Set preloading for the WidgetTray recycler view.

b/20339302

Introduced a constant that we can use to tweak how much extra screen
space worth of widgets are preloaded.

Change-Id: I47f0fec134b4e268140482694a1903cf902731c6

src/com/android/launcher3/widget/WidgetsContainerView.java

@@ -24,6 +24,7 @@ import android.graphics.Rect;
 import android.graphics.drawable.Drawable;
 import android.support.v7.widget.LinearLayoutManager;
 import android.support.v7.widget.RecyclerView;
+import android.support.v7.widget.RecyclerView.State;
 import android.util.AttributeSet;
 import android.util.Log;
 import android.view.View;
@@ -62,6 +63,9 @@ public class WidgetsContainerView extends FrameLayout implements Insettable,
 
     private static final int SPRING_MODE_DELAY_MS = 150;
 
+    /* Coefficient multiplied to the screen height for preloading widgets. */
+    private static final int PRELOAD_SCREEN_HEIGHT_MULTIPLE = 1;
+
     /* Global instances that are used inside this container. */
     private Launcher mLauncher;
     private DragController mDragController;
@@ -114,8 +118,15 @@ public class WidgetsContainerView extends FrameLayout implements Insettable,
         }
         mView = (RecyclerView) findViewById(R.id.widgets_list_view);
         mView.setAdapter(mAdapter);
-        mView.setLayoutManager(new LinearLayoutManager(getContext()));
 
+        // This extends the layout space so that preloading happen for the {@link RecyclerView}
+        mView.setLayoutManager(new LinearLayoutManager(getContext()) {
+            @Override
+            protected int getExtraLayoutSpace(State state) {
+                return super.getExtraLayoutSpace(state)
+                        + WidgetsContainerView.this.getHeight() * PRELOAD_SCREEN_HEIGHT_MULTIPLE;
+            }
+        });
         mPadding.set(getPaddingLeft(), getPaddingTop(), getPaddingRight(),
                 getPaddingBottom());
     }