source: trunk/doc/src/i18n.qdoc@ 337

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/*!
    \group i18n
    \title Internationalization with Qt
    \ingroup topics

    \brief Information about Qt's support for internationalization and multiple languages.

    \keyword internationalization
    \keyword i18n

    The internationalization of an application is the process of making
    the application usable by people in countries other than one's own.

    \tableofcontents

    In some cases internationalization is simple, for example, making a US
    application accessible to Australian or British users may require
    little more than a few spelling corrections. But to make a US
    application usable by Japanese users, or a Korean application usable
    by German users, will require that the software operate not only in
    different languages, but use different input techniques, character
    encodings and presentation conventions.

    Qt tries to make internationalization as painless as possible for
    developers. All input widgets and text drawing methods in Qt offer
    built-in support for all supported languages. The built-in font engine
    is capable of correctly and attractively rendering text that contains
    characters from a variety of different writing systems at the same
    time.

    Qt supports most languages in use today, in particular:
    \list
    \o All East Asian languages (Chinese, Japanese and Korean)
    \o All Western languages (using Latin script)
    \o Arabic
    \o Cyrillic languages (Russian, Ukrainian, etc.)
    \o Greek
    \o Hebrew
    \o Thai and Lao
    \o All scripts in Unicode 4.0 that do not require special processing
    \endlist

    On Windows, Unix/X11 with FontConfig (client side font support)
    and Qt for Embedded Linux the following languages are also supported:
    \list
    \o Bengali
    \o Devanagari
    \o Dhivehi (Thaana)
    \o Gujarati
    \o Gurmukhi
    \o Kannada
    \o Khmer
    \o Malayalam
    \o Myanmar
    \o Syriac
    \o Tamil
    \o Telugu
    \o Tibetan
    \endlist

    Many of these writing systems exhibit special features:

    \list

    \o \bold{Special line breaking behavior.} Some of the Asian languages are
    written without spaces between words. Line breaking can occur either
    after every character (with exceptions) as in Chinese, Japanese and
    Korean, or after logical word boundaries as in Thai.

    \o \bold{Bidirectional writing.} Arabic and Hebrew are written from right to
    left, except for numbers and embedded English text which is written
    left to right. The exact behavior is defined in the
    \l{http://www.unicode.org/unicode/reports/tr9/}{Unicode Technical Annex #9}.

    \o \bold{Non-spacing or diacritical marks (accents or umlauts in European
    languages).} Some languages such as Vietnamese make extensive use of
    these marks and some characters can have more than one mark at the
    same time to clarify pronunciation.

    \o \bold{Ligatures.} In special contexts, some pairs of characters get
    replaced by a combined glyph forming a ligature. Common examples are
    the fl and fi ligatures used in typesetting US and European books.

    \endlist

    Qt tries to take care of all the special features listed above. You
    usually don't have to worry about these features so long as you use
    Qt's input widgets (e.g. QLineEdit, QTextEdit, and derived classes)
    and Qt's display widgets (e.g. QLabel).

    Support for these writing systems is transparent to the
    programmer and completely encapsulated in \l{rich text
    processing}{Qt's text engine}. This means that you don't need to
    have any knowledge about the writing system used in a particular
    language, except for the following small points:

    \list

    \o QPainter::drawText(int x, int y, const QString &str) will always
    draw the string with its left edge at the position specified with
    the x, y parameters. This will usually give you left aligned strings.
    Arabic and Hebrew application strings are usually right
    aligned, so for these languages use the version of drawText() that
    takes a QRect since this will align in accordance with the language.

    \o When you write your own text input controls, use \l
    QFontMetrics::charWidth() to determine the width of a character in a
    string. In some languages (e.g. Arabic or languages from the Indian
    subcontinent), the width and shape of a glyph changes depending on the
    surrounding characters. Writing input controls usually requires a
    certain knowledge of the scripts it is going to be used in. Usually
    the easiest way is to subclass QLineEdit or QTextEdit.

    \endlist

    The following sections give some information on the status of the
    internationalization (i18n) support in Qt. See also the \l{Qt
    Linguist manual}.

    \section1 Step by Step

    Writing cross-platform international software with Qt is a gentle,
    incremental process. Your software can become internationalized in
    the following stages:

    \section2 Use QString for All User-Visible Text

    Since QString uses the Unicode 4.0 encoding internally, every
    language in the world can be processed transparently using
    familiar text processing operations. Also, since all Qt functions
    that present text to the user take a QString as a parameter,
    there is no \c{char *} to QString conversion overhead.

