path: root/doc/src/declarative/extending.qdoc

/****************************************************************************
**
** Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies).
** All rights reserved.
** Contact: Nokia Corporation (qt-info@nokia.com)
**
** This file is part of the documentation of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:FDL$
** Commercial Usage
** Licensees holding valid Qt Commercial licenses may use this file in
** accordance with the Qt Commercial License Agreement provided with the
** Software or, alternatively, in accordance with the terms contained in a
** written agreement between you and Nokia.
**
** GNU Free Documentation License
** 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.
**
** If you have questions regarding the use of this file, please contact
** Nokia at qt-info@nokia.com.
** $QT_END_LICENSE$
**
****************************************************************************/

/*!
\page qml-extending.html
\title Extending QML in C++

The QML syntax declaratively describes how to construct an in-memory object
tree.  In Qt, QML is mainly used to describe a visual scene graph, but it is
not conceptually limited to this: the QML format is an abstract description of
any object tree.  All the QML element types included in Qt are implemented using
the C++ extension mechanisms describe on this page.  Programmers can use these
APIs to add new types that interact with the existing Qt types, or to repurpose
QML for their own independent use.

\tableofcontents

\section1 Adding Types
\target adding-types

\snippet examples/declarative/cppextensions/referenceexamples/adding/example.qml 0

The QML snippet shown above instantiates one \c Person instance and sets
the \c name and \c shoeSize properties on it.  Everything in QML ultimately comes down
to either instantiating an object instance, or assigning a property a value.

QML relies heavily on Qt's meta object system and can only instantiate classes
that derive from QObject. For visual element types, this will usually mean a subclass
of QDeclarativeItem; for models used with the view elements, a subclass of QAbstractItemModel;
and for abitrary objects with properties, a direct subclass of QObject.

The QML engine has no intrinsic knowledge of any class types.  Instead the
programmer must register the C++ types with their corresponding QML names.

Custom C++ types are registered using a template function:

\quotation

\code
template<typename T>
int qmlRegisterType(const char *uri, int versionMajor, int versionMinor, const char *qmlName)
\endcode

Calling qmlRegisterType() registers the C++ type \a T with the QML
system, and makes it available in QML under the name \a qmlName in
library \a uri version \a versionMajor.versionMinor.  The \a qmlName
can be the same as the C++ type name.

Type \a T must be a concrete type that inherits QObject and has a default
constructor.

\endquotation

Types can be registered by libraries, application code, or by plugins
(see QDeclarativeExtensionPlugin).

Once registered, all \l {Qt's Property System}{properties} of the
supported types are available in QML.  QML has intrinsic support for
properties of these types:

\list
\o bool
\o unsigned int, int
\o float, double, qreal
\o QString
\o QUrl
\o QColor
\o QDate, QTime, QDateTime
\o QPoint, QPointF
\o QSize, QSizeF
\o QRect, QRectF
\o QVariant
\endlist

When a property of a supported type is added to a C++ class, in a QML
element based on the C++ class, a \e{value-changed} signal handler
will be available. See \l{Signal Support} below.

QML is typesafe.  Attempting to assign an invalid value to a property
will generate an error.  For example, assuming the \e{name} property
of the \c Person element had a type of QString, this would cause an
error:

\code
Person {
    // Will NOT work
    name: 12
}
\endcode

\l {Extending QML - Adding Types Example} shows the complete code used to create
the \c Person type.

\section1 Object and List Property Types

\snippet examples/declarative/cppextensions/referenceexamples/properties/example.qml 0

The QML snippet shown above assigns a \c Person object to the \c BirthdayParty's
\c host property, and assigns three \c Person objects to the guests property.

QML can set properties of types that are more complex than basic intrinsics like
integers and strings.  Properties can also be object pointers, Qt interface
pointers,  lists of object points, and lists of Qt interface pointers.  As QML
is typesafe it ensures that only valid types are assigned to these properties,
just like it does for primitive types.

Properties that are pointers to objects or Qt interfaces are declared with the
Q_PROPERTY() macro, just like other properties.  The \c host property
declaration looks like this:

\snippet examples/declarative/cppextensions/referenceexamples/properties/birthdayparty.h 1

As long as the property type, in this case \c Person, is registered with QML the
property can be assigned.

