Fix DnD logic for determining current drag screen / panel

=> we want to use the extremum of finger location and object
   center for snapping to a new screen (max when snapping to right,
   min when snapping to left)
=> when selecting a panel within the current screen, we want to
   rely strictly on the object center to determine selection; there
   is plenty of space to be consistent here, and extremum logic
   could cause thrashing back and forth with no page movement.

Test: Manual. See videos in b/221104663. On two-panel and one-panel
      test dragging a widget between different screens with
      different starting positions of where the widget goes down.

Bug 221104663

Change-Id: I8f0f13f07b753752087a90c4bacb76cfbf7aa5bf

1 files changed, 20 insertions, 10 deletions

diff --git a/src/com/android/launcher3/Workspace.java b/src/com/android/launcher3/Workspace.java
index b3f5c0334f..df97d2fd06 100644
--- a/src/com/android/launcher3/Workspace.java
+++ b/src/com/android/launcher3/Workspace.java
@@ -2484,21 +2484,27 @@ public class Workspace extends PagedView<WorkspacePageIndicator>
             }
         }
 
-        int nextPage = getNextPage();
-        IntSet pageIndexesToVerify = IntSet.wrap(nextPage - 1, nextPage + 1);
-        if (isTwoPanelEnabled()) {
-            // If two panel is enabled, users can also drag items to nextPage + 2
-            pageIndexesToVerify.add(nextPage + 2);
-        }
-
-        int touchX = (int) Math.min(centerX, d.x);
-        int touchY = d.y;
+        // Note, centerX represents the center of the object that is being dragged, visually. d.x
+        // represents the location of the finger within the dragged item.
+        float touchX;
+        float touchY = d.y;
 
-        // Go through the pages and check if the dragged item is inside one of them
+        // Go through the pages and check if the dragged item is inside one of them. This block
+        // is responsible for  determining whether we need to snap to a different screen.
+        int nextPage = getNextPage();
+        IntSet pageIndexesToVerify = IntSet.wrap(nextPage - 1, nextPage
+                + (isTwoPanelEnabled() ? 2 : 1));
         for (int pageIndex : pageIndexesToVerify) {
             if (layout != null || isPageInTransition()) {
                 break;
             }
+
+            // When deciding whether to perform a page switch, we need to consider the most extreme
+            // X coordinate between the finger location and the center of the object being dragged.
+            // This is either the max or the min of the two depending on whether dragging to the
+            // left / right, respectively.
+            touchX = ((((pageIndex < nextPage) && !mIsRtl) || pageIndex > nextPage && mIsRtl)
+                    ? Math.min(d.x, centerX) : Math.max(d.x, centerX));
             layout = verifyInsidePage(pageIndex, touchX, touchY);
         }
 
@@ -2507,12 +2513,16 @@ public class Workspace extends PagedView<WorkspacePageIndicator>
         // on one panel just choose the current page.
         if (layout == null && nextPage >= 0 && nextPage < getPageCount()) {
             if (isTwoPanelEnabled()) {
+                // When determining which panel to use within a single screen, we always use
+                // the centroid of the object rather than the finger.
+                touchX = centerX;
                 nextPage = getScreenCenter(getScrollX()) > touchX
                         ? (mIsRtl ? nextPage + 1 : nextPage) // left side
                         : (mIsRtl ? nextPage : nextPage + 1); // right side
             }
             layout = (CellLayout) getChildAt(nextPage);
         }
+
         if (layout != mDragTargetLayout) {
             setCurrentDropLayout(layout);
             setCurrentDragOverlappingLayout(layout);