Echo Plugin Example

EchoWindow Class Implementation

We start with a look at the constructor:

 EchoWindow::EchoWindow()
 {
     createGUI();
     setLayout(layout);
     setWindowTitle("Echo Plugin Example");

     if (!loadPlugin()) {
         QMessageBox::information(this, "Error", "Could not load the plugin");
         lineEdit->setEnabled(false);
         button->setEnabled(false);
     }
 }

We create the widgets and set a title for the window. We then load the plugin. 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 errorString(); we will look more closely at QPluginLoader later.

Here is the implementation of sendEcho():

 void EchoWindow::sendEcho()
 {
     QString text = echoInterface->echo(lineEdit->text());
     label->setText(text);
 }

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

Here is the implementation of createGUI():

 void EchoWindow::createGUI()
 {
     lineEdit = new QLineEdit;
     label = new QLabel;
     label->setFrameStyle(QFrame::Box | QFrame::Plain);
     button = new QPushButton(tr("Send Message"));

     connect(lineEdit, SIGNAL(editingFinished()),
             this, SLOT(sendEcho()));
     connect(button, SIGNAL(clicked()),
             this, SLOT(sendEcho()));

     layout = new QGridLayout;
     layout->addWidget(new QLabel(tr("Message:")), 0, 0);
     layout->addWidget(lineEdit, 0, 1);
     layout->addWidget(new QLabel(tr("Answer:")), 1, 0);
     layout->addWidget(label, 1, 1);
     layout->addWidget(button, 2, 1, Qt::AlignRight);
     layout->setSizeConstraint(QLayout::SetFixedSize);
 }

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

Here is the loadPlugin() function:

 bool EchoWindow::loadPlugin()
 {
     QDir pluginDirectory(qApp->applicationDirPath() + "/plugin/");
     QString fileName = pluginDirectory.entryList(
         QStringList() << "libechoplugin*").first();

     QPluginLoader pluginLoader(pluginDirectory.absoluteFilePath(fileName));
     QObject *plugin = pluginLoader.instance();
     if (plugin) {
         echoInterface = qobject_cast<EchoInterface *>(plugin);
     if(echoInterface)
         return true;
     }
     return false;
 }

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 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 EchoInterface and return true. In the case that the plugin loaded does not implement the EchoInterface, instance() will return null, but this cannot happen in our example.

 class EchoInterface
 {
 public:
     virtual ~EchoInterface() {}
     virtual QString echo(const QString &message) = 0;
 };

 Q_DECLARE_INTERFACE(EchoInterface,
                     "com.trolltech.Plugin.EchoInterface/1.0");

 class EchoPlugin : public QObject, EchoInterface
 {
     Q_OBJECT
     Q_INTERFACES(EchoInterface)

 public:
     QString echo(const QString &message);
 };

 QString EchoPlugin::echo(const QString &message)
 {
     return message;
 }

 Q_EXPORT_PLUGIN2(echoplugin, EchoPlugin);

 int main(int argv, char *args[])
 {
     QApplication app(argv, args);

     EchoWindow window;
     window.show();

     return app.exec();
 }

 TEMPLATE    = subdirs
 SUBDIRS     = echowindow \
               plugin

 TEMPLATE        = lib
 CONFIG         += plugin
 INCLUDEPATH    += ../echowindow
 HEADERS         = echoplugin.h
 SOURCES         = echoplugin.cpp
 TARGET          = echoplugin