WebAssembly is a binary format that allows sand-boxed executable code in web pages. This format is nearly as fast as native machine code, and is now supported by all major web browsers.

Qt for WebAssembly is a platform plugin that lets you build Qt applications, which can be integrated into your web pages. It doesn't require any client-side installations and reduces the server-side resource usage.

Getting Started with Qt for WebAssembly

Install Emscripten

emscripten is a toolchain for compiling to asm.js and Webassembly. It lets you run Qt on the web at near-native speed without plugins.

Refer to the emscripten documentation for the installation procedure.

After installation, you should have emscripten in your path. Check this with the following command:

em++ --version

Build Qt from the sources

Download the Qt 5.12 sources and build Qt from source. Specify that we are cross-compiling for wasm using emscripten:

$ ~/Qt/5.12.0/Src/configure -xplatform wasm-emscripten -nomake examples -prefix $PWD/qtbase
$ make module-qtbase module-qtdeclarative [other modules]

The Qt build is a static build, and does not support threads.

Build your application

$ /path/to/qt-wasm/qtbase/bin/qmake
$ make

This generates the following files:

When deploying the app, the compression is typically handled on the server side. We recommend to compress the wasm binaries because this typically reduces the size of the binary by 50 %.

Some examples

• An example displaying a Qt logo using OpenGL calls
• An industry automation use case
• A gallery of available controls in Qt Quick Controls
• A simple text editor, written with Qt Widgets

Limitations

The most important limitation is that the Qt build is static, and does not support threads.

For further info, refer to Known_issues_and_platform_notes.

External resources

• Qt for WebAssembly Technology Preview
• Qt and WebAssembly
• Qt for WebAssembly wiki
• Getting Started with Qt for WebAssembly
• Remote UIs with WebGL and WebAssembly
• WebAssembly Resource site

Licenses

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