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diff --git a/doc/src/frameworks-technologies/statemachine.qdoc b/doc/src/frameworks-technologies/statemachine.qdoc new file mode 100644 index 0000000..7b8e91d --- /dev/null +++ b/doc/src/frameworks-technologies/statemachine.qdoc @@ -0,0 +1,548 @@ +/**************************************************************************** +** +** Copyright (C) 2009 Nokia Corporation and/or its subsidiary(-ies). +** Contact: Nokia Corporation (qt-info@nokia.com) +** +** This file is part of the documentation of the Qt Toolkit. +** +** $QT_BEGIN_LICENSE:LGPL$ +** No Commercial Usage +** This file contains pre-release code and may not be distributed. +** You may use this file in accordance with the terms and conditions +** contained in the Technology Preview License Agreement accompanying +** this package. +** +** GNU Lesser General Public License Usage +** Alternatively, this file may be used under the terms of the GNU Lesser +** General Public License version 2.1 as published by the Free Software +** Foundation and appearing in the file LICENSE.LGPL included in the +** packaging of this file. Please review the following information to +** ensure the GNU Lesser General Public License version 2.1 requirements +** will be met: http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html. +** +** In addition, as a special exception, Nokia gives you certain +** additional rights. These rights are described in the Nokia Qt LGPL +** Exception version 1.1, included in the file LGPL_EXCEPTION.txt in this +** package. +** +** If you have questions regarding the use of this file, please contact +** Nokia at qt-info@nokia.com. +** +** +** +** +** +** +** +** +** $QT_END_LICENSE$ +** +****************************************************************************/ + +/*! + \group statemachine + \title State Machine Classes +*/ + +/*! + \page statemachine-api.html + \title The State Machine Framework + \brief An overview of the State Machine framework for constructing and executing state graphs. + + \ingroup frameworks-technologies + + \tableofcontents + + The State Machine framework provides classes for creating and executing + state graphs. The concepts and notation are based on those from Harel's + \l{Statecharts: A visual formalism for complex systems}{Statecharts}, which + is also the basis of UML state diagrams. The semantics of state machine + execution are based on \l{State Chart XML: State Machine Notation for + Control Abstraction}{State Chart XML (SCXML)}. + + Statecharts provide a graphical way of modeling how a system reacts to + stimuli. This is done by defining the possible \e states that the system can + be in, and how the system can move from one state to another (\e transitions + between states). A key characteristic of event-driven systems (such as Qt + applications) is that behavior often depends not only on the last or current + event, but also the events that preceded it. With statecharts, this + information is easy to express. + + The State Machine framework provides an API and execution model that can be + used to effectively embed the elements and semantics of statecharts in Qt + applications. The framework integrates tightly with Qt's meta-object system; + for example, transitions between states can be triggered by signals, and + states can be configured to set properties and invoke methods on QObjects. + Qt's event system is used to drive the state machines. + + \section1 Classes in the State Machine Framework + + These classes are provided by qt for creating event-driven state machines. + + \annotatedlist statemachine + + \section1 A Simple State Machine + + To demonstrate the core functionality of the State Machine API, let's look + at a small example: A state machine with three states, \c s1, \c s2 and \c + s3. The state machine is controlled by a single QPushButton; when the button + is clicked, the machine transitions to another state. Initially, the state + machine is in state \c s1. The statechart for this machine is as follows: + + \img statemachine-button.png + \omit + \caption This is a caption + \endomit + + The following snippet shows the code needed to create such a state machine. + First, we create the state machine and states: + + \snippet doc/src/snippets/statemachine/main.cpp 0 + + Then, we create the transitions by using the QState::addTransition() + function: + + \snippet doc/src/snippets/statemachine/main.cpp 1 + + Next, we add the states to the machine and set the machine's initial state: + + \snippet doc/src/snippets/statemachine/main.cpp 2 + + Finally, we start the state machine: + + \snippet doc/src/snippets/statemachine/main.cpp 3 + + The state machine executes asynchronously, i.e. it becomes part of your + application's event