lib/web_ui/lib/src/engine/semantics/image.dart

// 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 engine;

/// Represents semantic objects that deliver information in a visual manner.
///
/// Uses aria img role to convey this semantic information to the element.
///
/// Screen-readers takes advantage of "aria-label" to describe the visual.
class ImageRoleManager extends RoleManager {
  ImageRoleManager(SemanticsObject semanticsObject)
      : super(Role.image, semanticsObject);

  /// The element with role="img" and aria-label could block access to all
  /// children elements, therefore create an auxiliary element and  describe the
  /// image in that if the semantic object have child nodes.
  html.Element _auxiliaryImageElement;

  @override
  void update() {
    if (semanticsObject.isVisualOnly && semanticsObject.hasChildren) {
      if (_auxiliaryImageElement == null) {
        _auxiliaryImageElement = html.Element.tag('flt-semantics-img');
        // Absolute positioning and sizing of leaf text elements confuses
        // VoiceOver. So we let the browser size the value node. The node will
        // still have a bigger tap area. However, if the node is a parent to
        // other nodes, then VoiceOver behaves as expected with absolute
        // positioning and sizing.
        if (semanticsObject.hasChildren) {
          _auxiliaryImageElement.style
            ..position = 'absolute'
            ..top = '0'
            ..left = '0'
            ..width = '${semanticsObject.rect.width}px'
            ..height = '${semanticsObject.rect.height}px';
        }
        _auxiliaryImageElement.style.fontSize = '6px';
        semanticsObject.element.append(_auxiliaryImageElement);
      }

      _auxiliaryImageElement.setAttribute('role', 'img');
      _setLabel(_auxiliaryImageElement);
    } else if (semanticsObject.isVisualOnly) {
      semanticsObject.setAriaRole('img', true);
      _setLabel(semanticsObject.element);
      _cleanUpAuxiliaryElement();
    } else {
      _cleanUpAuxiliaryElement();
      _cleanupElement();
    }
  }

  void _setLabel(html.Element element) {
    if (semanticsObject.hasLabel) {
      element.setAttribute('aria-label', semanticsObject.label);
    }
  }

  void _cleanUpAuxiliaryElement() {
    if (_auxiliaryImageElement != null) {
      _auxiliaryImageElement.remove();
      _auxiliaryImageElement = null;
    }
  }

  void _cleanupElement() {
    semanticsObject.setAriaRole('img', false);
    semanticsObject.element.attributes.remove('aria-label');
  }

  @override
  void dispose() {
    _cleanUpAuxiliaryElement();
    _cleanupElement();
  }
}