    Strings that are in "programmer space" (such as QObject names
    and file format texts) need not use QString; the traditional
    \c{char *} or the QByteArray class will suffice.

    You're unlikely to notice that you are using Unicode;
    QString, and QChar are just like easier versions of the crude
    \c{const char *} and char from traditional C.

    \section2 Use tr() for All Literal Text

    Wherever your program uses "quoted text" for text that will
    be presented to the user, ensure that it is processed by the \l
    QCoreApplication::translate() function. Essentially all that is necessary
    to achieve this is to use QObject::tr(). For example, assuming the
    \c LoginWidget is a subclass of QWidget:

    \snippet doc/src/snippets/code/doc_src_i18n.qdoc 0

    This accounts for 99% of the user-visible strings you're likely to
    write.

    If the quoted text is not in a member function of a
    QObject subclass, use either the tr() function of an
    appropriate class, or the QCoreApplication::translate() function
    directly:

    \snippet doc/src/snippets/code/doc_src_i18n.qdoc 1

    If you need to have translatable text completely
    outside a function, there are two macros to help: QT_TR_NOOP()
    and QT_TRANSLATE_NOOP(). They merely mark the text for
    extraction by the \c lupdate utility described below.
    The macros expand to just the text (without the context).

    Example of QT_TR_NOOP():

    \snippet doc/src/snippets/code/doc_src_i18n.qdoc 2

    Example of QT_TRANSLATE_NOOP():

    \snippet doc/src/snippets/code/doc_src_i18n.qdoc 3

    If you disable the \c{const char *} to QString automatic
    conversion by compiling your software with the macro \c
    QT_NO_CAST_FROM_ASCII defined, you'll be very likely to catch any
    strings you are missing. See QString::fromLatin1() for more
    information. Disabling the conversion can make programming a bit
    cumbersome.

    If your source language uses characters outside Latin1, you
    might find QObject::trUtf8() more convenient than
    QObject::tr(), as tr() depends on the
    QTextCodec::codecForTr(), which makes it more fragile than
    QObject::trUtf8().

    \section2 Use QKeySequence() for Accelerator Values

    Accelerator values such as Ctrl+Q or Alt+F need to be translated
    too. If you hardcode Qt::CTRL + Qt::Key_Q for "quit" in your
    application, translators won't be able to override it. The
    correct idiom is

    \snippet examples/mainwindows/application/mainwindow.cpp 20

    \section2 Use QString::arg() for Dynamic Text

    The QString::arg() functions offer a simple means for substituting
    arguments:
    \snippet doc/src/snippets/code/doc_src_i18n.qdoc 4

    In some languages the order of arguments may need to change, and this
    can easily be achieved by changing the order of the % arguments. For
    example:

    \snippet doc/src/snippets/code/doc_src_i18n.qdoc 5

    produces the correct output in English and Norwegian:
    \snippet doc/src/snippets/code/doc_src_i18n.qdoc 6

    \section2 Produce Translations

    Once you are using tr() throughout an application, you can start
    producing translations of the user-visible text in your program.

    The \l{Qt Linguist manual} provides further information about
    Qt's translation tools, \e{Qt Linguist}, \c lupdate and \c
    lrelease.

    Translation of a Qt application is a three-step process:

    \list 1

    \o Run \c lupdate to extract translatable text from the C++
    source code of the Qt application, resulting in a message file
    for translators (a \c .ts file). The utility recognizes the tr()
    construct and the \c{QT_TR*_NOOP()} macros described above and
    produces \c .ts files (usually one per language).

    \o Provide translations for the source texts in the \c .ts file, using
    \e{Qt Linguist}. Since \c .ts files are in XML format, you can also
    edit them by hand.

    \o Run \c lrelease to obtain a light-weight message file (a \c .qm
    file) from the \c .ts file, suitable only for end use. Think of the \c
    .ts files as "source files", and \c .qm files as "object files". The
    translator edits the \c .ts files, but the users of your application
    only need the \c .qm files. Both kinds of files are platform and
    locale independent.

    \endlist

    Typically, you will repeat these steps for every release of your
    application. The \c lupdate utility does its best to reuse the
    translations from previous releases.

