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

// Copyright 2016 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:async';

import 'package:flutter/foundation.dart';
import 'package:flutter/gestures.dart' show kDoubleTapTimeout, kDoubleTapSlop;
import 'package:flutter/gestures.dart';
import 'package:flutter/rendering.dart';
import 'package:flutter/scheduler.dart';
import 'package:flutter/services.dart';

import 'basic.dart';
import 'container.dart';
import 'editable_text.dart';
import 'framework.dart';
import 'gesture_detector.dart';
import 'overlay.dart';
import 'ticker_provider.dart';
import 'transitions.dart';

export 'package:flutter/services.dart' show TextSelectionDelegate;

/// A duration that controls how often the drag selection update callback is
/// called.
const Duration _kDragSelectionUpdateThrottle = Duration(milliseconds: 50);

/// Which type of selection handle to be displayed.
///
/// With mixed-direction text, both handles may be the same type. Examples:
///
/// * LTR text: 'the <quick brown> fox':
///
///   The '<' is drawn with the [left] type, the '>' with the [right]
///
/// * RTL text: 'XOF <NWORB KCIUQ> EHT':
///
///   Same as above.
///
/// * mixed text: '<the NWOR<B KCIUQ fox'
///
///   Here 'the QUICK B' is selected, but 'QUICK BROWN' is RTL. Both are drawn
///   with the [left] type.
///
/// See also:
///
///  * [TextDirection], which discusses left-to-right and right-to-left text in
///    more detail.
enum TextSelectionHandleType {
  /// The selection handle is to the left of the selection end point.
  left,

  /// The selection handle is to the right of the selection end point.
  right,

  /// The start and end of the selection are co-incident at this point.
  collapsed,
}

/// The text position that a give selection handle manipulates. Dragging the
/// [start] handle always moves the [start]/[baseOffset] of the selection.
enum _TextSelectionHandlePosition { start, end }

/// Signature for reporting changes to the selection component of a
/// [TextEditingValue] for the purposes of a [TextSelectionOverlay]. The
/// [caretRect] argument gives the location of the caret in the coordinate space
/// of the [RenderBox] given by the [TextSelectionOverlay.renderObject].
///
/// Used by [TextSelectionOverlay.onSelectionOverlayChanged].
typedef TextSelectionOverlayChanged = void Function(TextEditingValue value, Rect caretRect);

/// Signature for when a pointer that's dragging to select text has moved again.
///
/// The first argument [startDetails] contains the details of the event that
/// initiated the dragging.
///
/// The second argument [updateDetails] contains the details of the current
/// pointer movement. It's the same as the one passed to [DragGestureRecognizer.onUpdate].
///
/// This signature is different from [GestureDragUpdateCallback] to make it
/// easier for various text fields to use [TextSelectionGestureDetector] without
/// having to store the start position.
typedef DragSelectionUpdateCallback = void Function(DragStartDetails startDetails, DragUpdateDetails updateDetails);

/// An interface for building the selection UI, to be provided by the
/// implementor of the toolbar widget.
///
/// Override text operations such as [handleCut] if needed.
abstract class TextSelectionControls {
  /// Builds a selection handle of the given type.
  ///
  /// The top left corner of this widget is positioned at the bottom of the
  /// selection position.
  Widget buildHandle(BuildContext context, TextSelectionHandleType type, double textLineHeight);

  /// Builds a toolbar near a text selection.
  ///
  /// Typically displays buttons for copying and pasting text.
  Widget buildToolbar(BuildContext context, Rect globalEditableRegion, Offset position, TextSelectionDelegate delegate);

  /// Returns the size of the selection handle.
  Size get handleSize;

  /// Whether the current selection of the text field managed by the given
  /// `delegate` can be removed from the text field and placed into the
  /// [Clipboard].
  ///
  /// By default, false is returned when nothing is selected in the text field.
  ///
  /// Subclasses can use this to decide if they should expose the cut
  /// functionality to the user.
  bool canCut(TextSelectionDelegate delegate) {
    return !delegate.textEditingValue.selection.isCollapsed;
  }

  /// Whether the current selection of the text field managed by the given
  /// `delegate` can be copied to the [Clipboard].
  ///
  /// By default, false is returned when nothing is selected in the text field.
  ///
  /// Subclasses can use this to decide if they should expose the copy
  /// functionality to the user.
  bool canCopy(TextSelectionDelegate delegate) {
    return !delegate.textEditingValue.selection.isCollapsed;
  }