QML also supports assigning Qt interfaces.  To assign to a property whose type
is a Qt interface pointer, the interface must also be registered with QML.  As
they cannot be instantiated directly, registering a Qt interface is different
from registering a new QML type. The following function is used instead:

\quotation
\code
template<typename T>
int qmlRegisterInterface(const char *typeName)
\endcode

This registers the C++ interface \a T with the QML system as \a typeName.

Following registration, QML can coerce objects that implement this interface
for assignment to appropriately typed properties.
\endquotation

The \c guests property is a list of \c Person objects.  Properties that are lists
of objects or Qt interfaces are also declared with the Q_PROPERTY() macro, just
like other properties.  List properties must have the type \c {QDeclarativeListProperty<T>}.
As with object properties, the type \a T must be registered with QML.

The \c guest property declaration looks like this:

\snippet examples/declarative/cppextensions/referenceexamples/properties/birthdayparty.h 2

\l {Extending QML - Object and List Property Types Example} shows the complete
code used to create the \c BirthdayParty type.

\section1 Inheritance and Coercion

\snippet examples/declarative/cppextensions/referenceexamples/coercion/example.qml 0

The QML snippet shown above assigns a \c Boy object to the \c BirthdayParty's
\c host property, and assigns three other objects to the \c guests property.

QML supports C++ inheritance hierarchies and can freely coerce between known,
valid object types.  This enables the creation of common base classes that allow
the assignment of specialized classes to object or list properties.  In the
snippet shown, both the \c host and the \c guests properties retain the \c Person
type used in the previous section, but the assignment is valid as both the \c Boy
and \c Girl objects inherit from \c Person.

To assign to a property, the property's type must have been registered with QML.
Both the qmlRegisterType() and qmlRegisterInterface() template functions already
shown can be used to register a type with QML.  Additionally, if a type that acts purely
as a base class that cannot be instantiated from QML needs to be
registered, the following function can be used:

\quotation
\code
    template<typename T>
    int qmlRegisterType()
\endcode

This registers the C++ type \a T with the QML system. The parameterless call to the template
function qmlRegisterType() does not define a mapping between the
C++ class and a QML element name, so the type is not instantiable from QML, but
it is available for type coercion.

Type \a T must inherit QObject, but there are no restrictions on whether it is
concrete or the signature of its constructor.
\endquotation

QML will automatically coerce C++ types when assigning to either an object
property, or to a list property.  Only if coercion fails does an assignment
error occur.

\l {Extending QML - Inheritance and Coercion Example} shows the complete
code used to create the \c Boy and \c Girl types.

\section1 Default Property

\snippet examples/declarative/cppextensions/referenceexamples/default/example.qml 0

The QML snippet shown above assigns a collection of objects to the
\c BirthdayParty's default property.

The \e {default property} is a syntactic convenience that allows a type designer to
specify a single property as the type's default.  The default property is
assigned to whenever no explicit property is specified.  As a convenience, it is
behaviorally identical to assigning to the default property explicitly by name.

From C++, type designers mark the default property using a Q_CLASSINFO()
annotation:

\quotation
\code
Q_CLASSINFO("DefaultProperty", "property")
\endcode

This marks \a property as the class's default property.  \a property must be either
an object property, or a list property.

A default property is optional.  A derived class inherits its base class's
default property, but may override it in its own declaration.  \a property can
refer to a property declared in the class itself, or a property inherited from a
base class.
\endquotation

\l {Extending QML - Default Property Example} shows the complete code used to
specify a default property.

\section1 Grouped Properties

\snippet examples/declarative/cppextensions/referenceexamples/grouped/example.qml 1

The QML snippet shown above assigns a number of properties to the \c Boy object,
including four properties using the grouped property syntax.

Grouped properties collect similar properties together into a single named
block.  Grouped properties can be used to present a nicer API to developers, and
may also simplify the implementation of common property collections across
different types through implementation reuse.

