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/****************************************************************************
**
** Copyright (C) 2009 Nokia Corporation and/or its subsidiary(-ies).
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/*!
\example animation/moveblocks
\title Move Blocks Example
The Move Blocks example shows how to animate items in a
QGraphicsScene using a QStateMachine with a custom transition.
\image moveblocks-example.png
The example animates the blue blocks that you can see in the image
above. The animation moves the blocks between four preset positions.
The example consists of the following classes:
\list
\o \c StateSwitcher inherits QState and can add
\c {StateSwitchTransition}s to other states.
When entered, it will randomly transition to one of these
states.
\o \c StateSwitchTransition is a custom transition that
triggers on \c{StateSwitchEvent}s.
\o \c StateSwitchEvent is a QEvent that trigger \c{StateSwitchTransition}s.
\o \c QGraphicsRectWidget is a QGraphicsWidget that simply
paints its background in a solid \l{Qt::}{blue} color.
\endlist
The blocks are instances of \c QGraphicsRectWidget and are
animated in a QGraphicsScene. We do this by building a state
graph, which we insert animations into. The graph is then executed
in a QStateMachine. All this is done in \c main().
Let's look at the \c main() function first.
\section1 The \c main() Function
After QApplication has been initialized, we set up the
QGraphicsScene with its \c{QGraphicsRectWidget}s.
\snippet examples/animation/moveblocks/main.cpp 1
After adding the scene to a QGraphicsView, it is time to build the
state graph. Let's first look at a statechart of what we are
trying to build.
\image move-blocks-chart.png
Note that the \c group has seven sub states, but we have only
included three of them in the diagram. The code that builds this
graph will be examined line-by-line, and will show how the graph
works. First off, we construct the \c group state:
\snippet examples/animation/moveblocks/main.cpp 2
The timer is used to add a delay between each time the blocks are
moved. The timer is started when \c group is entered. As we will
see later, \c group has a transition back to the \c StateSwitcher
when the timer times out. \c group is the initial state in the
machine, so an animation will be scheduled when the example is
started.
\snippet examples/animation/moveblocks/main.cpp 3
\dots
\snippet examples/animation/moveblocks/main.cpp 4
\c createGeometryState() returns a QState that will set the
geometry of our items upon entry. It also assigns \c group as the
parent of this state.
A QPropertyAnimation inserted into a transition will use the
values assigned to a QState (with QState::assignProperty()), i.e.,
the animation will interpolate between the current values of the
properties and the values in the target state. We add animated
transitions to the state graph later.
\snippet examples/animation/moveblocks/main.cpp 5
We move the items in parallel. Each item is added to \c
animationGroup, which is the animation that is inserted into the
transitions.
\snippet examples/animation/moveblocks/main.cpp 6
The sequential animation group, \c subGroup, helps us insert a
delay between the animation of each item.
\snippet examples/animation/moveblocks/main.cpp 7
\dots
\snippet examples/animation/moveblocks/main.cpp 8
A StateSwitchTransition is added to the state switcher
in \c StateSwitcher::addState(). We also add the animation in this
function. Since QPropertyAnimation uses the values from the
states, we can insert the same QPropertyAnimation instance in all
\c {StateSwitchTransition}s.
As mentioned previously, we add a transition to the state switcher
that triggers when the timer times out.
\snippet examples/animation/moveblocks/main.cpp 9
Finally, we can create the state machine, add our initial state,
and start execution of the state graph.
\section2 The \c createGemetryState() Function
In \c createGeometryState(), we set up the geometry for each
graphics item.
\snippet examples/animation/moveblocks/main.cpp 13
As mentioned before, QAbstractTransition will set up an animation
added with \l{QAbstractTransition::}{addAnimation()} using
property values set with \l{QState::}{assignProperty()}.
\section1 The StateSwitcher Class
\c StateSwitcher has state switch transitions to each \l{QState}s
we created with \c createGemetryState(). Its job is to transition
to one of these states at random when it is entered.
All functions in \c StateSwitcher are inlined. We'll step through
its definition.
\snippet examples/animation/moveblocks/main.cpp 10
\c StateSwitcher is a state designed for a particular purpose and
will always be a top-level state. We use \c m_stateCount to keep
track of how many states we are managing, and \c m_lastIndex to
remember which state was the last state to which we transitioned.
\snippet examples/animation/moveblocks/main.cpp 11
We select the next state we are going to transition to, and post a
\c StateSwitchEvent, which we know will trigger the \c
StateSwitchTransition to the selected state.
\snippet examples/animation/moveblocks/main.cpp 12
This is where the magic happens. We assign a number to each state
added. This number is given to both a StateSwitchTransition and to
StateSwitchEvents. As we have seen, state switch events will
trigger a transition with the same number.
\section1 The StateSwitchTransition Class
\c StateSwitchTransition inherits QAbstractTransition and triggers
on \c{StateSwitchEvent}s. It contains only inline functions, so
let's take a look at its \l{QAbstractTransition::}{eventTest()}
function, which is the only function that we define..
\snippet examples/animation/moveblocks/main.cpp 14
\c eventTest is called by QStateMachine when it checks whether a
transition should be triggered--a return value of true means that
it will. We simply check if our assigned number is equal to the
event's number (in which case we fire away).
\section1 The StateSwitchEvent Class
\c StateSwitchEvent inherits QEvent, and holds a number that has
been assigned to a state and state switch transition by \c
StateSwitcher. We have already seen how it is used to trigger \c
\c{StateSwitchTransition}s in \c StateSwitcher.
\snippet examples/animation/moveblocks/main.cpp 15
We only have inlined functions in this class, so a look at its
definition will do.
\section1 The QGraphicsRectWidget Class
QGraphicsRectWidget inherits QGraphicsWidget and simply paints its
\l{QWidget::}{rect()} blue. We inline \l{QWidget::}{paintEvent()},
which is the only function we define. Here is the
QGraphicsRectWidget class definition:
\snippet examples/animation/moveblocks/main.cpp 16
\section1 Moving On
The technique shown in this example works equally well for all
\l{QPropertyAnimation}s. As long as the value to be animated is a
Qt property, you can insert an animation of it into a state graph.
QState::addAnimation() takes a QAbstractAnimation, so any type
of animation can be inserted into the graph.
*/
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