webassembly: Javascript <-> C++ Interop

Enable bidirectional communication between the embedding web page and Qt application code.

Topics

• Javascript <-> C++ control flow
• Type converserions (QString <-> JavaScript string)

emscripten documentation

Control flow

• C++ -> Javascript: use EM_ASM to inject js.
• Javascript -> C++: use Module.ccall().
• The linker may not include functions not called from C++ (javascript use is invisible to it.)
• Tag functions with EMSCRIPTEN_KEEPALIVE to mark them as used.
• Add functions to -s EXPORTED_FUNCTIONS='["_main"]' linker line to mark them as used

For Qt we would like to have a solution that can be contained in the Qt libraries, and that also works with static builds. EMSCRIPTEN_KEEPALIVE is not sufficient when the function is contained in a static library. EXPORTED_FUNCTIONS "leaks" from the library: there is a single entry on the final linker line where all functions has to be listed.

Proposed solution for the HTML5 platform plugin:

• Tag the function with EMSCRIPTEN_KEEPALIVE
• Call it once from platform plugin init, with a "no-op" flag set.

Strings

It is not clear what assumptions we can make about the internal encoding of Javascript strings.  Convert via utf8.

Javascript -> QString:

1. Module.stringToUTF8()
2. QString::fromUtf8();

 QString -> Javascript

1. QString::toUtf8()
2. Module.Pointer_stringify()

This incurs at least two string scan/copies in each direction, and allocation of a temporary UTF8 buffer. Should be OK for small strings.

Prototype Code

https://github.com/msorvig/qt-webassembly-examples/tree/master/emscripten_interop