Archived:Using QStateMachine and QState

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Qt Quick should be used for all UI development on mobile devices. The approach described in this article (using C++ for the Qt app UI) is deprecated.

Article Metadata

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Tested with

Devices(s): All (S60)

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Compatibility

Platform(s): S60 3rd Edition, FP1
S60 3rd Edition, FP2
S60 5th Edition

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Article

Keywords: QtStateMachine, QtState

Created: User:Kbwiki (10 May 2010)

Last edited: hamishwillee (11 Oct 2012)

Description

The Qt state machine framework provides classes for creating and executing state graphs in a hierarchical finite state machine. The state machine is represented by QStateMachine and the individual states are represented by QState. The corresponding transitions (and signals on which transitions occur) are set up with the QState::addTransition function. Once all transitions are set, the state machine can be started using QStateMachine::start(). The QState::assignProperty() function can be used to define assignment for a property of a QObject, performed when the state is entered.

In the following example, whenever a state transition occurs, the text property of a QLabel is changed to provide information about the new state.

Solution

MainWindow.h

 class MainWindow : public QMainWindow
 {
     Q_OBJECT
 public:
     MainWindow( QWidget *parent = 0 );
     ~MainWindow();
 
 private:
      QStateMachine machine;
 };

MainWindow.cpp

 MainWindow::MainWindow( QWidget *parent )
     : QMainWindow( parent )
 {
 
 QWidget* centralWidget = new QWidget(this);
 
 // Create a layout
 QVBoxLayout *layout = new QVBoxLayout( centralWidget );
 
 // Create push buttons to control the state machine
 QPushButton* produceButton = new QPushButton( tr("Produce"), centralWidget );   
 QPushButton* consumeButton = new QPushButton( tr("Consume"), centralWidget );
 
 // Create a label which will display text when state change occurs
 QLabel* myLabel = new QLabel( tr("Unknown state"), centralWidget );
 
  // Add all the widgets to layout
 layout->addWidget( produceButton );
 layout->addWidget( consumeButton );
 layout->addWidget( myLabel );
 
 centralWidget->setLayout( layout );
 setCentralWidget( centralWidget );
 
 // Create states and add them to machine
 QState *produce = new QState();
 QState *consume = new QState();
 machine.addState( produce );
 machine.addState( consume );
 
 // Add state transitions and assign property to control the to the text label
 produce->addTransition( produceButton, SIGNAL(clicked()), consume );
 consume->addTransition( consumeButton, SIGNAL(clicked()), produce );
 produce->assignProperty(myLabel, "text", tr("Produce Item"));
 consume->assignProperty(myLabel, "text", tr("Consume Item"));
 
 // Start the machine
 machine.setInitialState( produce );
 machine.start();
 }