// Copyright 2013 The Flutter Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

// @dart = 2.6
part of ui;

/// The possible actions that can be conveyed from the operating system
/// accessibility APIs to a semantics node.
class SemanticsAction {
  const SemanticsAction._(this.index) : assert(index != null);

  static const int _kTapIndex = 1 << 0;
  static const int _kLongPressIndex = 1 << 1;
  static const int _kScrollLeftIndex = 1 << 2;
  static const int _kScrollRightIndex = 1 << 3;
  static const int _kScrollUpIndex = 1 << 4;
  static const int _kScrollDownIndex = 1 << 5;
  static const int _kIncreaseIndex = 1 << 6;
  static const int _kDecreaseIndex = 1 << 7;
  static const int _kShowOnScreenIndex = 1 << 8;
  static const int _kMoveCursorForwardByCharacterIndex = 1 << 9;
  static const int _kMoveCursorBackwardByCharacterIndex = 1 << 10;
  static const int _kSetSelectionIndex = 1 << 11;
  static const int _kCopyIndex = 1 << 12;
  static const int _kCutIndex = 1 << 13;
  static const int _kPasteIndex = 1 << 14;
  static const int _kDidGainAccessibilityFocusIndex = 1 << 15;
  static const int _kDidLoseAccessibilityFocusIndex = 1 << 16;
  static const int _kCustomAction = 1 << 17;
  static const int _kDismissIndex = 1 << 18;
  static const int _kMoveCursorForwardByWordIndex = 1 << 19;
  static const int _kMoveCursorBackwardByWordIndex = 1 << 20;

  /// The numerical value for this action.
  ///
  /// Each action has one bit set in this bit field.
  final int/*!*/ index;

  /// The equivalent of a user briefly tapping the screen with the finger
  /// without moving it.
  static const SemanticsAction tap = SemanticsAction._(_kTapIndex);

  /// The equivalent of a user pressing and holding the screen with the finger
  /// for a few seconds without moving it.
  static const SemanticsAction longPress = SemanticsAction._(_kLongPressIndex);

  /// The equivalent of a user moving their finger across the screen from right
  /// to left.
  ///
  /// This action should be recognized by controls that are horizontally
  /// scrollable.
  static const SemanticsAction scrollLeft =
      SemanticsAction._(_kScrollLeftIndex);

  /// The equivalent of a user moving their finger across the screen from left
  /// to right.
  ///
  /// This action should be recognized by controls that are horizontally
  /// scrollable.
  static const SemanticsAction scrollRight =
      SemanticsAction._(_kScrollRightIndex);

  /// The equivalent of a user moving their finger across the screen from
  /// bottom to top.
  ///
  /// This action should be recognized by controls that are vertically
  /// scrollable.
  static const SemanticsAction scrollUp = SemanticsAction._(_kScrollUpIndex);

  /// The equivalent of a user moving their finger across the screen from top
  /// to bottom.
  ///
  /// This action should be recognized by controls that are vertically
  /// scrollable.
  static const SemanticsAction scrollDown =
      SemanticsAction._(_kScrollDownIndex);

  /// A request to increase the value represented by the semantics node.
  ///
  /// For example, this action might be recognized by a slider control.
  static const SemanticsAction increase = SemanticsAction._(_kIncreaseIndex);

  /// A request to decrease the value represented by the semantics node.
  ///
  /// For example, this action might be recognized by a slider control.
  static const SemanticsAction decrease = SemanticsAction._(_kDecreaseIndex);

  /// A request to fully show the semantics node on screen.
  ///
  /// For example, this action might be send to a node in a scrollable list that
  /// is partially off screen to bring it on screen.
  static const SemanticsAction showOnScreen =
      SemanticsAction._(_kShowOnScreenIndex);

  /// Move the cursor forward by one character.
  ///
  /// This is for example used by the cursor control in text fields.
  ///
  /// The action includes a boolean argument, which indicates whether the cursor
  /// movement should extend (or start) a selection.
  static const SemanticsAction moveCursorForwardByCharacter =
      SemanticsAction._(_kMoveCursorForwardByCharacterIndex);

