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/*!
\page wasm.html
\title Qt for WebAssembly
\brief Runs Qt applications in a secure sandbox in the browser.

Qt for Webassembly lets you to run Qt applications on the web.

WebAssembly (abbreviated Wasm) is a binary instruction format intended to
be executed in a virtual machine, for example in a web browser.

With Qt for WebAssembly, you can distribute your application as a web
application that runs in a browser sandbox. This approach is suitable for
web distributed applications that do not require full access to host device
capabilities.

\note Qt for WebAssembly is in Tech Preview.

\ingroup experimentalplatform

\section1 Getting Started with Qt for WebAssembly

Building Qt applications for WebAssembly is similar to building Qt for other platforms. You
need to install an SDK (Emscripten), install Qt (or build Qt from source), and finally, build
the application. Some differences exist, for example, Qt for WebAssambly supports fewer modules
and less features than other Qt builds.

\section2 Installing Emscripten

\l{https://emscripten.org/docs/introducing_emscripten/index.html}{emscripten}
is a toolchain for compiling to WebAssembly. It lets you run Qt on the web at
near-native speed without browser plugins.

Refer to the \l{https://emscripten.org/docs/getting_started/index.html}
{emscripten documentation} for more information about installing the
Emscripten SDK.

After installation, you should have the Emscripten compiler in your path.
Check this with the following command:
\badcode
    em++ --version
\endcode

Each minor version of Qt targets a specific minimum Emcsripten version, which will
not change for the lifetime of that minor version. Qt's binary packages are
built using this version of the Emscripten SDK. Install the minimum Emcsripten version
that corresponds to the Qt version you use, especially if you use the binary packages.

Later versions of Emscripten may work (and often do), but may introduce behavior
changes which require changes to Qt.

The minimum versions are:
\list
   \li Qt 6.2: 2.0.14
   \li Qt 6.3: 3.1.6
\endlist

Use \c emsdk to install specific \c emscripten versions. For example, to install
it for Qt 6.2 enter:
\list
    \li ./emsdk install 2.0.14
    \li ./emsdk activate 2.0.14
\endlist

On Windows, emscripten is in your path after installation. On macOS or Linux
you need to add it to your path, like this:

\badcode
    source /path/to/emsdk/emsdk_env.sh
\endcode

Check this with the following command:

\badcode
    em++ --version
\endcode

\section2 Installing Qt

Download Qt from the Downloads section of your Qt account. We provide builds
for Linux, macOS, and Windows as development platforms.

The binary builds are designed to run on as many browsers as possible,
and do not enable features such as threads or SIMD support.

\section2 Building Qt from Source

Building from source lets you set Qt configuration options such as thread
support, OpenGL ES level, or SIMD support. Download the Qt sources from the
Downloads section of your Qt account.

Configure Qt as a cross-compile build for the \c wasm-emscripten platform.
This sets the \c -static, \c -no-feature-thread, and \c{-no-make examples}
configure options. You can enable thread support with the \c -feature-thread,
configure option. Shared library builds are not supported.

You need a host build of the same version of Qt and specify that path in the \e
QT_HOST_PATH CMake variable or by using the \c -qt-host-path configure argument.

Although it should be detected, you may optionally set the
\e{CMAKE_TOOLCHAIN_FILE} CMake variable to the Emscripten.cmake toolchain file
that comes with Emscripten SDK. This can be done by setting the environment
variable \e{CMAKE_TOOLCHAIN_FILE} or by passing
\c{CMAKE_TOOLCHAIN_FILE=/path/to/Emscripten.cmake} to configure.

\badcode
    ./configure -qt-host-path /path/to/Qt -platform wasm-emscripten -prefix $PWD/qtbase
\endcode

On Windows, make sure you have MinGW in your \c PATH
and configure with the following:

\badcode
    configure -qt-host-path C:\Path\to\Qt -no-warnings-are-errors -platform wasm-emscripten -prefix %CD%\qtbase
\endcode

Then build the required modules:

\badcode
    cmake --build . -t qtbase -t qtdeclarative [-t another_module]
\endcode

\section2 Building Applications on the Command Line

Qt for WebAssembly supports building applications using qmake and make, or CMake with ninja or make.

\badcode
   $ /path/to/qt-wasm/qtbase/bin/qt-cmake .
   $ cmake --build .
\endcode

Building the application generates several output files, including a .wasm file that contains
the application and Qt code (statically linked), a .html file that can be opened in the browser
to run the application.

\note Emscripten produces relatively large .wasm files at the "-g" debug level. Consider
linking with "-g2" for debug builds.

\section2 Running Applications

Running the application requires a web server. The build output files are all
static content, so any web server will do. Some use cases might require special
server configuration, such as providing https certificates or setting http headers
required to enable multithreading support.

