source: trunk/doc/src/frameworks-technologies/accessible.qdoc@ 846

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/*!
    \group accessibility
    \title Accessibility Classes
*/

/*!
    \page accessible.html
    \title Accessibility
    \brief How to make your applications accessible to those with disabilities.

    \ingroup technology-apis
    \ingroup qt-basic-concepts
    \ingroup best-practices

    \tableofcontents

    \section1 Introduction

    Accessibility in computer software is making applications usable
    for people with disabilities. This could be achieved by providing
    keyboard shortcuts, a high-contrast user interface that uses
    specially selected colors and fonts, or support for assistive tools
    such as screen readers and braille displays.

    An application does not usually communicate directly with
    assistive tools but through an assistive technology, which is a
    bridge for exchange of information between the applications and
    the tools. Information about user interface elements, such
    as buttons and scroll bars, is exposed to the assistive technologies.
    Qt supports Microsoft Active Accessibility (MSAA) on Windows and
    Mac OS X Accessibility on Mac OS X.
    On Unix/X11, support is preliminary. The individual technologies
    are abstracted from Qt, and there is only a single interface to
    consider. We will use MSAA throughout this document when we need
    to address technology related issues.

    In this overview document, we will examine the overall Qt
    accessibility architecture, and how to implement accessibility for
    custom widgets and elements. 

    \section1 Architecture

    Providing accessibility is a collaboration between accessibility
    compliant applications, the assistive technology, and the
    assistive tools. 

    \image accessibilityarchitecture.png

    Accessibility compliant applications are called AT-Servers while
    assistive tools are called AT-Clients. A Qt application will
    typically be an AT-Server, but specialized programs might also
    function like AT-Clients. We will refer to clients and servers
    when talking about AT-Clients and AT-Servers in the rest of this
    document.

    We will from now on focus on the Qt accessibility interface and
    how it is implemented to create Qt applications that support
    accessibility.

    \section2 Accessibility in Qt
    
    These classes provide support for accessible applications.

    \annotatedlist accessibility

    When we communicate with the assistive technologies, we need to
    describe Qt's user interface in a way that they can understand. Qt
    applications use QAccessibleInterface to expose information about the
    individual UI elements. Currently, Qt provides support for its widgets
    and widget parts, e.g., slider handles, but the interface could
    also be implemented for any QObject if necessary. QAccessible
    contains enums that describe the UI. The description is mainly
    based on MSAA and is independent of Qt. We will examine the enums
    in the course of this document.

    The structure of the UI is represented as a tree of
    QAccessibleInterface subclasses. You can think of this as a
    representation of a UI like the QObject tree built by Qt. Objects
    can be widgets or widget parts (such as scroll bar handles). We
    examine the tree in detail in the next section.

    Servers notify clients through \l{QAccessible::}{updateAccessibility()}
    about changes in objects by sending events, and the clients
    register to receive the events. The available events are defined
    by the QAccessible::Event enum. The clients may then query for
    the object that generated the event through
    QAccessible::queryAccessibleInterface().

    Three of the enums in QAccessible help clients query and alter
    accessible objects:

    \list
        \o \l{QAccessible::}{Role}: Describes the role the object
            fills in the user interface, e.g., if it is a main
            window, a text caret, or a cell in an item view.
        \o \l{QAccessible::}{Action}: The actions that the
            clients can perform on the objects, e.g., pushing a
            button.
        \o \l{QAccessible::}{Relation}: Describes the relationship
            between objects in the object tree.
            This is used for navigation.
    \endlist

    The clients also have some possibilities to get the content of
    objects, e.g., a button's text; the object provides strings
    defined by the QAccessible::Text enum, that give information
    about content.

    The objects can be in a number of different states as defined by
    the \l{QAccessible::}{State} enum. Examples of states are whether
    the object is disabled, if it has focus, or if it provides a pop-up
    menu.

    \section2 The Accessible Object Tree

    As mentioned, a tree structure is built from the accessible
    objects of an application. By navigating through the tree, the
    clients can access all elements in the UI. Object relations give
    clients information about the UI. For instance, a slider handle is
    a child of the slider to which it belongs. QAccessible::Relation
    describes the various relationships the clients can ask objects
    for.

    Note that there are no direct mapping between the Qt QObject tree
    and the accessible object tree. For instance, scroll bar handles
    are accessible objects but are not widgets or objects in Qt.

