The Qt accessibility module

• Originally designed in Qt 3.0 to support MSAA
• Later extended to support Mac OS X accessibillity
• Extended in Qt 4 to support at-spi

Design goals

• Make it easy for developers to make applications accessible
• No or very little impact on existing code
• No penalties for users who don't require accessibility
• Load on demand

Qt 4 Accessibility

• Accessibility is on by default
• All Widgets are fully accessible
• Subclasses inherit accessibility from their superclasses
• Accessible interfaces can be plugins
• Accessible bridges are plugins

Advantages:

• Most custom widgets will be accessible via their superclass
• Independent patch releases possible

Qt 4 Accessible Interface

// properties and state

virtual QString text(Text t, int child) const = 0;

virtual QRect rect(int child) const = 0;

virtual Role role(int child) const = 0;

virtual int state(int child) const = 0;

// hierarchy

virtual int childCount() const = 0;

virtual int indexOfChild(const QAccessibleInterface *) const = 0;

virtual int childAt(int x, int y) const = 0;

// relations

virtual int relationTo(int child, const QAccessibleInterface *other, int otherChild) const = 0;

// navigation

virtual int navigate(Relation relation, int index, QAccessibleInterface **iface) const = 0;

// action

virtual int userActionCount(int child) const = 0;

virtual QString actionText(int action, Text t, int child) const = 0;

virtual bool doAction(int action, int child, const QVariantList &ms = QVariantList()) = 0;

Qt 4 Accessible Interface

Advantages:

• Low memory overhead - instances only store an object pointer
• Children do not have to be objects
• Implementation is generic and not widget specific - no trouble with BIC
• New widgets can be made accessible without modifying assistive technologies
• Lifetime of accessible interface != lifetime of widget

Qt 4 Accessible Interface

• Accessible help-texts can be set in Qt Designer
• Accessible help-texts can be localized
• Backend classes (document/view classes) are "accessible-ready"

Accessible Plugins

class AccessibleFactory : public QAccessiblePlugin

{

public:

    AccessibleFactory() {}

    QStringList keys() const;

    QAccessibleInterface *create(const QString &classname, QObject *object);

};

Accessible Plugins

QStringList AccessibleFactory::keys() const

{

    QStringList list;

    list << "MyWidget1" << "MyWidget2";

    return list;

}

QAccessibleInterface *AccessibleFactory::create(const QString &classname, QObject *object)

{

    if (classname == "MyWidget1")

        return new MyWidget1Iface(object);

    if (classname == "MyWidget2")

        return new MyWidget2Iface(object);

    return 0;

}

Q_EXPORT_PLUGIN(AccessibleFactory)

at-spi Integration

                Qt Application

                 |

                 | links to

                 |

          Qt Accessibility Module

                 |

                 | Plugin (in-process)

                 |

                Qt ATK Bridge

                 |

                 | links to

                 |

                      ATK

                 |

                 | Plugin (in-process)

                 |

                    at-spi

                 |

                 | CORBA

                 |

          assistive technologies

Qt 4.x accessibility

• Qt is open for new backends via accessibility bridges
• Ready for new out-of-process communication channels (D-BUS)

D-BUS: sending a signal

    QDBusConnection con;

    QDBusMessage message = QDBusMessage::signal(

                "/org/harald/AccessibleRoot",

                "org.harald.accessibility", "Ping");

    message << QString("Give me a ping") << 42;

    con.send(message);

D-BUS: receiving a signal

class AccessibleRoot: public QObject

{

public slots:

    void pingReceived();

};

{

    QDBusConnection con;

    con.connect("/org/harald/AccessibleRoot",

                  "org.harald.accessibility", "Ping",

                &o, SLOT(pingReceived()));

}

D-BUS: invoking a method

    QDBusConnection con;

    QDBusMessage message = QDBusMessage::methodCall(

                ":1.15", "/org/harald/AccessibleRoot",

                "org.harald.accessibility", "Pong");

    QDBusMessage reply = con.sendWithReply(message);

    qDebug() << reply;

D-BUS: receiving method calls

class AccessibleRoot: public QObject

{

public:

    Q_SCRIPTABLE void doSomething(int i, QString s, double d);

};

{

    AccessibleRoot root;

    QDBusConnection con;

    con.registerObject("/com/harald/AccessibleRoot",

                      "com.harald.accessibility", &root)   

}

Qt D-BUS bridge

                Qt Application

                 |

                 | links to

                 |

          Qt Accessibility Module

                 |

                 | Plugin (in-process)

                 |

                Qt D-BUS bridge

                 |

                 | talks to

                 |

          assistive technologies