    Before you run \c lupdate, you should prepare a project file. Here's
    an example project file (\c .pro file):

    \snippet doc/src/snippets/code/doc_src_i18n.qdoc 7

    When you run \c lupdate or \c lrelease, you must give the name of the
    project file as a command-line argument.

    In this example, four exotic languages are supported: Danish,
    Finnish, Norwegian and Swedish. If you use \l{qmake}, you usually
    don't need an extra project file for \c lupdate; your \c qmake
    project file will work fine once you add the \c TRANSLATIONS
    entry.

    In your application, you must \l QTranslator::load() the translation
    files appropriate for the user's language, and install them using \l
    QCoreApplication::installTranslator().

    \c linguist, \c lupdate and \c lrelease are installed in the \c bin
    subdirectory of the base directory Qt is installed into. Click Help|Manual
    in \e{Qt Linguist} to access the user's manual; it contains a tutorial
    to get you started.

    \target qt-itself
    Qt itself contains over 400 strings that will also need to be
    translated into the languages that you are targeting. You will find
    translation files for French, German and Simplified Chinese in
    \c{$QTDIR/translations}, as well as a template for translating to
    other languages. (This directory also contains some additional
    unsupported translations which may be useful.)

    Typically, your application's \c main() function will look like
    this:

    \snippet doc/src/snippets/code/doc_src_i18n.qdoc 8

    Note the use of QLibraryInfo::location() to locate the Qt translations.
    Developers should request the path to the translations at run-time by
    passing QLibraryInfo::TranslationsPath to this function instead of
    using the \c QTDIR environment variable in their applications.

    \section2 Support for Encodings

    The QTextCodec class and the facilities in QTextStream make it easy to
    support many input and output encodings for your users' data. When an
    application starts, the locale of the machine will determine the 8-bit
    encoding used when dealing with 8-bit data: such as for font
    selection, text display, 8-bit text I/O, and character input.

    The application may occasionally require encodings other than the
    default local 8-bit encoding. For example, an application in a
    Cyrillic KOI8-R locale (the de-facto standard locale in Russia) might
    need to output Cyrillic in the ISO 8859-5 encoding. Code for this
    would be:

    \snippet doc/src/snippets/code/doc_src_i18n.qdoc 9

    For converting Unicode to local 8-bit encodings, a shortcut is
    available: the QString::toLocal8Bit() function returns such 8-bit
    data. Another useful shortcut is QString::toUtf8(), which returns
    text in the 8-bit UTF-8 encoding: this perfectly preserves
    Unicode information while looking like plain ASCII if the text is
    wholly ASCII.

    For converting the other way, there are the QString::fromUtf8() and
    QString::fromLocal8Bit() convenience functions, or the general code,
    demonstrated by this conversion from ISO 8859-5 Cyrillic to Unicode
    conversion:

    \snippet doc/src/snippets/code/doc_src_i18n.qdoc 10

    Ideally Unicode I/O should be used as this maximizes the portability
    of documents between users around the world, but in reality it is
    useful to support all the appropriate encodings that your users will
    need to process existing documents. In general, Unicode (UTF-16 or
    UTF-8) is best for information transferred between arbitrary people,
    while within a language or national group, a local standard is often
    more appropriate. The most important encoding to support is the one
    returned by QTextCodec::codecForLocale(), as this is the one the user
    is most likely to need for communicating with other people and
    applications (this is the codec used by local8Bit()).

    Qt supports most of the more frequently used encodings natively. For a
    complete list of supported encodings see the \l QTextCodec
    documentation.

    In some cases and for less frequently used encodings it may be
    necessary to write your own QTextCodec subclass. Depending on the
    urgency, it may be useful to contact Qt's technical support team or
    ask on the \c qt-interest mailing list to see if someone else is
    already working on supporting the encoding.

    \keyword localization

    \section2 Localize

    Localization is the process of adapting to local conventions, for
    example presenting dates and times using the locally preferred
    formats. Such localizations can be accomplished using appropriate tr()
    strings.

    \snippet doc/src/snippets/code/doc_src_i18n.qdoc 11

    In the example, for the US we would leave the translation of
    "AMPM" as it is and thereby use the 12-hour clock branch; but in
    Europe we would translate it as something else and this will make
    the code use the 24-hour clock branch.

    For localized numbers use the QLocale class.

