qt-everywhere-src-5.14.1/qtdoc/doc/src/platforms/qtwebassembly-platform-notes.qdoc

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/*!
    \page qtwebassembly-platform-notes.html
    \title Qt WebAssembly Platform Notes

    \brief Contains information about issues that are specific to the Qt WebAssembly module.

    WebAssembly (or webasm) is a bytecode format intended to be executed
    in a virtual machine inside a web browser. This allows an application
    to be deployed to a device with a compliant web browser without going
    through any installation steps. The application will run inside a
    secure sandbox in the web browser. This makes it appropriate for
    applications that do not need full access to the device capabilities,
    but benefits from a swift and uncomplicated installation process.

    \section1 Supported target browsers and devices

    \section2 Desktop

    \list
        \li Chrome
        \li FireFox
        \li Safari
        \li Edge(Chrome)
    \endlist

    If the browser supports WebAssembly, then Qt should run.

    \note Qt has a fixed WebGL requirement, also for apps that do not
    use WebGL directly. Browsers often blacklist WebGL for older/unsupported
    GPUs.

    \section2 Mobile

    \list
        \li Android Browser
        \li Mobile Safari
    \endlist

    \note There is currently no support for text input using the virtual
    keyboard. Safari currently does not support wasm modules of the size
    Qt produces.

    Qt does not make direct use of operating system features and it makes
    no difference if, for example, FireFox runs on Windows or macOS. Qt
    does use some operating system adaptations, for example for ctrl/cmd
    key handling on macOS.

    \section2 Supported Qt Modules

    Qt for WebAssembly supports a subset of the Qt modules. The list below
    lists the currently tested modules. The list can be pasted as arguments
    to \c make.

    \badcode
        module-qtbase module-qtdeclarative module-qtquickcontrols2 module-qtwebsockets module-qtsvg module-qtcharts module-qtmqtt
    \endcode

    Other modules are untested and may work.  Are not supported: QtMultimedia
    and QtWebView

    \section3 Known issues

    Refer to the \l{https://wiki.qt.io/Qt_for_WebAssembly#Known issues}{wiki}.

    \section3 Known limitations

    \list
    \li Debugging: Qt debug and logging output is printed on the JavaScript
        console, which can be accessed via browser "Developer Tools" or similar.
    \li Nested event loops are not supported. Applications should not call, for
        example, QDialog::exec() or create a new QEventLoop object.
    \li Qt renders application content to a canvas element, and does not use
        (other) native DOM elements. This means accessibility (screen readers)
        are not supported and that text inputs won't trigger virtual keyboards.
    \li WebGL is required, even for applications which do not use OpenGL themselves.
        Most modern browsers support WebGL, but note that some browsers blacklist
        certain older GPUs. The Qt loader will detect this and display an error message.
    \li Child OpenGL windows are not supported. The window compositor (in the Qt
        for Wasm platform plugin) supports raster windows only.
    \li Qt will detect OpenGL support as OpenGL ES. In reality the browser will
        be providing WebGL. WebGL is based on ES and is very similar, but there
        are some incompatibilities. See WebGL and OpenGL Differences.
    \li Applications do not have access to system fonts. Font files must be
        distributed with the application, for example in Qt resources. Qt for
        WebAssembly itself embeds one such font.
    \li High-DPI and scaling: High-DPI rendering is supported, and so is setting
        the overall UI visual size using the browser zoom feature. Browser font
        size (and type) settings have no effect on Qt applications.
    \li There may be artifacts of uninitialized graphics memory on some Qt Quick
        Controls 2 components, such as checkboxes.
    \li Network access: the web sandbox limits network access to a subset of what
        is available for native apps.
        \list
        \li QNetworkAccessManager http requests to the web page origin server, or
            to a server which supports CORS.
        \li QWebScoket connections to any host.
        \li TCP and UDP socked tunneling using over WebSockets using a websockify
            server [implemented by Emscripten, not tested].
            \list
            \li Websockify v0.8.0 can be used to tunnel TCP connections with QT5.12
                but it is MANDATORY to specify the base64 or binary subprotocols before
                calling QWebSocket::open().
            \li For example:

                QWebSocket socket;

                QUrl url{QString("ws://server:port")};

                QNetworkRequest request{url};

                request.setRawHeader("Sec-WebSocket-Protocol", "binary");

                socket.open(request);
            \endlist
        \endlist

    \li Link-time warnings of the form: "cannot represent a NaN literal '0x7fdae4bde910'
        with custom bit pattern", are expected.
    \li Expected footprint (download size): Wasm modules as produced by the compiler can
        be large, but compress well.
    \endlist
    \table
    \header
        \li Example
        \li gzip
        \li brotli
    \row
        \li helloglwindow (QtCore + QtGui)
        \li 2.8M
        \li 2.1M
    \row
        \li wiggly widget (QtCore + QtGui + QtWidgets)
        \li 4.3M
        \li 3.2M
    \row
        \li SensorTag (QtCore + QtGui + QtWidgets + QtQuick + QtCharts)
        \li 8.6M
        \li 6.3M
    \endtable

    Compression is typically handled on the web server side, using standard compression
    features: the server compresses automatically or picks up pre-compressed versions of
    the files. There's generally no need to have special handling of wasm files.
*/