  /// Whether the current [Clipboard] content can be pasted into the text field
  /// managed by the given `delegate`.
  ///
  /// Subclasses can use this to decide if they should expose the paste
  /// functionality to the user.
  bool canPaste(TextSelectionDelegate delegate) {
    // TODO(goderbauer): return false when clipboard is empty, https://github.com/flutter/flutter/issues/11254
    return true;
  }

  /// Whether the current selection of the text field managed by the given
  /// `delegate` can be extended to include the entire content of the text
  /// field.
  ///
  /// Subclasses can use this to decide if they should expose the select all
  /// functionality to the user.
  bool canSelectAll(TextSelectionDelegate delegate) {
    return delegate.textEditingValue.text.isNotEmpty && delegate.textEditingValue.selection.isCollapsed;
  }

  /// Copy the current selection of the text field managed by the given
  /// `delegate` to the [Clipboard]. Then, remove the selected text from the
  /// text field and hide the toolbar.
  ///
  /// This is called by subclasses when their cut affordance is activated by
  /// the user.
  void handleCut(TextSelectionDelegate delegate) {
    final TextEditingValue value = delegate.textEditingValue;
    Clipboard.setData(ClipboardData(
      text: value.selection.textInside(value.text),
    ));
    delegate.textEditingValue = TextEditingValue(
      text: value.selection.textBefore(value.text)
          + value.selection.textAfter(value.text),
      selection: TextSelection.collapsed(
        offset: value.selection.start
      ),
    );
    delegate.bringIntoView(delegate.textEditingValue.selection.extent);
    delegate.hideToolbar();
  }

  /// Copy the current selection of the text field managed by the given
  /// `delegate` to the [Clipboard]. Then, move the cursor to the end of the
  /// text (collapsing the selection in the process), and hide the toolbar.
  ///
  /// This is called by subclasses when their copy affordance is activated by
  /// the user.
  void handleCopy(TextSelectionDelegate delegate) {
    final TextEditingValue value = delegate.textEditingValue;
    Clipboard.setData(ClipboardData(
      text: value.selection.textInside(value.text),
    ));
    delegate.textEditingValue = TextEditingValue(
      text: value.text,
      selection: TextSelection.collapsed(offset: value.selection.end),
    );
    delegate.bringIntoView(delegate.textEditingValue.selection.extent);
    delegate.hideToolbar();
  }

  /// Paste the current clipboard selection (obtained from [Clipboard]) into
  /// the text field managed by the given `delegate`, replacing its current
  /// selection, if any. Then, hide the toolbar.
  ///
  /// This is called by subclasses when their paste affordance is activated by
  /// the user.
  ///
  /// This function is asynchronous since interacting with the clipboard is
  /// asynchronous. Race conditions may exist with this API as currently
  /// implemented.
  // TODO(ianh): https://github.com/flutter/flutter/issues/11427
  Future<void> handlePaste(TextSelectionDelegate delegate) async {
    final TextEditingValue value = delegate.textEditingValue; // Snapshot the input before using `await`.
    final ClipboardData data = await Clipboard.getData(Clipboard.kTextPlain);
    if (data != null) {
      delegate.textEditingValue = TextEditingValue(
        text: value.selection.textBefore(value.text)
            + data.text
            + value.selection.textAfter(value.text),
        selection: TextSelection.collapsed(
          offset: value.selection.start + data.text.length
        ),
      );
    }
    delegate.bringIntoView(delegate.textEditingValue.selection.extent);
    delegate.hideToolbar();
  }

  /// Adjust the selection of the text field managed by the given `delegate` so
  /// that everything is selected.
  ///
  /// Does not hide the toolbar.
  ///
  /// This is called by subclasses when their select-all affordance is activated
  /// by the user.
  void handleSelectAll(TextSelectionDelegate delegate) {
    delegate.textEditingValue = TextEditingValue(
      text: delegate.textEditingValue.text,
      selection: TextSelection(
        baseOffset: 0,
        extentOffset: delegate.textEditingValue.text.length,
      ),
    );
    delegate.bringIntoView(delegate.textEditingValue.selection.extent);
  }
}