A grouped property block is implemented as a read-only object property.  The
\c shoe property shown is declared like this:

\snippet examples/declarative/cppextensions/referenceexamples/grouped/person.h 1

The \c ShoeDescription type declares the properties available to the grouped
property block - in this case the \c size, \c color, \c brand and \c price properties.

Grouped property blocks may declared and accessed be recusively.

\l {Extending QML - Grouped Properties Example} shows the complete code used to
implement the \c shoe property grouping.

\section1 Attached Properties

\snippet examples/declarative/cppextensions/referenceexamples/attached/example.qml 1

The QML snippet shown above assigns a date to the \c rsvp property using the attached
property syntax.

Attached properties allow unrelated types to annotate other types with some
additional properties, generally for their own use.  Attached properties are
identified through the use of the attacher type name, in the case shown
\c BirthdayParty, as a prefix to the property name.

In the example shown, \c BirthdayParty is called the attaching type, and the
\c Boy instance the attachee object instance.

For the attaching type, an attached property block is implemented as a new
QObject derived type, called the attachment object.  The properties on the
attachment object are those that become available for use as the attached
property block.

Any QML type can become an attaching type by declaring the
\c qmlAttachedProperties() public function and declaring that the class has
QML_HAS_ATTACHED_PROPERTIES:

\quotation
\code
class MyType : public QObject {
    Q_OBJECT
public:

    ...

    static AttachedPropertiesType *qmlAttachedProperties(QObject *object);
};

QML_DECLARE_TYPEINFO(MyType, QML_HAS_ATTACHED_PROPERTIES)
\endcode
This returns an attachment object, of type \a AttachedPropertiesType, for the
attachee \a object instance.  It is customary, though not strictly required, for
the attachment object to be parented to \a object to prevent memory leaks.

\a AttachedPropertiesType must be a QObject derived type.  The properties on
this type will be accessible through the attached properties syntax.

This method will be called at most once for each attachee object instance.  The
QML engine will cache the returned instance pointer for subsequent attached
property accesses.  Consequently the attachment object may not be deleted until
\a object is destroyed.
\endquotation

Conceptually, attached properties are a \e type exporting a set of additional
properties that can be set on \e any other object instance.  Attached properties
cannot be limited to only attaching to a sub-set of object instances, although
their effect may be so limited.

For example, a common usage scenario is for a type to enhance the properties
available to its children in order to gather instance specific data.  Here we
add a \c rsvp field to all the guests coming to a birthday party:
\code
BirthdayParty {
    Boy { BirthdayParty.rsvp: "2009-06-01" }
}
\endcode
However, as a type cannot limit the instances to which the attachment object
must attach, the following is also allowed, even though adding a birthday party
rsvp in this context will have no effect.
\code
GraduationParty {
    Boy { BirthdayParty.rsvp: "2009-06-01" }
}
\endcode

From C++, including the attaching type implementation, the attachment object for
an instance can be accessed using the following method:

\quotation
\code
template<typename T>
QObject *qmlAttachedPropertiesObject<T>(QObject *attachee, bool create = true);
\endcode
This returns the attachment object attached to \a attachee by the attaching type
\a T.  If type \a T is not a valid attaching type, this method always returns 0.

If \a create is true, a valid attachment object will always be returned,
creating it if it does not already exist.  If \a create is false, the attachment
object will only be returned if it has previously been created.
\endquotation

\l {Extending QML - Attached Properties Example} shows the complete code used to
implement the rsvp attached property.

\section1 Memory Management and QVariant types

It is an element's responsibility to ensure that it does not access or return
pointers to invalid objects.  QML makes the following guarentees:

\list
\o An object assigned to a QObject (or QObject-derived) pointer property will be
valid at the time of assignment.  

Following assignment, it is the responsibility of the class to subsequently guard 
this pointer, either through a class specific method or the generic QPointer class.

\o An object assigned to a QVariant will be valid at the time of assignment.  

When assigning an object to a QVariant property, QML will always use a QMetaType::QObjectStar 
typed QVariant.  It is the responsibility of the class to guard the pointer.  A 
general rule when writing a class that uses QVariant properties is to check the 
type of the QVariant when it is set and if the type is not handled by your class, 
reset it to an invalid variant.