    AT-Clients have access to the accessibility object tree through
    the root object in the tree, which is the QApplication. They can
    query other objects through QAccessible::navigate(), which fetches
    objects based on \l{QAccessible::}{Relation}s. The children of any
    node is 1-based numbered. The child numbered 0 is the object
    itself. The children of all interfaces are numbered this way,
    i.e., it is not a fixed numbering from the root node in the entire
    tree.

    Qt provides accessible interfaces for its widgets. Interfaces for
    any QObject subclass can be requested through
    QAccessible::queryInterface(). A default implementation is
    provided if a more specialized interface is not defined. An
    AT-Client cannot acquire an interface for accessible objects that
    do not have an equivalent QObject, e.g., scroll bar handles, but
    they appear as normal objects through interfaces of parent
    accessible objects, e.g., you can query their relationships with
    QAccessible::relationTo().

    To illustrate, we present an image of an accessible object tree.
    Beneath the tree is a table with examples of object relationships.

    \image accessibleobjecttree.png

    The labels in top-down order are: the QAccessibleInterface class
    name, the widget for which an interface is provided, and the
    \l{QAccessible::}{Role} of the object. The Position, PageLeft and 
    PageRight correspond to the slider handle, the slider groove left
    and the slider groove right, respectively. These accessible objects
    do not have an equivalent QObject.

    \table 40%
        \header
            \o Source Object
            \o Target Object
            \o Relation
        \row
            \o Slider
            \o Indicator
            \o Controller
        \row
            \o Indicator
            \o Slider
            \o Controlled
        \row
            \o Slider
            \o Application
            \o Ancestor
        \row
            \o Application
            \o Slider
            \o Child
        \row
            \o PushButton
            \o Indicator
            \o Sibling
    \endtable

    \section2 The Static QAccessible Functions

    The accessibility is managed by QAccessible's static functions,
    which we will examine shortly. They produce QAccessible
    interfaces, build the object tree, and initiate the connection
    with MSAA or the other platform specific technologies. If you are
    only interested in learning how to make your application
    accessible, you can safely skip over this section to
    \l{Implementing Accessibility}.

    The communication between clients and the server is initiated when
    \l{QAccessible::}{setRootObject()} is called. This is done when
    the QApplication instance is instantiated and you should not have
    to do this yourself.

    When a QObject calls \l{QAccessible::}{updateAccessibility()},
    clients that are listening to events are notified of the
    change. The function is used to post events to the assistive
    technology, and accessible \l{QAccessible::Event}{events} are
    posted by \l{QAccessible::}{updateAccessibility()}.

    \l{QAccessible::}{queryAccessibleInterface()} returns accessible
    interfaces for \l{QObject}s. All widgets in Qt provide interfaces;
    if you need interfaces to control the behavior of other \l{QObject}
    subclasses, you must implement the interfaces yourself, although
    the QAccessibleObject convenience class implements parts of the
    functionality for you.

    The factory that produces accessibility interfaces for QObjects is
    a function of type QAccessible::InterfaceFactory. It is possible
    to have several factories installed. The last factory installed
    will be the first to be asked for interfaces.
    \l{QAccessible::}{queryAccessibleInterface()} uses the factories
    to create interfaces for \l{QObject}s. Normally, you need not be
    concerned about factories because you can implement plugins that
    produce interfaces. We will give examples of both approaches
    later.

    \section2 Enabling Accessibility Support

    By default, Qt applications are run with accessibility support
    enabled on Windows and Mac OS X. On Unix/X11 platforms, applications
    must be launched in an environment with the \c QT_ACCESSIBILITY
    variable set to 1. For example, this is set in the following way with
    the bash shell:

    \snippet doc/src/snippets/code/doc_src_qt4-accessibility.qdoc environment

    Accessibility features are built into Qt by default when the libraries
    are configured and built.

    \section1 Implementing Accessibility

    To provide accessibility support for a widget or other user
    interface element, you need to implement the QAccessibleInterface
    and distribute it in a QAccessiblePlugin. It is also possible to
    compile the interface into the application and provide a
    QAccessible::InterfaceFactory for it. The factory can be used if
    you link statically or do not want the added complexity of
    plugins.  This can be an advantage if you, for instance, are
    delivering a 3-rd party library.

