Say hello to the Declarative State Machine Framework

The Declarative State Machine Framework extends Qt's State Machine
Framework (QSM) into QML to provide types for creating and executing
state graphs in QML. This gives you the power of deterministic state
machines, but declaratively and without having to write all of the
boilerplate code. It is an alternative to the existing QML State type,
intended for more complex models.

[ChangeLog][QtQML] The Declarative State Machine Framework extends Qt's
State Machine Framework (QSM) into QML. This gives you the power of
deterministic state machines, but declaratively.

Change-Id: I02390ba7f1baed50935364530925bd75087299cb
Reviewed-by: Sebastian Sauer <sebastian.sauer@kdab.com>
Reviewed-by: BogDan Vatra <bogdan@kde.org>
Reviewed-by: Brett Stottlemyer <bstottle@ford.com>

1 files changed, 317 insertions, 0 deletions

diff --git a/src/qml/doc/src/statemachine.qdoc b/src/qml/doc/src/statemachine.qdoc
new file mode 100644
index 0000000000..5a37ef8026
--- /dev/null
+++ b/src/qml/doc/src/statemachine.qdoc
@@ -0,0 +1,317 @@
+/****************************************************************************
+**
+** Copyright (C) 2013 Digia Plc and/or its subsidiary(-ies).
+** Contact: http://www.qt-project.org/legal
+**
+** This file is part of the documentation of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:FDL$
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and Digia.  For licensing terms and
+** conditions see http://qt.digia.com/licensing.  For further information
+** use the contact form at http://qt.digia.com/contact-us.
+**
+** GNU Free Documentation License Usage
+** Alternatively, this file may be used under the terms of the GNU Free
+** Documentation License version 1.3 as published by the Free Software
+** Foundation and appearing in the file included in the packaging of
+** this file.  Please review the following information to ensure
+** the GNU Free Documentation License version 1.3 requirements
+** will be met: http://www.gnu.org/copyleft/fdl.html.
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+/*!
+  \group qmlstatemachine
+  \page qmlstatemachine.html
+  \title The Declarative State Machine Framework
+  \brief An overview of the Declarative State Machine Framework for constructing and executing state graphs.
+
+  \ingroup frameworks-technologies
+
+  \tableofcontents
+
+  The Declarative State Machine Framework provides classes for creating and executing
+  state graphs in QML. The concepts and notation are based on those from Harel's
+  \l{http://www.wisdom.weizmann.ac.il/~dharel/SCANNED.PAPERS/Statecharts.pdf}
+  {Statecharts: A visual formalism for complex systems}, 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 Declarative State Machine Framework provides an API and execution model that can
+  be used to effectively embed the elements and semantics of statecharts in Qml
+  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 Declarative 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.
+
+  For user interfaces with multiple visual states, independent of the
+  application's logical state, consider using QML States and Transitions.
+
+  \section1 QML types in the Declarative State Machine Framework
+
+  These QML types are provided by QML for creating event-driven state machines.
+
+  \annotatedlist qmlstatemachine
+
+  \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
+    \omit
+    \caption This is a caption
+    \endomit
+
+  The following snippet shows the code needed to create such a state machine.
+
+  \snippet qml/statemachine/statemachine-button.qml 0
+
+  The state machine executes asynchronously, i.e. it becomes 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 a top-level
+  final state, the machine will emit the \l{StateBase::finished}{finished}
+  signal and halt.
+
+  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 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.
+
+    \image 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 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 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 qml/statemachine/statemachine-button-nested-ignore-quit.qml 0
+
+  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 (HistoryState 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
+    \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 qml/statemachine/statemachine-button-history.qml 0
+
+  \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
+    \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.
+
+    \image statemachine-parallel.png
+    \omit
+    \caption This is a caption
+    \endomit
+
+  To create a parallel state group, set childMode to QState.ParallelStates.
+
+  \qml
+    StateBase {
+        id: s1
+        childMode: QState.ParallelStates
+        StateBase {
+            id: s11
+        }
+        StateBase {
+            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, 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 FinalState object); when a final child state
+  is entered, the parent state emits the StateBase::finished signal. The
+  following diagram shows a composite state \c s1 which does some processing
+  before entering a final state:
+
+    \image statemachine-finished.png
+    \omit
+    \caption This is a caption
+    \endomit
+
+  When \c s1 's final state is entered, \c s1 will automatically emit
+  \l{StateBase::finished}{finished}. We use a signal transition to cause this event to
+  trigger a state change:
+
+  \qml
+    StateBase {
+        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; 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 StateBase::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:
+
+  \qml
+    Button {
+        id: button
+        text: "button"
+        StateMachine {
+            id: stateMachine
+            initialState: s1
+            running: true
+            StateBase {
+                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, however, s1 would be exited and re-entered each
+  time (e.g. the QAbstractState::entered and QAbstractState::exited
+  signals would be emitted).
+*/