source: trunk/doc/src/internationalization/i18n.qdoc@ 968

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/*!
    \group i18n
    \title Qt Classes for Internationalization

    See \l{Internationalization with Qt} for information on how to use these classes
    in your applications.
*/

/*!
    \page internationalization.html
    \title Internationalization with Qt
    \brief Information about Qt's support for internationalization and multiple languages.
    \nextpage Writing Source Code for Translation

    \ingroup qt-basic-concepts
        
    \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

    \section1 Relevant Qt Classes and APIs

    These classes support internationalizing of Qt applications.

    \annotatedlist i18n

    \section1 Languages and Writing Systems

    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 5.1 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
    \o N'Ko
    \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 QTextLayout.
    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, which QTextLayout takes into account.
    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 5.1 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