source: trunk/doc/src/files-and-resources/resources.qdoc@ 860

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/*!
    \group io
    \title Input/Output and Networking
    \ingroup groups

    \brief Classes providing file input and output along with directory and
    network handling.

    These classes are used to handle input and output to and from external
    devices, processes, files etc. as well as manipulating files and directories.
*/

/*!
    \page resources.html
    \title The Qt Resource System
    \ingroup qt-network
    \brief A platform-independent mechanism for storing binary files in an application.

    \keyword resource system

    The Qt resource system is a platform-independent mechanism for
    storing binary files in the application's executable. This is
    useful if your application always needs a certain set of files
    (icons, translation files, etc.) and you don't want to run the
    risk of losing the files.

    The resource system is based on tight cooperation between \l qmake,
    \l rcc (Qt's resource compiler), and QFile. It obsoletes Qt 3's
    \c qembed tool and the
    \l{http://qt.nokia.com/doc/qq/qq05-iconography.html#imagestorage}{image
    collection} mechanism.

    \section1 Resource Collection Files (\c{.qrc})

    The resources associated with an application are specified in a
    \c .qrc file, an XML-based file format that lists files on the
    disk and optionally assigns them a resource name that the
    application must use to access the resource.

    Here's an example \c .qrc file:

    \quotefile mainwindows/application/application.qrc

    The resource files listed in the \c .qrc file are files that are
    part of the application's source tree. The specified paths are
    relative to the directory containing the \c .qrc file. Note that
    the listed resource files must be located in the same directory as
    the \c .qrc file, or one of its subdirectories.

    Resource data can either be compiled into the binary and thus accessed
    immediately in application code, or a binary resource can be created
    and at a later point in application code registered with the resource
    system.

    By default, resources are accessible in the application under the
    same file name as they have in the source tree, with a \c :/ prefix,
    or by a \link QUrl URL\endlink with a \c qrc scheme.

    For example, the file path \c :/images/cut.png or the URL
    \c qrc:///images/cut.png would give access to the
    \c cut.png file, whose location in the application's source tree
    is \c images/cut.png. This can be changed using the \c file tag's
    \c alias attribute:

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

    The file is then accessible as \c :/cut-img.png from the
    application. It is also possible to specify a path prefix for all
    files in the \c .qrc file using the \c qresource tag's \c prefix
    attribute:

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

    In this case, the file is accessible as \c
    :/myresources/cut-img.png.

    Some resources, such as translation files and icons, many need to
    change based on the user's locale. This is done by adding a \c lang
    attribute to the \c qresource tag, specifying a suitable locale
    string. For example:

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

    If the user's locale is French (i.e., QLocale::system().name() returns
    "fr_FR"), \c :/cut.jpg becomes a reference to the \c cut_fr.jpg
    image. For other locales, \c cut.jpg is used.

    See the QLocale documentation for a description of the format to use
    for locale strings.


    \section2 External Binary Resources

    For an external binary resource to be created you must create the resource
    data (commonly given the \c .rcc extension) by passing the -binary switch to
    \l rcc. Once the binary resource is created you can register the resource
    with the QResource API.

    For example, a set of resource data specified in a \c .qrc file can be
    compiled in the following way:

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

    In the application, this resource would be registered with code like this:

    \snippet doc/src/snippets/code/doc_src_resources.qdoc 4

    \section2 Compiled-In Resources

    For a resource to be compiled into the binary the \c .qrc file must be
    mentioned in the application's \c .pro file so that \c qmake knows
    about it. For example:

    \snippet examples/mainwindows/application/application.pro 0

    \c qmake will produce make rules to generate a file called \c
    qrc_application.cpp that is linked into the application. This
    file contains all the data for the images and other resources as
    static C++ arrays of compressed binary data. The \c
    qrc_application.cpp file is automatically regenerated whenever
    the \c .qrc file changes or one of the files that it refers to
    changes. If you don't use \c .pro files, you can either invoke
    \c rcc manually or add build rules to your build system.

    \image resources.png Building resources into an application

    Currently, Qt always stores the data directly in the executable,
    even on Windows and Mac OS X, where the operating system provides
    native support for resources. This might change in a future Qt
    release.

    \section1 Compression

    Resources are compressed by default (in the \c ZIP format). It is
    possible to turn off compression. This can be useful if your
    resources already contain a compressed format, such as \c .png
    files. You do this by giving the \c {-no-compress} command line
    argument.

    \code
        rcc -no-compress myresources.qrc 
    \endcode

    \c rcc also gives you some control over the compression. You can
    specify the compression level and the threshold level to consider
    while compressing files, for example:

    \code
        rcc -compress 2 -threshold 3 myresources.qrc
    \endcode

    \section1 Using Resources in the Application

    In the application, resource paths can be used in most places
    instead of ordinary file system paths. In particular, you can
    pass a resource path instead of a file name to the QIcon, QImage,
    or QPixmap constructor:

    \snippet examples/mainwindows/application/mainwindow.cpp 21

    See the \l{mainwindows/application}{Application} example for an
    actual application that uses Qt's resource system to store its
    icons.

    In memory, resources are represented by a tree of resource
    objects. The tree is automatically built at startup and used by
    QFile for resolving paths to resources. You can use a QDir initialized
    with ":/" to navigate through the resource tree from the root.

    Qt's resources support the concept of a search path list. If you then
    refer to a resource with \c : instead of \c :/ as the prefix, the
    resource will be looked up using the search path list. The search
    path list is empty at startup; call QDir::addSearchPath() to
    add paths to it.

    If you have resources in a static library, you might need to
    force initialization of your resources by calling \l
    Q_INIT_RESOURCE() with the base name of the \c .qrc file. For
    example:

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

    Similarly, if you must unload a set of resources explicitly
    (because a plugin is being unloaded or the resources are not valid
    any longer), you can force removal of your resources by calling
    Q_CLEANUP_RESOURCE() with the same base name as above.
*/