Qt SCXML Pinball Example#
Encapsulates the internal logic of an application in an SCXML file.
Pinball demonstrates a clear separation between the user interface, which may be easily replaced, and the internal logic encapsulated in an SCXML file, which could also be used with another user interface.
Running the Example#
To run the example from Qt Creator, open the Welcome mode and select the example from Examples. For more information, visit Building and Running an Example.
The Pinball example mimics a pinball game. The targets on the pinball table are substituted by GUI controls, mainly by push buttons. Display elements, including current score, highscore, and targets’ lights, are substituted by labels. Usually, the state of the targets’ lights changes very often during a game: the lights get turned on or off permanently or they blink at varying speed indicating a game (or a certain target) entered a temporary state. The state of each target light is presented as an enabled or a disabled label. There is no real ball, but clicking a target’s button represents hitting a real pinball target with a ball.
Our pinball contains the following features:
When not in
hurryState, the letters already hit should blink at intermediate speed (500ms). Letters not hit yet should stay off.
hurryState, the letters already hit should stay on. Letters not hit yet should blink fast (200ms). In addition, the HURRY light should blink at the same speed.
When the jackpot gets collected, the JACKPOT light should stay on.
SCXML Part: Internal Logic Description#
The pinball.scxml file describes the internal logic implemented for the pinball game. In this example, we have chosen the ECMAScript data model:
The ECMAScript data model enables declaring variables with initial values that can be modified later. We declare the
"score" variables with the initial values of 0:
We define a root parallel state
"global", with two child states,
internalState, which are also parallel. Because the top
global state is parallel, all of its direct children are active when it is active. In this example, the role of
global is to collect the child states and make them both active at a time.
Maintaining Light State#
guiControl element is responsible for maintaining the current state of each light control that is visible on the pinball table. Each light has a corresponding state.
For example, the light of the letter C corresponds to the
cLight state. Each light state has two child states indicating whether the light is on or off:
As mentioned before, the
guiControl state is always active, and since it is of parallel type, all its direct children are always active too. Therefore, the
cLight state is always active. However, only one of its children,
cLightOff, is active at a time. The same applies to the other children of the
guiControl state. In addition, we define transitions between on and off substates. For example, whenever the active state is
cLightOn and a
turnOffC event is received, we change the active substate of
cLightOff. Whenever the active state is
cLightOff and we receive a
turnOnC event, we change the active substate of
In our application, we use instances of
QLabel class in C++ to represent real lights on the table. When the light transitions into the on or off state, we enable or disable the particular label accordingly. The connection between the state machine and the GUI part of the application will be shown in the C++ code later on. For now, it is enough to realize that changes to active states inside the state machine will serve as the external interface of the state machine that the other parts of the application (such as the GUI part) can listen to.
All of the mentioned events that switch the state of a light will be generated by this state machine inside the
internalState in reaction to running timers or external triggers.
Maintaining Game State#
internalState state consists of two main parts:
logicalState state holds the definitions for the modes that the game is able to go into and for the logical states of collected targets. The
workflow state implements a generator for light blinking and calculates most of the new states the machine should go into depending on incoming events and on currently active states. As mentioned already,
internalState is always active, and since it is of a parallel type,
workflow are always active too.
Maintaining Game Modes#
modeState state consists of two substates,
offState state describes what should happen before the pinball game is started and when it is over, while
onState represents the logic appropriate for the active game.
When the pinball application starts or a game ends, the machine goes into
offState. Entering that state invokes some actions, which are enclosed inside an
<onentry> element. First, we update the
highScore variable in case the current
highScore value is less than current
score value. This is being checked inside the
"cond" attribute of the
<if> element (note that we need to escape the “<” character with “<”). Even in the
off state, we want to show the last reached score, so we do not clear it here; we will do that when we enter the
on state. Next, we raise two events:
resetLetters to logically reset all letters that might have been hit during the last game and
update to immediately activate the blinking and updating of all lights. When the machine is in
offState, it is ready to transition into the
onState if only the
startTriggered event occurs, which is described by the <transition> element. This event is expected to be generated externally after clicking the START button on the pinball table.
When the state machine enters
onState, it first clears the current score variable. The
onState state is of the parallel type and has two direct child states:
jackpotState. They are active as long as their parent,
onState, is active. Both
jackpotState contain two substates that reflect their off and on states. Only one substate of
hurryState and one substate of
jackpotState can be active at a time. Initially, the off substates are active.
Whenever we enter
hurryStateOn, we generate the same two events we generate when entering the
update. In addition, when we enter the
hurryStateOn state, we send a delayed event,
goToHurryOff, with a delay of five seconds, marked with
hurryId. This means that after five seconds we just switch the state back to
hurryStateOff without granting the bonus points. In this way, we implement the five-second hurry feature of the pinball table. We also define transitions from
hurryStateOn when the
goToHurryOn event occurs and from
hurryStateOff when the
goToHurryOff event occurs. When we exit the
hurryStateOn state, we cancel the possibly pending delayed event that was marked with
hurryId. This is important in case the five seconds have not elapsed yet, but players have collected all the five letters in the hurry state. We then collect the jackpot and want the pending timer to finish.
The substates of
jackpotState generate the request to update the state of lights. The
jackpotStateOff state defines the transition to
jackpotStateOn when the
goForJackpot event occurs. The opposite transition is not needed, because when the jackpot gets collected, the corresponding light remains lit until the end of game. When a new game starts, the
jackpotState is entered again which causes its initial active substate to be
In addition, the
onState state defines one transition in reaction to the
ballOutTriggered event which instructs the machine to go into the
ballOutTriggered event is expected to be an event posted into the state machine from outside of the state machine. This event should be generated when the ball gets out of playing area of the table. In our example we mimic it by the clicking BALL OUT button. Posting the event from outside of state machine will be shown in the C++ code later on.
GUI Part: User Interface Description#
The GUI part of the application consists of a mainwindow.ui file which describes the static user interface of the game.
C++ Part: Glue GUI with SCXML#
The C++ part of the application consists of a
MainWindow class which glues the GUI part with the SCXML part. The class is declared in mainwindow.h.
MainWindow class holds the pointer to the
QScxmlStateMachine *m_machine which is the state machine class automatically generated by Qt out of SCMXL file and the pointer to the
Ui::MainWindow *m_ui which describes the GUI part. It also declares two helper methods.
The constructor of the
MainWindow class instantiates the GUI part of the application and stores the pointer to the passed
QScxmlStateMachine. It also initializes the GUI part and glues the GUI part to the state machine by connecting their communication interfaces together.
initAndConnect() method connects the state with the corresponding GUI widget by binding its activity with the enabling of the widget, so that whenever the state is active, its corresponding widget is enabled and whenever the state is inactive, the widget is disabled. We do that for all lights, targets, and description labels.
We also intercept the
updateScore event sent by the state machine, in order to update the score displays with the values passed with the event.
The info about hitting any GUI target needs to be passed to the state machine and we do that by connecting all target buttons’
clicked signals to the lambda expressions which submit the corresponding event into the state machine.
main() function in the main.cpp file, we instantiate the
app application object,
Pinball state machine, and
MainWindow GUI class. We initialize and start the state machine, show the main window, and execute the application.