/****************************************************************************
**
** Copyright (C) 2012 Nokia Corporation and/or its subsidiary(-ies).
** Contact: http://www.qt-project.org/
**
** This file is part of the documentation of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:FDL$
** 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.
**
** Other Usage
** Alternatively, this file may be used in accordance with the terms
** and conditions contained in a signed written agreement between you
** and Nokia.
**
**
**
**
**
** $QT_END_LICENSE$
**
****************************************************************************/

/*!
\page qml-network.html
\ingroup qml-features
\title Resource Loading and Network Transparency in QML
\brief about loading files and resources accross a network

QML supports network transparency by using URLs (rather than file names) for all
references from a QML document to other content. This means that anywhere a URL source is expected,
QML can handle remote resources as well as local ones, for example in the following image source:

\qml
Image {
    source: "http://www.example.com/images/logo.png"
}
\endqml

Since a \e relative URL is the same
as a relative file, development of QML on regular file systems remains simple:

\qml
Image {
    source: "images/logo.png"
}
\endqml

Network transparency is supported throughout QML, for example:

\list
\li Fonts - the \c source property of FontLoader is a URL
\li WebViews - the \c url property of WebView (obviously!)
\endlist

Even QML types themselves can be on the network - if the \l {QML Viewer} is used to load
\tt http://example.com/mystuff/Hello.qml and that content refers to a type "World", the engine
will load \tt http://example.com/mystuff/qmldir and resolve the type just as it would for a local file.
For example if the qmldir file contains the line "World World.qml", it will load
\tt http://example.com/mystuff/World.qml
Any other resources that \tt Hello.qml referred to, usually by a relative URL, would
similarly be loaded from the network.


\section1 Relative vs. Absolute URLs

Whenever an object has a property of type URL (QUrl), assigning a string to that
property will actually assign an absolute URL - by resolving the string against
the URL of the document where the string is used.

For example, consider this content in \tt{http://example.com/mystuff/test.qml}:

\qml
Image {
    source: "images/logo.png"
}
\endqml

The \l Image source property will be assigned \tt{http://example.com/mystuff/images/logo.png},
but while the QML is being developed, in say \tt C:\\User\\Fred\\Documents\\MyStuff\\test.qml, it will be assigned
\tt C:\\User\\Fred\\Documents\\MyStuff\\images\\logo.png.

If the string assigned to a URL is already an absolute URL, then "resolving" does
not change it and the URL is assigned directly.


\section1 Progressive Loading

Because of the declarative nature of QML and the asynchronous nature of network resources,
objects which reference network resource generally change state as the network resource loads.
For example, an Image with a network source will initially have
a \c width and \c height of 0, a \c status of \c Loading, and a \c progress of 0.0.
While the content loads, the \c progress will increase until
the content is fully loaded from the network,
at which point the \c width and \c height become the content size, the \c status becomes \c Ready, and the \c progress reaches 1.0.
Applications can bind to these changing states to provide visual progress indicators where appropriate, or simply
bind to the \c width and \c height as if the content was a local file, adapting as those bound values change.

Note that when objects reference local files they immediately have the \c Ready status, but applications wishing
to remain network transparent should not rely on this. Future versions of QML may also use asynchronous local file I/O
to improve performance.


\section1 Accessing Network Services

QML types such as XmlListModel, and JavaScript classes like XMLHttpRequest are intended
entirely for accessing network services, which usually respond with references to
content by URLs that can then be used directly in QML. For example, using these facilities
to access an on-line photography service would provide the QML application with URLs to
photographs, which can be directly set on an \l Image \c source property.

See the \tt examples/declarative/flickr for a real demonstration of this.


\section1 Configuring the Network Access Manager

All network access from QML is managed by a QNetworkAccessManager set on the QQmlEngine which executes the QML.
By default, this is an unmodified Qt QNetworkAccessManager. You may set a different manager by
providing a QQmlNetworkAccessManagerFactory and setting it via
QQmlEngine::setNetworkAccessManagerFactory().
For example, the \l {QML Viewer} sets a QQmlNetworkAccessManagerFactory which
creates QNetworkAccessManager that trusts HTTP Expiry headers to avoid network cache checks,
allows HTTP Pipelining, adds a persistent HTTP CookieJar, a simple disk cache, and supports proxy settings.


\section1 QRC Resources

One of the URL schemes built into Qt is the "qrc" scheme. This allows content to be compiled into
the executable using \l{The Qt Resource System}. Using this, an executable can reference QML content
that is compiled into the executable:

\code
    QQuickView *canvas = new QQuickView;
    canvas->setUrl(QUrl("qrc:/dial.qml"));
\endcode

The content itself can then use relative URLs, and so be transparently unaware that the content is
compiled into the executable.


\section1 Limitations

The \c import statement is only network transparent if it has an "as" clause.

More specifically:
\list
\li \c{import "dir"} only works on local file systems
\li \c{import libraryUri} only works on local file systems
\li \c{import "dir" as D} works network transparently
\li \c{import libraryUrl as U} works network transparently
\endlist

\section1 XMLHttpRequest

\target XMLHttpRequest

QML script supports the XMLHttpRequest object, which can be used to asynchronously obtain
data from over a network.

The XMLHttpRequest API implements the same \l {http://www.w3.org/TR/XMLHttpRequest/}{W3C standard}
as many popular web browsers with following exceptions:
\list
\li QML's XMLHttpRequest does not enforce the same origin policy.
\li QML's XMLHttpRequest does not support \e synchronous requests.
\endlist

Additionally, the \c responseXML XML DOM tree currently supported by QML is a reduced subset
of the \l {http://www.w3.org/TR/DOM-Level-3-Core/}{DOM Level 3 Core} API supported in a web
browser.  The following objects and properties are supported by the QML implementation:

\table
\header
\li \b {Node}
\li \b {Document}
\li \b {Element}
\li \b {Attr}
\li \b {CharacterData}
\li \b {Text}

\row
\li
\list
\li nodeName
\li nodeValue
\li nodeType
\li parentNode
\li childNodes
\li firstChild
\li lastChild
\li previousSibling
\li nextSibling
\li attributes
\endlist

\li
\list
\li xmlVersion
\li xmlEncoding
\li xmlStandalone
\li documentElement
\endlist

\li
\list
\li tagName
\endlist

\li
\list
\li name
\li value
\li ownerElement
\endlist

\li
\list
\li data
\li length
\endlist

\li
\list
\li isElementContentWhitespace
\li wholeText
\endlist

\endtable

The \l{declarative/xml/xmlhttprequest}{XMLHttpRequest example} demonstrates how to
use the XMLHttpRequest object to make a request and read the response headers.

*/