  /// Move the cursor backward by one character.
  ///
  /// This is for example used by the cursor control in text fields.
  ///
  /// The action includes a boolean argument, which indicates whether the cursor
  /// movement should extend (or start) a selection.
  static const SemanticsAction moveCursorBackwardByCharacter =
      SemanticsAction._(_kMoveCursorBackwardByCharacterIndex);

  /// Set the text selection to the given range.
  ///
  /// The provided argument is a Map<String, int> which includes the keys `base`
  /// and `extent` indicating where the selection within the `value` of the
  /// semantics node should start and where it should end. Values for both
  /// keys can range from 0 to length of `value` (inclusive).
  ///
  /// Setting `base` and `extent` to the same value will move the cursor to
  /// that position (without selecting anything).
  static const SemanticsAction setSelection =
      SemanticsAction._(_kSetSelectionIndex);

  /// Copy the current selection to the clipboard.
  static const SemanticsAction copy = SemanticsAction._(_kCopyIndex);

  /// Cut the current selection and place it in the clipboard.
  static const SemanticsAction cut = SemanticsAction._(_kCutIndex);

  /// Paste the current content of the clipboard.
  static const SemanticsAction paste = SemanticsAction._(_kPasteIndex);

  /// Indicates that the nodes has gained accessibility focus.
  ///
  /// This handler is invoked when the node annotated with this handler gains
  /// the accessibility focus. The accessibility focus is the
  /// green (on Android with TalkBack) or black (on iOS with VoiceOver)
  /// rectangle shown on screen to indicate what element an accessibility
  /// user is currently interacting with.
  ///
  /// The accessibility focus is different from the input focus. The input focus
  /// is usually held by the element that currently responds to keyboard inputs.
  /// Accessibility focus and input focus can be held by two different nodes!
  static const SemanticsAction didGainAccessibilityFocus =
      SemanticsAction._(_kDidGainAccessibilityFocusIndex);

  /// Indicates that the nodes has lost accessibility focus.
  ///
  /// This handler is invoked when the node annotated with this handler
  /// loses the accessibility focus. The accessibility focus is
  /// the green (on Android with TalkBack) or black (on iOS with VoiceOver)
  /// rectangle shown on screen to indicate what element an accessibility
  /// user is currently interacting with.
  ///
  /// The accessibility focus is different from the input focus. The input focus
  /// is usually held by the element that currently responds to keyboard inputs.
  /// Accessibility focus and input focus can be held by two different nodes!
  static const SemanticsAction didLoseAccessibilityFocus =
      SemanticsAction._(_kDidLoseAccessibilityFocusIndex);

  /// Indicates that the user has invoked a custom accessibility action.
  ///
  /// This handler is added automatically whenever a custom accessibility
  /// action is added to a semantics node.
  static const SemanticsAction customAction = SemanticsAction._(_kCustomAction);

  /// A request that the node should be dismissed.
  ///
  /// A [Snackbar], for example, may have a dismiss action to indicate to the
  /// user that it can be removed after it is no longer relevant. On Android,
  /// (with TalkBack) special hint text is spoken when focusing the node and
  /// a custom action is availible in the local context menu. On iOS,
  /// (with VoiceOver) users can perform a standard gesture to dismiss it.
  static const SemanticsAction dismiss = SemanticsAction._(_kDismissIndex);

  /// Move the cursor forward by one word.
  ///
  /// This is for example used by the cursor control in text fields.
  ///
  /// The action includes a boolean argument, which indicates whether the cursor
  /// movement should extend (or start) a selection.
  static const SemanticsAction moveCursorForwardByWord =
      SemanticsAction._(_kMoveCursorForwardByWordIndex);

  /// Move the cursor backward by one word.
  ///
  /// This is for example used by the cursor control in text fields.
  ///
  /// The action includes a boolean argument, which indicates whether the cursor
  /// movement should extend (or start) a selection.
  static const SemanticsAction moveCursorBackwardByWord =
      SemanticsAction._(_kMoveCursorBackwardByWordIndex);

