     */
+    public static class ViewAdapter

[Bernhard Bauer, 2016/07/19 14:46:52]

Pull this out into a separate class?

[dgn, 2016/07/19 23:33:56]

Done.

+            implements DisplayStyleObserver, View.OnAttachStateChangeListener {
+        private final View mView;
+        private final UiConfig mConfig;
+        private final DisplayStyleObserver mListener;
+
+        private boolean mIsViewAttached;
+
+        /**
+         * @param view the view whose lifecycle is tracked to determine when to not fire the
+         *             observer.
+         * @param config the {@link UiConfig} object to subscribe to.
+         * @param listener the listener to adapt. It's {#onDisplayStyleChanged} will be called when
+         *                 the configuration changes, provided that {@code view} is attached to the
+         *                 window.
+         */
+        public ViewAdapter(View view, UiConfig config, DisplayStyleObserver listener) {
+            mView = view;
+            mConfig = config;
+            mListener = listener;
+
+            // TODO(dgn): getParent() is not a good way to test that, but isAttachedToWindow()
+            // requires API 19.
+            mIsViewAttached = view.getParent() != null;
+
+            mView.addOnAttachStateChangeListener(this);
+            mConfig.addListener(this);

[Bernhard Bauer, 2016/07/19 14:46:52]

You could just have the caller attached the returned object, then you don't to
pass it in here. Actually, that would mean you wouldn't have a constructor call
with the return value ignored anymore, so you could remove the static method as
well.

[dgn, 2016/07/19 23:33:56]

I use it in refreshDisplayStyle() to pull the current display style.

[Bernhard Bauer, 2016/07/20 08:56:24]

Hm... you could have UiConfig.addObserver() immediately notify the new observer,
and cache the last style in this class. Then you could immediately notify your
observer when we attach back to a window. This would effectively make everything
push-based. It would also have the side effect of avoiding notifications if all
we did was detach and reattach without the display style actually changing.

[dgn, 2016/07/20 11:38:39]

Done.

+
+            refreshDisplayStyle();
+        }
+
+        @Override
+        public void onDisplayStyleChanged(@UiConfig.DisplayStyle int newDisplayStyle) {
+            if (!mIsViewAttached) return;
+            mListener.onDisplayStyleChanged(newDisplayStyle);
+        }
+
+        @Override
+        public void onViewAttachedToWindow(View v) {
+            mIsViewAttached = true;
+            refreshDisplayStyle();
+        }
+
+        @Override
+        public void onViewDetachedFromWindow(View v) {
+            mIsViewAttached = false;
+        }
+
+        private void refreshDisplayStyle() {
+            onDisplayStyleChanged(mConfig.getDisplayStyle());
+        }
+    }
+}