Added support for passing in velocity and offset into Draggable.onDragCompleted (#22267)

* Added support for passing in velocity and offset into Draggable.onDragCompleted.

* Fixed documentation of DragCompletedCallback.

* Spun off previous onDragCompleted breaking changes into new callback called Draggable.onDragEnd.

* Revert "Fixed documentation of DragCompletedCallback."

This reverts commit 069051f5be0d4ec6a1f4b3f072e535ca87e5d740.

* Revert "Added support for passing in velocity and offset into Draggable.onDragCompleted."

This reverts commit 7ef744aa5645429b7bc92527226203ee8bff68ec.

* DraggableDetails constructor is now declared first as per Flutter code style.

* Draggable.onDragEnd will only call back if its widget is currently mounted to the tree.

* Moved "});" in DraggableDetails constructor to new line, vertically aligned with the constructor name, as per Flutter code style.

* Added space between if statement in drag_target.dart.

* widget.onDragEnd call is now formated as per flutter code style.

* Added more details to DraggableDetails documentation.

* Added brackets to if statement block as per Flutter code style.

* Fixed minor nits in DraggableDetails documentation.

* Made DraggableDetails constructor public. Also added documentation for its constructor.

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

@@ -36,6 +36,15 @@ typedef DragTargetBuilder<T> = Widget Function(BuildContext context, List<T> can
 /// Used by [Draggable.onDraggableCanceled].
 typedef DraggableCanceledCallback = void Function(Velocity velocity, Offset offset);
 
+/// Signature for when the draggable is dropped.
+///
+/// The velocity and offset at which the pointer was moving when the draggable
+/// was dropped is available in the [DraggableDetails]. Also included in the
+/// `details` is whether the draggable's [DragTarget] accepted it.
+///
+/// Used by [Draggable.onDragEnd]
+typedef DragEndCallback = void Function(DraggableDetails details);
+
 /// Signature for when a [Draggable] leaves a [DragTarget].
 ///
 /// Used by [DragTarget.onLeave].
@@ -100,6 +109,7 @@ class Draggable<T> extends StatefulWidget {
     this.maxSimultaneousDrags,
     this.onDragStarted,
     this.onDraggableCanceled,
+    this.onDragEnd,
     this.onDragCompleted,
     this.ignoringFeedbackSemantics = true,
   }) : assert(child != null),
@@ -229,6 +239,16 @@ class Draggable<T> extends StatefulWidget {
   /// callback is still in the tree.
   final VoidCallback onDragCompleted;
 
+  /// Called when the draggable is dropped.
+  ///
+  /// The velocity and offset at which the pointer was moving when it was
+  /// dropped is available in the [DraggableDetails]. Also included in the
+  /// `details` is whether the draggable's [DragTarget] accepted it.
+  ///
+  /// This function will only be called while this widget is still mounted to
+  /// the tree (i.e. [State.mounted] is true).
+  final DragEndCallback onDragEnd;
+
   /// Creates a gesture recognizer that recognizes the start of the drag.
   ///
   /// Subclasses can override this function to customize when they start
@@ -266,6 +286,7 @@ class LongPressDraggable<T> extends Draggable<T> {
     int maxSimultaneousDrags,
     VoidCallback onDragStarted,
     DraggableCanceledCallback onDraggableCanceled,
+    DragEndCallback onDragEnd,
     VoidCallback onDragCompleted,
     this.hapticFeedbackOnStart = true,
     bool ignoringFeedbackSemantics = true,
@@ -281,6 +302,7 @@ class LongPressDraggable<T> extends Draggable<T> {
     maxSimultaneousDrags: maxSimultaneousDrags,
     onDragStarted: onDragStarted,
     onDraggableCanceled: onDraggableCanceled,
+    onDragEnd: onDragEnd,
     onDragCompleted: onDragCompleted,
     ignoringFeedbackSemantics: ignoringFeedbackSemantics,
   );
@@ -372,6 +394,13 @@ class _DraggableState<T> extends State<Draggable<T>> {
           _activeCount -= 1;
           _disposeRecognizerIfInactive();
         }
+        if (mounted && widget.onDragEnd != null) {
+          widget.onDragEnd(DraggableDetails(
+              wasAccepted: wasAccepted,
+              velocity: velocity,
+              offset: offset
+          ));
+        }
         if (wasAccepted && widget.onDragCompleted != null)
           widget.onDragCompleted();
         if (!wasAccepted && widget.onDraggableCanceled != null)
@@ -396,6 +425,38 @@ class _DraggableState<T> extends State<Draggable<T>> {
   }
 }
 
+/// Represents the details when a specific pointer event occurred on
+/// the [Draggable].
+///
+/// This includes the [Velocity] at which the pointer was moving and [Offset]
+/// when the draggable event occurred, and whether its [DragTarget] accepted it.
+///
+/// Also, this is the details object for callbacks that use [DragEndCallback].
+class DraggableDetails {
+  /// Creates details for a [DraggableDetails].
+  ///
+  /// If [wasAccepted] is not specified, it will default to `false`.
+  ///
+  /// The [velocity] or [offset] arguments must not be `null`.
+  DraggableDetails({
+    this.wasAccepted = false,
+    @required this.velocity,
+    @required this.offset
+  }) : assert(velocity != null),
+       assert(offset != null);
+
+  /// Determines whether the [DragTarget] accepted this draggable.
+  final bool wasAccepted;
+
+  /// The velocity at which the pointer was moving when the specific pointer
+  /// event occurred on the draggable.
+  final Velocity velocity;
+
+  /// The global position when the specific pointer event occurred on
+  /// the draggable.
+  final Offset offset;
+}
+
 /// A widget that receives data when a [Draggable] widget is dropped.
 ///
 /// When a draggable is dragged on top of a drag target, the drag target is