loop. + + \section1 Doing Useful Work on State Entry and Exit + + The above state machine merely transitions from one state to another, it + doesn't perform any operations. The QState::assignProperty() function can be + used to have a state set a property of a QObject when the state is + entered. In the following snippet, the value that should be assigned to a + QLabel's text property is specified for each state: + + \snippet doc/src/snippets/statemachine/main.cpp 4 + + When any of the states is entered, the label's text will be changed + accordingly. + + The QState::entered() signal is emitted when the state is entered, and the + QState::exited() signal is emitted when the state is exited. In the + following snippet, the button's showMaximized() slot will be called when + state \c s3 is entered, and the button's showMinimized() slot will be called + when \c s3 is exited: + + \snippet doc/src/snippets/statemachine/main.cpp 5 + + Custom states can reimplement QAbstractState::onEntry() and + QAbstractState::onExit(). + + \section1 State Machines That Finish + + The state machine defined in the previous section never finishes. In order + for a state machine to be able to finish, it needs to have a top-level \e + final state (QFinalState object). When the state machine enters a top-level + final state, the machine will emit the QStateMachine::finished() signal and + halt. + + All you need to do to introduce a final state in the graph is create a + QFinalState object and use it as the target of one or more transitions. + + \section1 Sharing Transitions By Grouping States + + Assume we wanted the user to be able to quit the application at any time by + clicking a Quit button. In order to achieve this, we need to create a final + state and make it the target of a transition associated with the Quit + button's clicked() signal. We could add a transition from each of \c s1, \c + s2 and \c s3; however, this seems redundant, and one would also have to + remember to add such a transition from every new state that is added in the + future. + + We can achieve the same behavior (namely that clicking the Quit button quits + the state machine, regardless of which state the state machine is in) by + grouping states \c s1, \c s2 and \c s3. This is done by creating a new + top-level state and making the three original states children of the new + state. The following diagram shows the new state machine. + + \img statemachine-button-nested.png + \omit + \caption This is a caption + \endomit + + The three original states have been renamed \c s11, \c s12 and \c s13 to + reflect that they are now children of the new top-level state, \c s1. Child + states implicitly inherit the transitions of their parent state. This means + it is now sufficient to add a single transition from \c s1 to the final + state \c s2. New states added to \c s1 will also automatically inherit this + transition. + + All that's needed to group states is to specify the proper parent when the + state is created. You also need to specify which of the child states is the + initial one (i.e. which child state the state machine should enter when the + parent state is the target of a transition). + + \snippet doc/src/snippets/statemachine/main2.cpp 0 + + \snippet doc/src/snippets/statemachine/main2.cpp 1 + + In this case we want the application to quit when the state machine is + finished, so the machine's finished() signal is connected to the + application's quit() slot. + + A child state can override an inherited transition. For example, the + following code adds a transition that effectively causes the Quit button to + be ignored when the state machine is in state \c s12. + + \snippet doc/src/snippets/statemachine/main2.cpp 2 + + A transition can have any state as its target, i.e. the target state does + not have to be on the same level in the state hierarchy as the source state. + + \section1 Using History States to Save and Restore the Current State + + Imagine that we wanted to add an "interrupt" mechanism to the example + discussed in the previous section; the user should be able to click a button + to have the state machine perform some non-related task, after which the + state machine should resume whatever it was doing before (i.e. return to the + old state, which is one of \c s11, \c s12 and \c s13 in this case). + + Such behavior can easily be modeled using \e{history states}. A history + state (QHistoryState object) is a pseudo-state that represents the child + state that the parent state was in the last time the parent state was + exited. + + A history state is created as a child of the state for which we wish to + record the current child state; when the state machine detects the presence + of such a state at runtime, it automatically records