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.
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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:
offState
. In that state, all lights on the table blink slowly (at intervals of 1 second).
onState
. All lights are turned off and the pinball table is ready to be played.
onState
and the players click the
BALL OUT
button, the game ends and enters
offState
. If the players' score is higher than the current highscore, the highscore is updated.
hurryState
and must collect them again within 5 seconds. If the time has passed and the letters were not collected again, the players must start collecting the letters from scratch.
hurryState
.
hurryState
.
hurryState
.
hurryState
(
JACKPOT
).
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.
The pinball.scxml file describes the internal logic implemented for the pinball game. In this example, we have chosen the ECMAScript data model:
<scxml xmlns="http://www.w3.org/2005/07/scxml" version="1.0"
name="Pinball" datamodel="ecmascript">
The ECMAScript data model enables declaring variables with initial values that can be modified later. We declare the
"highscore"
and
"score"
variables with the initial values of 0:
<datamodel>
<data id="highScore" expr="0"/>
<data id="score" expr="0"/>
</datamodel>
We define a root parallel state
"global"
, with two child states,
guiControl
and
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.
The
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:
<parallel id="global">
<parallel id="guiControl">
<state id="cLight">
<state id="cLightOn">
<transition event="turnOffC" target="cLightOff"/>
</state>
<state id="cLightOff">
<transition event="turnOnC" target="cLightOn"/>
</state>
</state>
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,
cLightOn
or
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
和
turnOffC
event is received, we change the active substate of
cLight
to
cLightOff
. Whenever the active state is
cLightOff
and we receive a
turnOnC
event, we change the active substate of
cLight
to
cLightOn
.
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++ 代码 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.
The
internalState
state consists of two main parts:
logicalState
and
workflow
.
The
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,
logicalState
and
workflow
are always active too.
The
logicalState
state consist of two parts:
letterState
and
modeState
.
As previously mentioned,
logicalState
is always active, and since it is of parallel type, the
letterState
and
modeState
children are always active too. Now let us look at the first part, the
letterState
, which contains one parallel
lettersState
:
<state id="letterState">
<parallel id="lettersState">
<state id="letter.C">
<state id="cLetterOff">
<transition event="cLetterTriggered" cond="In('onState')" target="cLetterOn"/>
</state>
<final id="cLetterOn"/>
</state>
<state id="letter.R">
...
</state>
<state id="letter.A">
...
</state>
<state id="letter.Z">
...
</state>
<state id="letter.Y">
...
</state>
<transition event="resetLetters" target="lettersState"/>
</parallel>
</state>
The
lettersState
state maintains the logical state of the buttons pretending to be targets that were clicked by the players. The letter state for the letter
C
holds whether the target for the letter
C
was hit, while the light state for the letter
C
holds whether the light for the target for the letter
C
should be currently on or off. In a real pinball game, these states are usually orthogonal, which means that if you have not hit a target yet, the target is blinking, indicating that it is currently worth hitting. This blinking means that the light state switches between on and off at short intervals, while the target state is continouosly off, because it has not been hit yet. The author of a pinball table can decide that after a target is hit (that is, after the target state switches to on) the target's light is continuously turned off or on or the intervals between lights blinking become shorter or longer.
As mentioned before,
letterState
is always active, which means that its only child
lettersState
should always be active too. However, there is one exception: for a short while the
lettersState
may end up being
not active
. This happens when the transition for
lettersState
is being performed. This transition is triggered when the
resetLetters
event occurs, and it instructs the state machine to exit
lettersState
and all its descendant states and reenter
lettersState
and set up all its descendant states with their initial states. In short, the
resetLetters
event resets the
lettersState
and all its descendant states to the default configuration.
The
lettersState
contains five direct substates that correspond to five different letters. The content for other letters' states than C is not shown here, but it is analogous to the content for C's state.
The
letter.C
state contains two substates reflecting its off and on states:
cLetterOff
and
cLetterOn
。
letter.C
state inside its parallel parent
lettersState
is always active (under the condition that
lettersState
is active, as described before). However, only one of its child states is active at a time:
cLetterOff
or
cLetterOn
. The initial substate of the
letter.C
state is
cLetterOff
meaning that whenever the
letter.C
state is being activated (which happens initially and after the
resetLetters
event) its active substate will be set to
cLetterOff
.
The
cLetterOff
state defines a transition, which will be triggered by the
cLetterTriggered
event. This transition activates
cLetterOn
, the other child of
letter.C
, only when the machine is in
onState
(that is, when the pinball game is running). The
cLetterTriggered
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 hits the letter
C
target. In our example we mimic it by the clicking the letter
C
button.