\o An object assigned to a QObject (or QObject-derived) list property will be 
valid at the time of assignment.  

Following assignment, it is the responsibility of the class to subsequently guard 
this pointer, either through a class specific method or the generic QPointer class.
\endlist

Elements should assume that any QML assigned object can be deleted at any time, and
respond accordingly.  If documented as such an element need not continue to work in 
this situation, but it must not crash.

\section1 Signal Support

\snippet examples/declarative/cppextensions/referenceexamples/signal/example.qml 0
\snippet examples/declarative/cppextensions/referenceexamples/signal/example.qml 1

The QML snippet shown above associates the evaluation of a JavaScript expression
with the emission of a Qt signal.

All Qt signals on a registered class become available as special "signal
properties" within QML to which the user can assign a single JavaScript
expression.  The signal property's name is a transformed version of the Qt
signal name: "on" is prepended, and the first letter of the signal name upper
cased.  For example, the signal used in the example above has the following
C++ signature:

\snippet examples/declarative/cppextensions/referenceexamples/signal/birthdayparty.h 0

In classes with multiple signals with the same name, only the final signal
is accessible as a signal property.  Note that signals with the same name
but different parameters cannot be distinguished.

Signal parameters become accessible by name to the assigned script.  An
unnamed parameter cannot be accessed, so care should be taken to name all the
signal parameters in the C++ class declaration.  The intrinsic types
listed in \l {Adding Types}, as well registered object types are permitted as
signal parameter types.  Using other types is not an error, but the parameter
value will not be accessible from script.

\l {Extending QML - Signal Support Example} shows the complete code used to
implement the onPartyStarted signal property.

If you want to use signals from items not created in QML, you can access their
signals with the \l {Connections} element.

Additionally, if a property is added to a C++ class, all QML elements
based on that C++ class will have a \e{value-changed} signal handler
for that property. The name of the signal handler is
\e{on<Property-name>Changed}, with the first letter of the property
name being upper case.

\note The QML signal handler will always be named
on<Property-name>Changed, regardless of the name used for the NOTIFY
signal in C++. We recommend using <property-name>Changed() for the
NOTIFY signal in C++.

See also \l {Extending types from QML}.

\section1 Property Value Sources

\snippet examples/declarative/cppextensions/referenceexamples/valuesource/example.qml 0
\snippet examples/declarative/cppextensions/referenceexamples/valuesource/example.qml 1

The QML snippet shown above applies a property value source to the \c announcment property.
A property value source generates a value for a property that changes over time.

Property value sources are most commonly used to do animation.  Rather than
constructing an animation object and manually setting the animation's "target"
property, a property value source can be assigned directly to a property of any
type and automatically set up this association.

The example shown here is rather contrived: the \c announcment property of the
\c BirthdayParty object is a string that is printed every time it is assigned and
the \c HappyBirthdaySong value source generates the lyrics of the song
"Happy Birthday".

\snippet examples/declarative/cppextensions/referenceexamples/valuesource/birthdayparty.h 0

Normally, assigning an object to a string property would not be allowed.  In
the case of a property value source, rather than assigning the object instance
itself, the QML engine sets up an association between the value source and
the property.

Property value sources are special types that derive from the
QDeclarativePropertyValueSource base class.  This base class contains a single method,
QDeclarativePropertyValueSource::setTarget(), that the QML engine invokes when
associating the property value source with a property.  The relevant part of
the \c HappyBirthdaySong type declaration looks like this:

\snippet examples/declarative/cppextensions/referenceexamples/valuesource/happybirthdaysong.h 0
\snippet examples/declarative/cppextensions/referenceexamples/valuesource/happybirthdaysong.h 1
\snippet examples/declarative/cppextensions/referenceexamples/valuesource/happybirthdaysong.h 2

In all other respects, property value sources are regular QML types.  They must
be registered with the QML engine using the same macros as other types, and can
contain properties, signals and methods just like other types.

