diff options
Diffstat (limited to 'doc/src/declarative/extending.qdoc')
| -rw-r--r-- | doc/src/declarative/extending.qdoc | 998 |
1 files changed, 998 insertions, 0 deletions
diff --git a/doc/src/declarative/extending.qdoc b/doc/src/declarative/extending.qdoc new file mode 100644 index 0000000..e1c6469 --- /dev/null +++ b/doc/src/declarative/extending.qdoc @@ -0,0 +1,998 @@ +/**************************************************************************** +** +** 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. +** +** $QT_BEGIN_LICENSE:LGPL$ +** No Commercial Usage +** This file contains pre-release code and may not be distributed. +** You may use this file in accordance with the terms and conditions +** contained in the Technology Preview License Agreement accompanying +** this package. +** +** GNU Lesser General Public License Usage +** Alternatively, this file may be used under the terms of the GNU Lesser +** General Public License version 2.1 as published by the Free Software +** Foundation and appearing in the file LICENSE.LGPL included in the +** packaging of this file. Please review the following information to +** ensure the GNU Lesser General Public License version 2.1 requirements +** will be met: http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html. +** +** In addition, as a special exception, Nokia gives you certain additional +** rights. These rights are described in the Nokia Qt LGPL Exception +** version 1.1, included in the file LGPL_EXCEPTION.txt in this package. +** +** 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/extending/adding/example.qml 0 + +The QML snippet shown above instantiates one \c Person instance and sets +the name and 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. + +The QML engine has no intrinsic knowledge of any class types. Instead the +programmer must define the C++ types, and their corresponding QML name. + +Custom C++ types are declared QML types using a macro and a template function: + +\quotation + +\code +#define QML_DECLARE_TYPE(T) +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. + +Generally the QML_DECLARE_TYPE() macro should be included immediately following +the type declaration (usually in its header file), and the template function qmlRegisterType() +called by the implementation. + +Type \a T must be a concrete type that inherits QObject and has a default +constructor. +\endquotation + +Types can be registered by libraries (such as Qt does), application code, +or by plugins (see QDeclarativeExtensionPlugin). + +Once registered, all of the \l {Qt's Property System}{properties} of a supported +type are available for use within 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 + +QML is typesafe. Attempting to assign an invalid value to a property will +generate an error. For example, assuming the 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/extending/properties/example.qml 0 + +The QML snippet shown above assigns a \c Person object to the \c BirthdayParty's +celebrant 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 celebrant property +declaration looks like this: + +\snippet examples/declarative/extending/properties/birthdayparty.h 1 + +As long as the property type, in this case 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 macro and function are used instead: + +\quotation +\code +#define QML_DECLARE_INTERFACE(T) +template<typename T> +int qmlRegisterInterface(const char *typeName) +\endcode + +Registers the C++ interface \a T with the QML system as \a typeName. + +Generally the QML_DECLARE_INTERFACE() macro should be included immediately +following the interface declaration (usually in its header file), and the +qmlRegisterInterface() template function called by the implementation. + +Following registration, QML can coerce objects that implement this interface +for assignment to appropriately typed properties. +\endquotation + +The 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 guest property declaration looks like this: + +\snippet examples/declarative/extending/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/extending/coercion/example.qml 0 + +The QML snippet shown above assigns a \c Boy object to the \c BirthdayParty's +celebrant property, and assigns three other objects to the guests property. + +QML supports C++ inheritance heirarchies 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 celebrant and the guests properties retain the Person +type used in the previous section, but the assignment is valid as both the Boy +and Girl objects inherit from 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 these macro and function can be used: + +\quotation +\code + #define QML_DECLARE_TYPE(T) + template<typename T> + int qmlRegisterType() +\endcode + +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. + +Generally the QML_DECLARE_TYPE() macro should be included immediately following +the type declaration (usually in its header file), and the +qmlRegisterType() template function called from the implementation. + +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/extending/default/example.qml 0 + +The QML snippet shown above assigns a collection of objects to the +\c BirthdayParty's default property. + +The 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 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 + +Mark \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/extending/grouped/example.qml 1 + +The QML snippet shown above assigns a number 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 +shoe property shown is declared like this: + +\snippet examples/declarative/extending/grouped/person.h 1 + +The ShoeDescription type declares the properties available to the grouped +property block - in this case the size, color, brand and 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/extending/attached/example.qml 1 + +The QML snippet shown above assigns the 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 suffix to the property name. + +In the example shown, \c BirthdayParty is called the attaching type, and the +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) +QML_DECLARE_TYPE(MyType) +\endcode +Return 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 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 +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 elements 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 an 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/extending/signal/example.qml 0 +\snippet examples/declarative/extending/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/extending/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. + +\section1 Property Value Sources + +\snippet examples/declarative/extending/valuesource/example.qml 0 +\snippet examples/declarative/extending/valuesource/example.qml 1 + +The QML snippet shown above assigns a property value to the speaker 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 speaker property of the +BirthdayParty object is a string that is printed every time it is assigned and +the HappyBirthday value source generates the lyrics of the song +"Happy Birthday". + +\snippet examples/declarative/extending/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 HappyBirthday type declaration looks like this: + +\snippet examples/declarative/extending/valuesource/happybirthday.h 0 +\snippet examples/declarative/extending/valuesource/happybirthday.h 1 +\snippet examples/declarative/extending/valuesource/happybirthday.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 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 HappyBirthday property value source. + +\section1 Property Binding + +\snippet examples/declarative/extending/binding/example.qml 0 +\snippet examples/declarative/extending/binding/example.qml 1 + +The QML snippet shown above uses a property binding to ensure the +HappyBirthday's name property remains up to date with the celebrant. + +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 +\c {Qt's Property System}{NOTIFY signal} for this determination. + +Here is the celebrant property declaration: + +\snippet examples/declarative/extending/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/extending/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/extending/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 classes are delay created 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 + #define QML_REGISTER_EXTENDED_TYPE(URI, VMAJ, VFROM, VTO, QDeclarativeName,T, ExtendedT) + #define QML_REGISTER_EXTENDED_NOCREATE_TYPE(T, ExtendedT) +\endcode +macros should be used instead of the regular \c QML_REGISTER_TYPE or +\c QML_REGISTER_NOCREATE_TYPE. The arguments are identical to the corresponding +non-extension object macro, 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 + +\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. 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. +*/ |