The
cLetterOn
state is defined as a final state, which means that whenever this state is activated the
done.state.letter.C
event will be automatically posted by the state machine. This event will be used later for updating the current score.
Moreover, when all
lettersState
children reach their final state, the state machine will automatically post the
done.state.lettersState
event. This event will be used later, too, for updating the current score and for turning on or off the hurry state.
The
modeState
state consists of two substates,
offState
and
onState
.
The
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.
<state id="offState">
<onentry>
<if cond="highScore < score">
<assign location="highScore" expr="score"/>
</if>
<raise event="resetLetters"/>
<raise event="update"/>
</onentry>
<transition event="startTriggered" target="onState"/>
</state>
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"
属性在
<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.
<parallel id="onState">
<onentry>
<assign location="score" expr="0"/>
</onentry>
<state id="hurryState">
<state id="hurryStateOff">
<onentry>
<raise event="resetLetters"/>
<raise event="update"/>
</onentry>
<transition event="goToHurryOn" target="hurryStateOn"/>
</state>
<state id="hurryStateOn">
<onentry>
<send event="goToHurryOff" id="hurryId" delay="5s"/>
<raise event="resetLetters"/>
<raise event="update"/>
</onentry>
<transition event="goToHurryOff" target="hurryStateOff"/>
<onexit>
<cancel sendid="hurryId"/>
</onexit>
</state>
</state>
<state id="jackpotState">
<state id="jackpotStateOff">
<onentry>
<raise event="update"/>
</onentry>
<transition event="goForJackpot" target="jackpotStateOn"/>
</state>
<state id="jackpotStateOn">
<onentry>
<raise event="update"/>
</onentry>
</state>
</state>
<transition event="ballOutTriggered" target="offState"/>
</parallel>
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:
hurryState
and
jackpotState
. They are active as long as their parent,
onState
, is active. Both
hurryState
and
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
hurryStateOff
or
hurryStateOn
, we generate the same two events we generate when entering the
onState
state:
resetLetters
and
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
hurryStateOff
to
hurryStateOn
when the
goToHurryOn
event occurs and from
hurryStateOn
to
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 secons 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
jackpotStateOff
.
此外，
onState
state defines one transition in reaction to the
ballOutTriggered
event which instructs the machine to go into the
offState
。
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++ 代码
later on.
The
workflow
state is responsible for generating the blinking lights. The generator is defined in its
lightImpulseGenerator
substate. In addition, it is responsible for reacting to events that have been posted so far from the other parts of the state machine.
<state id="workflow">
<state id="lightImpulseGenerator">
<state id="lightImpulseOn"/>
<state id="lightImpulseOff"/>
<onentry>
<raise event="update"/>
</onentry>
<transition event="scheduleNewImpulse">
<cancel sendid="lightId"/>
<if cond="In('offState')">
<send event="lightImpulse" id="lightId" delay="1s"/>
<elseif cond="In('hurryStateOff')"/>
<send event="lightImpulse" id="lightId" delay="500ms"/>
<else/>
<send event="lightImpulse" id="lightId" delay="200ms"/>
</if>
</transition>
<transition event="update">
<raise event="scheduleNewImpulse"/>
<raise event="updateLights"/>
</transition>
<transition event="lightImpulse" cond="In('lightImpulseOn')" target="lightImpulseOff"/>
<transition event="lightImpulse" cond="In('lightImpulseOff')" target="lightImpulseOn"/>
</state>
<transition event="done.state.letter.*">
...
</transition>
<transition event="done.state.lettersState">
...
</transition>
<transition event="updateLights">
...
</transition>
<transition event="updateLightsAccordingToLettersState">
...
</transition>
<transition event="turnOnLights">
...
</transition>
<transition event="turnOffLights">
...
</transition>
</state>
The
lightImpulseGenerator
contains two child states:
lightImpulseOn
and
lightImpulseOff
, with only one active at a time.
Whenever the delayed
lightImpulse
event is being delivered, it immediately causes the transition from
lightImpluseOn
into
lightImpulseOff
or vice versa, depending on the state the machine was in. In effect, the
lightImpulseGenerator
toggles between its on and off state. These transitions are defined inside
lightImpulseGenerator
, so it means that during this toggling the machine also exits
lightImpulseGenerator
and reenters it immediately afterwards. Entering
lightImpulseGenerator
causes the generation of the
update
event. The
update
event triggers a targetless transition and posts two other events:
scheduleNewImpulse
and
updateLights
. The first one,
scheduleNewImpulse
, returns back to the
lightImpulseGenerator
, which posts a delayed
lightImpulse
event. After the delay, the
lightImpulse
event gets delivered back to
lightImpulseGenerator
, which causes it to toggle its substate again. In this way, the machine enters into a cycle. The current delay of the
lightImpulse
event depends on the state in which the machine was in the time of posting the delayed event. If a
scheduleNewImpulse
event occurs on demand, before the next delayed
lightImpulse
event gets delivered, we cancel any possible pending events.
