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QDeclarativeComponent Class Reference

The QDeclarativeComponent class encapsulates a QML component definition. More...

 #include <QDeclarativeComponent>

Inherits: QObject.

This class was introduced in Qt 4.7.

Public Types

enum Status { Null, Ready, Loading, Error }

Properties

  • 1 property inherited from QObject

Public Functions

QDeclarativeComponent ( QDeclarativeEngine * engine, QObject * parent = 0 )
QDeclarativeComponent ( QDeclarativeEngine * engine, const QString & fileName, QObject * parent = 0 )
QDeclarativeComponent ( QDeclarativeEngine * engine, const QUrl & url, QObject * parent = 0 )
virtual ~QDeclarativeComponent ()
virtual QObject * beginCreate ( QDeclarativeContext * context )
virtual void completeCreate ()
virtual QObject * create ( QDeclarativeContext * context = 0 )
QDeclarativeContext * creationContext () const
QList<QDeclarativeError> errors () const
bool isError () const
bool isLoading () const
bool isNull () const
bool isReady () const
void loadUrl ( const QUrl & url )
qreal progress () const
void setData ( const QByteArray & data, const QUrl & url )
Status status () const
QUrl url () const
  • 29 public functions inherited from QObject

Signals

void progressChanged ( qreal progress )
void statusChanged ( QDeclarativeComponent::Status status )

Protected Functions

Q_REVISION ()
  • 7 protected functions inherited from QObject

Additional Inherited Members

  • 1 public slot inherited from QObject
  • 5 static public members inherited from QObject

Detailed Description

The QDeclarativeComponent class encapsulates a QML component definition.

Components are reusable, encapsulated QML elements with well-defined interfaces. They are often defined in Component Files.

A QDeclarativeComponent instance can be created from a QML file. For example, if there is a main.qml file like this:

 import QtQuick 1.0

 Item {
     width: 200
     height: 200
 }

The following code loads this QML file as a component, creates an instance of this component using create(), and then queries the Item's width value:

 QDeclarativeEngine *engine = new QDeclarativeEngine;
 QDeclarativeComponent component(engine, QUrl::fromLocalFile("main.qml"));

 QObject *myObject = component.create();
 QDeclarativeItem *item = qobject_cast<QDeclarativeItem*>(myObject);
 int width = item->width();  // width = 200

Network Components

If the URL passed to QDeclarativeComponent is a network resource, or if the QML document references a network resource, the QDeclarativeComponent has to fetch the network data before it is able to create objects. In this case, the QDeclarativeComponent will have a Loading status. An application will have to wait until the component is Ready before calling QDeclarativeComponent::create().

The following example shows how to load a QML file from a network resource. After creating the QDeclarativeComponent, it tests whether the component is loading. If it is, it connects to the QDeclarativeComponent::statusChanged() signal and otherwise calls the continueLoading() method directly. Note that QDeclarativeComponent::isLoading() may be false for a network component if the component has been cached and is ready immediately.

 MyApplication::MyApplication()
 {
     // ...
     component = new QDeclarativeComponent(engine, QUrl("http://www.example.com/main.qml"));
     if (component->isLoading())
         QObject::connect(component, SIGNAL(statusChanged(QDeclarativeComponent::Status)),
                          this, SLOT(continueLoading()));
     else
         continueLoading();
 }

 void MyApplication::continueLoading()
 {
     if (component->isError()) {
         qWarning() << component->errors();
     } else {
         QObject *myObject = component->create();
     }
 }

See also Using QML Bindings in C++ Applications and Integrating QML Code with Existing Qt UI Code.

Member Type Documentation

enum QDeclarativeComponent::Status

Specifies the loading status of the QDeclarativeComponent.

ConstantValueDescription
QDeclarativeComponent::Null0This QDeclarativeComponent has no data. Call loadUrl() or setData() to add QML content.
QDeclarativeComponent::Ready1This QDeclarativeComponent is ready and create() may be called.
QDeclarativeComponent::Loading2This QDeclarativeComponent is loading network data.
QDeclarativeComponent::Error3An error has occurred. Call errors() to retrieve a list of {QDeclarativeError}{errors}.

Property Documentation

progress : const qreal

The progress of loading the component, from 0.0 (nothing loaded) to 1.0 (finished).

Access functions:

qreal progress () const

Notifier signal:

void progressChanged ( qreal progress )

status : const Status

The component's current status.