/// An object that manages a pair of text selection handles.
///
/// The selection handles are displayed in the [Overlay] that most closely
/// encloses the given [BuildContext].
class TextSelectionOverlay {
  /// Creates an object that manages overly entries for selection handles.
  ///
  /// The [context] must not be null and must have an [Overlay] as an ancestor.
  TextSelectionOverlay({
    @required TextEditingValue value,
    @required this.context,
    this.debugRequiredFor,
    @required this.layerLink,
    @required this.renderObject,
    this.selectionControls,
    this.selectionDelegate,
    this.dragStartBehavior = DragStartBehavior.start,
  }) : assert(value != null),
       assert(context != null),
       _value = value {
    final OverlayState overlay = Overlay.of(context);
    assert(overlay != null,
      'No Overlay widget exists above $context.\n'
      'Usually the Navigator created by WidgetsApp provides the overlay. Perhaps your '
      'app content was created above the Navigator with the WidgetsApp builder parameter.');
    _toolbarController = AnimationController(duration: fadeDuration, vsync: overlay);
  }

  /// The context in which the selection handles should appear.
  ///
  /// This context must have an [Overlay] as an ancestor because this object
  /// will display the text selection handles in that [Overlay].
  final BuildContext context;

  /// Debugging information for explaining why the [Overlay] is required.
  final Widget debugRequiredFor;

  /// The object supplied to the [CompositedTransformTarget] that wraps the text
  /// field.
  final LayerLink layerLink;

  // TODO(mpcomplete): what if the renderObject is removed or replaced, or
  // moves? Not sure what cases I need to handle, or how to handle them.
  /// The editable line in which the selected text is being displayed.
  final RenderEditable renderObject;

  /// Builds text selection handles and toolbar.
  final TextSelectionControls selectionControls;

  /// The delegate for manipulating the current selection in the owning
  /// text field.
  final TextSelectionDelegate selectionDelegate;

  /// Determines the way that drag start behavior is handled.
  ///
  /// If set to [DragStartBehavior.start], handle drag behavior will
  /// begin upon the detection of a drag gesture. If set to
  /// [DragStartBehavior.down] it will begin when a down event is first detected.
  ///
  /// In general, setting this to [DragStartBehavior.start] will make drag
  /// animation smoother and setting it to [DragStartBehavior.down] will make
  /// drag behavior feel slightly more reactive.
  ///
  /// By default, the drag start behavior is [DragStartBehavior.start].
  ///
  /// See also:
  ///
  ///  * [DragGestureRecognizer.dragStartBehavior], which gives an example for the different behaviors.
  final DragStartBehavior dragStartBehavior;

  /// Controls the fade-in and fade-out animations for the toolbar and handles.
  static const Duration fadeDuration = Duration(milliseconds: 150);

  AnimationController _toolbarController;
  Animation<double> get _toolbarOpacity => _toolbarController.view;

  TextEditingValue _value;

  /// A pair of handles. If this is non-null, there are always 2, though the
  /// second is hidden when the selection is collapsed.
  List<OverlayEntry> _handles;

  /// A copy/paste toolbar.
  OverlayEntry _toolbar;

  TextSelection get _selection => _value.selection;

  /// Shows the handles by inserting them into the [context]'s overlay.
  void showHandles() {
    assert(_handles == null);
    _handles = <OverlayEntry>[
      OverlayEntry(builder: (BuildContext context) => _buildHandle(context, _TextSelectionHandlePosition.start)),
      OverlayEntry(builder: (BuildContext context) => _buildHandle(context, _TextSelectionHandlePosition.end)),
    ];
    Overlay.of(context, debugRequiredFor: debugRequiredFor).insertAll(_handles);
  }

  /// Shows the toolbar by inserting it into the [context]'s overlay.
  void showToolbar() {
    assert(_toolbar == null);
    _toolbar = OverlayEntry(builder: _buildToolbar);
    Overlay.of(context, debugRequiredFor: debugRequiredFor).insert(_toolbar);
    _toolbarController.forward(from: 0.0);
  }

  /// Updates the overlay after the selection has changed.
  ///
  /// If this method is called while the [SchedulerBinding.schedulerPhase] is
  /// [SchedulerPhase.persistentCallbacks], i.e. during the build, layout, or
  /// paint phases (see [WidgetsBinding.drawFrame]), then the update is delayed
  /// until the post-frame callbacks phase. Otherwise the update is done
  /// synchronously. This means that it is safe to call during builds, but also
  /// that if you do call this during a build, the UI will not update until the
  /// next frame (i.e. many milliseconds later).
  void update(TextEditingValue newValue) {
    if (_value == newValue)
      return;
    _value = newValue;
    if (SchedulerBinding.instance.schedulerPhase == SchedulerPhase.persistentCallbacks) {
      SchedulerBinding.instance.addPostFrameCallback(_markNeedsBuild);
    } else {
      _markNeedsBuild();
    }
  }

