Reorganise the drag related gesture files. (#9487)

Move the Drag*Details classes into drag_details.dart.
Move the one-gesture drag recognizers into monodrag.dart.
Move Drag into drag.dart.

Adjust the comments to claim that Drag is used by other things than
MultiDragGestureRecognizer. (Right now this is a lie but it will
hopefully be true soon.)

packages/flutter/lib/gestures.dart

@@ -12,10 +12,12 @@ export 'src/gestures/binding.dart';
 export 'src/gestures/constants.dart';
 export 'src/gestures/converter.dart';
 export 'src/gestures/drag.dart';
+export 'src/gestures/drag_details.dart';
 export 'src/gestures/events.dart';
 export 'src/gestures/hit_test.dart';
 export 'src/gestures/long_press.dart';
 export 'src/gestures/lsq_solver.dart';
+export 'src/gestures/monodrag.dart';
 export 'src/gestures/multidrag.dart';
 export 'src/gestures/multitap.dart';
 export 'src/gestures/pointer_router.dart';

packages/flutter/lib/src/gestures/drag_details.dart

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:ui' show Offset;
+
+import 'package:flutter/foundation.dart';
+
+import 'velocity_tracker.dart';
+
+/// Details object for callbacks that use [GestureDragDownCallback].
+///
+/// See also:
+///
+///  * [DragGestureRecognizer.onDown], which uses [GestureDragDownCallback].
+///  * [DragStartDetails], the details for [GestureDragStartCallback].
+///  * [DragUpdateDetails], the details for [GestureDragUpdateCallback].
+///  * [DragEndDetails], the details for [GestureDragEndCallback].
+class DragDownDetails {
+  /// Creates details for a [GestureDragDownCallback].
+  ///
+  /// The [globalPosition] argument must not be null.
+  DragDownDetails({ this.globalPosition: Offset.zero }) {
+    assert(globalPosition != null);
+  }
+
+  /// The global position at which the pointer contacted the screen.
+  ///
+  /// Defaults to the origin if not specified in the constructor.
+  final Offset globalPosition;
+
+  @override
+  String toString() => '$runtimeType($globalPosition)';
+}
+
+/// Signature for when a pointer has contacted the screen and might begin to
+/// move.
+///
+/// The `details` object provides the position of the touch.
+///
+/// See [DragGestureRecognizer.onDown].
+typedef void GestureDragDownCallback(DragDownDetails details);
+
+/// Details object for callbacks that use [GestureDragStartCallback].
+///
+/// See also:
+///
+///  * [DragGestureRecognizer.onStart], which uses [GestureDragStartCallback].
+///  * [DragDownDetails], the details for [GestureDragDownCallback].
+///  * [DragUpdateDetails], the details for [GestureDragUpdateCallback].
+///  * [DragEndDetails], the details for [GestureDragEndCallback].
+class DragStartDetails {
+  /// Creates details for a [GestureDragStartCallback].
+  ///
+  /// The [globalPosition] argument must not be null.
+  DragStartDetails({ this.globalPosition: Offset.zero }) {
+    assert(globalPosition != null);
+  }
+
+  /// The global position at which the pointer contacted the screen.
+  ///
+  /// Defaults to the origin if not specified in the constructor.
+  final Offset globalPosition;
+
+  @override
+  String toString() => '$runtimeType($globalPosition)';
+}
+
+/// Signature for when a pointer has contacted the screen and has begun to move.
+///
+/// The `details` object provides the position of the touch when it first
+/// touched the surface.
+///
+/// See [DragGestureRecognizer.onStart].
+typedef void GestureDragStartCallback(DragStartDetails details);
+
+/// Details object for callbacks that use [GestureDragUpdateCallback].
+///
+/// See also:
+///
+///  * [DragGestureRecognizer.onUpdate], which uses [GestureDragUpdateCallback].
+///  * [DragDownDetails], the details for [GestureDragDownCallback].
+///  * [DragStartDetails], the details for [GestureDragStartCallback].
+///  * [DragEndDetails], the details for [GestureDragEndCallback].
+class DragUpdateDetails {
+  /// Creates details for a [DragUpdateDetails].
+  ///
+  /// The [delta] argument must not be null.
+  ///
+  /// If [primaryDelta] is non-null, then its value must match one of the
+  /// coordinates of [delta] and the other coordinate must be zero.
+  ///
+  /// The [globalPosition] argument must be provided and must not be null.
+  DragUpdateDetails({
+    this.delta: Offset.zero,
+    this.primaryDelta,
+    @required this.globalPosition
+  }) {
+    assert(primaryDelta == null
+        || (primaryDelta == delta.dx && delta.dy == 0.0)
+        || (primaryDelta == delta.dy && delta.dx == 0.0));
+  }
+
+  /// The amount the pointer has moved since the previous update.
+  ///
+  /// If the [GestureDragUpdateCallback] is for a one-dimensional drag (e.g.,
+  /// a horizontal or vertical drag), then this offset contains only the delta
+  /// in that direction (i.e., the coordinate in the other direction is zero).
+  ///
+  /// Defaults to zero if not specified in the constructor.
+  final Offset delta;
+
+  /// The amount the pointer has moved along the primary axis since the previous
+  /// update.
+  ///
+  /// If the [GestureDragUpdateCallback] is for a one-dimensional drag (e.g.,
+  /// a horizontal or vertical drag), then this value contains the component of
+  /// [delta] along the primary axis (e.g., horizontal or vertical,
+  /// respectively). Otherwise, if the [GestureDragUpdateCallback] is for a
+  /// two-dimensional drag (e.g., a pan), then this value is null.
+  ///
+  /// Defaults to null if not specified in the constructor.
+  final double primaryDelta;
+
+  /// The pointer's global position when it triggered this update.
+  final Offset globalPosition;
+
+  @override
+  String toString() => '$runtimeType($delta)';
+}
+
+/// Signature for when a pointer that is in contact with the screen and moving
+/// has moved again.
+///
+/// The `details` object provides the position of the touch and the distance it
+/// has travelled since the last update.
+///
+/// See [DragGestureRecognizer.onUpdate].
+typedef void GestureDragUpdateCallback(DragUpdateDetails details);
+
+/// Details object for callbacks that use [GestureDragEndCallback].
+///
+/// See also:
+///
+///  * [DragGestureRecognizer.onEnd], which uses [GestureDragEndCallback].
+///  * [DragDownDetails], the details for [GestureDragDownCallback].
+///  * [DragStartDetails], the details for [GestureDragStartCallback].
+///  * [DragUpdateDetails], the details for [GestureDragUpdateCallback].
+class DragEndDetails {
+  /// Creates details for a [GestureDragEndCallback].
+  ///
+  /// The [velocity] argument must not be null.
+  DragEndDetails({
+    this.velocity: Velocity.zero,
+    this.primaryVelocity,
+  }) {
+    assert(velocity != null);
+    assert(primaryVelocity == null
+        || primaryVelocity == velocity.pixelsPerSecond.dx
+        || primaryVelocity == velocity.pixelsPerSecond.dy);
+  }
+
+  /// The velocity the pointer was moving when it stopped contacting the screen.
+  ///
+  /// Defaults to zero if not specified in the constructor.
+  final Velocity velocity;
+
+  /// The velocity the pointer was moving along the primary axis when it stopped
+  /// contacting the screen, in logical pixels per second.
+  ///
+  /// If the [GestureDragEndCallback] is for a one-dimensional drag (e.g., a
+  /// horizontal or vertical drag), then this value contains the component of
+  /// [velocity] along the primary axis (e.g., horizontal or vertical,
+  /// respectively). Otherwise, if the [GestureDragEndCallback] is for a
+  /// two-dimensional drag (e.g., a pan), then this value is null.
+  ///
+  /// Defaults to null if not specified in the constructor.
+  final double primaryVelocity;
+
+  @override
+  String toString() => '$runtimeType($velocity)';
+}