the current (real) + child state when the parent state is exited. A transition to the history + state is in fact a transition to the child state that the state machine had + previously saved; the state machine automatically "forwards" the transition + to the real child state. + + The following diagram shows the state machine after the interrupt mechanism + has been added. + + \img statemachine-button-history.png + \omit + \caption This is a caption + \endomit + + The following code shows how it can be implemented; in this example we + simply display a message box when \c s3 is entered, then immediately return + to the previous child state of \c s1 via the history state. + + \snippet doc/src/snippets/statemachine/main2.cpp 3 + + \section1 Using Parallel States to Avoid a Combinatorial Explosion of States + + Assume that you wanted to model a set of mutually exclusive properties of a + car in a single state machine. Let's say the properties we are interested in + are Clean vs Dirty, and Moving vs Not moving. It would take four mutually + exclusive states and eight transitions to be able to represent and freely + move between all possible combinations. + + \img statemachine-nonparallel.png + \omit + \caption This is a caption + \endomit + + If we added a third property (say, Red vs Blue), the total number of states + would double, to eight; and if we added a fourth property (say, Enclosed vs + Convertible), the total number of states would double again, to 16. + + Using parallel states, the total number of states and transitions grows + linearly as we add more properties, instead of exponentially. Furthermore, + states can be added to or removed from the parallel state without affecting + any of their sibling states. + + \img statemachine-parallel.png + \omit + \caption This is a caption + \endomit + + To create a parallel state group, pass QState::ParallelStates to the QState + constructor. + + \snippet doc/src/snippets/statemachine/main3.cpp 0 + + When a parallel state group is entered, all its child states will be + simultaneously entered. Transitions within the individual child states + operate normally. However, any of the child states may take a transition + outside the parent state. When this happens, the parent state and all of its + child states are exited. + + \section1 Detecting that a Composite State has Finished + + A child state can be final (a QFinalState object); when a final child state + is entered, the parent state emits the QState::finished() signal. The + following diagram shows a composite state \c s1 which does some processing + before entering a final state: + + \img statemachine-finished.png + \omit + \caption This is a caption + \endomit + + When \c s1 's final state is entered, \c s1 will automatically emit + finished(). We use a signal transition to cause this event to trigger a + state change: + + \snippet doc/src/snippets/statemachine/main3.cpp 1 + + Using final states in composite states is useful when you want to hide the + internal details of a composite state; i.e. the only thing the outside world + should be able to do is enter the state, and get a notification when the + state has completed its work. This is a very powerful abstraction and + encapsulation mechanism when building complex (deeply nested) state + machines. (In the above example, you could of course create a transition + directly from \c s1 's \c done state rather than relying on \c s1 's + finished() signal, but with the consequence that implementation details of + \c s1 are exposed and depended on). + + For parallel state groups, the QState::finished() signal is emitted when \e + all the child states have entered final states. + + \section1 Events, Transitions and Guards + + A QStateMachine runs its own event loop. For signal transitions + (QSignalTransition objects), QStateMachine automatically posts a + QSignalEvent to itself when it intercepts the corresponding signal; + similarly, for QObject event transitions (QEventTransition objects) a + QWrappedEvent is posted. + + You can post your own events to the state machine using + QStateMachine::postEvent(). + + When posting a custom event to the state machine, you typically also have + one or more custom transitions that can be triggered from events of that + type. To create such a transition, you subclass QAbstractTransition and + reimplement QAbstractTransition::eventTest(), where you check if an event + matches your event type (and optionally other criteria, e.g. attributes of + the event object). + + Here we define our own custom event type, \c StringEvent, for posting + strings to the state machine: + + \snippet doc/src/snippets/statemachine/main4.cpp 0 + + Next, we define a transition that only triggers when the event's string + matches a particular