When using a QML WorkerScript, are the requests (sent via postMessage()) queued (and executed on a single handler thread) or is there the possibility/danger that two successive postMessage() will be executed on two threads concurrently?
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1 Answer
Short answer
All WorkerScript should execute in the same thread.
Not so short answer
All WorkerScript in a QQmlEngine should execute in the same thread.
Long answer
When you create a WorkerScript in QML you instantiate the QQuickWorkerScript C++ class. This class uses the QQuickWorkerScriptEngine class to handles all the thready things.
Now, if you look at QQuickWorkerScript::engine() and QQmlEnginePrivate::getWorkerScriptEngine() you will see that all WorkerScript objects will share the same QQuickWorkerScriptEngine instance as long as they share the same QQmlEngine.
Also QQuickWorkerScriptEngine is a QThread (public inheritance) and contains a member variable named d of type QQuickWorkerScriptEnginePrivate *. d is running in the thread handled by QQuickWorkerScriptEngine (see d->moveToThread(this) in QQuickWorkerScriptEngine constructor). And it is this d that will effectively run the asynchronous work in QQuickWorkerScriptEnginePrivate::event().
PS
This kind of contradicts Qt documentation that states:
Use WorkerScript to run operations in a new thread.
Which might make you think that each WorkerScript is a new thread.
1 Comment
pmf
"Which might make you think that each WorkerScript is a new thread" Yeah, that's what has prompted me to ask the question. For the future, a thread pool size for WorkerScript would be nice for cases where more than one executor is desirable.