Qt State Machine Framework

Qt State Machine Framework09/25/09

About Me (Kent Hansen)

• Working on Qt since 2005

• QtScript

• Qt State Machine framework

• Plays harmonica and Irish whistle

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Goals For This Talk

• Introduce the Qt State Machine Framework (SMF)

• Show how to incorporate it in your Qt application

• Inspire you to think about how you would use it

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Agenda

• State machines – what and why?

• Statecharts tour

• Qt State Machine tour

• Wrap-up

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Qt State Machine Framework (SMF)

• Introduced in Qt 4.6

• Part of QtCore module (ubiquitous)

• Originated from Qt-SCXML research project

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Qt State Machine Framework (SMF)

• Provides C++ API for creating hierarchical finite

state machines (HFSMs)

• Provides “interpreter” for executing HFSMs

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State Chart XML (SCXML)

• “A general-purpose event-based state machine

language”

• W3C draft (http://www.w3.org/TR/scxml/)–Defines tags and attributes–Defines algorithm for interpretation

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Statecharts – Some use cases

• State-based (“fluid”) UIs

• Asynchronous communication

• AI

• Gesture recognition

• Controller of Model-View-Controller

• Your new, fancy product (e.g. “Hot dog oven”)

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Mmm, hot dogs...

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Why State Machines in Qt? (I)

?

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Why State Machines in Qt? (II)

• Program = Structure + Behavior

• C++: Structure is language construct

• C++: Event-driven behavior is not language

construct

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Why State Machines in Qt? (III)

• Qt already has event-based infrastructure

• Event representation (QEvent)

• Event dispatch

• Event handlers

• So what's the problem?

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The spaghetti code incident (I)

“On button clicked:if X, do thiselse, do that”

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The spaghetti code incident (II)

“On button clicked:if X, do thiselse, if Y or Z

if I and not J do thatelse, do that other thing

else, go and have a nap”

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ifs can get iffy; whiles can get wiley

• if-statements --> state is implicit

• Control flow and useful work jumbled together

• Hard to understand and maintain

• Hard to extend

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Can we do better...?

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Qt SMF Mission

It shouldn't be your job to implement a general-

purpose HFSM framework!

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There's flow...

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… and there's control

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What's in it for YOU?

• Write more robust code

• Have your design and implementation speak the

same language

• Cope with incremental complexity

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Qt + State Machines = Very Good Fit

• Natural extension to Qt's application model

• Integration with meta-object system

• Nested states fit nicely with Qt ownership model

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The right tool for the job...

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When NOT to use Qt SMF?

• Lexical analysis, parsing, image decoding

• When performance is critical

• When abstraction level becomes too low

• Not everything should be implemented as a (Qt)

state machine!

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Agenda

• State machines – what and why?

• Statecharts overview

• Qt State Machine tour

• Wrap-up

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Statecharts overview

• Composite (nested) states

• Behavorial inheritance

• History states

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A composite state

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A composite state decomposed

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“Get ready” state decomposed

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“Speak” state decomposed

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Composite states

• “Zoom in”: Consider more details

• “Zoom out”: Abstract away

• Black box vs white box

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Like Father, Like Son...

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Behavioral Inheritance

• States implicitly “inherit” the transitions of their

ancestor state(s)

• Enables grouping and specialization

• Analogue: Class-based inheritance

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Behavioral Inheritance example (I)

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Behavioral Inheritance example (II)

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History matters...

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History states

• Provide “pause and resume” functionality

• State machine remembers active state

• State machine restores last active state

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History state example from real life

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History state example

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Agenda

• State machines – what and why?

• Statecharts overview

• Qt State Machine tour

• Wrap-up

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Qt State Machine tour

• API introduction w/ small example

• Events and transitions

• Working with states

• Using state machines in your application

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Qt State Machine API

• Classes for representing states

• Classes for representing transitions

• Classes for state machine-specific events

• QStateMachine class (container & interpreter)

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My First State Machine

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My First State Machine

“Show me the code!”

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State machine set-up recipe

• Create QStateMachine

• Create states

• Create transitions between states

• Hook up to state signals (entered(), finished())

• Set the initial state

• Start the machine

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State machine event processing

• State machine runs its own event loop

• QEvent-based

• Use QStateMachine::postEvent() to post an event

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Transitions (I)

• Abstract base class: QAbstractTransition–bool eventTest(QEvent*);

• Has zero or more target states

• Add to source state using QState::addTransition()

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Transitions (II)

• Convenience for Qt signal transitions:

addTransition(object, signal, targetState)

• Standard Qt event (e.g. QMouseEvent) transitions

also supported– QEventTransition

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Responding to state changes

• QAbstractState::entered() signal

• QAbstractState::exited() signal

• QAbstractTransition::triggered() signal

• QState::finished() signal

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Composite states

• Follows Qt object hierarchy

• Pass parent state to state constructor

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QState *s1 = new QState();

QState *s11 = new QState(s1);

QState *s12 = new QState(s1);

QFinalState *s13 = new QFinalState(s1);

Paralell state group

• Set the state's childMode property

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QState *s1 = new QState();

s1->setChildMode(QState::ParallelStates);

QState *s11 = new QState(s1);

QState *s12 = new QState(s1);

History states

• QHistoryState class

• Create as child of composite state

• Use the history state as target of a transition

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QState *s1 = new QState();

QHistoryState *s1h = new QHistoryState(s1);

…

s2->addTransition(foo, SIGNAL(bar()), s1h);

How to use state machines...?

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Scenario: Game (I)

• Many different types of game objects

• Each type's behavior modeled as composite state

• Events trigger transitions– Input (e.g. key press)

• States operate on the game object–Setting properties (e.g. velocity)–Calling slots

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Scenario: Game (II)

• Each game object has its own state machine

• The machines run independently

• Separate, top-level state machine that

“orchestrates”–Game menus & modes–Start/quit

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Scenario: Game (III)

• Presence of a state machine is encapsulated

• Up to each type of object

• “Simple” objects don't need to use a state machine

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States and animations

• Integrates with Qt animation framework (also new

in Qt 4.6)

• QAbstractTransition::addAnimation()

• Almost all Qt animation examples use Qt SMF

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My tips (I)

• Use the meta-object system integration–assignProperty(object, propertyName, value)–entered() and exited() signals

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My tips (II)

• Use composition–Build complex behavior from simple states–Take advantage of behavioral inheritance!–Don't subclass unnecessarily

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My tips (III)

• Always draw the statechart first–Visualizing the design from C++ is hard–The statechart is the design document

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Agenda

• State machines – what and why?

• Statecharts tour

• Qt State Machine tour

• Wrap-up

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Summary (I)

• Statecharts are a powerful tool for modeling

complex, event-driven systems–General-purpose–Well-defined semantics

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Summary (II)

• With the Qt State Machine Framework, you can

build and run statecharts

• Write more robust code

• You need to consider when/where/how to use it

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The Future (Research)

• Qt-SCXML to become part of Qt?

• Qt state machine compiler

• Visual design tool?

• Your feedback matters!

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Relevant resources

● http://labs.qt.nokia.com

● http://lists.trolltech.com

● Qt Quarterly issue 30

● irc.freenode.net: #qt-labs

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Thank You!

Questions?

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