Handle dragging improvements (#114042)

Improves the feel of dragging text selection vertically between lines on mobile.

Author: justinmc

packages/flutter/lib/src/widgets/text_selection.dart

@@ -637,7 +637,13 @@ class TextSelectionOverlay {
     );
   }
 
-  late Offset _dragEndPosition;
+  // The contact position of the gesture at the current end handle location.
+  // Updated when the handle moves.
+  late double _endHandleDragPosition;
+
+  // The distance from _endHandleDragPosition to the center of the line that it
+  // corresponds to.
+  late double _endHandleDragPositionToCenterOfLine;
 
   void _handleSelectionEndHandleDragStart(DragStartDetails details) {
     if (!renderObject.attached) {
@@ -646,10 +652,17 @@ class TextSelectionOverlay {
 
     // This adjusts for the fact that the selection handles may not
     // perfectly cover the TextPosition that they correspond to.
-    final Offset offsetFromHandleToTextPosition = _getOffsetToTextPositionPoint(_selectionOverlay.endHandleType);
-    _dragEndPosition = details.globalPosition + offsetFromHandleToTextPosition;
-
-    final TextPosition position = renderObject.getPositionForPoint(_dragEndPosition);
+    _endHandleDragPosition = details.globalPosition.dy;
+    final Offset endPoint =
+        renderObject.localToGlobal(_selectionOverlay.selectionEndpoints.last.point);
+    final double centerOfLine = endPoint.dy - renderObject.preferredLineHeight / 2;
+    _endHandleDragPositionToCenterOfLine = centerOfLine - _endHandleDragPosition;
+    final TextPosition position = renderObject.getPositionForPoint(
+      Offset(
+        details.globalPosition.dx,
+        centerOfLine,
+      ),
+    );
 
     _selectionOverlay.showMagnifier(
       _buildMagnifier(
@@ -660,14 +673,33 @@ class TextSelectionOverlay {
     );
   }
 
+  /// Given a handle position and drag position, returns the position of handle
+  /// after the drag.
+  ///
+  /// The handle jumps instantly between lines when the drag reaches a full
+  /// line's height away from the original handle position. In other words, the
+  /// line jump happens when the contact point would be located at the same
+  /// place on the handle at the new line as when the gesture started.
+  double _getHandleDy(double dragDy, double handleDy) {
+    final double distanceDragged = dragDy - handleDy;
+    final int dragDirection = distanceDragged < 0.0 ? -1 : 1;
+    final int linesDragged =
+        dragDirection * (distanceDragged.abs() / renderObject.preferredLineHeight).floor();
+    return handleDy + linesDragged * renderObject.preferredLineHeight;
+  }
+
   void _handleSelectionEndHandleDragUpdate(DragUpdateDetails details) {
     if (!renderObject.attached) {
       return;
     }
-    _dragEndPosition += details.delta;
 
-    final TextPosition position = renderObject.getPositionForPoint(_dragEndPosition);
-    final TextSelection currentSelection = TextSelection.fromPosition(position);
+    _endHandleDragPosition = _getHandleDy(details.globalPosition.dy, _endHandleDragPosition);
+    final Offset adjustedOffset = Offset(
+      details.globalPosition.dx,
+      _endHandleDragPosition + _endHandleDragPositionToCenterOfLine,
+    );
+
+    final TextPosition position = renderObject.getPositionForPoint(adjustedOffset);
 
     if (_selection.isCollapsed) {
       _selectionOverlay.updateMagnifier(_buildMagnifier(
@@ -676,6 +708,7 @@ class TextSelectionOverlay {
         renderEditable: renderObject,
       ));
 
+      final TextSelection currentSelection = TextSelection.fromPosition(position);
       _handleSelectionHandleChanged(currentSelection, isEnd: true);
       return;
     }
@@ -716,7 +749,13 @@ class TextSelectionOverlay {
     ));
   }
 