When a property value source object is assigned to a property, QML first tries
to assign it normally, as though it were a regular QML type.  Only if this
assignment fails does the engine call the \l {QDeclarativePropertyValueSource::}{setTarget()} method.  This allows
the type to also be used in contexts other than just as a value source.

\l {Extending QML -  Property Value Source Example} shows the complete code used
implement the \c HappyBirthdaySong property value source.

\section1 Property Binding

\snippet examples/declarative/cppextensions/referenceexamples/binding/example.qml 0
\snippet examples/declarative/cppextensions/referenceexamples/binding/example.qml 1

The QML snippet shown above uses a property binding to ensure the
\c HappyBirthdaySong's \c name property remains up to date with the \c host.

Property binding is a core feature of QML.  In addition to assigning literal
values, property bindings allow the developer to assign an arbitrarily complex
JavaScript expression that may include dependencies on other property values.
Whenever the expression's result changes - through a change in one of its
constituent values - the expression is automatically reevaluated and
the new result assigned to the property.

All properties on custom types automatically support property binding.  However,
for binding to work correctly, QML must be able to reliably determine when a
property has changed so that it knows to reevaluate any bindings that depend on
the property's value.  QML relies on the presence of a
\l {Qt's Property System}{NOTIFY signal} for this determination.

Here is the \c host property declaration:

\snippet examples/declarative/cppextensions/referenceexamples/binding/birthdayparty.h 0

The NOTIFY attribute is followed by a signal name.  It is the responsibility of
the class implementer to ensure that whenever the property's value changes, the
NOTIFY signal is emitted.  The signature of the NOTIFY signal is not important to QML.

To prevent loops or excessive evaluation, developers should ensure that the
signal is only emitted whenever the property's value is actually changed.  If
a property, or group of properties, is infrequently used it is permitted to use
the same NOTIFY signal for several properties.  This should be done with care to
ensure that performance doesn't suffer.

To keep QML reliable, if a property does not have a NOTIFY signal, it cannot be
used in a binding expression.  However, the property can still be assigned
a binding as QML does not need to monitor the property for change in that
scenario.

Consider a custom type, \c TestElement, that has two properties, "a" and "b".
Property "a" does not have a NOTIFY signal, and property "b" does have a NOTIFY
signal.

\code
TestElement {
    // This is OK
    a: b
}
TestElement {
    // Will NOT work
    b: a
}
\endcode

The presence of a NOTIFY signal does incur a small overhead.  There are cases
where a property's value is set at object construction time, and does not
subsequently change.  The most common case of this is when a type uses
\l {Grouped Properties}, and the grouped property object is allocated once, and
only freed when the object is deleted.  In these cases, the CONSTANT attribute
may be added to the property declaration instead of a NOTIFY signal.

\snippet examples/declarative/cppextensions/referenceexamples/binding/person.h 0

Extreme care must be taken here or applications using your type may misbehave.
The CONSTANT attribute should only be used for properties whose value is set,
and finalized, only in the class constructor.  All other properties that want
to be used in bindings should have a NOTIFY signal instead.

\l {Extending QML -  Binding Example} shows the BirthdayParty example updated to
include NOTIFY signals for use in binding.

\section1 Extension Objects

\snippet examples/declarative/cppextensions/referenceexamples/extended/example.qml 0

The QML snippet shown above adds a new property to an existing C++ type without
modifying its source code.

When integrating existing classes and technology into QML, their APIs will often
need to be tweaked to fit better into the declarative environment.  Although
the best results are usually obtained by modifying the original classes
directly, if this is either not possible or is complicated by some other
concerns, extension objects allow limited extension possibilities without
direct modifications.

Extension objects are used to add additional properties to an existing type.
Extension objects can only add properties, not signals or methods.  An extended
type definition allows the programmer to supply an additional type - known as the
extension type - when registering the target class whose properties are
transparently merged with the original target class when used from within QML.

An extension class is a regular QObject, with a constructor that takes a QObject
pointer.  When needed (extension class creation is delayed until the first extended
property is accessed) the extension class is created and the target object is
passed in as the parent.  When an extended property on the original is accessed,
the appropriate property on the extension object is used instead.

