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diff --git a/doc/src/examples/echoplugin.qdoc b/doc/src/examples/echoplugin.qdoc new file mode 100644 index 0000000000..7c56b21ecf --- /dev/null +++ b/doc/src/examples/echoplugin.qdoc @@ -0,0 +1,208 @@ +/**************************************************************************** +** +** Copyright (C) 2011 Nokia Corporation and/or its subsidiary(-ies). +** All rights reserved. +** Contact: Nokia Corporation (qt-info@nokia.com) +** +** This file is part of the documentation of the Qt Toolkit. +** +** $QT_BEGIN_LICENSE:FDL$ +** No Commercial Usage +** This file contains pre-release code and may not be distributed. +** You may use this file in accordance with the terms and conditions +** contained in the Technology Preview License Agreement accompanying +** this package. +** +** GNU Free Documentation License +** Alternatively, this file may be used under the terms of the GNU Free +** Documentation License version 1.3 as published by the Free Software +** Foundation and appearing in the file included in the packaging of this +** file. +** +** If you have questions regarding the use of this file, please contact +** Nokia at qt-info@nokia.com. +** $QT_END_LICENSE$ +** +****************************************************************************/ + +/*! + \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. +*/ |