  /// Causes the overlay to update its rendering.
  ///
  /// This is intended to be called when the [renderObject] may have changed its
  /// text metrics (e.g. because the text was scrolled).
  void updateForScroll() {
    _markNeedsBuild();
  }

  void _markNeedsBuild([ Duration duration ]) {
    if (_handles != null) {
      _handles[0].markNeedsBuild();
      _handles[1].markNeedsBuild();
    }
    _toolbar?.markNeedsBuild();
  }

  /// Whether the handles are currently visible.
  bool get handlesAreVisible => _handles != null;

  /// Whether the toolbar is currently visible.
  bool get toolbarIsVisible => _toolbar != null;

  /// Hides the overlay.
  void hide() {
    if (_handles != null) {
      _handles[0].remove();
      _handles[1].remove();
      _handles = null;
    }
    _toolbar?.remove();
    _toolbar = null;

    _toolbarController.stop();
  }

  /// Final cleanup.
  void dispose() {
    hide();
    _toolbarController.dispose();
  }

  Widget _buildHandle(BuildContext context, _TextSelectionHandlePosition position) {
    if ((_selection.isCollapsed && position == _TextSelectionHandlePosition.end) ||
         selectionControls == null)
      return Container(); // hide the second handle when collapsed
    return _TextSelectionHandleOverlay(
      onSelectionHandleChanged: (TextSelection newSelection) { _handleSelectionHandleChanged(newSelection, position); },
      onSelectionHandleTapped: _handleSelectionHandleTapped,
      layerLink: layerLink,
      renderObject: renderObject,
      selection: _selection,
      selectionControls: selectionControls,
      position: position,
      dragStartBehavior: dragStartBehavior,
    );
  }

  Widget _buildToolbar(BuildContext context) {
    if (selectionControls == null)
      return Container();

    // Find the horizontal midpoint, just above the selected text.
    final List<TextSelectionPoint> endpoints = renderObject.getEndpointsForSelection(_selection);
    final Offset midpoint = Offset(
      (endpoints.length == 1) ?
        endpoints[0].point.dx :
        (endpoints[0].point.dx + endpoints[1].point.dx) / 2.0,
      endpoints[0].point.dy - renderObject.preferredLineHeight,
    );

    final Rect editingRegion = Rect.fromPoints(
      renderObject.localToGlobal(Offset.zero),
      renderObject.localToGlobal(renderObject.size.bottomRight(Offset.zero)),
    );

    return FadeTransition(
      opacity: _toolbarOpacity,
      child: CompositedTransformFollower(
        link: layerLink,
        showWhenUnlinked: false,
        offset: -editingRegion.topLeft,
        child: selectionControls.buildToolbar(context, editingRegion, midpoint, selectionDelegate),
      ),
    );
  }

  void _handleSelectionHandleChanged(TextSelection newSelection, _TextSelectionHandlePosition position) {
    TextPosition textPosition;
    switch (position) {
      case _TextSelectionHandlePosition.start:
        textPosition = newSelection.base;
        break;
      case _TextSelectionHandlePosition.end:
        textPosition =newSelection.extent;
        break;
    }
    selectionDelegate.textEditingValue = _value.copyWith(selection: newSelection, composing: TextRange.empty);
    selectionDelegate.bringIntoView(textPosition);
  }

  void _handleSelectionHandleTapped() {
    if (_value.selection.isCollapsed) {
      if (_toolbar != null) {
        _toolbar?.remove();
        _toolbar = null;
      } else {
        showToolbar();
      }
    }
  }
}

/// This widget represents a single draggable text selection handle.
class _TextSelectionHandleOverlay extends StatefulWidget {
  const _TextSelectionHandleOverlay({
    Key key,
    @required this.selection,
    @required this.position,
    @required this.layerLink,
    @required this.renderObject,
    @required this.onSelectionHandleChanged,
    @required this.onSelectionHandleTapped,
    @required this.selectionControls,
    this.dragStartBehavior = DragStartBehavior.start,
  }) : super(key: key);

  final TextSelection selection;
  final _TextSelectionHandlePosition position;
  final LayerLink layerLink;
  final RenderEditable renderObject;
  final ValueChanged<TextSelection> onSelectionHandleChanged;
  final VoidCallback onSelectionHandleTapped;
  final TextSelectionControls selectionControls;
  final DragStartBehavior dragStartBehavior;