string (a \e guarded transition): + + \snippet doc/src/snippets/statemachine/main4.cpp 1 + + In the eventTest() reimplementation, we first check if the event type is the + desired one; if so, we cast the event to a StringEvent and perform the + string comparison. + + The following is a statechart that uses the custom event and transition: + + \img statemachine-customevents.png + \omit + \caption This is a caption + \endomit + + Here's what the implementation of the statechart looks like: + + \snippet doc/src/snippets/statemachine/main4.cpp 2 + + Once the machine is started, we can post events to it. + + \snippet doc/src/snippets/statemachine/main4.cpp 3 + + An event that is not handled by any relevant transition will be silently + consumed by the state machine. It can be useful to group states and provide + a default handling of such events; for example, as illustrated in the + following statechart: + + \img statemachine-customevents2.png + \omit + \caption This is a caption + \endomit + + For deeply nested statecharts, you can add such "fallback" transitions at + the level of granularity that's most appropriate. + + \section1 Using Restore Policy To Automatically Restore Properties + + In some state machines it can be useful to focus the attention on assigning properties in states, + not on restoring them when the state is no longer active. If you know that a property should + always be restored to its initial value when the machine enters a state that does not explicitly + give the property a value, you can set the global restore policy to + QStateMachine::RestoreProperties. + + \code + QStateMachine machine; + machine.setGlobalRestorePolicy(QStateMachine::RestoreProperties); + \endcode + + When this restore policy is set, the machine will automatically restore all properties. If it + enters a state where a given property is not set, it will first search the hierarchy of ancestors + to see if the property is defined there. If it is, the property will be restored to the value + defined by the closest ancestor. If not, it will be restored to its initial value (i.e. the + value of the property before any property assignments in states were executed.) + + Take the following code: + \code + QStateMachine machine; + machine.setGlobalRestorePolicy(QStateMachine::RestoreProperties); + + QState *s1 = new QState(); + s1->assignProperty(object, "fooBar", 1.0); + machine.addState(s1); + machine.setInitialState(s1); + + QState *s2 = new QState(); + machine.addState(s2); + \endcode + + Lets say the property \c fooBar is 0.0 when the machine starts. When the machine is in state + \c s1, the property will be 1.0, since the state explicitly assigns this value to it. When the + machine is in state \c s2, no value is explicitly defined for the property, so it will implicitly + be restored to 0.0. + + If we are using nested states, the parent defines a value for the property which is inherited by + all descendants that do not explicitly assign a value to the property. + \code + QStateMachine machine; + machine.setGlobalRestorePolicy(QStateMachine::RestoreProperties); + + QState *s1 = new QState(); + s1->assignProperty(object, "fooBar", 1.0); + machine.addState(s1); + machine.setInitialState(s1); + + QState *s2 = new QState(s1); + s2->assignProperty(object, "fooBar", 2.0); + s1->setInitialState(s2); + + QState *s3 = new QState(s1); + \endcode + + Here \c s1 has two children: \c s2 and \c s3. When \c s2 is entered, the property \c fooBar + will have the value 2.0, since this is explicitly defined for the state. When the machine is in + state \c s3, no value is defined for the state, but \c s1 defines the property to be 1.0, so this + is the value that will be assigned to \c fooBar. + + \section1 Animating Property Assignments + + The State Machine API connects with the Animation API in Qt to allow automatically animating + properties as they are assigned in states. + + Say we have the following code: + \code + QState *s1 = new QState(); + QState *s2 = new QState(); + + s1->assignProperty(button, "geometry", QRectF(0, 0, 50, 50)); + s2->assignProperty(button, "geometry", QRectF(0, 0, 100, 100)); + + s1->addTransition(button, SIGNAL(clicked()), s2); + \endcode + + Here we define two states of a user interface. In \c s1 the \c button is small, and in \c s2 + it is bigger. If we click the button to transition from \c s1 to \c s2, the geometry of the button + will be set immediately when a given state has been entered. If we want the transition to be + smooth, however, all we need to do is make a QPropertyAnimation and add this to the transition + object. + + \code + QState *s1 = new QState(); + QState *s2 = new QState(); + + s1->assignProperty(button, "geometry", QRectF(0, 0, 50, 50)); + s2->assignProperty(button, "geometry", QRectF(0, 