path: root/src/statemachine/doc/qtstatemachine-cpp-guide.qdoc

// Copyright (C) 2021 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only

/*!
  \page qtstatemachine-cpp-guide.html
  \title Qt State Machine C++ Guide
  \brief An overview of the State Machine framework for constructing and
  executing state graphs with the C++ API.
  \ingroup technology-apis
  \ingroup explanation

  The State Machine framework provides classes for creating and executing
  state graphs. This page illustrates the framework's key features in C++.

  \sa {Qt State Machine Overview}
  \sa {Qt State Machine QML Guide}

  \section1 C++ Classes in the State Machine Framework

  For the full list of C++ classes in the State Machine framework see
  \l {Qt State Machine C++ Classes}

  \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:

    \image statemachine-button.png

  The following snippet shows the code needed to create such a state machine.
  First, we create the state machine and states:

  \snippet statemachine/main.cpp 0

  Then, we create the transitions by using the QState::addTransition()
  function:

  \snippet statemachine/main.cpp 1

  Next, we add the states to the machine and set the machine's initial state:

  \snippet statemachine/main.cpp 2

  Finally, we start the state machine:

  \snippet 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 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 \l {QPushButton::}{showMaximized()} slot
  will be called when state \c s3 is entered, and the button's \l {QPushButton::}{showMinimized()}
  slot will be called when \c s3 is exited:

  \snippet 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 \l{QPushButton::}{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.

    \image statemachine-button-nested.png

  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 statemachine/main2.cpp 0

  \snippet statemachine/main2.cpp 1

  In this case we want the application to quit when the state machine is
  finished, so the machine's \l {QStateMachine::}{finished()} signal is connected to the
  application's \l {QCoreApplication::}{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 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.

    \image statemachine-button-history.png

  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 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.

    \image statemachine-nonparallel.png

  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.

    \image statemachine-parallel.png

  To create a parallel state group, pass QState::ParallelStates to the QState
  constructor.

  \snippet 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:

    \image statemachine-finished.png

  When \c s1 's final state is entered, \c s1 will automatically emit
  \l {QState::}{finished()}. We use a signal transition to cause this event to trigger a
  state change:

  \snippet 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
  \l {QState::}{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, &QPushButton::clicked);
    s1->addTransition(trans);

    QMessageBox msgBox;
    msgBox.setText("The button was clicked; carry on.");
    QObject::connect(trans, QSignalTransition::triggered, &msgBox, &QMessageBox::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 \l [CPP] {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 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 statemachine/main4.cpp 1

  In the \l [CPP] {QAbstractTransition::}{eventTest()} reimplementation, we first check if the event type is the
  desired one; if so, we cast the event to a \c StringEvent and perform the
  string comparison.

  The following is a statechart that uses the custom event and transition:

    \image statemachine-customevents.png

  Here's what the implementation of the statechart looks like:

  \snippet statemachine/main4.cpp 2

  Once the machine is started, we can post events to it.

  \snippet 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:

    \image statemachine-customevents2.png

  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 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 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 Animations and States Machines

  The State Machine API connects with the \l {The Animation Framework} to allow automatically
  animating properties as they are assigned in states.

  The state machine provides a special state that can play an animation.
  A QState can also set properties when the state is entered or exited, and
  this special animation state will interpolate between these values when given a
  QPropertyAnimation.

  We can associate one or more animations to a transition between states
  using a QSignalTransition or QEventTransition class. These classes
  are both derived from QAbstractTransition, which defines the
  convenience function \l [CPP] {QAbstractTransition::}{addAnimation()} that
  enables the appending of one or more animations triggered when the
  transition occurs.

  We also have the possibility to associate properties with the
  states rather than setting the start and end values ourselves.

  Say we have the following code:

  \snippet 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 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 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 \l {QState::}{propertiesAssigned()} signal.
  The \l {QState::}{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 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 \l {QState::}{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 \l {QState::}{propertiesAssigned()} signal, as described in the previous section.

  The State Machine API guarantees that a property assigned by the state machine either:
  \list
  \li Has a value explicitly assigned to the property.
  \li 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.

  \section1 Nesting State Machines

  QStateMachine is a subclass of QState. This allows for a state machine to be a child state of
  another machine. QStateMachine reimplements QState::onEntry() and calls QStateMachine::start(),
  so that when the child state machine is entered, it will automatically start running.

  The parent state machine treats the child machine as an \e atomic state in the state machine
  algorithm. The child state machine is self-contained; it maintains its own event queue and
  configuration. In particular, note that the \l{QStateMachine::}{configuration()}
  of the child machine is not part of the parent machine's configuration (only the child machine
  itself is).

  States of the child state machine cannot be specified as targets of transitions in the parent
  state machine; only the child state machine itself can. Conversely, states of the parent state
  machine cannot be specified as targets of transitions in the child state machine. The child
  state machine's \l{QState::}{finished}() signal can be used to trigger a transition
  in the parent machine.
*/