  @override
  _TextSelectionHandleOverlayState createState() => _TextSelectionHandleOverlayState();

  ValueListenable<bool> get _visibility {
    switch (position) {
      case _TextSelectionHandlePosition.start:
        return renderObject.selectionStartInViewport;
      case _TextSelectionHandlePosition.end:
        return renderObject.selectionEndInViewport;
    }
    return null;
  }
}

class _TextSelectionHandleOverlayState
    extends State<_TextSelectionHandleOverlay> with SingleTickerProviderStateMixin {
  Offset _dragPosition;

  AnimationController _controller;
  Animation<double> get _opacity => _controller.view;

  @override
  void initState() {
    super.initState();

    _controller = AnimationController(duration: TextSelectionOverlay.fadeDuration, vsync: this);

    _handleVisibilityChanged();
    widget._visibility.addListener(_handleVisibilityChanged);
  }

  void _handleVisibilityChanged() {
    if (widget._visibility.value) {
      _controller.forward();
    } else {
      _controller.reverse();
    }
  }

  @override
  void didUpdateWidget(_TextSelectionHandleOverlay oldWidget) {
    super.didUpdateWidget(oldWidget);
    oldWidget._visibility.removeListener(_handleVisibilityChanged);
    _handleVisibilityChanged();
    widget._visibility.addListener(_handleVisibilityChanged);
  }

  @override
  void dispose() {
    widget._visibility.removeListener(_handleVisibilityChanged);
    _controller.dispose();
    super.dispose();
  }

  void _handleDragStart(DragStartDetails details) {
    _dragPosition = details.globalPosition + Offset(0.0, -widget.selectionControls.handleSize.height);
  }

  void _handleDragUpdate(DragUpdateDetails details) {
    _dragPosition += details.delta;
    final TextPosition position = widget.renderObject.getPositionForPoint(_dragPosition);

    if (widget.selection.isCollapsed) {
      widget.onSelectionHandleChanged(TextSelection.fromPosition(position));
      return;
    }

    TextSelection newSelection;
    switch (widget.position) {
      case _TextSelectionHandlePosition.start:
        newSelection = TextSelection(
          baseOffset: position.offset,
          extentOffset: widget.selection.extentOffset,
        );
        break;
      case _TextSelectionHandlePosition.end:
        newSelection = TextSelection(
          baseOffset: widget.selection.baseOffset,
          extentOffset: position.offset,
        );
        break;
    }

    if (newSelection.baseOffset >= newSelection.extentOffset)
      return; // don't allow order swapping.

    widget.onSelectionHandleChanged(newSelection);
  }

  void _handleTap() {
    widget.onSelectionHandleTapped();
  }

  @override
  Widget build(BuildContext context) {
    final List<TextSelectionPoint> endpoints = widget.renderObject.getEndpointsForSelection(widget.selection);
    Offset point;
    TextSelectionHandleType type;

    switch (widget.position) {
      case _TextSelectionHandlePosition.start:
        point = endpoints[0].point;
        type = _chooseType(endpoints[0], TextSelectionHandleType.left, TextSelectionHandleType.right);
        break;
      case _TextSelectionHandlePosition.end:
        // [endpoints] will only contain 1 point for collapsed selections, in
        // which case we shouldn't be building the [end] handle.
        assert(endpoints.length == 2);
        point = endpoints[1].point;
        type = _chooseType(endpoints[1], TextSelectionHandleType.right, TextSelectionHandleType.left);
        break;
    }

    final Size viewport = widget.renderObject.size;
    point = Offset(
      point.dx.clamp(0.0, viewport.width),
      point.dy.clamp(0.0, viewport.height),
    );

    return CompositedTransformFollower(
      link: widget.layerLink,
      showWhenUnlinked: false,
      child: FadeTransition(
        opacity: _opacity,
        child: GestureDetector(
          dragStartBehavior: widget.dragStartBehavior,
          onPanStart: _handleDragStart,
          onPanUpdate: _handleDragUpdate,
          onTap: _handleTap,
          child: Stack(
            // Always let the selection handles draw outside of the conceptual
            // box where (0,0) is the top left corner of the RenderEditable.
            overflow: Overflow.visible,
            children: <Widget>[
              Positioned(
                left: point.dx,
                top: point.dy,
                child: widget.selectionControls.buildHandle(
                  context,
                  type,
                  widget.renderObject.preferredLineHeight,
                ),
              ),
            ],
          ),
        ),
      ),
    );
  }