0, 100, 100)); + + QSignalTransition *transition = s1->addTransition(button, SIGNAL(clicked()), s2); + transition->addAnimation(new QPropertyAnimation(button, "geometry")); + \endcode + + Adding an animation for the property in question means that the property assignment will no + longer take immediate effect when the state has been entered. Instead, the animation will start + playing when the state has been entered and smoothly animate the property assignment. Since we + do not set the start value or end value of the animation, these will be set implicitly. The + start value of the animation will be the property's current value when the animation starts, and + the end value will be set based on the property assignments defined for the state. + + If the global restore policy of the state machine is set to QStateMachine::RestoreProperties, + it is possible to also add animations for the property restorations. + + \section1 Detecting That All Properties Have Been Set In A State + + When animations are used to assign properties, a state no longer defines the exact values that a + property will have when the machine is in the given state. While the animation is running, the + property can potentially have any value, depending on the animation. + + In some cases, it can be useful to be able to detect when the property has actually been assigned + the value defined by a state. For this, we can use the state's polished() signal. + \code + QState *s1 = new QState(); + s1->assignProperty(button, "geometry", QRectF(0, 0, 50, 50)); + + QState *s2 = new QState(); + + s1->addTransition(s1, SIGNAL(polished()), s2); + \endcode + + The machine will be in state \c s1 until the \c geometry property has been set. Then it will + immediately transition into \c s2. If the transition into \c s1 has an animation for the \c + geometry property, then the machine will stay in \c s1 until the animation has finished. If there + is no animation, it will simply set the property and immediately enter state \c s2. + + Either way, when the machine is in state \c s2, the property \c geometry has been assigned the + defined value. + + If the global restore policy is set to QStateMachine::RestoreProperties, the state will not emit + the polished() signal until these have been executed as well. + + \section1 What happens if a state is exited before the animation has finished + + If a state has property assignments, and the transition into the state has animations for the + properties, the state can potentially be exited before the properties have been assigned to the + values defines by the state. This is true in particular when there are transitions out from the + state that do not depend on the state being polished, as described in the previous section. + + The State Machine API guarantees that a property assigned by the state machine either: + \list + \o Has a value explicitly assigned to the property. + \o Is currently being animated into a value explicitly assigned to the property. + \endlist + + When a state is exited prior to the animation finishing, the behavior of the state machine depends + on the target state of the transition. If the target state explicitly assigns a value to the + property, no additional action will be taken. The property will be assigned the value defined by + the target state. + + If the target state does not assign any value to the property, there are two + options: By default, the property will be assigned the value defined by the state it is leaving + (the value it would have been assigned if the animation had been permitted to finish playing.) If + a global restore policy is set, however, this will take precedence, and the property will be + restored as usual. + + \section1 Default Animations + + As described earlier, you can add animations to transitions to make sure property assignments + in the target state are animated. If you want a specific animation to be used for a given property + regardless of which transition is taken, you can add it as a default animation to the state + machine. This is in particular useful when the properties assigned (or restored) by specific + states is not known when the machine is constructed. + + \code + QState *s1 = new QState(); + QState *s2 = new QState(); + + s2->assignProperty(object, "fooBar", 2.0); + s1->addTransition(s2); + + QStateMachine machine; + machine.setInitialState(s1); + machine.addDefaultAnimation(new QPropertyAnimation(object, "fooBar")); + \endcode + + When the machine is in state \c s2, the machine will play the default animation for the + property \c fooBar since this property is assigned by \c s2. + + Note that animations explicitly set on transitions will take precedence over any default + animation for the given property. +*/ |