qt5base-lts/examples/widgets/doc/src/echoplugin.qdoc

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/*!
    \example tools/echoplugin
    \title Echo Plugin Example
    \ingroup examples-widgets-tools

    \brief This example shows how to create a Qt plugin.

    \image echopluginexample.png

    There are two kinds of plugins in Qt: plugins that extend Qt
    itself and plugins that extend applications written in Qt. In this
    example, we show the procedure of implementing plugins that extend
    applications. When you create a plugin you declare an interface,
    which is a class with only pure virtual functions. This interface
    is inherited by the class that implements the plugin. The class is
    stored in a shared library and can therefore be loaded by
    applications at run-time. When loaded, the plugin is dynamically
    cast to the interface using Qt's \l{Meta-Object
    System}{meta-object system}. The plugin \l{How to Create Qt
    Plugins}{overview document} gives a high-level introduction to
    plugins.

    We have implemented a plugin, the \c EchoPlugin, which implements
    the \c EchoInterface. The interface consists of \c echo(), which
    takes a QString as argument. The \c EchoPlugin returns the string
    unaltered (i.e., it works as the familiar echo command found in
    both Unix and Windows).

    We test the plugin in \c EchoWindow: when you push the QPushButton
    (as seen in the image above), the application sends the text in
    the QLineEdit to the plugin, which echoes it back to the
    application. The answer from the plugin is displayed in the
    QLabel.


    \section1 EchoWindow Class Definition

    The \c EchoWindow class lets us test the \c EchoPlugin through a
    GUI.

    \snippet tools/echoplugin/echowindow/echowindow.h 0

    We load the plugin in \c loadPlugin() and cast it to \c
    EchoInterface. When the user clicks the \c button we take the
    text in \c lineEdit and call the interface's \c echo() with it.


    \section1 EchoWindow Class Implementation

    We start with a look at the constructor:

    \snippet tools/echoplugin/echowindow/echowindow.cpp 0

    We create the widgets and set a title for the window. We then load
    the plugin. \c loadPlugin() returns false if the plugin could not
    be loaded, in which case we disable the widgets. If you wish a
    more detailed error message, you can use
    \l{QPluginLoader::}{errorString()}; we will look more closely at
    QPluginLoader later.

    Here is the implementation of \c sendEcho():

    \snippet tools/echoplugin/echowindow/echowindow.cpp 1

    This slot is called when the user pushes \c button or presses
    enter in \c lineEdit. We call \c echo() of the echo interface. In
    our example this is the \c EchoPlugin, but it could be any plugin
    that inherit the \c EchoInterface. We take the QString returned
    from \c echo() and display it in the \c label.

    Here is the implementation of \c createGUI():

    \snippet tools/echoplugin/echowindow/echowindow.cpp 2

    We create the widgets and lay them out in a grid layout. We
    connect the label and line edit to our \c sendEcho() slot.

    Here is the \c loadPlugin() function:

    \snippet tools/echoplugin/echowindow/echowindow.cpp 3

    Access to plugins at run-time is provided by QPluginLoader. You
    supply it with the filename of the shared library the plugin is
    stored in and call \l{QPluginLoader::}{instance()}, which loads
    and returns the root component of the plugin (i.e., it resolves
    the type of the plugin and creates a QObject instance of it). If
    the plugin was not successfully loaded, it will be null, so we
    return false. If it was loaded correctly, we can cast the plugin
    to our \c EchoInterface and return true. In the case that the
    plugin loaded does not implement the \c EchoInterface, \c
    instance() will return null, but this cannot happen in our
    example. Notice that the location of the plugin is not the same
    for all platforms.


    \section1 EchoInterface Class Definition

    The \c EchoInterface defines the functions that the plugin will
    provide. An interface is a class that only consists of pure
    virtual functions. If non virtual functions were present in the
    class you would get misleading compile errors in the moc files.

    \snippet tools/echoplugin/echowindow/echointerface.h 0

    We declare \c echo(). In our \c EchoPlugin we use this method to
    return, or echo, \a message.

    We use the Q_DECLARE_INTERFACE macro to let \l{Meta-Object
    System}{Qt's meta object system} aware of the interface. We do
    this so that it will be possible to identify plugins that
    implements the interface at run-time. The second argument is a
    string that must identify the interface in a unique way.


    \section1 EchoPlugin Class Definition

    We inherit both QObject and \c EchoInterface to make this class a
    plugin. The Q_INTERFACES macro tells Qt which interfaces the class
    implements. In our case we only implement the \c EchoInterface.
    If a class implements more than one interface, they are given as
    a space separated list. The Q_PLUGIN_METADATA macro is included next
    to the Q_OBJECT macro. It contains the plugins IID and a filename
    pointing to a json file containing the metadata for the plugin.
    The json file is compiled into the plugin and does not need to be installed.

    \snippet tools/echoplugin/plugin/echoplugin.h 0

    \section1 EchoPlugin Class Implementation

    Here is the implementation of \c echo():

    \snippet tools/echoplugin/plugin/echoplugin.cpp 0

    We simply return the functions parameter.

    \section1 The \c main() function

    \snippet tools/echoplugin/echowindow/main.cpp 0

    We create an \c EchoWindow and display it as a top-level window.

    \section1 The Profiles

    When creating plugins the profiles need to be adjusted.
    We show here what changes need to be done.

    The profile in the echoplugin directory uses the \c subdirs
    template and simply includes includes to directories in which
    the echo window and echo plugin lives:

    \snippet tools/echoplugin/echoplugin.pro 0

    The profile for the echo window does not need any plugin specific
    settings. We move on to the plugin profile:

    \snippet tools/echoplugin/plugin/plugin.pro 0

    We need to set the TEMPLATE as we now want to make a library
    instead of an executable. We also need to tell qmake that we are
    creating a plugin. The \c EchoInterface that the plugin implements
    lives in the \c echowindow directory, so we need to add that
    directory to the include path. We set the TARGET of the project,
    which is the name of the library file in which the plugin will be
    stored; qmake appends the appropriate file extension depending on
    the platform. By convention the target should have the same name
    as the plugin (set with Q_EXPORT_PLUGIN2)

    \section1 Further Reading and Examples

    The \l {qtplugin-defining-plugins}{Defining Plugins} page presents an overview of the macros needed to
    create plugins.

    We give an example of a plugin that extends Qt in the \l{Style
    Plugin Example}{style plugin} example. The \l{Plug & Paint
    Example}{plug and paint} example shows how to create static
    plugins.
*/