<transition event="done.state.letter.*">
<if cond="In('hurryStateOff')">
<assign location="score" expr="score + 1000"/>
<elseif cond="In('hurryStateOn')"/>
<assign location="score" expr="score + 10000"/>
</if>
<raise event="updateLights"/>
</transition>
<transition event="done.state.lettersState">
<if cond="In('hurryStateOff')">
<assign location="score" expr="score + 100000"/>
<raise event="goToHurryOn"/>
<elseif cond="In('hurryStateOn')"/>
<assign location="score" expr="score + 1000000"/>
<raise event="goToHurryOff"/>
<raise event="goForJackpot"/>
</if>
</transition>
Whenever we receive the event the name of which matches the
done.state.letter.*
, we update the current score. When the machine enters the final substate of the
letter.C
, it emits the
done.state.letter.C
event. The same happens for all other letters we have previously defined. We capture the events for all letters, that is why we have used an asterisk after a dot in the event name. The transition above is targetless, since we just listen for matching events and update the internal data accordingly without changing any active state. The new score is being increased by 1.000 or 10.000 points, depending on whether we currently are in
hurryStateOff
or
hurryStateOn
. After the score is updated, we generate the
updateLights
event in order to immediately update the letters' lights accordingly. We do not generate the
update
event here, since we do not want to toggle the light impulse now, but just update the lights according to the current impulse state.
We also intercept the
done.state.lettersState
event, which is being generated when all the letters have been hit. Depending on which state we are currently in, we grant the players either a small bonus of 100.000 or a big one of 1.000.000 (jackpot). In addition, we toggle the
hurryState
substate by sending the
goToHurryOn
or
goToHurryOff
event. When all letters have been collected while in
hurryStateOn
, we also raise the
goForJackpot
event which instructs the machine to activate the
jackpotStateOn
.
<transition event="updateLights">
<send event="updateScore">
<param name="highScore" expr="highScore"/>
<param name="score" expr="score"/>
</send>
<if cond="In('jackpotStateOn')">
<raise event="turnOnJackpot"/>
<else/>
<raise event="turnOffJackpot"/>
</if>
<if cond="In('lightImpulseOn')">
<if cond="In('offState')">
<raise event="turnOnLights"/>
<raise event="turnOnHurry"/>
<raise event="turnOnJackpot"/>
<raise event="turnOnGameOver"/>
<elseif cond="In('hurryStateOff')"/>
<raise event="updateLightsAccordingToLettersState"/>
<raise event="turnOffHurry"/>
<raise event="turnOffGameOver"/>
<else/>
<raise event="turnOnLights"/>
<raise event="turnOnHurry"/>
<raise event="turnOffGameOver"/>
</if>
<else/>
<if cond="In('offState')">
<raise event="turnOffLights"/>
<raise event="turnOffHurry"/>
<raise event="turnOffJackpot"/>
<elseif cond="In('hurryStateOff')"/>
<raise event="turnOffLights"/>
<else/>
<raise event="updateLightsAccordingToLettersState"/>
</if>
<raise event="turnOffHurry"/>
<raise event="turnOffGameOver"/>
</if>
</transition>
<transition event="updateLightsAccordingToLettersState">
<if cond="In('cLetterOn')">
<raise event="turnOnC"/>
<else/>
<raise event="turnOffC"/>
</if>
<if cond="In('rLetterOn')">
<raise event="turnOnR"/>
<else/>
<raise event="turnOffR"/>
</if>
<if cond="In('aLetterOn')">
<raise event="turnOnA"/>
<else/>
<raise event="turnOffA"/>
</if>
<if cond="In('zLetterOn')">
<raise event="turnOnZ"/>
<else/>
<raise event="turnOffZ"/>
</if>
<if cond="In('yLetterOn')">
<raise event="turnOnY"/>
<else/>
<raise event="turnOffY"/>
</if>
</transition>
<transition event="turnOnLights">
<raise event="turnOnC"/>
<raise event="turnOnR"/>
<raise event="turnOnA"/>
<raise event="turnOnZ"/>
<raise event="turnOnY"/>
</transition>
<transition event="turnOffLights">
<raise event="turnOffC"/>
<raise event="turnOffR"/>
<raise event="turnOffA"/>
<raise event="turnOffZ"/>
<raise event="turnOffY"/>
</transition>
When we receive the
updateLights
event, we first want to send a
updateScore
event outside of the state machine. We pass the current values of the
highScore
and
score
variables to the event. This event is received by the C++ part.
