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/****************************************************************************
**
** Copyright (C) 2015 The Qt Company Ltd.
** Contact: http://www.qt.io/licensing/
**
** This file is part of the documentation of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:FDL$
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****************************************************************************/
/*!
\qmlmodule QtQml.StateMachine 1.0
\title Declarative 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
import QtQuick 2.0
import QtQml.StateMachine 1.0
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
import QtQuick 2.0
import QtQml.StateMachine 1.0 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{Declarative State Machine QML Types}
\li \l{The State Machine Framework}
\endlist
*/
|