    All widgets and other user interface elements should have
    interfaces and plugins. If you want your application to support
    accessibility, you will need to consider the following:

    \list
        \o Qt already implements accessibility for its own widgets.
           We therefore recommend that you use Qt widgets where possible.
        \o A QAccessibleInterface needs to be implemented for each element
           that you want to make available to accessibility clients.
        \o You need to send accessibility events from the custom
           user interface elements that you implement.
    \endlist

    In general, it is recommended that you are somewhat familiar with
    MSAA, which Qt's accessibility support originally was built for.
    You should also study the enum values of QAccessible, which
    describe the roles, actions, relationships, and events that you
    need to consider.

    Note that you can examine how Qt's widgets implement their
    accessibility. One major problem with the MSAA standard is that
    interfaces are often implemented in an inconsistent way. This
    makes life difficult for clients and often leads to guesswork on
    object functionality.

    It is possible to implement interfaces by inheriting
    QAccessibleInterface and implementing its pure virtual functions.
    In practice, however, it is usually preferable to inherit
    QAccessibleObject or QAccessibleWidget, which implement part of
    the functionality for you. In the next section, we will see an
    example of implementing accessibility for a widget by inheriting
    the QAccessibleWidget class.

    \section2 The QAccessibleObject and QAccessibleWidget Convenience Classes

    When implementing an accessibility interface for widgets, one would
    as a rule inherit QAccessibleWidget, which is a convenience class
    for widgets. Another available convenience class, which is
    inherited by QAccessibleWidget, is the QAccessibleObject, which
    implements part of the interface for QObjects.

    The QAccessibleWidget provides the following functionality:

    \list
        \o It handles the navigation of the tree and
           hit testing of the objects. 
        \o It handles events, roles, and actions that are common for all
           \l{QWidget}s.
        \o It handles action and methods that can be performed on
           all widgets.
        \o It calculates bounding rectangles with
           \l{QAccessibleInterface::}{rect()}.
        \o It gives \l{QAccessibleInterface::}{text()} strings that are
           appropriate for a generic widget.
        \o It sets the \l{QAccessible::State}{states} that
           are common for all widgets.
    \endlist

    \section2 QAccessibleWidget Example

    Instead of creating a custom widget and implementing an interface
    for it, we will show how accessibility can be implemented for one of
    Qt's standard widgets: QSlider. Making this widget accessible
    demonstrates many of the issues that need to be faced when making
    a custom widget accessible.

    The slider is a complex control that functions as a
    \l{QAccessible::}{Controller} for its accessible children.
    This relationship must be known by the interface (for
    \l{QAccessibleInterface::}{relationTo()} and
    \l{QAccessibleInterface::}{navigate()}). This can be done
    using a controlling signal, which is a mechanism provided by
    QAccessibleWidget. We do this in the constructor:

    \snippet doc/src/snippets/accessibilityslidersnippet.cpp 0

    The choice of signal shown is not important; the same principles
    apply to all signals that are declared in this way. Note that we
    use QLatin1String to ensure that the signal name is correctly
    specified.

    When an accessible object is changed in a way that users need
    to know about, it notifies clients of the change by sending them
    an event via the accessible interface. This is how QSlider calls
    \l{QAccessibleInterface::}{updateAccessibility()} to indicate that
    its value has changed:

    \snippet doc/src/snippets/qabstractsliderisnippet.cpp 0
    \dots
    \snippet doc/src/snippets/qabstractsliderisnippet.cpp 1
    \dots
    \snippet doc/src/snippets/qabstractsliderisnippet.cpp 2

    Note that the call is made after the value of the slider has
    changed because clients may query the new value immediately after
    receiving the event.

    The interface must be able to calculate bounding rectangles of
    itself and any children that do not provide an interface of their
    own. The \c QAccessibleSlider has three such children identified by
    the private enum, \c SliderElements, which has the following values:
    \c PageLeft (the rectangle on the left hand side of the slider
    handle), \c PageRight (the rectangle on the right hand side of the
    handle), and \c Position (the slider handle). Here is the
    implementation of \l{QAccessibleInterface::}{rect()}:

    \snippet doc/src/snippets/accessibilityslidersnippet.cpp 1
    \dots
    \snippet doc/src/snippets/accessibilityslidersnippet.cpp 2
    \dots

    The first part of the function, which we have omitted, uses the
    current \l{QStyle}{style} to calculate the slider handle's
    bounding rectangle; it is stored in \c srect. Notice that child 0,
    covered in the default case in the above code, is the slider itself,
    so we can simply return the QSlider bounding rectangle obtained
    from the superclass, which is effectively the value obtained from
    QAccessibleWidget::rect().

