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/*!

   INCOMPLETE

\page qtprogrammers.html
\target qtprogrammers
\title QML for Qt programmers

\section1 Overview

While QML does not require Qt knowledge to use, if you \e are already familar with Qt,
much of your knowledge is directly relevant to learning and using QML. Of course,
an application with a UI defined in QML also uses Qt for all the non-UI logic.

\section1 Familiar Concepts

QML provides direct access to the following concepts from Qt:

\list
 \o QAction - the \l {QML Basic Types}{action} type
 \o QObject signals and slots - available as functions to call in JavaScript
 \o QObject properties - available as variables in JavaScript
 \o QWidget - QmlView is a QML-displaying widget
 \o Qt models - used directly in data binding (QAbstractItemModel and next generation QListModelInterface)
\endlist

Qt knowledge is \e required for \l {Extending QML}, and also for \l{Integrating QML with existing Qt UI code}.

\section1 QML Items compared with QWidgets

QML Items are very similar to QWidgets: they define the look and feel of the user interface. (Note that while QWidgets
haven't traditionally been used to define the look and feel of view delegates, QML Items can be used for this as well.)

There are three structurally different types of QWidget:

\list
 \o Simple widgets that are not used as parents (QLabel, QCheckBox, QToolButton, etc.)
 \o Parent widgets that are normally used as parents to other widgets (QGroupBox, QStackedWidget, QTabWidget, etc.)
 \o Compound widgets that are internally composed of child widgets (QComboBox, QSpinBox, QFileDialog, QTabWidget, etc.)
\endlist

QML Items also serve these purposes. Each is considered separately below.

\section2 Simple Widgets

The most important rule to remember while implementing a new QmlGraphicsItem in C++
is that it should not contain any look and feel policies - leave that to the
QML usage of the item.

As an example, imagine you wanted a reusable Button item. If you therefore
decided to write a QmlGraphicsItem subclass to implement a button,
just as QToolButton subclasses QWidget for this purpose, following the rule above, your
\c QmlGraphicsButton would not have any appearance - just the notions of enabled, triggering, etc.

But there is already an object in Qt that does this: QAction.

QAction is the UI-agnostic essence of QPushButton, QCheckBox, QMenu items, QToolButton,
and other visual widgets that are commonly bound to a QAction.

So, the job of implementing a checkbox abstraction for QML is already done - it's QAction.
The look and feel of an action - the appearance of the button, the transition between states,
and exactly how it respond to mouse, key, or touch input, should all be left for definition
in QML.

It is illustrative to note that QmlGraphicsTextEdit is built upon QTextControl,
QmlGraphicsWebView is built upon QWebPage, and ListView uses QListModelInterface,
just as QTextEdit, QWebView, and QListView are built upon
those same UI-agnostic components.

The encapsulation of the look and feel that QWidgets gives is important, and for this
the QML concept of \l {qmldocuments.html}{components} serves the same purpose. If you are building a complete
suite of applications which should have a consistent look and feel, you should build
a set of reusable components with the look and feel you desire.

So, to implement your reusable button, you would simply build a QML component.


\section2 Parent Widgets

Parent widgets each provide a generic way to interface to one or more arbitrary other widgets.
A QTabWidget provides an interface to multiple "pages", one of which is visible at any time,
and a mechnism for selecting among them (the QTabBar). A QScollArea provides scrollbars around
a widget that is otherwise too large to fit in available space.

Nearly all such components can be created directly in QML. Only a few cases
which require very particular event handling, such as Flickable, require C++ implementations.

As an example, imagine you decided to make a generic tab widget item to be used
through your application suite wherever information is in such quantity that it
needs to be divided up into pages.

A significant difference in the parenting concept with QML compare to QWidgets
is that while child items are positioned relative to their parents,
there is no requirement that they be wholy contained ("clipped") to
the parent (although the clipped property of the child Item does allow
this where it is needed).
This difference has rather far-reaching consequences, for example:

\list
    \o A shadow or highlight around a widget could be a child of that widget.
    \o Particle effects can flow outside the object where they originate.
    \o Transitioning animations can "hide" items by visibly moving them beyond the screen bounds.
\endlist


\section2 Compound Widgets

Some widgets provide functionality by composing other widgets as an "implementation detail",
providing a higher level API to the composition. QSpinBox for example is a line edit and some
buttons to increase/decrease the edited value. QFileDialog uses a whole host of widgets to
give the user a way of finding and selecting a file name.

When developing reusable QML Items, you may choose to do the same: build an item composed
of other items you have already defined.

The only caveat when doing this is to consider the possible animations and transitions that
users of the compound item might wish to employ. For example, a spinbox might need to smoothly
transition from an arbitrary Text item, or characters within a Text item, so your spinbox
item would need to be sufficiently flexible to allow such animation.

\section1 QML Items Compared With QGraphicsWidgets
*/