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QQmlComponent 类

The QQmlComponent class encapsulates a QML component definition. 更多...

头： #include <QQmlComponent>
qmake: QT += qml
Since: Qt 5.0
实例化： Component
继承： QObject

公共类型

enum CompilationMode { PreferSynchronous, Asynchronous }
enum Status { Null, Ready, Loading, Error }

特性

公共函数

QQmlComponent (QQmlEngine * engine , QObject * parent = nullptr)
QQmlComponent (QQmlEngine * engine , const QString & fileName , QObject * parent = nullptr)
QQmlComponent (QQmlEngine * engine , const QString & fileName , QQmlComponent::CompilationMode mode , QObject * parent = nullptr)
QQmlComponent (QQmlEngine * engine , const QUrl & url , QObject * parent = nullptr)
QQmlComponent (QQmlEngine * engine , const QUrl & url , QQmlComponent::CompilationMode mode , QObject * parent = nullptr)
virtual ~QQmlComponent () override
virtual QObject * beginCreate (QQmlContext * publicContext )
virtual void completeCreate ()
virtual QObject * create (QQmlContext * context = nullptr)
void create (QQmlIncubator & incubator , QQmlContext * context = nullptr, QQmlContext * forContext = nullptr)
QQmlContext * creationContext () const
QQmlEngine * engine () const
QList<QQmlError> errors () const
bool isError () const
bool isLoading () const
bool isNull () const
bool isReady () const
qreal progress () const
QQmlComponent::Status status () const
QUrl url () const

公共槽

void loadUrl (const QUrl & url )
void loadUrl (const QUrl & url , QQmlComponent::CompilationMode mode )
void setData (const QByteArray & data , const QUrl & url )

信号

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

静态公共成员

const QMetaObject staticMetaObject

额外继承成员

详细描述

The QQmlComponent class encapsulates a QML component definition.

Component 是可重用，具有良好定义接口的 QML 封装类型。

A QQmlComponent instance can be created from a QML file. For example, if there is a main.qml 文件像这样： 

import QtQuick 2.0
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 值： 

QQmlEngine *engine = new QQmlEngine;
QQmlComponent component(engine, QUrl::fromLocalFile("main.qml"));
QObject *myObject = component.create();
QQuickItem *item = qobject_cast<QQuickItem*>(myObject);
int width = item->width();  // width = 200

To create instances of a component in code where a QQmlEngine instance is not available, you can use qmlContext () 或 qmlEngine (). For example, in the scenario below, child items are being created within a QQuickItem subclass: 

void MyCppItem::init()
{
    QQmlEngine *engine = qmlEngine(this);
    // Or:
    // QQmlEngine *engine = qmlContext(this)->engine();
    QQmlComponent component(engine, QUrl::fromLocalFile("MyItem.qml"));
    QQuickItem *childItem = qobject_cast<QQuickItem*>(component.create());
    childItem->setParentItem(this);
}

Note that these functions will return null when called inside the constructor of a QObject subclass, as the instance will not yet have a context nor engine.

网络组件

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

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

MyApplication::MyApplication()
{
    // ...
    component = new QQmlComponent(engine, QUrl("http://www.example.com/main.qml"));
    if (component->isLoading())
        QObject::connect(component, SIGNAL(statusChanged(QQmlComponent::Status)),
                         this, SLOT(continueLoading()));
    else
        continueLoading();
}
void MyApplication::continueLoading()
{
    if (component->isError()) {
        qWarning() << component->errors();
    } else {
        QObject *myObject = component->create();
    }
}

成员类型文档编制

enum QQmlComponent:: CompilationMode

Specifies whether the QQmlComponent should load the component immediately, or asynchonously.

常量 值 描述
QQmlComponent::PreferSynchronous 0 Prefer loading/compiling the component immediately, blocking the thread. This is not always possible; for example, remote URLs will always load asynchronously.
QQmlComponent::Asynchronous 1 Load/compile the component in a background thread.

enum QQmlComponent:: Status

Specifies the loading status of the QQmlComponent .

常量 值 描述
QQmlComponent::Null 0 This QQmlComponent has no data. Call loadUrl () 或 setData () to add QML content.
QQmlComponent::Ready 1 This QQmlComponent is ready and create () may be called.
QQmlComponent::Loading 2 This QQmlComponent is loading network data.
QQmlComponent::Error 3 An error has occurred. Call errors () to retrieve a list of errors .

特性文档编制

progress : const qreal

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

访问函数：

qreal progress () const

通知程序信号：

void progressChanged (qreal progress )

status : const Status

The component's current status .

访问函数：

QQmlComponent::Status status () const

通知程序信号：

void statusChanged (QQmlComponent::Status status )

url : const QUrl

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

访问函数：

QUrl url () const

成员函数文档编制

QQmlComponent:: QQmlComponent ( QQmlEngine * engine , QObject * parent = nullptr)

创建 QQmlComponent with no data and give it the specified engine and parent . Set the data with setData ().