    \snippet doc/src/snippets/accessibilityslidersnippet.cpp 3

    Before the rectangle is returned it must be mapped to screen
    coordinates.

    The QAccessibleSlider must reimplement
    QAccessibleInterface::childCount() since it manages children
    without interfaces.

    The \l{QAccessibleInterface::}{text()} function returns the
    QAccessible::Text strings for the slider:

    \snippet doc/src/snippets/accessibilityslidersnippet.cpp 4

    The \c slider() function returns a pointer to the interface's
    QSlider. Some values are left for the superclass's implementation.
    Not all values are appropriate for all accessible objects, as you
    can see for QAccessible::Value case. You should just return an
    empty string for those values where no relevant text can be
    provided.

    The implementation of the \l{QAccessibleInterface::}{role()}
    function is straightforward:

    \snippet doc/src/snippets/accessibilityslidersnippet.cpp 5

    The role function should be reimplemented by all objects and
    describes the role of themselves and the children that do not
    provide accessible interfaces of their own.

    Next, the accessible interface needs to return the
    \l{QAccessible::State}{states} that the slider can be in. We look
    at parts of the \c state() implementation to show how just a few
    of the states are handled:

    \snippet doc/src/snippets/accessibilityslidersnippet.cpp 6
    \dots
    \snippet doc/src/snippets/accessibilityslidersnippet.cpp 7

    The superclass implementation of
    \l{QAccessibleInterface::}{state()}, uses the
    QAccessibleInterface::state() implementation. We simply need to
    disable the buttons if the slider is at its minimum or maximum.

    We have now exposed the information we have about the slider to
    the clients. For the clients to be able to alter the slider - for
    example, to change its value - we must provide information about
    the actions that can be performed and perform them upon request.
    We discuss this in the next section.

    \section2 Handling Action Requests from Clients

    QAccessible provides a number of \l{QAccessible::}{Action}s
    that can be performed on request from clients. If an
    accessible object supports actions, it should reimplement the
    following functions from QAccessibleInterface:

    \list
        \o \l{QAccessibleInterface::}{actionText()} returns
            strings that describe each action. The descriptions
            to be made available are one for each
            \l{QAccessible::}{Text} enum value.
        \o  \l{QAccessibleInterface::}{doAction()} executes requests
            from clients to perform actions.
    \endlist

    Note that a client can request any action from an object. If
    the object does not support the action, it returns false from
    \l{QAccessibleInterface::}{doAction()}.

    None of the standard actions take any parameters. It is possible
    to provide user-defined actions that can take parameters.
    The interface must then also reimplement
    \l{QAccessibleInterface::}{userActionCount()}. Since this is not
    defined in the MSAA specification, it is probably only useful to
    use this if you know which specific AT-Clients will use the
    application.

    QAccessibleInterface gives another technique for clients to handle
    accessible objects. It works basically the same way, but uses the
    concept of methods in place of actions. The available methods are
    defined by the QAccessible::Method enum. The following functions
    need to be reimplemented from QAccessibleInterface if the
    accessible object is to support methods:

    \list
        \o \l{QAccessibleInterface::}{supportedMethods()} returns
            a QSet of \l{QAccessible::}{Method} values that are 
            supported by the object.
        \o \l{QAccessibleInterface::}{invokeMethod()} executes
           methods requested by clients.
    \endlist

    The action mechanism will probably be substituted by providing
    methods in place of the standard actions. 

    To see examples on how to implement actions and methods, you
    could examine the QAccessibleObject and QAccessibleWidget
    implementations. You might also want to take a look at the
    MSAA documentation.

    \section2 Implementing Accessible Plugins

    In this section we will explain the procedure of implementing
    accessible plugins for your interfaces. A plugin is a class stored
    in a shared library that can be loaded at run-time. It is
    convenient to distribute interfaces as plugins since they will only
    be loaded when required.

