Cross-Platform Accessibility Support in Qt 4

Qt 4 allows developers to write cross-platform applications that are usable by visually impaired users as well as by users with other disabilities. Qt accessibility will make applications accessible to more users and opens the governmental market, where accessibility is often a requirement.

一般概述

The accessibility classes have been extended in various ways since Qt 3. We added new functions and new enum values, and revised the API to make it more consistent with the rest of Qt. We also added two properties to QWidget , accessibleName and accessibleDescription , that can be set in Qt Designer to provide basic help texts without having to write any code.

Qt's accessibility architecture is as follows. Qt offers one generic interface, QAccessibleInterface ，可用于包裹所有 Widget 和对象 (如 QPushButton )。此单一接口为辅助技术，提供所有必要元数据。Qt 为内置 Widget 作为插件提供此接口的实现。

A more detailed overview of the accessibility support in Qt can be found on the 可访问性页面。

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 QT_ACCESSIBILITY variable set to 1. For example, this is set in the following way with the bash shell:

export QT_ACCESSIBILITY=1

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

Creating New Accessible Interfaces

当开发自定义 Widget 时，可以创建自定义子类为 QAccessibleInterface 并将它们分发作为插件 (使用 QAccessiblePlugin ) 或将它们编译进应用程序。同样，Qt 的预定义可访问性支持可以构建作为插件 (默认)，或直接构建进 Qt 库。使用插件的主要优点是可访问性类仅加载进内存，若它们被实际使用；它们不会减慢没有使用辅助技术的常见情况。

除了 QAccessibleInterface , Qt includes two convenience classes, QAccessibleObject and QAccessibleWidget , that provide the lowest common denominator of metadata (e.g., widget geometry, window title, basic help text). You can use them as base classes when wrapping your custom QObject or QWidget 子类。

Another new feature in Qt 4 is that Qt can now support other backends in addition to the predefined ones. This is done by subclassing QAccessibleBridge .

范例代码

The first example illustrates how to provide accessibility information for a custom widget. We can use QAccessibleWidget as a base class and reimplement various functions:

class MyWidgetInterface : public QAccessibleWidget
{
public:
    MyWidgetInterface(QWidget *widget, Role role);
    QString text(Text text, int child) const;
    State state(int child) const;
    QString actionText(int action, Text text, int child) const;
    bool doAction(int action, int child, const QVariantList &params);
    ...
};

Here's how we would implement the doAction() function to call a function named click() on the wrapped MyWidget object when the user invokes the object's default action or "presses" it.

bool MyWidgetInterface::doAction(int action, int child,
                                 const QVariantList &params)
{
    if (child || !widget()->isEnabled())
        return false;
    switch (action) {
    case DefaultAction:
    case Press:
        {
            MyWidget *widget = qobject_cast<MyWidget *>(object());
            if (widget)
                widget->click();
        }
        return true;
    }
    return QAccessibleWidget::doAction(action, child, params);
}

To export the widget interface as a plugin, we must subclass QAccessibleFactory:

QStringList MyFactory::keys() const
{
    return QStringList() << "MyWidget" << "MyOtherWidget";
}
QAccessibleInterface *MyFactory::create(const QString &className,
                                        QObject *object)
{
    if (classname == "MyWidget")
        return new MyWidgetInterface(object);
    if (classname == "MyOtherWidget")
        return new MyOtherWidgetInterface(object);
    return 0;
}
Q_EXPORT_PLUGIN2(myfactory, MyFactory)