- Qt 5.15
- Qt Widgets
- Event Transitions Example
- Qt 5.15.18 Reference Documentation

# Event Transitions Example

The Event Transitions example shows how to use event transitions, a feature of The State Machine Framework.

The Event Transitions Example illustrates how states change when a user enters or leaves the area of a button. The states are handled by a QStateMachine object. The screen consists of a QVBoxLayout with a central button.

When the mouse is outside the button, the text in the button displays "Outside". When the mouse enters the button, it displays "Inside".

     class Window : public QWidget
     {
     public:
         Window(QWidget *parent = nullptr)
             : QWidget(parent)
         {
             QPushButton *button = new QPushButton(this);
             button->setSizePolicy(QSizePolicy::Expanding, QSizePolicy::Expanding);

             QVBoxLayout *layout = new QVBoxLayout;
             layout->addWidget(button);
             layout->setContentsMargins(80, 80, 80, 80);
             setLayout(layout);

The `Window` class's constructors begins by creating a button. This button is added to `layout`, which is a QVBoxLayout object. Then two states are created: `s1` is the state "Outside", and `s2` is the state "Inside".

             QStateMachine *machine = new QStateMachine(this);

             QState *s1 = new QState();
             s1->assignProperty(button, "text", "Outside");

             QState *s2 = new QState();
             s2->assignProperty(button, "text", "Inside");

State `s1` is the state "Outside" and state `s2` is state "Inside".

             QEventTransition *enterTransition = new QEventTransition(button, QEvent::Enter);
             enterTransition->setTargetState(s2);
             s1->addTransition(enterTransition);

When the button receives an event of type QEvent::Enter and the state machine is in state `s1`, the machine will transition to state `s2`.

             QEventTransition *leaveTransition = new QEventTransition(button, QEvent::Leave);
             leaveTransition->setTargetState(s1);
             s2->addTransition(leaveTransition);

When the button receives an event of type QEvent::Leave and the state machine is in state `s2`, the machine will transition back to state `s1`.

             QState *s3 = new QState();
             s3->assignProperty(button, "text", "Pressing...");

             QEventTransition *pressTransition = new QEventTransition(button, QEvent::MouseButtonPress);
             pressTransition->setTargetState(s3);
             s2->addTransition(pressTransition);

             QEventTransition *releaseTransition = new QEventTransition(button, QEvent::MouseButtonRelease);
             releaseTransition->setTargetState(s2);
             s3->addTransition(releaseTransition);

Next, state `s3` is created. `s3` will be entered when the button receives an event of type QEvent::MouseButtonPress and the state machine is in state `s2`. When the button receives an event of type QEvent::MouseButtonRelease and the state machine is in state `s3`, the machine will revert to state `s2`.

             machine->addState(s1);
             machine->addState(s2);
             machine->addState(s3);

             machine->setInitialState(s1);
             machine->start();
         }
     };

Finally, the states are added to the machine as top-level states, the initial state is set to be `s1` ("Outside"), and the machine is started.

     int main(int argc, char **argv)
     {
         QApplication app(argc, argv);
         Window window;
         window.resize(300, 300);
         window.show();

         return app.exec();
     }

The main() function constructs a Window object that displays the QVBoxLayout object `layout` with its `button`.

Example project @ code.qt.io