  /// The possible semantics actions.
  ///
  /// The map's key is the [index] of the action and the value is the action
  /// itself.
  static const Map<int, SemanticsAction> values = <int, SemanticsAction>{
    _kTapIndex: tap,
    _kLongPressIndex: longPress,
    _kScrollLeftIndex: scrollLeft,
    _kScrollRightIndex: scrollRight,
    _kScrollUpIndex: scrollUp,
    _kScrollDownIndex: scrollDown,
    _kIncreaseIndex: increase,
    _kDecreaseIndex: decrease,
    _kShowOnScreenIndex: showOnScreen,
    _kMoveCursorForwardByCharacterIndex: moveCursorForwardByCharacter,
    _kMoveCursorBackwardByCharacterIndex: moveCursorBackwardByCharacter,
    _kSetSelectionIndex: setSelection,
    _kCopyIndex: copy,
    _kCutIndex: cut,
    _kPasteIndex: paste,
    _kDidGainAccessibilityFocusIndex: didGainAccessibilityFocus,
    _kDidLoseAccessibilityFocusIndex: didLoseAccessibilityFocus,
    _kCustomAction: customAction,
    _kDismissIndex: dismiss,
    _kMoveCursorForwardByWordIndex: moveCursorForwardByWord,
    _kMoveCursorBackwardByWordIndex: moveCursorBackwardByWord,
  };

  @override
  String toString() {
    switch (index) {
      case _kTapIndex:
        return 'SemanticsAction.tap';
      case _kLongPressIndex:
        return 'SemanticsAction.longPress';
      case _kScrollLeftIndex:
        return 'SemanticsAction.scrollLeft';
      case _kScrollRightIndex:
        return 'SemanticsAction.scrollRight';
      case _kScrollUpIndex:
        return 'SemanticsAction.scrollUp';
      case _kScrollDownIndex:
        return 'SemanticsAction.scrollDown';
      case _kIncreaseIndex:
        return 'SemanticsAction.increase';
      case _kDecreaseIndex:
        return 'SemanticsAction.decrease';
      case _kShowOnScreenIndex:
        return 'SemanticsAction.showOnScreen';
      case _kMoveCursorForwardByCharacterIndex:
        return 'SemanticsAction.moveCursorForwardByCharacter';
      case _kMoveCursorBackwardByCharacterIndex:
        return 'SemanticsAction.moveCursorBackwardByCharacter';
      case _kSetSelectionIndex:
        return 'SemanticsAction.setSelection';
      case _kCopyIndex:
        return 'SemanticsAction.copy';
      case _kCutIndex:
        return 'SemanticsAction.cut';
      case _kPasteIndex:
        return 'SemanticsAction.paste';
      case _kDidGainAccessibilityFocusIndex:
        return 'SemanticsAction.didGainAccessibilityFocus';
      case _kDidLoseAccessibilityFocusIndex:
        return 'SemanticsAction.didLoseAccessibilityFocus';
      case _kCustomAction:
        return 'SemanticsAction.customAction';
      case _kDismissIndex:
        return 'SemanticsAction.dismiss';
      case _kMoveCursorForwardByWordIndex:
        return 'SemanticsAction.moveCursorForwardByWord';
      case _kMoveCursorBackwardByWordIndex:
        return 'SemanticsAction.moveCursorBackwardByWord';
    }
    assert(false, 'Unhandled index: $index');
    return '';
  }
}