    Creating an accessible plugin is achieved by inheriting
    QAccessiblePlugin, reimplementing \l{QAccessiblePlugin::}{keys()}
    and \l{QAccessiblePlugin::}{create()} from that class, and adding
    one or two macros. The \c .pro file must be altered to use the
    plugin template, and the library containing the plugin must be
    placed on a path where Qt searches for accessible plugins.

    We will go through the implementation of \c SliderPlugin, which is an
    accessible plugin that produces interfaces for the
    QAccessibleSlider we implemented in the \l{QAccessibleWidget Example}.
    We start with the \c key() function:

    \snippet doc/src/snippets/accessibilitypluginsnippet.cpp 0

    We simply need to return the class name of the single interface
    our plugin can create an accessible interface for. A plugin
    can support any number of classes; just add more class names
    to the string list. We move on to the \c create() function:

    \snippet doc/src/snippets/accessibilitypluginsnippet.cpp 1

    We check whether the interface requested is for the QSlider; if it
    is, we create and return an interface for it. Note that \c object
    will always be an instance of \c classname. You must return 0 if
    you do not support the class.
    \l{QAccessible::}{updateAccessibility()} checks with the
    available accessibility plugins until it finds one that does not
    return 0.

    Finally, you need to include macros in the cpp file:

    \snippet doc/src/snippets/accessibilitypluginsnippet.cpp 2

    The Q_EXPORT_PLUGIN2 macro exports the plugin in the \c
    SliderPlugin class into the \c acc_sliderplugin library. The first
    argument is the name of the plugin library file, excluding the
    file suffix, and the second is the class name. For more information
    on plugins, consult the plugins \l{How to Create Qt
    Plugins}{overview document}.

    You can omit the first macro unless you want the plugin
    to be statically linked with the application.

    \section2 Implementing Interface Factories

    If you do not want to provide plugins for your accessibility
    interfaces, you can use an interface factory
    (QAccessible::InterfaceFactory), which is the recommended way to
    provide accessible interfaces in a statically-linked application.

    A factory is a function pointer for a function that takes the same
    parameters as \l{QAccessiblePlugin}'s
    \l{QAccessiblePlugin::}{create()} - a QString and a QObject. It
    also works the same way. You install the factory with the
    \l{QAccessible::}{installFactory()} function. We give an example
    of how to create a factory for the \c SliderPlugin class:

    \snippet doc/src/snippets/accessibilityfactorysnippet.cpp 0
    \dots
    \snippet doc/src/snippets/accessibilityfactorysnippet.cpp 1

    \omit

    \section1 Implementing Bridges for Other Assistive Technologies

    An accessibility bridge provides the means for an assistive
    technology to talk to Qt. On Windows and Mac, the built-in bridges
    will be used. On UNIX, however, there are no built-in standard
    assistive technology, and it might therefore be necessary to
    implement an accessible bridge.

    A bridge is implemented by inheriting QAccessibleBridge for the
    technology to support. The class defines the interface that Qt
    needs an assistive technology to support:

    \list
        \o A root object. This is the root in the accessible
           object tree and is of type QAccessibleInterface.
        \o Receive events from from accessible objects.
    \endlist

    The root object is set with the
    \l{QAccessibleBridge::}{setRootObject()}. In the case of Qt, this
    will always be an interface for the QApplication instance of the
    application.

    Event notification is sent through
    \l{QAccessibleBridge::}{notifyAccessibilityUpdate()}. This
    function is called by \l{QAccessible::}{updateAccessibility()}. Even
    though the bridge needs only to implement these two functions, it
    must be able to communicate the entire QAccessibleInterface to the
    underlying technology. How this is achieved is, naturally, up to
    the individual bridge and none of Qt's concern.

    As with accessible interfaces, you distribute accessible bridges
    in plugins. Accessible bridge plugins are subclasses of the
    QAccessibleBridgePlugin class; the class defines the functions
    \l{QAccessibleBridgePlugin::}{create()} and
    \l{QAccessibleBridgePlugin::}{keys()}, which must me
    reimplemented. If Qt finds a built-in bridge to use, it will
    ignore any available plugins.

    \endomit

    \section1 Further Reading

    The \l{Cross-Platform Accessibility Support in Qt 4} document contains a more
    general overview of Qt's accessibility features and discusses how it is
    used on each platform.
    issues
*/