Qt 5.11 | Qt Widgets | Event Transitions Example

Qt 5.11.3 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 = 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`.

Files:

- statemachine/eventtransitions/eventtransitions.pro
- statemachine/eventtransitions/main.cpp