\section3 Emrun

Emscripten provides the \e emrun utility for test-running applications. Emrun
starts a web server, launches a browser, and will also capture and forward
stdout/stderr (which will normally go to the JavaScript console).

\badcode
    /path/to/emscripten/emrun --browser=firefox appname.html
\endcode

\section3 Python http.server

Another option is to start a development web server and then launch the web
browser separately. One of the simplest options is http.server from Python:

\badcode
    python -m http.server
\endcode

\section3 qtwasmserver

Qt provides a developer web server which uses \l{https://github.com/FiloSottile/mkcert}{mkcert}
to generate https certificates. This allows testing web features which require
a \l{https://developer.mozilla.org/en-US/docs/Web/Security/Secure_Contexts}{secure context}.
Note that delivery over http://localhost is also considered secure, without requiring a
certificate.

The web server also sets the \l{https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Cross-Origin-Opener-Policy}{COOP}
and \l{https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Cross-Origin-Embedder-Policy}{COEP}
headers to values which enalbles support for \l{https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/SharedArrayBuffer}{SharedArrayBuffer}
and multi-threading.

The qtwasmserver script starts one server which binds to localhost by default. You may
add additional addresses using the \e -a command-line argument, or use \e --all to bind
to all available addresses.

\badcode
    python /path/to/qtbase/util/wasm/qtwasmserver/qtwasmserver.py --all
\endcode

\section2 Building Applications using Qt Creator

    \l{https://doc.qt.io/qtcreator/creator-setup-webassembly.html}{Setting Up Qt Creator for WebAssembly}.

\section2 Deploying Applications on the web

Building an application generates several files (substitute "app" with
the application name in the following table).

\table
    \header
        \li Generated file
        \li Brief Description
    \row
        \li app.html
        \li HTML container
    \row
        \li qtloader.js
        \li JavaScript API for loading Qt apps
    \row
        \li app.js
        \li JS API for loading Qt apps
    \row
        \li app.wasm
        \li app binary
\endtable

You can deploy \e {app.html} as-is, or discard it in favor favor of a custom html
file. Smaller adjustments, such as changing the splash screen image from the Qt
logo to the app logo, is also possible. In both cases, \e {qtloader.js} provides a JavaScript
API for loading the application.

We recommend compressing the wasm file using e.g. gzip or brotli before deployment,
as this can provide a significant reductiton in file size.

Enabling certain features, such as multi-threading and SIMD, produces .wasm binaries
that are incompatible with browsers that do not support the enabled feature. It is
possible to work around this limitation by building multiple .wasm files and then
use JavaScript feature detection to select the correct one, but note that Qt does
not provide any functionality for doing this.

\section1 Supported Browsers
\section2 Desktop

Qt for WebAssembly is developed and tested on the following browsers:

\list
    \li Chrome
    \li Firefox
    \li Safari
    \li Edge
\endlist

Qt should run if the browser supports WebAssembly. Qt has a fixed WebGL
requirement, even if the application itself does not use hardware accelerated
graphics. Browsers that support WebAssembly often support WebGL, though
some browsers blacklist older or unsupported GPUs. s/\e {qtloader.js}
provides APIs to check if WebGL is available.

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 Mobile

Qt for WebAssembly applications runs on mobile browsers such as mobile
Safari and Android Chrome. However, there is no support for text input
using the native virtual keyboard.

\section1 Supported Qt Modules

Qt for WebAssembly supports a subset of the Qt modules and features. Tested
modules are listed below, other modules may or may not work.

\list
    \li \l {Qt Core}
    \li \l {Qt GUI}
    \li \l {Qt Network}
    \li \l {Qt Widgets}
    \li \l {Qt QML}
    \li \l {Qt Quick}
    \li \l {Qt Quick Controls}
    \li \l {Qt Quick Layouts}
    \li \l {Qt 5 Core Compatibility APIs}
    \li \l {Qt Concurrent}
    \li \l {Qt Image Formats}
    \li \l {Qt OpenGL}
    \li \l {Qt SVG}
    \li \l {Qt WebSockets}
\endlist

In all cases, module support may not be complete and and there may be
additional limitations, either due to the browser sandbox or due to
incompleteness of the Qt platform port. See \l {Developing with Qt for WebAssembly}
for further info.

\section1 Developing with Qt for WebAssembly
\section2 OpenGL and WebGL

Qt requires WebGL, also for applications which do not use OpenGL directly. All
relevant browsers support WebGL, but note that some browsers blacklist certain
older GPUs. The Qt loader will detect this and display an error message.