When an extended type is installed, one of the
\code
template<typename T, typename ExtendedT>
int qmlRegisterType(const char *uri, int versionMajor, int versionMinor, const char *qmlName)

template<typename T, typename ExtendedT>
int qmlRegisterType()
\endcode
functions should be used instead of the regular \c qmlRegisterType() variations.
The arguments are identical to the corresponding non-extension registration functions,
except for the ExtendedT parameter which is the type
of the extension object.

\section1 Optimization

Often to develop high performance elements it is helpful to know more about the
status of the QML engine. For example, it might be beneficial to delay
initializing some costly data structures until after all the properties have been
set.

The QML engine defines an interface class called QDeclarativeParserStatus, which contains a
number of virtual methods that are invoked at various stages during component
instantiation.  To receive these notifications, an element implementation inherits
QDeclarativeParserStatus and notifies the Qt meta system using the Q_INTERFACES() macro.

For example,

\code
class Example : public QObject, public QDeclarativeParserStatus
{
    Q_OBJECT
    Q_INTERFACES(QDeclarativeParserStatus)
public:
    virtual void componentComplete()
    {
        qDebug() << "Woohoo!  Now to do my costly initialization";
    }
};
\endcode

*/

/*!
\page qml-extending-types.html
\title Extending types from QML

Many of the elements available for use in QML are implemented in
\l {Extending QML in C++}{C++}.  These types are know as "core types".  QML
allows programmers to build new, fully functional elements without using C++.
Existing core types can be extended, and new types defined entirely in the QML
language.

\tableofcontents

\section1 Adding new properties

New properties can be added to an existing type.  These new properties are
available for use within QML, and also appear as regular Qt properties on the
C++ object, accessible through the regular property access mechanisms.

Like all properties in QML, custom properties are typed.  The type is used to
define the property's behavior, and also determines the C++ type of the created
Qt property.  The following table shows the list of types available when
declaring a new property, and the corresponding C++ type.

\table
\header \o QML Type Name \o C++ Type Name
\row \o int \o int
\row \o bool \o bool
\row \o double \o double
\row \o real \o double
\row \o string \o QString
\row \o url \o QUrl
\row \o color \o QColor
\row \o date \o QDateTime
\row \o variant \o QVariant
\endtable

QML supports two methods for adding a new property to a type: a new property
definition, and a property alias.

\section2 Property definitions

Property definitions add a new property to an existing type.  The storage of the
property is managed by QML.  The defined property may be read, written and bound
to and from.

The syntax for defining a new property is:
\code
    [default] property <type> <name>[: defaultValue]
\endcode

This declaration may appear anywhere within a type body, but it is customary to
include it at the top.  Attempting to declare two properties with the same name
in the same type block is an error.  However, a new property may reuse the name
of an existing property on the type.  This should be done with caution, as the
existing property will be hidden, and become inaccessible.

The <type> must be one of the QML type names shown in the above table.
Additionally, an optional default value of the property can be provided.  The
default value is a convenient shortcut, but is behaviorally identical to doing
it in two steps, like this:

\code
    // Use default value
    property int myProperty: 10

    // Longer, but behaviorally identical
    property int myProperty
    myProperty: 10
\endcode

If a default value is not supplied or set later in the file, each type has a
default value for when none is explictly set. Below are the default values 
of some of the types. For the remaining types the default values are undefined.

\table
\header \o QML Type \o Default Value
\row \o bool \o false
\row \o int \o 0
\row \o double, real \o 0.0
\row \o string, url \o "" (an empty string)
\row \o color \o #000000 (black)
\endtable

If specified, the optional "default" attribute marks the new property as the
types default property, overriding any existing default property.  Using the
default attribute twice in the same type block is an error.

The following example shows how to declare a new "innerColor" property that
controls the color of the inner rectangle.

\code
    Rectangle {
        property color innerColor: "black"

        color: "red"; width: 100; height: 100
        Rectangle {
            anchors.centerIn: parent
            width: parent.width - 10
            height: parent.height - 10
            color: innerColor
        }
    }
\endcode

Adding a property to an item automatically adds a \e{value-changed}
signal handler to the item. The signal hander is named
\c{on<Property_name>Changed}, with the first letter of the property
name being upper case.

