Qt 4.8: QDeclarativeComponent Class Reference

Qt Reference Documentation

- Qt 4.8
- ALL VERSIONS

- A
- A
- A
- Print

# QDeclarativeComponent Class Reference

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

     #include <QDeclarativeComponent>

Inherits: QObject.

This class was introduced in Qt 4.7.

- List of all members, including inherited members

## Public Types

enum | Status { Null, Ready, Loading, Error }

## Properties

- progress : const qreal
- status : const Status
- url : const QUrl

- 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 )

- 1 signal inherited from QObject

## Protected Functions

| Q_REVISION ()

- 8 protected functions inherited from QObject

### Additional Inherited Members

- 1 public slot inherited from QObject
- 7 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.

Constant | Value | Description
--- | --- | ---
`QDeclarativeComponent::Null` | `0` | This QDeclarativeComponent has no data. Call loadUrl() or setData() to add QML content.
`QDeclarativeComponent::Ready` | `1` | This QDeclarativeComponent is ready and create() may be called.
`QDeclarativeComponent::Loading` | `2` | This QDeclarativeComponent is loading network data.
`QDeclarativeComponent::Error` | `3` | An 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.