A threaded wrapper for QML WebSocket interface

The problem

This aims to be a work-around for QTBUG-63719, without this QML WebSocket API blocks main event loop and user interface while receiving messages over network — that is after the server started to send the message and before the message was fully received, the UI is frozen.

That is visible on Wi-Fi networks, under heavy WebSocket usage, and especially on 4G/mobile networks.

That issue could be observed in all current Qt versions at the date, including 5.9.2 and 5.10.0-beta.

See the test-original.qml file to see the problem, and test-threaded to see this workaround in action.

The solution

This repo provides a WebSocketThreaded import, which could be used as a drop-in replacement for WebSocket, but it has some drawbacks.

This work-around works by encapsulating QWebSocket inside a QThread and passing data as events — see QWebSocketThreaded file for that.

That class could also potentially be used from C++ code, but it is not exported and does not replicate full QWebSocket API — only the subset that was needed for QML WebSocket.

The drawbacks

• Starts a separate thread for each WebSocket instance
• QML WebSocketServer is not implemented
• sendTextMessage and sendBinaryMessage return -1 instead of the transferred size, but that shouldn't break anything as those are documented to return void (which is not actually true, btw).
• errorString is empty in most cases (FIXME)

Copyright

This code is based on the original Qt WebSockets code and is licensed accordingly (LGPL3/GPL2+).