packages/flutter/lib/src/gestures/multidrag.dart

@@ -11,6 +11,7 @@ import 'arena.dart';
 import 'binding.dart';
 import 'constants.dart';
 import 'drag.dart';
+import 'drag_details.dart';
 import 'events.dart';
 import 'recognizer.dart';
 import 'velocity_tracker.dart';
@@ -18,24 +19,6 @@ import 'velocity_tracker.dart';
 /// Signature for when [MultiDragGestureRecognizer] recognizes the start of a drag gesture.
 typedef Drag GestureMultiDragStartCallback(Offset position);
 
-/// Interface for receiving updates about drags from a [MultiDragGestureRecognizer].
-abstract class Drag {
-  /// The pointer has moved.
-  void update(DragUpdateDetails details) { }
-
-  /// The pointer is no longer in contact with the screen.
-  ///
-  /// The velocity at which the pointer was moving when it stopped contacting
-  /// the screen is available in the `details`.
-  void end(DragEndDetails details) { }
-
-  /// The input from the pointer is no longer directed towards this receiver.
-  ///
-  /// For example, the user might have been interrupted by a system-modal dialog
-  /// in the middle of the drag.
-  void cancel() { }
-}
-
 /// Per-pointer state for a [MultiDragGestureRecognizer].
 ///
 /// A [MultiDragGestureRecognizer] tracks each pointer separately. The state for