[NTP Snippets] Refresh the displayed snippets less frequently

chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/NewTabPageAdapter.java

Index: chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/NewTabPageAdapter.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/NewTabPageAdapter.java b/chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/NewTabPageAdapter.java
index b1d4b7147d97d61eddb932c477082d809041bc78..de7aeeefe90c611aa3b2fd9dc82756b9b79616d2 100644
--- a/chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/NewTabPageAdapter.java
+++ b/chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/NewTabPageAdapter.java
@@ -17,6 +17,8 @@
 import org.chromium.chrome.browser.ntp.snippets.SnippetArticleViewHolder;
 import org.chromium.chrome.browser.ntp.snippets.SnippetHeaderListItem;
 import org.chromium.chrome.browser.ntp.snippets.SnippetsBridge.SnippetsObserver;
+import org.chromium.chrome.browser.tab.EmptyTabObserver;
+import org.chromium.chrome.browser.tab.Tab;
 
 import java.util.ArrayList;
 import java.util.List;
@@ -79,6 +81,14 @@ public NewTabPageAdapter(NewTabPageManager manager, NewTabPageLayout newTabPageL
         mNewTabPageListItems.add(mAboveTheFoldListItem);
 
         mNewTabPageManager.setSnippetsObserver(this);
+
+        // Fetch new snippets when switching back to the NTP, from another tab or another app.
+        mNewTabPageManager.addTabObserver(new EmptyTabObserver() {
+            @Override
+            public void onShown(Tab tab) {
+                mNewTabPageManager.setSnippetsObserver(NewTabPageAdapter.this);
+            }
+        });
     }
 
     /** Returns callbacks to configure the interactions with the RecyclerView's items. */
@@ -90,15 +100,29 @@ public NewTabPageAdapter(NewTabPageManager manager, NewTabPageLayout newTabPageL
     public void onSnippetsReceived(List<SnippetArticle> listSnippets) {
         int newSnippetCount = listSnippets.size();
         Log.d(TAG, "Received %d new snippets.", newSnippetCount);
-        mNewTabPageListItems.clear();
-        mNewTabPageListItems.add(mAboveTheFoldListItem);
 
-        if (newSnippetCount > 0) {
-            mNewTabPageListItems.add(new SnippetHeaderListItem());
-            mNewTabPageListItems.addAll(listSnippets);
+        // At first, there might be no snippets available, we wait until they have been fetched.
+        if (newSnippetCount == 0) return;
+
+        // Copy thumbnails over
+        for (SnippetArticle newSnippet : listSnippets) {
+            int existingSnippetIdx = mNewTabPageListItems.indexOf(newSnippet);
+            if (existingSnippetIdx == -1) continue;
+
+            newSnippet.setThumbnailBitmap(
+                    ((SnippetArticle) mNewTabPageListItems.get(existingSnippetIdx))
+                            .getThumbnailBitmap());
         }
 
+        mNewTabPageListItems.clear();
+        mNewTabPageListItems.add(mAboveTheFoldListItem);
+        mNewTabPageListItems.add(new SnippetHeaderListItem());
+        mNewTabPageListItems.addAll(listSnippets);
+
         notifyDataSetChanged();
+
+        // We don't want to get notified of other changes.
+        mNewTabPageManager.setSnippetsObserver(null);
     }
 
     @Override
@@ -150,4 +174,8 @@ private void dismissItem(int position) {
 
         notifyItemRangeRemoved(position, numRemovedItems);
     }
+
+    List<NewTabPageListItem> getItemsForTesting() {
+        return mNewTabPageListItems;
+    }
 }