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 = 0)
          : 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.

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