Tweaking fast scroller to follow touch closer.

- Issue: The fast scroller currently does not follow the touch input
  because fundamentally, the fixed scrollbar height and the mapping
  of the scroll space to the scrollbar space is fundamentally incompatible.
- This CL changes the fast scroller to allow it to detach when the user
  fast-scrolls, then re-attaches after the user scrolls the screen and
  the current scroll position for the scrollbar picks up the thumb position.
- Since the scroll position and the fast scroller thumb is now detached,
  we can change the distribution of the fast scroll letters to make it
  independent of the rows for each section and instead uniformly distribute
  it along the scrollbar, which allows for more stability.
- There are edge cases where this fails, especially when there are few
  apps, which we can investigate further.
  
Bug: 20035978

Change-Id: I8322f862107e6f330deff692885233706564bffd

1 files changed, 23 insertions, 17 deletions

diff --git a/src/com/android/launcher3/BaseRecyclerView.java b/src/com/android/launcher3/BaseRecyclerView.java
index 0fae427e8..f0d8b3b3d 100644
--- a/src/com/android/launcher3/BaseRecyclerView.java
+++ b/src/com/android/launcher3/BaseRecyclerView.java
@@ -92,9 +92,15 @@ public abstract class BaseRecyclerView extends RecyclerView
             // TODO(winsonc): If we want to animate the section heads while scrolling, we can
             //                initiate that here if the recycler view scroll state is not
             //                RecyclerView.SCROLL_STATE_IDLE.
+
+            onUpdateScrollbar(dy);
         }
     }
 
+    public void reset() {
+        mScrollbar.reattachThumbToScroll();
+    }
+
     @Override
     protected void onFinishInflate() {
         super.onFinishInflate();
@@ -143,7 +149,7 @@ public abstract class BaseRecyclerView extends RecyclerView
                 mScrollbar.handleTouchEvent(ev, mDownX, mDownY, mLastY);
                 break;
         }
-        return mScrollbar.isDragging();
+        return mScrollbar.isDraggingThumb();
     }
 
     public void onRequestDisallowInterceptTouchEvent(boolean disallowIntercept) {
@@ -185,12 +191,10 @@ public abstract class BaseRecyclerView extends RecyclerView
      *   AvailableScrollHeight = Total height of the all items - last page height
      *
      * This assumes that all rows are the same height.
-     *
-     * @param yOffset the offset from the top of the recycler view to start tracking.
      */
-    protected int getAvailableScrollHeight(int rowCount, int rowHeight, int yOffset) {
+    protected int getAvailableScrollHeight(int rowCount, int rowHeight) {
         int visibleHeight = getHeight() - mBackgroundPadding.top - mBackgroundPadding.bottom;
-        int scrollHeight = getPaddingTop() + yOffset + rowCount * rowHeight + getPaddingBottom();
+        int scrollHeight = getPaddingTop() + rowCount * rowHeight + getPaddingBottom();
         int availableScrollHeight = scrollHeight - visibleHeight;
         return availableScrollHeight;
     }
@@ -222,7 +226,7 @@ public abstract class BaseRecyclerView extends RecyclerView
     @Override
     protected void dispatchDraw(Canvas canvas) {
         super.dispatchDraw(canvas);
-        onUpdateScrollbar();
+        onUpdateScrollbar(0);
         mScrollbar.draw(canvas);
     }
 
@@ -234,24 +238,21 @@ public abstract class BaseRecyclerView extends RecyclerView
      * @param scrollPosState the current scroll position
      * @param rowCount the number of rows, used to calculate the total scroll height (assumes that
      *                 all rows are the same height)
-     * @param yOffset the offset to start tracking in the recycler view (only used for all apps)
      */
     protected void synchronizeScrollBarThumbOffsetToViewScroll(ScrollPositionState scrollPosState,
-            int rowCount, int yOffset) {
-        int availableScrollHeight = getAvailableScrollHeight(rowCount, scrollPosState.rowHeight,
-                yOffset);
-        int availableScrollBarHeight = getAvailableScrollBarHeight();
-
+            int rowCount) {
         // Only show the scrollbar if there is height to be scrolled
+        int availableScrollBarHeight = getAvailableScrollBarHeight();
+        int availableScrollHeight = getAvailableScrollHeight(rowCount, scrollPosState.rowHeight);
         if (availableScrollHeight <= 0) {
-            mScrollbar.setScrollbarThumbOffset(-1, -1);
+            mScrollbar.setThumbOffset(-1, -1);
             return;
         }
 
         // Calculate the current scroll position, the scrollY of the recycler view accounts for the
         // view padding, while the scrollBarY is drawn right up to the background padding (ignoring
         // padding)
-        int scrollY = getPaddingTop() + yOffset +
+        int scrollY = getPaddingTop() +
                 (scrollPosState.rowIndex * scrollPosState.rowHeight) - scrollPosState.rowTopOffset;
         int scrollBarY = mBackgroundPadding.top +
                 (int) (((float) scrollY / availableScrollHeight) * availableScrollBarHeight);
@@ -261,9 +262,9 @@ public abstract class BaseRecyclerView extends RecyclerView
         if (Utilities.isRtl(getResources())) {
             scrollBarX = mBackgroundPadding.left;
         } else {
-            scrollBarX = getWidth() - mBackgroundPadding.right - mScrollbar.getWidth();
+            scrollBarX = getWidth() - mBackgroundPadding.right - mScrollbar.getThumbWidth();
         }
-        mScrollbar.setScrollbarThumbOffset(scrollBarX, scrollBarY);
+        mScrollbar.setThumbOffset(scrollBarX, scrollBarY);
     }
 
     /**
@@ -276,10 +277,15 @@ public abstract class BaseRecyclerView extends RecyclerView
      * Updates the bounds for the scrollbar.
      * <p>Override in each subclass of this base class.
      */
-    public abstract void onUpdateScrollbar();
+    public abstract void onUpdateScrollbar(int dy);
 
     /**
      * <p>Override in each subclass of this base class.
      */
     public void onFastScrollCompleted() {}
+
+    /**
+     * Returns information about the item that the recycler view is currently scrolled to.
+     */
+    protected abstract void getCurScrollState(ScrollPositionState stateOut);
 }
\ No newline at end of file