Signal handlers can have arbitrary JavaScript code assigned. The following 
example shows how to output to a text console a new value of property 
\c{innerColor} whenever the value of this property changes.

\code
    Rectangle {
        id: rect
        property color innerColor: "black"

        onInnerColorChanged: { console.log(rect.innerColor); }
    }
\endcode

\target qml-property-aliases
\section2 Property aliases

Property aliases are a more advanced form of property declaration.  Unlike a
property definition, that allocates a new, unique storage space for the
property, a property alias connects the newly declared property (called the
aliasing property) to an existing property (the aliased property).  Read
operations on the aliasing property act as read operations on the aliased
property, and write operations on the aliasing property as write operations on
the aliased property.

A property alias declaration looks a lot like a property definition:
\code
    [default] property alias <name>: <alias reference>
\endcode

As the aliasing property has the same type as the aliased property, an explicit
type is omitted, and the special "alias" keyword is used.  Instead of a default
value, a property alias includes a compulsary alias reference.  The alias
reference is used to locate the aliased property.  While similar to a property
binding, the alias reference syntax is highly restricted.

An alias reference takes one of the following forms
\code
    <id>.<property>
    <id>
\endcode

where <id> must refer to an object id within the same component as the type
declaring the alias, and, optionally, <property> refers to a property on that object.

Here is the property definition example rewritten to use property aliases.
\code
Rectangle {
    property alias innerColor: innerRect.color

    color: "red"; width: 100; height: 100
    Rectangle {
        id: innerRect
        anchors.centerIn: parent
        width: parent.width - 10
        height: parent.height - 10
        color: "black"
    }
}
\endcode

Aliases are most useful when \l {Defining new Components}.  Consequently
they have several apparent limitations that only make sense in this context.

Aliases are only activated once the component specifying them is completed.  The
most obvious consequence of this is that the component itself cannot generally
use the aliased property directly.  For example, this will not work:

\code
    // Does NOT work
    property alias innerColor: innerRect.color
    innerColor: "black"
\endcode

This behavior is required to allow type developers to redefine the behavior
of existing property names while continuing to use the existing behavior within
the type they are building, something that is not possible with property
definitions.  In the example used so far, this could allows the developer to fix
the external rectangle's color as "red" and redefine the "color" property to
refer to the inner rectangle, like this:

\code
Rectangle {
    property alias color: innerRect.color

    color: "red"; width: 100; height: 100
    Rectangle {
        id: innerRect
        anchors.centerIn: parent
        width: parent.width - 10
        height: parent.height - 10
        color: "black"
    }
}
\endcode

Users of this type would not be able to affect the color of the red rectangle,
but would find using the "color" property, rather than the strange new
"innerColor" property, much more familiar.

A second, much less significant, consequence of the delayed activation of
aliases is that an alias reference cannot refer to another aliasing property
declared within the same component.  This will not work:

\code
    // Does NOT work
    id: root
    property alias innerColor: innerRect.color
    property alias innerColor2: root.innerColor
\endcode

From outside the component, aliasing properties appear as regular Qt properties
and consequently can be used in alias references.

\section1 Adding new signals

New signals can be added to an existing type.  These new signals are available
for use within QML, and also appear as regular Qt signals on the C++ object that
can be used in Qt signal/slot connections.

The syntax for defining a new signal is:
\code
signal <name>[([<type> <parameter name>[, ...]])]
\endcode

This declaration may appear anywhere within a type body, but it is customary to
include it at the top.  Attempting to declare two signals or methods with the
same name in the same type block is an error.  However, a new signal may reuse
the name of an existing signal on the type.  This should be done with caution,
as the existing signal may be hidden and become inaccessible.

The options for parameter types are the same as for property types (see
\l {Adding new properties}.  If this signal has no parameters, the parameter
list may be omitted entirely.