Access functions:

Status status () const

Notifier signal:

void statusChanged ( QDeclarativeComponent::Status status )

url : const QUrl

The component URL. This is the URL passed to either the constructor, or the loadUrl() or setData() methods.

Access functions:

QUrl url () const

Member Function Documentation

QDeclarativeComponent::QDeclarativeComponent ( QDeclarativeEngine * engine, QObject * parent = 0 )

Create a QDeclarativeComponent with no data and give it the specified engine and parent. Set the data with setData().

QDeclarativeComponent::QDeclarativeComponent ( QDeclarativeEngine * engine, const QString & fileName, QObject * parent = 0 )

Create a QDeclarativeComponent from the given fileName and give it the specified parent and engine.

See also loadUrl().

QDeclarativeComponent::QDeclarativeComponent ( QDeclarativeEngine * engine, const QUrl & url, QObject * parent = 0 )

Create a QDeclarativeComponent from the given url and give it the specified parent and engine.

Ensure that the URL provided is full and correct, in particular, use QUrl::fromLocalFile() when loading a file from the local filesystem.

See also loadUrl().

QDeclarativeComponent::~QDeclarativeComponent () [virtual]

Destruct the QDeclarativeComponent.

QDeclarativeComponent::Q_REVISION () [protected]

QObject * QDeclarativeComponent::beginCreate ( QDeclarativeContext * context ) [virtual]

This method provides more advanced control over component instance creation. In general, programmers should use QDeclarativeComponent::create() to create a component.

Create an object instance from this component. Returns 0 if creation failed. context specifies the context within which to create the object instance.

When QDeclarativeComponent constructs an instance, it occurs in three steps:

  1. The object hierarchy is created, and constant values are assigned.
  2. Property bindings are evaluated for the the first time.
  3. If applicable, QDeclarativeParserStatus::componentComplete() is called on objects.

QDeclarativeComponent::beginCreate() differs from QDeclarativeComponent::create() in that it only performs step 1. QDeclarativeComponent::completeCreate() must be called to complete steps 2 and 3.

This breaking point is sometimes useful when using attached properties to communicate information to an instantiated component, as it allows their initial values to be configured before property bindings take effect.

void QDeclarativeComponent::completeCreate () [virtual]

This method provides more advanced control over component instance creation. In general, programmers should use QDeclarativeComponent::create() to create a component.

Complete a component creation begin with QDeclarativeComponent::beginCreate().

QObject * QDeclarativeComponent::create ( QDeclarativeContext * context = 0 ) [virtual]

Create an object instance from this component. Returns 0 if creation failed. context specifies the context within which to create the object instance.

If context is 0 (the default), it will create the instance in the engine' s root context.

QDeclarativeContext * QDeclarativeComponent::creationContext () const

Returns the QDeclarativeContext the component was created in. This is only valid for components created directly from QML.

QList<QDeclarativeError> QDeclarativeComponent::errors () const

Return the list of errors that occurred during the last compile or create operation. An empty list is returned if isError() is not set.

bool QDeclarativeComponent::isError () const

Returns true if status() == QDeclarativeComponent::Error.

bool QDeclarativeComponent::isLoading () const

Returns true if status() == QDeclarativeComponent::Loading.

bool QDeclarativeComponent::isNull () const

Returns true if status() == QDeclarativeComponent::Null.

bool QDeclarativeComponent::isReady () const

Returns true if status() == QDeclarativeComponent::Ready.

void QDeclarativeComponent::loadUrl ( const QUrl & url )

Load the QDeclarativeComponent from the provided url.

Ensure that the URL provided is full and correct, in particular, use QUrl::fromLocalFile() when loading a file from the local filesystem.

void QDeclarativeComponent::progressChanged ( qreal progress ) [signal]

Emitted whenever the component's loading progress changes. progress will be the current progress between 0.0 (nothing loaded) and 1.0 (finished).

void QDeclarativeComponent::setData ( const QByteArray & data, const QUrl & url )

Sets the QDeclarativeComponent to use the given QML data. If url is provided, it is used to set the component name and to provide a base path for items resolved by this component.

void QDeclarativeComponent::statusChanged ( QDeclarativeComponent::Status status ) [signal]

Emitted whenever the component's status changes. status will be the new status.

Responsable bénévole de la rubrique Qt : Thibaut Cuvelier -