Next, depending on whether we are in
jackpotStateOn
or
jackpotStateOff
, we send the
turnOnJackpot
或
turnOffJackpot
event, which instructs the
guiControl
state to transition to
jackpotLightOn
or
jackpotLightOff
，分别。
When the machine is in
idle
state, (that is, in the off state) or when the game is on, but no interaction occurs, the
updateLights
event is delivered periodically during the game, each time with the
lightImpulseOn
or
lightImpulseOff
state toggled. Depending on the current state of the light impulse and on the active state (
offState
,
hurryStateOff
or
hurryStateOn
), we turn on or off all the lights according to the description of the pinball table.
The GUI part of the application consists of a mainwindow.ui file which describes the static user interface of the game.
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
.
class MainWindow; } class QScxmlStateMachine; QT_END_NAMESPACE class MainWindow : public QWidget { Q_OBJECT public: explicit MainWindow(QScxmlStateMachine *machine, QWidget *parent = nullptr); ~MainWindow(); private: void initAndConnect(const QString &state, QWidget *widget); QT_PREPEND_NAMESPACE(Ui::MainWindow) *m_ui; QScxmlStateMachine *m_machine; };
The
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.
#include "mainwindow.h" #include "ui_mainwindow.h" #include <QScxmlStateMachine> #include <QStringListModel> QT_USE_NAMESPACE MainWindow::MainWindow(QScxmlStateMachine *machine, QWidget *parent) : QWidget(parent), m_ui(new Ui::MainWindow), m_machine(machine) { m_ui->setupUi(this); // lights initAndConnect(QLatin1String("cLightOn"), m_ui->cLabel); initAndConnect(QLatin1String("rLightOn"), m_ui->rLabel); initAndConnect(QLatin1String("aLightOn"), m_ui->aLabel); initAndConnect(QLatin1String("zLightOn"), m_ui->zLabel); initAndConnect(QLatin1String("yLightOn"), m_ui->yLabel); initAndConnect(QLatin1String("hurryLightOn"), m_ui->hurryLabel); initAndConnect(QLatin1String("jackpotLightOn"), m_ui->jackpotLabel); initAndConnect(QLatin1String("gameOverLightOn"), m_ui->gameOverLabel); // help labels initAndConnect(QLatin1String("offState"), m_ui->offStateLabel); initAndConnect(QLatin1String("hurryStateOff"), m_ui->normalStateLabel); initAndConnect(QLatin1String("hurryStateOn"), m_ui->hurryStateLabel); initAndConnect(QLatin1String("jackpotStateOn"), m_ui->jackpotStateLabel); // context enablement initAndConnect(QLatin1String("offState"), m_ui->startButton); initAndConnect(QLatin1String("onState"), m_ui->cButton); initAndConnect(QLatin1String("onState"), m_ui->rButton); initAndConnect(QLatin1String("onState"), m_ui->aButton); initAndConnect(QLatin1String("onState"), m_ui->zButton); initAndConnect(QLatin1String("onState"), m_ui->yButton); initAndConnect(QLatin1String("onState"), m_ui->ballOutButton); // datamodel update m_machine->connectToEvent("updateScore", [this] (const QScxmlEvent &event) { const QVariant data = event.data(); const QString highScore = data.toMap().value("highScore").toString(); m_ui->highScoreLabel->setText(highScore); const QString score = data.toMap().value("score").toString(); m_ui->scoreLabel->setText(score); }); // gui interaction connect(m_ui->cButton, &QAbstractButton::clicked, [this] { m_machine->submitEvent("cLetterTriggered"); }); connect(m_ui->rButton, &QAbstractButton::clicked, [this] { m_machine->submitEvent("rLetterTriggered"); }); connect(m_ui->aButton, &QAbstractButton::clicked, [this] { m_machine->submitEvent("aLetterTriggered"); }); connect(m_ui->zButton, &QAbstractButton::clicked, [this] { m_machine->submitEvent("zLetterTriggered"); }); connect(m_ui->yButton, &QAbstractButton::clicked, [this] { m_machine->submitEvent("yLetterTriggered"); }); connect(m_ui->startButton, &QAbstractButton::clicked, [this] { m_machine->submitEvent("startTriggered"); }); connect(m_ui->ballOutButton, &QAbstractButton::clicked, [this] { m_machine->submitEvent("ballOutTriggered"); }); }
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.
The
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.
#include "mainwindow.h" #include "pinball.h" #include <QApplication> int main(int argc, char **argv) { QApplication app(argc, argv); Pinball machine; MainWindow mainWindow(&machine); machine.start(); mainWindow.show(); return app.exec(); }
在
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.