-  late Offset _dragStartPosition;
+  // The contact position of the gesture at the current start handle location.
+  // Updated when the handle moves.
+  late double _startHandleDragPosition;
+
+  // The distance from _startHandleDragPosition to the center of the line that
+  // it corresponds to.
+  late double _startHandleDragPositionToCenterOfLine;
 
   void _handleSelectionStartHandleDragStart(DragStartDetails details) {
     if (!renderObject.attached) {
@@ -725,10 +764,17 @@ class TextSelectionOverlay {
 
     // This adjusts for the fact that the selection handles may not
     // perfectly cover the TextPosition that they correspond to.
-    final Offset offsetFromHandleToTextPosition = _getOffsetToTextPositionPoint(_selectionOverlay.startHandleType);
-    _dragStartPosition = details.globalPosition + offsetFromHandleToTextPosition;
-
-    final TextPosition position = renderObject.getPositionForPoint(_dragStartPosition);
+    _startHandleDragPosition = details.globalPosition.dy;
+    final Offset startPoint =
+        renderObject.localToGlobal(_selectionOverlay.selectionEndpoints.first.point);
+    final double centerOfLine = startPoint.dy - renderObject.preferredLineHeight / 2;
+    _startHandleDragPositionToCenterOfLine = centerOfLine - _startHandleDragPosition;
+    final TextPosition position = renderObject.getPositionForPoint(
+      Offset(
+        details.globalPosition.dx,
+        centerOfLine,
+      ),
+    );
 
     _selectionOverlay.showMagnifier(
       _buildMagnifier(
@@ -743,8 +789,13 @@ class TextSelectionOverlay {
     if (!renderObject.attached) {
       return;
     }
-    _dragStartPosition += details.delta;
-    final TextPosition position = renderObject.getPositionForPoint(_dragStartPosition);
+
+    _startHandleDragPosition = _getHandleDy(details.globalPosition.dy, _startHandleDragPosition);
+    final Offset adjustedOffset = Offset(
+      details.globalPosition.dx,
+      _startHandleDragPosition + _startHandleDragPositionToCenterOfLine,
+    );
+    final TextPosition position = renderObject.getPositionForPoint(adjustedOffset);
 
     if (_selection.isCollapsed) {
       _selectionOverlay.updateMagnifier(_buildMagnifier(
@@ -753,7 +804,8 @@ class TextSelectionOverlay {
         renderEditable: renderObject,
       ));
 
-      _handleSelectionHandleChanged(TextSelection.fromPosition(position), isEnd: false);
+      final TextSelection currentSelection = TextSelection.fromPosition(position);
+      _handleSelectionHandleChanged(currentSelection, isEnd: false);
       return;
     }
 
@@ -813,32 +865,6 @@ class TextSelectionOverlay {
     }
   }
 
-  // Returns the offset that locates a drag on a handle to the correct line of text.
-  Offset _getOffsetToTextPositionPoint(TextSelectionHandleType type) {
-    final Size handleSize = selectionControls!.getHandleSize(
-      renderObject.preferredLineHeight,
-    );
-
-    // Try to shift center of handle to top by half of handle height.
-    final double halfHandleHeight = handleSize.height / 2;
-
-    // [getHandleAnchor] is used to shift the selection endpoint to the top left
-    // point of the handle rect when building the handle widget.
-    // The endpoint is at the bottom of the selection rect, which is also at the
-    // bottom of the line of text.
-    // Try to shift the top of the handle to the selection endpoint by the dy of
-    // the handle's anchor.
-    final double handleAnchorDy = selectionControls!.getHandleAnchor(type, renderObject.preferredLineHeight).dy;
-
-    // Try to shift the selection endpoint to the center of the correct line by
-    // using half of the line height.
-    final double halfPreferredLineHeight = renderObject.preferredLineHeight / 2;
-
-    // The x offset is accurate, so we only need to adjust the y position.
-    final double offsetYFromHandleToTextPosition = handleAnchorDy - halfHandleHeight - halfPreferredLineHeight;
-    return Offset(0.0, offsetYFromHandleToTextPosition);
-  }
-
   void _handleSelectionHandleChanged(TextSelection newSelection, {required bool isEnd}) {
     final TextPosition textPosition = isEnd ? newSelection.extent : newSelection.base;
     selectionDelegate.userUpdateTextEditingValue(