/// A Boolean value that can be associated with a semantics node.
class SemanticsFlag {
  static const int _kHasCheckedStateIndex = 1 << 0;
  static const int _kIsCheckedIndex = 1 << 1;
  static const int _kIsSelectedIndex = 1 << 2;
  static const int _kIsButtonIndex = 1 << 3;
  static const int _kIsTextFieldIndex = 1 << 4;
  static const int _kIsFocusedIndex = 1 << 5;
  static const int _kHasEnabledStateIndex = 1 << 6;
  static const int _kIsEnabledIndex = 1 << 7;
  static const int _kIsInMutuallyExclusiveGroupIndex = 1 << 8;
  static const int _kIsHeaderIndex = 1 << 9;
  static const int _kIsObscuredIndex = 1 << 10;
  static const int _kScopesRouteIndex = 1 << 11;
  static const int _kNamesRouteIndex = 1 << 12;
  static const int _kIsHiddenIndex = 1 << 13;
  static const int _kIsImageIndex = 1 << 14;
  static const int _kIsLiveRegionIndex = 1 << 15;
  static const int _kHasToggledStateIndex = 1 << 16;
  static const int _kIsToggledIndex = 1 << 17;
  static const int _kHasImplicitScrollingIndex = 1 << 18;
  static const int _kIsMultilineIndex = 1 << 19;
  static const int _kIsReadOnlyIndex = 1 << 20;
  static const int _kIsFocusableIndex = 1 << 21;
  static const int _kIsLinkIndex = 1 << 22;

  const SemanticsFlag._(this.index) : assert(index != null);

  /// The numerical value for this flag.
  ///
  /// Each flag has one bit set in this bit field.
  final int/*!*/ index;

  /// The semantics node has the quality of either being "checked" or "unchecked".
  ///
  /// This flag is mutually exclusive with [hasToggledState].
  ///
  /// For example, a checkbox or a radio button widget has checked state.
  ///
  /// See also:
  ///
  ///   * [SemanticsFlag.isChecked], which controls whether the node is "checked" or "unchecked".
  static const SemanticsFlag hasCheckedState =
      SemanticsFlag._(_kHasCheckedStateIndex);

  /// Whether a semantics node that [hasCheckedState] is checked.
  ///
  /// If true, the semantics node is "checked". If false, the semantics node is
  /// "unchecked".
  ///
  /// For example, if a checkbox has a visible checkmark, [isChecked] is true.
  ///
  /// See also:
  ///
  ///   * [SemanticsFlag.hasCheckedState], which enables a checked state.
  static const SemanticsFlag isChecked = SemanticsFlag._(_kIsCheckedIndex);

  /// Whether a semantics node is selected.
  ///
  /// If true, the semantics node is "selected". If false, the semantics node is
  /// "unselected".
  ///
  /// For example, the active tab in a tab bar has [isSelected] set to true.
  static const SemanticsFlag isSelected = SemanticsFlag._(_kIsSelectedIndex);

  /// Whether the semantic node represents a button.
  ///
  /// Platforms has special handling for buttons, for example Android's TalkBack
  /// and iOS's VoiceOver provides an additional hint when the focused object is
  /// a button.
  static const SemanticsFlag isButton = SemanticsFlag._(_kIsButtonIndex);

  /// Whether the semantic node represents a link.
  ///
  /// Platforms have special handling for links, for example, iOS's VoiceOver
  /// provides an additional hint when the focused object is a link.
  static const SemanticsFlag isLink = SemanticsFlag._(_kIsLinkIndex);

  /// Whether the semantic node represents a text field.
  ///
  /// Text fields are announced as such and allow text input via accessibility
  /// affordances.
  static const SemanticsFlag isTextField = SemanticsFlag._(_kIsTextFieldIndex);

  /// Whether the semantic node is read only.
  ///
  /// Only applicable when [isTextField] is true.
  static const SemanticsFlag isReadOnly = SemanticsFlag._(_kIsReadOnlyIndex);

  /// Whether the semantic node is able to hold the user's focus.
  ///
  /// The focused element is usually the current receiver of keyboard inputs.
  static const SemanticsFlag isFocusable = SemanticsFlag._(_kIsFocusableIndex);

  /// Whether the semantic node currently holds the user's focus.
  ///
  /// The focused element is usually the current receiver of keyboard inputs.
  static const SemanticsFlag isFocused = SemanticsFlag._(_kIsFocusedIndex);

