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/****************************************************************************
**
** Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies).
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****************************************************************************/
/*!
\example tools/echoplugin
\title Echo Plugin Example
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 examples/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 examples/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 examples/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 examples/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 examples/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 examples/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 comma separated list.
\snippet examples/tools/echoplugin/plugin/echoplugin.h 0
\section1 EchoPlugin Class Implementation
Here is the implementation of \c echo():
\snippet examples/tools/echoplugin/plugin/echoplugin.cpp 0
We simply return the functions parameter.
\snippet examples/tools/echoplugin/plugin/echoplugin.cpp 1
We use the Q_EXPORT_PLUGIN2 macro to let Qt know that the \c
EchoPlugin class is a plugin. The first parameter is the name of
the plugin; it is usual to give the plugin and the library file it
is stored in the same name.
\section1 The \c main() function
\snippet examples/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 examples/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 examples/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
You can find an overview of the macros needed to create plugins
\l{Macros for Defining Plugins}{here}.
We give an example of a plugin that extend 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.
*/
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