  TextSelectionHandleType _chooseType(
    TextSelectionPoint endpoint,
    TextSelectionHandleType ltrType,
    TextSelectionHandleType rtlType,
  ) {
    if (widget.selection.isCollapsed)
      return TextSelectionHandleType.collapsed;

    assert(endpoint.direction != null);
    switch (endpoint.direction) {
      case TextDirection.ltr:
        return ltrType;
      case TextDirection.rtl:
        return rtlType;
    }
    return null;
  }
}

/// A gesture detector to respond to non-exclusive event chains for a text field.
///
/// An ordinary [GestureDetector] configured to handle events like tap and
/// double tap will only recognize one or the other. This widget detects both:
/// first the tap and then, if another tap down occurs within a time limit, the
/// double tap.
///
/// See also:
///
///  * [TextField], a Material text field which uses this gesture detector.
///  * [CupertinoTextField], a Cupertino text field which uses this gesture
///    detector.
class TextSelectionGestureDetector extends StatefulWidget {
  /// Create a [TextSelectionGestureDetector].
  ///
  /// Multiple callbacks can be called for one sequence of input gesture.
  /// The [child] parameter must not be null.
  const TextSelectionGestureDetector({
    Key key,
    this.onTapDown,
    this.onForcePressStart,
    this.onForcePressEnd,
    this.onSingleTapUp,
    this.onSingleTapCancel,
    this.onSingleLongTapStart,
    this.onSingleLongTapMoveUpdate,
    this.onSingleLongTapEnd,
    this.onDoubleTapDown,
    this.onDragSelectionStart,
    this.onDragSelectionUpdate,
    this.onDragSelectionEnd,
    this.behavior,
    @required this.child,
  }) : assert(child != null),
       super(key: key);

  /// Called for every tap down including every tap down that's part of a
  /// double click or a long press, except touches that include enough movement
  /// to not qualify as taps (e.g. pans and flings).
  final GestureTapDownCallback onTapDown;

  /// Called when a pointer has tapped down and the force of the pointer has
  /// just become greater than [ForcePressGestureDetector.startPressure].
  final GestureForcePressStartCallback onForcePressStart;

  /// Called when a pointer that had previously triggered [onForcePressStart] is
  /// lifted off the screen.
  final GestureForcePressEndCallback onForcePressEnd;

  /// Called for each distinct tap except for every second tap of a double tap.
  /// For example, if the detector was configured [onSingleTapDown] and
  /// [onDoubleTapDown], three quick taps would be recognized as a single tap
  /// down, followed by a double tap down, followed by a single tap down.
  final GestureTapUpCallback onSingleTapUp;

  /// Called for each touch that becomes recognized as a gesture that is not a
  /// short tap, such as a long tap or drag. It is called at the moment when
  /// another gesture from the touch is recognized.
  final GestureTapCancelCallback onSingleTapCancel;

  /// Called for a single long tap that's sustained for longer than
  /// [kLongPressTimeout] but not necessarily lifted. Not called for a
  /// double-tap-hold, which calls [onDoubleTapDown] instead.
  final GestureLongPressStartCallback onSingleLongTapStart;

  /// Called after [onSingleLongTapStart] when the pointer is dragged.
  final GestureLongPressMoveUpdateCallback onSingleLongTapMoveUpdate;

  /// Called after [onSingleLongTapStart] when the pointer is lifted.
  final GestureLongPressEndCallback onSingleLongTapEnd;

  /// Called after a momentary hold or a short tap that is close in space and
  /// time (within [kDoubleTapTimeout]) to a previous short tap.
  final GestureTapDownCallback onDoubleTapDown;

  /// Called when a mouse starts dragging to select text.
  final GestureDragStartCallback onDragSelectionStart;

  /// Called repeatedly as a mouse moves while dragging.
  ///
  /// The frequency of calls is throttled to avoid excessive text layout
  /// operations in text fields. The throttling is controlled by the constant
  /// [_kDragSelectionUpdateThrottle].
  final DragSelectionUpdateCallback onDragSelectionUpdate;