  /// The semantics node has the quality of either being "enabled" or
  /// "disabled".
  ///
  /// For example, a button can be enabled or disabled and therefore has an
  /// "enabled" state. Static text is usually neither enabled nor disabled and
  /// therefore does not have an "enabled" state.
  static const SemanticsFlag hasEnabledState =
      SemanticsFlag._(_kHasEnabledStateIndex);

  /// Whether a semantic node that [hasEnabledState] is currently enabled.
  ///
  /// A disabled element does not respond to user interaction. For example, a
  /// button that currently does not respond to user interaction should be
  /// marked as disabled.
  static const SemanticsFlag isEnabled = SemanticsFlag._(_kIsEnabledIndex);

  /// Whether a semantic node is in a mutually exclusive group.
  ///
  /// For example, a radio button is in a mutually exclusive group because
  /// only one radio button in that group can be marked as [isChecked].
  static const SemanticsFlag isInMutuallyExclusiveGroup =
      SemanticsFlag._(_kIsInMutuallyExclusiveGroupIndex);

  /// Whether a semantic node is a header that divides content into sections.
  ///
  /// For example, headers can be used to divide a list of alphabetically
  /// sorted words into the sections A, B, C, etc. as can be found in many
  /// address book applications.
  static const SemanticsFlag isHeader = SemanticsFlag._(_kIsHeaderIndex);

  /// Whether the value of the semantics node is obscured.
  ///
  /// This is usually used for text fields to indicate that its content
  /// is a password or contains other sensitive information.
  static const SemanticsFlag isObscured = SemanticsFlag._(_kIsObscuredIndex);

  /// Whether the semantics node is the root of a subtree for which a route name
  /// should be announced.
  ///
  /// When a node with this flag is removed from the semantics tree, the
  /// framework will select the last in depth-first, paint order node with this
  /// flag.  When a node with this flag is added to the semantics tree, it is
  /// selected automatically, unless there were multiple nodes with this flag
  /// added.  In this case, the last added node in depth-first, paint order
  /// will be selected.
  ///
  /// From this selected node, the framework will search in depth-first, paint
  /// order for the first node with a [namesRoute] flag and a non-null,
  /// non-empty label. The [namesRoute] and [scopesRoute] flags may be on the
  /// same node. The label of the found node will be announced as an edge
  /// transition. If no non-empty, non-null label is found then:
  ///
  ///   * VoiceOver will make a chime announcement.
  ///   * TalkBack will make no announcement
  ///
  /// Semantic nodes annotated with this flag are generally not a11y focusable.
  ///
  /// This is used in widgets such as Routes, Drawers, and Dialogs to
  /// communicate significant changes in the visible screen.
  static const SemanticsFlag scopesRoute = SemanticsFlag._(_kScopesRouteIndex);

  /// Whether the semantics node label is the name of a visually distinct
  /// route.
  ///
  /// This is used by certain widgets like Drawers and Dialogs, to indicate
  /// that the node's semantic label can be used to announce an edge triggered
  /// semantics update.
  ///
  /// Semantic nodes annotated with this flag will still recieve a11y focus.
  ///
  /// Updating this label within the same active route subtree will not cause
  /// additional announcements.
  static const SemanticsFlag namesRoute = SemanticsFlag._(_kNamesRouteIndex);

  /// Whether the semantics node is considered hidden.
  ///
  /// Hidden elements are currently not visible on screen. They may be covered
  /// by other elements or positioned outside of the visible area of a viewport.
  ///
  /// Hidden elements cannot gain accessibility focus though regular touch. The
  /// only way they can be focused is by moving the focus to them via linear
  /// navigation.
  ///
  /// Platforms are free to completely ignore hidden elements and new platforms
  /// are encouraged to do so.
  ///
  /// Instead of marking an element as hidden it should usually be excluded from
  /// the semantics tree altogether. Hidden elements are only included in the
  /// semantics tree to work around platform limitations and they are mainly
  /// used to implement accessibility scrolling on iOS.
  static const SemanticsFlag isHidden = SemanticsFlag._(_kIsHiddenIndex);