Qt detects WebGL as OpenGL ES, with the following version mapping:

\table
    \header
        \li OpenGL
        \li WebGL
    \row
        \li OpengL ES 2
        \li WebGL 1
    \row
        \li OpengL ES 3
        \li WebGL 2
\endtable

OpengL ES 2 is selected by default, OpenGL ES 3 can be enabled by configuring Qt with
the -feature-opengles3 option.

Qt for WebAssembly does support mixing raster and OpenGL content. Supported
use cases are pure raster apps using QWigets, and pure OpenGL apps using
Qt Quick or QOpenGLWindow. QOpenGLWidget is not supported at this point (QTBUG-66944).

Web and Desktop OpenGL differences are documented in:
\l {https://www.khronos.org/registry/webgl/specs/latest/1.0/#6}{WebGL and OpenGL Differences}
There are additional differences between WebGL 1.0 and WebGL 2.0 which are documented in:
\l {https://www.khronos.org/registry/webgl/specs/latest/2.0}{WebGL 2.0 Specification}

\section2 Multithreading

Qt supports multithreading on WebAssembly, however this feature is experimental
and is not enabled by default. Thread support can be enabled by building Qt from
source and using the "-feature-thread" configure flag.

The Qt for WebAssembly binary packages do not support multithreading.

Emscripten implements support for pthreads using web workers, and this abstraction
is not completely leak free. See \l {https://emscripten.org/docs/porting/pthreads.html}{Pthreads Support}
for further info.

Multithreading requires browser support for SharedArrayBuffer, see \l {https://caniuse.com/sharedarraybuffer}
{ caniuse sharedarraybuffer } for current supported status. If suppoerted, SharedArrayBuffer will
be enabled provided the web server sets the COOP and and COEP headers:
\list
\li Cross-Origin-Opener-Policy: same-origin
\li Cross-Origin-Embedder-Policy: require-corp
\endlist

\section2 SIMD

The LLVM compiler supports generating \l{https://emscripten.org/docs/porting/simd.html}{WebAssembly SIMD}.
Pass the -msimd128 flag at compile time to enable. This enables LLVM autovectorization, which
makes it possible to benefit from SIMD without making source code modifications.

You can target WebAssembly SIMD directly using either GCC/Clang SIMD Vector Extensions or WASM
SIMD128 intrinsics. For more information, see the Emscripten
\l{https://emscripten.org/docs/porting/simd.html}{ SIMD documentation }.

In addition, Emscripten supports emulating/translating x86 SSE instructions to Wasm SIMD instructions.
Enable by building Qt from source and configure with the "-sse2" option. This adds support for SSE1,
SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, and 128-bit AVX instructions, and enables Qt's SSE code paths.
Use of SSE SIMD instructions that have no native Wasm SIMD equivalent may cause reduced performance.

Note that SIMD-enabled binaries are incompatible with browsers that do not support WebAssembly SIMD,
also if the SIMD code paths are not called at run-time. SIMD support may need to be enabled
in the browsers advanced configurations, such as 'about:config' or 'chrome:flags'

\section2 Networking

Qt provides limited support for networking. In general, network protocols which are
already in use on the web can be use also from Qt, while others are not directly
available due to the web sandbox.

The following protocols are supported:

\list
\li QNetworkAccessManager http requests to the web page origin server, or to a
server which supports CORS. This includes XMLHttpRequest from QML.
\li QWebSocket connections to any host. Note that web pages served over the secure https protocol
allows websockets connections over secure wss protocol only.
\endlist

All other network protocols are not supported.

In addition Emscripten supports \l{https://emscripten.org/docs/porting/networking.html}
{emulated POSIX TCP sockets over WebSockets}. This is not currently supported by Qt's socket
classes.

\section2 Local File Access

File system access is sandboxed on the web, and this has implications for how the
application works with files. The Web platform provides APIs for accessing the local
file system in a way which is under user control, as well as APIs for accessing
persistent storage. Emscripten and Qt wraps these features and provides APIs which
are easier to use from C++ and Qt-based applications.

The web platform provides features for accessing local files and persistent storage:
\list
\li <input type="file"> for showing a native open-file dialog where the user can pick a file.
\li IndexedDB provides persistent local storage (not accessible outside the browser)
\endlist

Emscripten provides several file systems with a POSIX like API. These include:
\list
\li the MEMFS ephemeral file system which stores files in-memory
\li the IDBFS persistent file system which stores files using IndexedDB
\endlist

Emscripten mounts a temporary MEMFS filesystem to "/" at app startup. This means
that QFile can be used, and will read and write files to memory by default. Qt
provides other API as well:

\list
\li QSettings has an IndexedDB-based backend; Note that QSettings is
asynchronous on WebAssembly.
\li \l {https://doc.qt.io/qt-5/qfiledialog.html#getOpenFileContent}{QFileDialog::getOpenFileContent() }
opens a native file dialog where the user can pick a file
\li \l {https://doc.qt.io/qt-5/qfiledialog.html#saveFileContent}{QFileDialog::saveFileContent()}
saves a file to the local file system via file download}
\endlist

\section2 Clipboard Access

Qt supports copying and pasting text to the system clipboard, with some differences due
to the web sandbox. In general clipboard access require user permission, which can be obtained
by handling an input event (e.g. CTRL+c), or by using the
\l{https://developer.mozilla.org/en-US/docs/Web/API/Clipboard_API}{Clipboard API}.

