path: root/src/qml/doc/src/statemachine.qdoc

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/*!
  \qmlmodule QtQml.StateMachine 1.\QtMinorVersion
  \title Qt QML State Machine QML Types
  \brief Provides QML types to create and execute state graphs.

  The following is a list of QML types provided by the module:
*/


/*!
  \page qmlstatemachine.html
  \title The Declarative State Machine Framework
  \brief Overview of the Declarative State Machine Framework for constructing and executing state graphs.

  \ingroup frameworks-technologies

  \tableofcontents

  The Declarative State Machine Framework provides types for creating and
  executing state graphs in QML. It is similar to the C++ State Machine
  framework based on Harel's
  \l{Statecharts: A visual formalism for complex systems}, which
  is also the basis for UML state diagrams. Like its
  \l{The State Machine Framework}{C++ counterpart}, the framework provides an
  API and execution model based on \l{State Chart XML: State Machine Notation for
  Control Abstraction}{State Chart XML (SCXML)}
  to embed the elements and semantics of statecharts in QML applications.

  For user interfaces with multiple visual states, independent of the
  application's logical state, consider using QML States and Transitions.

  These following QML types are provided by framework to create event-driven
  state machines:

  \annotatedlist statemachine-qmltypes

  \section1 Using Both QtQuick and QtQml.StateMachine Imports

  \warning If you're attempting to import both \l{QtQuick} and
    \e{QtQml.StateMachine} in one single QML file, make sure to import
    \e{QtQml.StateMachine} \e{last}. This way, the \e{State} type is provided
    by the Declarative State Machine Framework and not by \l{QtQuick}:

  \qml \QtMinorVersion
    import QtQuick 2.\1
    import QtQml.StateMachine 1.\1

    StateMachine {
        State {
            // okay, is of type QtQml.StateMachine.State
        }
    }
  \endqml

  Alternatively, you can import \e{QtQml.StateMachine} into a separate
  namespace to avoid any ambiguity with QtQuick's \e{State} item:

  \qml \QtMinorVersion
    import QtQuick 2.\1
    import QtQml.StateMachine 1.\1 as DSM

    DSM.StateMachine {
        DSM.State {
            // ...
        }
    }
  \endqml

  \section1 A Simple State Machine

  To demonstrate the core functionality of the State Machine API, let's look
  at an example: A state machine with three states, \c s1, \c s2 and \c
  s3. The state machine is controlled by a single Button; when the button
  is clicked, the machine transitions to another state. Initially, the state
  machine is in state \c s1. The following is a state chart showing the
  different states in our example.

  \image statemachine-button.png

  The following snippet shows the code needed to create such a state machine.

  \snippet qml/statemachine/statemachine-button.qml 0

  The state machine runs asynchronously to become part of your application's
  event loop.

  \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 (FinalState object). When the state machine enters the top-level
  final state, the machine emits the \l{State::finished}{finished}
  signal and halts.

  All you need to do to introduce a final state in the graph is create a
  FinalState object and use it as the target of one or more transitions.

  \section1 Sharing Transitions

  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 \e clicked() signal. We could add a transition for each state;
  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 childrens 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 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 (the child state the state machine should enter when the
  parent state is the target of a transition).

  \snippet qml/statemachine/statemachine-button-nested.qml 0

  In this case we want the application to quit when the state machine is
  finished, so the machine's \e finished() signal is connected to the
  application's \e 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 qml/statemachine/statemachine-button-nested-ignore-quit.qml 0

  A transition can have any state as its target irrespective of where the
  target state is in the state hierarchy.

  \section1 Using History States

  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 the three states in this case).

  Such behavior can easily be modeled using \e{history states}. A history
  state (HistoryState object) is a pseudo-state that represents the child
  state that the parent state was in before it exited last.

  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 exits. 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 qml/statemachine/statemachine-button-history.qml 0

  \section1 Using Parallel 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 represent the states and freely
  move between all possible combinations as shown in the following state chart.

    \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.

  This exponential increase can be reduced using parallel states, which enables
  linear growth in the number of states and transitions as we add more
  properties. Furthermore, states can be added to or removed from the parallel
  state without affecting any of their sibling states. The following state
  chart shows the different paralles states for the car example.

    \image statemachine-parallel.png

  To create a parallel state group, set childMode to QState.ParallelStates.

  \qml
    State {
        id: s1
        childMode: QState.ParallelStates
        State {
            id: s11
        }
        State {
            id: s12
        }
    }
  \endqml

  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, as the machine itself
  is single threaded. For 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.

  \section1 Exiting a Composite State

  A child state can be final (a FinalState object); when a final child state
  is entered, the parent state emits the State::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{State::finished}{finished}. We use a signal transition to cause this event to
  trigger a state change:

  \qml
    State {
        id: s1
        SignalTransition {
            targetState: s2
            signal: s1.finished
        }
    }
  \endqml

  Using final states in composite states is useful when you want to hide the
  internal details of a composite state. The outside world should be able to
  enter the state and get a notification when the state has completed its work,
  without the need to know the internal details. 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 State::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
  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. For example:

  \qml
    Button {
        id: button
        text: "button"
        StateMachine {
            id: stateMachine
            initialState: s1
            running: true
            State {
                id: s1
                SignalTransition {
                    signal: button.clicked
                    onTriggered: console.log("button pressed")
                }
            }
        }
    }
  \endqml

  The "button pressed" message 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, s1 would be exited and re-entered each
  time (the QAbstractState::entered and QAbstractState::exited
  signals would be emitted).

  \section1 Related Information

  \list
   \li \l{Qt QML State Machine QML Types}
   \li \l{The State Machine Framework}
  \endlist
*/