  /// Whether the semantics node represents an image.
  ///
  /// Both TalkBack and VoiceOver will inform the user the semantics node
  /// represents an image.
  static const SemanticsFlag isImage = SemanticsFlag._(_kIsImageIndex);

  /// Whether the semantics node is a live region.
  ///
  /// A live region indicates that updates to semantics node are important.
  /// Platforms may use this information to make polite announcements to the
  /// user to inform them of updates to this node.
  ///
  /// An example of a live region is a [SnackBar] widget. On Android, A live
  /// region causes a polite announcement to be generated automatically, even
  /// if the user does not have focus of the widget.
  static const SemanticsFlag isLiveRegion =
      SemanticsFlag._(_kIsLiveRegionIndex);

  /// The semantics node has the quality of either being "on" or "off".
  ///
  /// This flag is mutually exclusive with [hasCheckedState].
  ///
  /// For example, a switch has toggled state.
  ///
  /// See also:
  ///
  ///    * [SemanticsFlag.isToggled], which controls whether the node is "on" or "off".
  static const SemanticsFlag hasToggledState =
      SemanticsFlag._(_kHasToggledStateIndex);

  /// If true, the semantics node is "on". If false, the semantics node is
  /// "off".
  ///
  /// For example, if a switch is in the on position, [isToggled] is true.
  ///
  /// See also:
  ///
  ///   * [SemanticsFlag.hasToggledState], which enables a toggled state.
  static const SemanticsFlag isToggled = SemanticsFlag._(_kIsToggledIndex);

  /// Whether the platform can scroll the semantics node when the user attempts
  /// to move focus to an offscreen child.
  ///
  /// For example, a [ListView] widget has implicit scrolling so that users can
  /// easily move to the next visible set of children. A [TabBar] widget does
  /// not have implicit scrolling, so that users can navigate into the tab
  /// body when reaching the end of the tab bar.
  static const SemanticsFlag hasImplicitScrolling =
      SemanticsFlag._(_kHasImplicitScrollingIndex);

  /// Whether the value of the semantics node is coming from a multi-line text
  /// field.
  ///
  /// This isused for text fields to distinguish single-line text field from
  /// multi-line ones.
  static const SemanticsFlag isMultiline = SemanticsFlag._(_kIsMultilineIndex);

  /// The possible semantics flags.
  ///
  /// The map's key is the [index] of the flag and the value is the flag itself.
  /// The possible semantics flags.
  ///
  /// The map's key is the [index] of the flag and the value is the flag itself.
  static const Map<int, SemanticsFlag> values = <int, SemanticsFlag>{
    _kHasCheckedStateIndex: hasCheckedState,
    _kIsCheckedIndex: isChecked,
    _kIsSelectedIndex: isSelected,
    _kIsButtonIndex: isButton,
    _kIsLinkIndex: isLink,
    _kIsTextFieldIndex: isTextField,
    _kIsFocusableIndex: isFocusable,
    _kIsFocusedIndex: isFocused,
    _kHasEnabledStateIndex: hasEnabledState,
    _kIsEnabledIndex: isEnabled,
    _kIsInMutuallyExclusiveGroupIndex: isInMutuallyExclusiveGroup,
    _kIsHeaderIndex: isHeader,
    _kIsObscuredIndex: isObscured,
    _kScopesRouteIndex: scopesRoute,
    _kNamesRouteIndex: namesRoute,
    _kIsHiddenIndex: isHidden,
    _kIsImageIndex: isImage,
    _kIsLiveRegionIndex: isLiveRegion,
    _kHasToggledStateIndex: hasToggledState,
    _kIsToggledIndex: isToggled,
    _kHasImplicitScrollingIndex: hasImplicitScrolling,
    _kIsMultilineIndex: isMultiline,
    _kIsReadOnlyIndex: isReadOnly,
  };

