[NTP Snippets] Fill space below the last snippet if necessary

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

Index: chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/NewTabPageRecyclerView.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/NewTabPageRecyclerView.java b/chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/NewTabPageRecyclerView.java
index f09d1c43398b0dfd220e95c5b0166a01496f2bd5..e5b2d9d16d54dc539ffc6aba7b9a73ee8288b10b 100644
--- a/chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/NewTabPageRecyclerView.java
+++ b/chrome/android/java/src/org/chromium/chrome/browser/ntp/cards/NewTabPageRecyclerView.java
@@ -5,6 +5,7 @@
 package org.chromium.chrome.browser.ntp.cards;
 
 import android.content.Context;
+import android.content.res.Resources;
 import android.support.v7.widget.LinearLayoutManager;
 import android.support.v7.widget.RecyclerView;
 import android.util.AttributeSet;
@@ -14,13 +15,33 @@
 import android.view.inputmethod.EditorInfo;
 import android.view.inputmethod.InputConnection;
 
+import org.chromium.chrome.R;
+
 /**
  * Simple wrapper on top of a RecyclerView that will acquire focus when tapped.  Ensures the
  * New Tab page receives focus when clicked.
  */
 public class NewTabPageRecyclerView extends RecyclerView {
-    private GestureDetector mGestureDetector;
-    private LinearLayoutManager mLayoutManager;
+    /**
+     * Minimum height of the bottom spacing item.
+     */
+    private static final int MIN_BOTTOM_SPACING = 0;
+
+    /**
+     * Position in the adapter of the item we snap the scroll at, when switching between above and
+     * below the fold.
+     */
+    private static final int SNAP_ITEM_ADAPTER_POSITION = 1;
+
+    private final GestureDetector mGestureDetector;
+    private final LinearLayoutManager mLayoutManager;
+    private final int mToolbarHeight;
+
+    /**
+     * Total height of the items being dismissed.  Tracked to allow the bottom space to compensate
+     * for their removal animation and avoid moving the scroll position.
+     */
+    private int mCompensationHeight;
 
     /**
      * Constructor needed to inflate from XML.
@@ -39,6 +60,10 @@ public boolean onSingleTapUp(MotionEvent e) {
                 });
         mLayoutManager = new LinearLayoutManager(getContext());
         setLayoutManager(mLayoutManager);
+
+        Resources res = context.getResources();
+        mToolbarHeight = res.getDimensionPixelSize(R.dimen.toolbar_height_no_shadow)
+                + res.getDimensionPixelSize(R.dimen.toolbar_progress_bar_height);
     }
 
     public boolean isFirstItemVisible() {
@@ -78,4 +103,50 @@ public InputConnection onCreateInputConnection(EditorInfo outAttrs) {
     public LinearLayoutManager getLinearLayoutManager() {
         return mLayoutManager;
     }
+
+    /**
+     * Updates the space added at the end of the list to make sure the above/below the fold
+     * distinction can be preserved.
+     */
+    public void refreshBottomSpacing() {
+        ViewHolder bottomSpacingViewHolder =
+                findViewHolderForAdapterPosition(getAdapter().getItemCount() - 1);
+
+        // It might not be in the layout yet if it's not visible or ready to be displayed.
+        if (bottomSpacingViewHolder == null) return;
+
+        assert bottomSpacingViewHolder.getItemViewType() == NewTabPageListItem.VIEW_TYPE_SPACING;
+        bottomSpacingViewHolder.itemView.requestLayout();
+    }
+
+    /**
+     * Calculates the height of the bottom spacing item, such that there is always enough content
+     * below the fold to push the header up to to the top of the screen.
+     */
+    int calculateBottomSpacing() {
+        int firstVisiblePos = mLayoutManager.findFirstVisibleItemPosition();
+
+        // We have enough items to fill the view, since the snap point item is not even visible.
+        if (firstVisiblePos > SNAP_ITEM_ADAPTER_POSITION) return MIN_BOTTOM_SPACING;
+
+        // The spacing item is the last item, the last content item is directly above that.
+        int lastContentItemPosition = getAdapter().getItemCount() - 2;
+        int contentHeight =
+                findViewHolderForAdapterPosition(lastContentItemPosition).itemView.getBottom()
+                - findViewHolderForAdapterPosition(SNAP_ITEM_ADAPTER_POSITION).itemView.getTop();
+        int bottomSpacing = getHeight() - mToolbarHeight - contentHeight + mCompensationHeight;
+
+        return Math.max(MIN_BOTTOM_SPACING, bottomSpacing);
+    }
+
+    /** Called when an item is in the process of being removed from the view. */
+    void onItemDismissStarted(View itemView) {
+        mCompensationHeight += itemView.getHeight();
+    }
+
+    /** Called when an item has finished being removed from the view. */
+    void onItemDismissFinished(View itemView) {
+        mCompensationHeight -= itemView.getHeight();
+        assert mCompensationHeight >= 0;
+    }
 }