FCL/sf/mw/qt: comparison doc/src/frameworks-technologies/statemachine.qdoc

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+****************************************************************************/
+/*!
+\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.
+The state graph in the State Machine framework is hierarchical. States can be nested inside of
+other states, and the current configuration of the state machine consists of the set of states
+which are currently active. All the states in a valid configuration of the state machine will
+have a common ancestor.
+\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 which exits the parent
+state. When this happens, the parent state and all of its child states are exited.
+The parallelism in the State Machine framework follows an interleaved semantics. All parallel
+operations will be executed in a single, atomic step of the event processing, so no event can
+interrupt the parallel operations. However, events will still be processed sequentially, since
+the machine itself is single threaded. As an example: Consider the situation where there are two
+transitions that exit the same parallel state group, and their conditions become true
+simultaneously. In this case, the event that is processed last of the two will not have any
+effect, since the first event will already have caused the machine to exit from the parallel
+state.
+\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 Targetless Transitions
+A transition need not have a target state. A transition without a target can
+be triggered the same way as any other transition; the difference is that
+when a targetless transition is triggered, it doesn't cause any state
+changes. This allows you to react to a signal or event when your machine is
+in a certain state, without having to leave that state. Example:
+\code
+QStateMachine machine;
+QState *s1 = new QState(&machine);
+QPushButton button;
+QSignalTransition *trans = new QSignalTransition(&button, SIGNAL(clicked()));
+s1->addTransition(trans);
+QMessageBox msgBox;
+msgBox.setText("The button was clicked; carry on.");
+QObject::connect(trans, SIGNAL(triggered()), &msgBox, SLOT(exec()));
+machine.setInitialState(s1);
+\endcode
+The message box will be displayed each time the button is clicked, but the
+state machine will remain in its current state (s1). If the target state
+were explicitly set to s1, however, s1 would be exited and re-entered each
+time (e.g. the QAbstractState::entered() and QAbstractState::exited()
+signals would be emitted).
+\section1 Events, Transitions and Guards
+A QStateMachine runs its own event loop. For signal transitions
+(QSignalTransition objects), QStateMachine automatically posts a
+QStateMachine::SignalEvent to itself when it intercepts the corresponding
+signal; similarly, for QObject event transitions (QEventTransition objects)
+a QStateMachine::WrappedEvent 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:
+\snippet doc/src/snippets/statemachine/main5.cpp 0
+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.
+\snippet doc/src/snippets/statemachine/main5.cpp 2
+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:
+\snippet doc/src/snippets/statemachine/main5.cpp 3
+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.
+\snippet doc/src/snippets/statemachine/main5.cpp 4
+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.
+Say we have the following code:
+\snippet doc/src/snippets/statemachine/main5.cpp 5
+When \c button is clicked, the machine will transition into state \c s2, which will set the
+geometry of the button, and then pop up a message box to alert the user that the geometry has
+been changed.
+In the normal case, where animations are not used, this will operate as expected. However, if
+an animation for the \c geometry of \c button is set on the transition between \c s1 and \c s2,
+the animation will be started when \c s2 is entered, but the \c geometry property will not
+actually reach its defined value before the animation is finished running. In this case, the
+message box will pop up before the geometry of the button has actually been set.
+To ensure that the message box does not pop up until the geometry actually reaches its final
+value, we can use the state's propertiesAssigned() signal. The propertiesAssigned() signal will be
+emitted when the property is assigned its final value, whether this is done immediately or
+after the animation has finished playing.
+\snippet doc/src/snippets/statemachine/main5.cpp 6
+In this example, when \c button is clicked, the machine will enter \c s2. It will remain in state
+\c s2 until the \c geometry property has been set to \c QRect(0, 0, 50, 50). Then it will
+transition into \c s3. When \c s3 is entered, the message box will pop up. If the transition into
+\c s2 has an animation for the \c geometry property, then the machine will stay in \c s2 until the
+animation has finished playing. If there is no such animation, it will simply set the property and
+immediately enter state \c s3.
+Either way, when the machine is in state \c s3, you are guaranteed that 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 propertiesAssigned() 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 propertiesAssigned signal, 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.
+*/