QQmlComponent:: QQmlComponent ( QQmlEngine * engine , const QString & fileName , QObject * parent = nullptr)

创建 QQmlComponent 从给定 fileName and give it the specified parent and engine .

另请参阅 loadUrl ().

QQmlComponent:: QQmlComponent ( QQmlEngine * engine , const QString & fileName , QQmlComponent::CompilationMode mode , QObject * parent = nullptr)

创建 QQmlComponent 从给定 fileName and give it the specified parent and engine 。若 mode is 异步 , the component will be loaded and compiled asynchronously.

另请参阅 loadUrl ().

QQmlComponent:: QQmlComponent ( QQmlEngine * engine , const QUrl & url , QObject * parent = nullptr)

创建 QQmlComponent 从给定 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.

Relative paths will be resolved against QQmlEngine::baseUrl() , which is the current working directory unless specified.

另请参阅 loadUrl ().

QQmlComponent:: QQmlComponent ( QQmlEngine * engine , const QUrl & url , QQmlComponent::CompilationMode mode , QObject * parent = nullptr)

创建 QQmlComponent 从给定 url and give it the specified parent and engine 。若 mode is 异步 , the component will be loaded and compiled asynchronously.

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

Relative paths will be resolved against QQmlEngine::baseUrl() , which is the current working directory unless specified.

另请参阅 loadUrl ().

[override virtual] QQmlComponent:: ~QQmlComponent ()

Destruct the QQmlComponent .

[虚拟] QObject *QQmlComponent:: beginCreate ( QQmlContext * publicContext )

This method provides advanced control over component instance creation. In general, programmers should use QQmlComponent::create () to create object instances.

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

当 QQmlComponent 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 first time.
  3. If applicable, QQmlParserStatus::componentComplete () is called on objects.

QQmlComponent::beginCreate() differs from QQmlComponent::create () in that it only performs step 1. QQmlComponent::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.

The ownership of the returned object instance is transferred to the caller.

另请参阅 completeCreate () 和 QQmlEngine::ObjectOwnership .

[虚拟] void QQmlComponent:: completeCreate ()

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

This function completes the component creation begun with QQmlComponent::beginCreate () and must be called afterwards.

另请参阅 beginCreate ().

[虚拟] QObject *QQmlComponent:: create ( QQmlContext * context = nullptr)

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

若 context is nullptr (the default), it will create the instance in the 根上下文 of the engine.

The ownership of the returned object instance is transferred to the caller.

If the object being created from this component is a visual item, it must have a visual parent, which can be set by calling QQuickItem::setParentItem ()。见 概念 - Qt Quick 中的可视父级 了解更多细节。

另请参阅 QQmlEngine::ObjectOwnership .

void QQmlComponent:: create ( QQmlIncubator & incubator , QQmlContext * context = nullptr, QQmlContext * forContext = nullptr)

Create an object instance from this component using the provided incubator . context specifies the context within which to create the object instance.

若 context is 0 (the default), it will create the instance in the engine's 根上下文 .

forContext specifies a context that this object creation depends upon. If the forContext is being created asynchronously, and the QQmlIncubator::IncubationMode is QQmlIncubator::AsynchronousIfNested , this object will also be created asynchronously. If forContext is 0 (the default), the context will be used for this decision.

The created object and its creation status are available via the incubator .

另请参阅 QQmlIncubator .

QQmlContext *QQmlComponent:: creationContext () const

返回 QQmlContext the component was created in. This is only valid for components created directly from QML.

QQmlEngine *QQmlComponent:: engine () const

返回 QQmlEngine of this component.

该函数在 Qt 5.12 引入。

QList < QQmlError > QQmlComponent:: errors () const

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

bool QQmlComponent:: isError () const

返回 true 若 status () == QQmlComponent::Error .

bool QQmlComponent:: isLoading () const

返回 true 若 status () == QQmlComponent::Loading .

bool QQmlComponent:: isNull () const

返回 true 若 status () == QQmlComponent::Null .

bool QQmlComponent:: isReady () const

返回 true 若 status () == QQmlComponent::Ready .

[slot] void QQmlComponent:: loadUrl (const QUrl & url )

加载 QQmlComponent 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.

Relative paths will be resolved against QQmlEngine::baseUrl() , which is the current working directory unless specified.

[slot] void QQmlComponent:: loadUrl (const QUrl & url , QQmlComponent::CompilationMode mode )

加载 QQmlComponent from the provided url 。若 mode is 异步 , the component will be loaded and compiled asynchronously.

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

Relative paths will be resolved against QQmlEngine::baseUrl() , which is the current working directory unless specified.

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

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

注意： 通知程序信号对于特性 progress .

[slot] void QQmlComponent:: setData (const QByteArray & data , const QUrl & url )

设置 QQmlComponent to use the given QML data 。若 url is provided, it is used to set the component name and to provide a base path for items resolved by this component.

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

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

注意： 通知程序信号对于特性 status .