  /// Called when a mouse that was previously dragging is released.
  final GestureDragEndCallback onDragSelectionEnd;

  /// How this gesture detector should behave during hit testing.
  ///
  /// This defaults to [HitTestBehavior.deferToChild].
  final HitTestBehavior behavior;

  /// Child below this widget.
  final Widget child;

  @override
  State<StatefulWidget> createState() => _TextSelectionGestureDetectorState();
}

class _TextSelectionGestureDetectorState extends State<TextSelectionGestureDetector> {
  // Counts down for a short duration after a previous tap. Null otherwise.
  Timer _doubleTapTimer;
  Offset _lastTapOffset;
  // True if a second tap down of a double tap is detected. Used to discard
  // subsequent tap up / tap hold of the same tap.
  bool _isDoubleTap = false;

  @override
  void dispose() {
    _doubleTapTimer?.cancel();
    _dragUpdateThrottleTimer?.cancel();
    super.dispose();
  }

  // The down handler is force-run on success of a single tap and optimistically
  // run before a long press success.
  void _handleTapDown(TapDownDetails details) {
    if (widget.onTapDown != null) {
      widget.onTapDown(details);
    }
    // This isn't detected as a double tap gesture in the gesture recognizer
    // because it's 2 single taps, each of which may do different things depending
    // on whether it's a single tap, the first tap of a double tap, the second
    // tap held down, a clean double tap etc.
    if (_doubleTapTimer != null && _isWithinDoubleTapTolerance(details.globalPosition)) {
      // If there was already a previous tap, the second down hold/tap is a
      // double tap down.
      if (widget.onDoubleTapDown != null) {
        widget.onDoubleTapDown(details);
      }

      _doubleTapTimer.cancel();
      _doubleTapTimeout();
      _isDoubleTap = true;
    }
  }

  void _handleTapUp(TapUpDetails details) {
    if (!_isDoubleTap) {
      if (widget.onSingleTapUp != null) {
        widget.onSingleTapUp(details);
      }
      _lastTapOffset = details.globalPosition;
      _doubleTapTimer = Timer(kDoubleTapTimeout, _doubleTapTimeout);
    }
    _isDoubleTap = false;
  }

  void _handleTapCancel() {
    if (widget.onSingleTapCancel != null) {
      widget.onSingleTapCancel();
    }
  }

  DragStartDetails _lastDragStartDetails;
  DragUpdateDetails _lastDragUpdateDetails;
  Timer _dragUpdateThrottleTimer;

  void _handleDragStart(DragStartDetails details) {
    assert(_lastDragStartDetails == null);
    _lastDragStartDetails = details;
    if (widget.onDragSelectionStart != null) {
      widget.onDragSelectionStart(details);
    }
  }

  void _handleDragUpdate(DragUpdateDetails details) {
    _lastDragUpdateDetails = details;
    // Only schedule a new timer if there's no one pending.
    _dragUpdateThrottleTimer ??= Timer(_kDragSelectionUpdateThrottle, _handleDragUpdateThrottled);
  }

  /// Drag updates are being throttled to avoid excessive text layouts in text
  /// fields. The frequency of invocations is controlled by the constant
  /// [_kDragSelectionUpdateThrottle].
  ///
  /// Once the drag gesture ends, any pending drag update will be fired
  /// immediately. See [_handleDragEnd].
  void _handleDragUpdateThrottled() {
    assert(_lastDragStartDetails != null);
    assert(_lastDragUpdateDetails != null);
    if (widget.onDragSelectionUpdate != null) {
      widget.onDragSelectionUpdate(_lastDragStartDetails, _lastDragUpdateDetails);
    }
    _dragUpdateThrottleTimer = null;
    _lastDragUpdateDetails = null;
  }

  void _handleDragEnd(DragEndDetails details) {
    assert(_lastDragStartDetails != null);
    if (_dragUpdateThrottleTimer != null) {
      // If there's already an update scheduled, trigger it immediately and
      // cancel the timer.
      _dragUpdateThrottleTimer.cancel();
      _handleDragUpdateThrottled();
    }
    if (widget.onDragSelectionEnd != null) {
      widget.onDragSelectionEnd(details);
    }
    _dragUpdateThrottleTimer = null;
    _lastDragStartDetails = null;
    _lastDragUpdateDetails = null;
  }