Here are three examples of signal declarations:
\code
    Item {
        signal clicked
        signal hovered()
        signal performAction(string action, variant actionArgument)
    }
\endcode

Adding a signal to an item automatically adds a signal handler to it.
The signal hander is named on<Signal name>, with the first letter of the
signal name being upper cased. The above example item would now have the
following signal handlers:

\list
   \o onClicked
   \o onHovered
   \o onPerformAction
\endlist

\section1 Adding new methods

New methods can be added to an existing type.  These new methods are available
for use within QML, and also appear as regular Qt slots on the C++ object that
can be used in Qt signal/slot connections.

\code
function <name>([<parameter name>[, ...]]) { <body> }
\endcode

This declaration may appear anywhere within a type body, but it is customary to
include it at the top.  Attempting to declare two methods or signals with the
same name in the same type block is an error.  However, a new method may reuse
the name of an existing method on the type.  This should be done with caution,
as the existing method may be hidden and become inaccessible.

Methods parameters are not typed.  In C++ these parameters are of type QVariant.
The body of the method is written in JavaScript and may access the parameters by
name.

This example adds a new method that behaves like a child:
\code
Item {
    function say(text) {
        console.log("You said " + text);
    }
}
\endcode

\section1 Defining new Components
\target components

A component is a reusable type with a well-defined interface built entirely in
QML.  Components appear as regular QML elements, and can be used interchangably
with core types.  Components allow developers to create new types to be reused
in other projects without the use of C++.  Components can also help to reduce
duplication inside one project by limiting the need for large numbers of
copy-and-pasted blocks.

Any snippet of QML code can become a component, just by placing it in the file "<Name>.qml" 
where <Name> is the new element name, and begins with an uppercase letter.  Note that
the case of all characters in the <Name> are significant on some filesystems, notably
UNIX filesystems.  It is recommended that the case of the filename matches the case of
the component name in QML exactly, regardless of the platform the QML will be deployed to.

These QML files automatically become available as new QML element types
to other QML components and applications in the same directory.

For example, here we show how a component named "Box" is defined and used
multiple times by an application.

\table
\row
\o application.qml
\code
Rectangle {
    width: 100; height: 400;
    Box { x: 0; y: 0 }
    Box { x: 0; y: 150 }
    Box { x: 0; y: 300 }
}
\endcode
\o Box.qml
\code
Rectangle {
    width: 100; height: 100;
    color: "blue"
}
\endcode
\endtable

Components may be collected into \l {Modules} that gives the
developer more freedom than just putting files in the same directory.

\section2 Building reusable components

A component type built to be reused by others must have a well defined
interface.  In QML, an interface consists of a defined collection of
properties, signals and methods.  Users of a component have access to all the
properties, signals and methods defined on the root element of the component.

In the component example above, the root element of the "Box" component is a
Rect.  As the Rect type has a "color" property, this property is accessible to
users of the Box component.  For example, the application.qml can be modified
to show three different colored boxes like this:
\code
Rectangle {
    width: 100; height: 400;
    Box { x: 0; y: 0; color: "red"; }
    Box { x: 0; y: 150; color: "yellow";  }
    Box { x: 0; y: 300; color: "green"; }
}
\endcode

As expected, adding additional properties to the root element of Box, makes them
available externally.  Here we add a "text" property:

\table
\row
\o application.qml
\code
Rectangle {
    width: 100; height: 400;
    Box { x: 0; y: 0; color: "red"; text: "stop" }
    Box { x: 0; y: 150; color: "yellow"; text: "slow" }
    Box { x: 0; y: 300; color: "green"; text: "go" }
}
\endcode
\o Box.qml
\code
Rectangle {
    property alias text: myText.text
    width: 100; height: 100;
    color: "blue"
    Text {
        id: myText
        anchors.centerIn: parent
    }
}
\endcode
\endtable

Methods and signals may be added in the same way.

As all external methods, signals and properties are accessible to external
users, developers should ensure that setting these properties does not have
any undesirable side-effects.  For most resiliance, root level properties should
only be used for literal default values.  When a root level property must be
used inside the component - such as the children property - property aliases
can be used to redirect this property to a "safe" location for external users.
Try to think of the root level properties as being "owned" by the components
user, rather than the component itself.
*/