Browsers that support the Clipboard API are preferred. Note that a requirement for this API
is that the web page is served over a secure connection (e.g. https), and that some browsers
my require changing configuration flags.

At the time of writing the following browsers support the Clipboard API, see
\l{https://caniuse.com/?search=Clipboard}{caniuse} for the current support level.

\list
    \li Chrome supports the Clipboard API
    \li Firefox supports the Clipboard API behind a flag:
        dom.events.asyncClipboard.dataTransfer
\endlist

\section2 Debugging and Profiling

Wasm debugging is done on browser javascript console, debugging applications on Wasm
directly within Qt Creator is not possible.

\list
\li Qt debug and logging output is printed on the JavaScript console, which can be
accessed via browser "Developer Tools" or similar.
\li Source maps for stepping through code, can be created by reconfiguring Qt with
the --device-option QT_WASM_SOURCE_MAP=1, and building a debug build.
\list
    \li \l {https://developer.mozilla.org/en-US/docs/Tools/Debugger/How_to/Use_a_source_map}
    \li \l {https://developers.google.com/web/tools/chrome-devtools/javascript/source-maps#source_maps_in_devtools_sources_panel}
\endlist
\li Mobile browsers can use remote debugging
\list
    \li \l {https://developer.mozilla.org/en-US/docs/Tools/Remote_Debugging}
    \li \l {https://developers.google.com/web/tools/chrome-devtools/remote-debugging}
    \li \l {https://developer.apple.com/safari/tools/}
\endlist
\li To stop execution on a certain line and popup the browser debugger
programmatically, you can add the function emscripten_debugger(); to the application
source code.
\li Profiling can be accomplished by using a debug build and the javascript console
profiling features. Qt adds --profiling-funcs to the linker arguments in debug builds,
which preserve function names in profiling
\endlist

\section2 Known Issues

\list
\li Nested event loops are not supported. Applications should not call API like QDialog::exec()
and QEventLoop::exec()
\li Drag and Drop is not supported
\li Printing is not supported
\li QDnsLookup lookups, QTcpSocket, QSsl do not work and are not supported due to the web sandbox
\li Accessibility is not supported: Qt draws application content to a canvas element and does not
use (other) native DOM elements.
\li Fonts: Wasm sandbox does not allow access to system fonts. Font files must be
distributed with the application, for example in Qt resources or downloading.
Qt for WebAssembly itself embeds one such font.
\li There may be artifacts of uninitialized graphics memory on some
Qt Quick Controls 2 components, such as checkboxes. This can sometimes
be seen on HighDPi displays.
\li Native styles for Windows and macOS are not supported as Wasm as a platform
is not providing that capability
\endlist

\section1 Other Topics
\section2 Qt Configure Options Reference

The following configure options are relevant when building Qt for WebAssembly from source.

\table
    \header
        \li Configure Argument
        \li Brief Description
    \row
        \li -sse2
        \li Enables SIMD support.
    \row
        \li -feature-thread
        \li Multi-threaded wasm
    \row
        \li -device-option QT_WASM_SOURCE_MAP=1
        \li Debugging option for creating source maps
    \row
        \li -feature-opengles3
        \li Use opengles3 in addition to the default opengles2
\endtable

\section2 Typical Download Sizes
Expected footprint (download size): Wasm modules as produced by the compiler can be
large, but compress well:

\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.

\section2 Examples

\list
        \li \l {https://www.qt.io/web-assembly-example-industrial-panel}
               {Industrial Panel Demo}
        \li \l {https://www.qt.io/web-assembly-example-slate}
               {QMainWindow app}
        \li \l {https://www.qt.io/web-assembly-example-gallery}
               {A gallery of available controls in Qt Quick Controls}
        \li \l {https://www.qt.io/web-assembly-example-pizza-shop}
               {Web app for prdering pizzas}
\endlist

\section1 External resources

\list
    \li \l {WebAssembly Resource site}
\endlist

\section1 License

Qt for WebAssembly is available under commercial licenses from \l{The Qt Company}.
In addition, it is available under the \l{GNU General Public License, version 3}.
See \l{Qt Licensing} for further details.

*/