  @override
  String toString() {
    switch (index) {
      case _kHasCheckedStateIndex:
        return 'SemanticsFlag.hasCheckedState';
      case _kIsCheckedIndex:
        return 'SemanticsFlag.isChecked';
      case _kIsSelectedIndex:
        return 'SemanticsFlag.isSelected';
      case _kIsButtonIndex:
        return 'SemanticsFlag.isButton';
      case _kIsLinkIndex:
        return 'SemanticsFlag.isLink';
      case _kIsTextFieldIndex:
        return 'SemanticsFlag.isTextField';
      case _kIsFocusableIndex:
        return 'SemanticsFlag.isFocusable';
      case _kIsFocusedIndex:
        return 'SemanticsFlag.isFocused';
      case _kHasEnabledStateIndex:
        return 'SemanticsFlag.hasEnabledState';
      case _kIsEnabledIndex:
        return 'SemanticsFlag.isEnabled';
      case _kIsInMutuallyExclusiveGroupIndex:
        return 'SemanticsFlag.isInMutuallyExclusiveGroup';
      case _kIsHeaderIndex:
        return 'SemanticsFlag.isHeader';
      case _kIsObscuredIndex:
        return 'SemanticsFlag.isObscured';
      case _kScopesRouteIndex:
        return 'SemanticsFlag.scopesRoute';
      case _kNamesRouteIndex:
        return 'SemanticsFlag.namesRoute';
      case _kIsHiddenIndex:
        return 'SemanticsFlag.isHidden';
      case _kIsImageIndex:
        return 'SemanticsFlag.isImage';
      case _kIsLiveRegionIndex:
        return 'SemanticsFlag.isLiveRegion';
      case _kHasToggledStateIndex:
        return 'SemanticsFlag.hasToggledState';
      case _kIsToggledIndex:
        return 'SemanticsFlag.isToggled';
      case _kHasImplicitScrollingIndex:
        return 'SemanticsFlag.hasImplicitScrolling';
      case _kIsMultilineIndex:
        return 'SemanticsFlag.isMultiline';
      case _kIsReadOnlyIndex:
        return 'SemanticsFlag.isReadOnly';
    }
    assert(false, 'Unhandled index: $index');
    return '';
  }
}

/// An object that creates [SemanticsUpdate] objects.
///
/// Once created, the [SemanticsUpdate] objects can be passed to
/// [Window.updateSemantics] to update the semantics conveyed to the user.
class SemanticsUpdateBuilder {
  /// Creates an empty [SemanticsUpdateBuilder] object.
  SemanticsUpdateBuilder();

  final List<engine.SemanticsNodeUpdate> _nodeUpdates =
      <engine.SemanticsNodeUpdate>[];