  void _forcePressStarted(ForcePressDetails details) {
    _doubleTapTimer?.cancel();
    _doubleTapTimer = null;
    if (widget.onForcePressStart != null)
      widget.onForcePressStart(details);
  }

  void _forcePressEnded(ForcePressDetails details) {
    if (widget.onForcePressEnd != null)
      widget.onForcePressEnd(details);
  }

  void _handleLongPressStart(LongPressStartDetails details) {
    if (!_isDoubleTap && widget.onSingleLongTapStart != null) {
      widget.onSingleLongTapStart(details);
    }
  }

  void _handleLongPressMoveUpdate(LongPressMoveUpdateDetails details) {
    if (!_isDoubleTap && widget.onSingleLongTapMoveUpdate != null) {
      widget.onSingleLongTapMoveUpdate(details);
    }
  }

  void _handleLongPressEnd(LongPressEndDetails details) {
    if (!_isDoubleTap && widget.onSingleLongTapEnd != null) {
      widget.onSingleLongTapEnd(details);
    }
    _isDoubleTap = false;
  }

  void _doubleTapTimeout() {
    _doubleTapTimer = null;
    _lastTapOffset = null;
  }

  bool _isWithinDoubleTapTolerance(Offset secondTapOffset) {
    assert(secondTapOffset != null);
    if (_lastTapOffset == null) {
      return false;
    }

    final Offset difference = secondTapOffset - _lastTapOffset;
    return difference.distance <= kDoubleTapSlop;
  }

  @override
  Widget build(BuildContext context) {
    final Map<Type, GestureRecognizerFactory> gestures = <Type, GestureRecognizerFactory>{};

    gestures[TapGestureRecognizer] = GestureRecognizerFactoryWithHandlers<TapGestureRecognizer>(
      () => TapGestureRecognizer(debugOwner: this),
      (TapGestureRecognizer instance) {
        instance
          ..onTapDown = _handleTapDown
          ..onTapUp = _handleTapUp
          ..onTapCancel = _handleTapCancel;
      },
    );

    if (widget.onSingleLongTapStart != null ||
        widget.onSingleLongTapMoveUpdate != null ||
        widget.onSingleLongTapEnd != null) {
      gestures[LongPressGestureRecognizer] = GestureRecognizerFactoryWithHandlers<LongPressGestureRecognizer>(
        () => LongPressGestureRecognizer(debugOwner: this, kind: PointerDeviceKind.touch),
        (LongPressGestureRecognizer instance) {
          instance
            ..onLongPressStart = _handleLongPressStart
            ..onLongPressMoveUpdate = _handleLongPressMoveUpdate
            ..onLongPressEnd = _handleLongPressEnd;
        },
      );
    }

    if (widget.onDragSelectionStart != null ||
        widget.onDragSelectionUpdate != null ||
        widget.onDragSelectionEnd != null) {
      // TODO(mdebbar): Support dragging in any direction (for multiline text).
      // https://github.com/flutter/flutter/issues/28676
      gestures[HorizontalDragGestureRecognizer] = GestureRecognizerFactoryWithHandlers<HorizontalDragGestureRecognizer>(
        () => HorizontalDragGestureRecognizer(debugOwner: this, kind: PointerDeviceKind.mouse),
        (HorizontalDragGestureRecognizer instance) {
          instance
            // Text selection should start from the position of the first pointer
            // down event.
            ..dragStartBehavior = DragStartBehavior.down
            ..onStart = _handleDragStart
            ..onUpdate = _handleDragUpdate
            ..onEnd = _handleDragEnd;
        },
      );
    }

    if (widget.onForcePressStart != null || widget.onForcePressEnd != null) {
      gestures[ForcePressGestureRecognizer] = GestureRecognizerFactoryWithHandlers<ForcePressGestureRecognizer>(
        () => ForcePressGestureRecognizer(debugOwner: this),
        (ForcePressGestureRecognizer instance) {
          instance
            ..onStart = widget.onForcePressStart != null ? _forcePressStarted : null
            ..onEnd = widget.onForcePressEnd != null ? _forcePressEnded : null;
        },
      );
    }

    return RawGestureDetector(
      gestures: gestures,
      excludeFromSemantics: true,
      behavior: widget.behavior,
      child: widget.child,
    );
  }
}