    Localizing images is not recommended. Choose clear icons that are
    appropriate for all localities, rather than relying on local puns or
    stretched metaphors. The exception is for images of left and right
    pointing arrows which may need to be reversed for Arabic and Hebrew
    locales.

    \section1 Dynamic Translation

    Some applications, such as Qt Linguist, must be able to support changes
    to the user's language settings while they are still running. To make
    widgets aware of changes to the installed QTranslators, reimplement the 
    widget's \l{QWidget::changeEvent()}{changeEvent()} function to check whether 
    the event is a \l{QEvent::LanguageChange}{LanguageChange} event, and update
    the text displayed by widgets using the \l{QObject::tr()}{tr()} function
    in the usual way. For example:

    \snippet doc/src/snippets/code/doc_src_i18n.qdoc 12

    All other change events should be passed on by calling the default
    implementation of the function.

    The list of installed translators might change in reaction to a 
    \l{QEvent::LocaleChange}{LocaleChange} event, or the application might
    provide a user interface that allows the user to change the current
    application language.

    The default event handler for QWidget subclasses responds to the
    QEvent::LanguageChange event, and will call this function when necessary;
    other application components can also force widgets to update themselves
    by posting the \l{QEvent::LanguageChange}{LanguageChange} event to them.

    \section1 Translating Non-Qt Classes

    It is sometimes necessary to provide internationalization support for
    strings used in classes that do not inherit QObject or use the Q_OBJECT
    macro to enable translation features. Since Qt translates strings at
    run-time based on the class they are associated with and \c lupdate
    looks for translatable strings in the source code, non-Qt classes must
    use mechanisms that also provide this information.

    One way to do this is to add translation support to a non-Qt class
    using the Q_DECLARE_TR_FUNCTIONS() macro; for example:

    \snippet doc/src/snippets/i18n-non-qt-class/myclass.h 0
    \dots
    \snippet doc/src/snippets/i18n-non-qt-class/myclass.h 1

    This provides the class with \l{QObject::}{tr()} functions that can
    be used to translate strings associated with the class, and makes it
    possible for \c lupdate to find translatable strings in the source
    code.

    Alternatively, the QCoreApplication::translate() function can be called
    with a specific context, and this will be recognized by \c lupdate and
    Qt Linguist.

    \section1 System Support

    Some of the operating systems and windowing systems that Qt runs on
    only have limited support for Unicode. The level of support available
    in the underlying system has some influence on the support that Qt can
    provide on those platforms, although in general Qt applications need
    not be too concerned with platform-specific limitations.

    \section2 Unix/X11

    \list
    \o  Locale-oriented fonts and input methods. Qt hides these and
        provides Unicode input and output.
    \o  Filesystem conventions such as
        \l{http://www.ietf.org/rfc/rfc2279.txt}{UTF-8}
        are under development in some Unix variants. All Qt file
        functions allow Unicode, but convert filenames to the local
        8-bit encoding, as this is the Unix convention (see
        QFile::setEncodingFunction() to explore alternative
        encodings).
    \o  File I/O defaults to the local 8-bit encoding,
        with Unicode options in QTextStream.
    \o  Many Unix distributions contain only partial support for some locales.
        For example, if you have a \c /usr/share/locale/ja_JP.EUC directory,
        this does not necessarily mean you can display Japanese text; you also
        need JIS encoded fonts (or Unicode fonts), and the
        \c /usr/share/locale/ja_JP.EUC directory needs to be complete. For
        best results, use complete locales from your system vendor.
    \endlist

    \section2 Windows

    \list
    \o  Qt provides full Unicode support, including input methods, fonts,
        clipboard, drag-and-drop and file names.
    \o  File I/O defaults to Latin1, with Unicode options in QTextStream.
        Note that some Windows programs do not understand big-endian
        Unicode text files even though that is the order prescribed by
        the Unicode Standard in the absence of higher-level protocols.
    \o  Unlike programs written with MFC or plain winlib, Qt programs
        are portable between Windows 98 and Windows NT.
        \e {You do not need different binaries to support Unicode.}
    \endlist

    \section2 Mac OS X

    For details on Mac-specific translation, refer to the Qt/Mac Specific Issues
    document \l{Qt for Mac OS X - Specific Issues#Translating the Application Menu and Native Dialogs}{here}.

    \section1 Relevant Qt Classes

    These classes are relevant to internationalizing Qt applications.
*/