  /// Update the information associated with the node with the given `id`.
  ///
  /// The semantics nodes form a tree, with the root of the tree always having
  /// an id of zero. The `childrenInTraversalOrder` and `childrenInHitTestOrder`
  /// are the ids of the nodes that are immediate children of this node. The
  /// former enumerates children in traversal order, and the latter enumerates
  /// the same children in the hit test order. The two lists must have the same
  /// length and contain the same ids. They may only differ in the order the
  /// ids are listed in. For more information about different child orders, see
  /// [DebugSemanticsDumpOrder].
  ///
  /// The system retains the nodes that are currently reachable from the root.
  /// A given update need not contain information for nodes that do not change
  /// in the update. If a node is not reachable from the root after an update,
  /// the node will be discarded from the tree.
  ///
  /// The `flags` are a bit field of [SemanticsFlag]s that apply to this node.
  ///
  /// The `actions` are a bit field of [SemanticsAction]s that can be undertaken
  /// by this node. If the user wishes to undertake one of these actions on this
  /// node, the [Window.onSemanticsAction] will be called with `id` and one of
  /// the possible [SemanticsAction]s. Because the semantics tree is maintained
  /// asynchronously, the [Window.onSemanticsAction] callback might be called
  /// with an action that is no longer possible.
  ///
  /// The `label` is a string that describes this node. The `value` property
  /// describes the current value of the node as a string. The `increasedValue`
  /// string will become the `value` string after a [SemanticsAction.increase]
  /// action is performed. The `decreasedValue` string will become the `value`
  /// string after a [SemanticsAction.decrease] action is performed. The `hint`
  /// string describes what result an action performed on this node has. The
  /// reading direction of all these strings is given by `textDirection`.
  ///
  /// The fields 'textSelectionBase' and 'textSelectionExtent' describe the
  /// currently selected text within `value`.
  ///
  /// For scrollable nodes `scrollPosition` describes the current scroll
  /// position in logical pixel. `scrollExtentMax` and `scrollExtentMin`
  /// describe the maximum and minimum in-rage values that `scrollPosition` can
  /// be. Both or either may be infinity to indicate unbound scrolling. The
  /// value for `scrollPosition` can (temporarily) be outside this range, for
  /// example during an overscroll.
  ///
  /// The `rect` is the region occupied by this node in its own coordinate
  /// system.
  ///
  /// The `transform` is a matrix that maps this node's coordinate system into
  /// its parent's coordinate system.
  void updateNode({
    int id,
    int flags,
    int actions,
    int maxValueLength,
    int currentValueLength,
    int textSelectionBase,
    int textSelectionExtent,
    int platformViewId,
    int scrollChildren,
    int scrollIndex,
    double scrollPosition,
    double scrollExtentMax,
    double scrollExtentMin,
    double elevation,
    double thickness,
    Rect rect,
    String label,
    String hint,
    String value,
    String increasedValue,
    String decreasedValue,
    TextDirection textDirection,
    Float64List transform,
    Int32List childrenInTraversalOrder,
    Int32List childrenInHitTestOrder,
    Int32List additionalActions,
  }) {
    if (transform.length != 16)
      throw ArgumentError('transform argument must have 16 entries.');
    _nodeUpdates.add(engine.SemanticsNodeUpdate(
      id: id,
      flags: flags,
      actions: actions,
      maxValueLength: maxValueLength,
      currentValueLength: currentValueLength,
      textSelectionBase: textSelectionBase,
      textSelectionExtent: textSelectionExtent,
      scrollChildren: scrollChildren,
      scrollIndex: scrollIndex,
      scrollPosition: scrollPosition,
      scrollExtentMax: scrollExtentMax,
      scrollExtentMin: scrollExtentMin,
      rect: rect,
      label: label,
      hint: hint,
      value: value,
      increasedValue: increasedValue,
      decreasedValue: decreasedValue,
      textDirection: textDirection,
      transform: engine.toMatrix32(transform),
      elevation: elevation,
      thickness: thickness,
      childrenInTraversalOrder: childrenInTraversalOrder,
      childrenInHitTestOrder: childrenInHitTestOrder,
      additionalActions: additionalActions,
      platformViewId: platformViewId,
    ));
  }

  void updateCustomAction(
      {/*required*/ int/*!*/ id, String/*?*/ label, String/*?*/ hint, int/*!*/ overrideId = -1}) {
    // TODO(yjbanov): implement.
  }

  /// Creates a [SemanticsUpdate] object that encapsulates the updates recorded
  /// by this object.
  ///
  /// The returned object can be passed to [Window.updateSemantics] to actually
  /// update the semantics retained by the system.
  SemanticsUpdate/*!*/ build() {
    return SemanticsUpdate._(
      nodeUpdates: _nodeUpdates,
    );
  }
}

/// An opaque object representing a batch of semantics updates.
///
/// To create a SemanticsUpdate object, use a [SemanticsUpdateBuilder].
///
/// Semantics updates can be applied to the system's retained semantics tree
/// using the [Window.updateSemantics] method.
abstract class SemanticsUpdate {
  /// This class is created by the engine, and should not be instantiated
  /// or extended directly.
  ///
  /// To create a SemanticsUpdate object, use a [SemanticsUpdateBuilder].
  factory SemanticsUpdate._({List<engine.SemanticsNodeUpdate> nodeUpdates}) =
      engine.SemanticsUpdate;

  /// Releases the resources used by this semantics update.
  ///
  /// After calling this function, the semantics update is cannot be used
  /// further.
  void dispose();
}