Qt Dispatcher

A Dispatcher object (similar to the one in WPF) for the Qt Framework, written in C++11.

This class can be used to invoke functions (including function objects and lambdas) on a specific Qt thread. Its primary purpose is to be able, from background worker threads, to cause actions to be performed on the UI thread which dispatched them.

A good example is when you want to notify the UI thread of some progress your background thread has made. If you tried to do that from a different thread, the behavior, in most cases, would be undefined, since (especially in multi-core systems) you'd be getting in the way of the UI thread doing whatever it needs to do, like dispatching events, drawing, etc.

So here's how to use this class:

QObject* someWidget = GetSomeUIWidget(...);  
Dispatcher dispatcher(someWidget); // now the dispatcher works on the thread Qt assigns to someWidget  
dispatcher.invoke([=] { doSomething(); }); // will be invoked on someWidget's thread,
                                           // and block this thread until the call
                                           // on the other thread is completed

The invoke function blocks the calling thread until the operation has been performed on the widget's thread. It may also return values, as in the following example:

int i = dispatcher.invoke([] { return 123; });

If the returned value is not a primitive, then it will either be copied or moved to the calling thread, depending on which constructor is available for its class.

invoke will also propagate any exceptions thrown in the other thread right into the calling thread, so you may wrap the call to invoke with a try/catch if you wish. If you know what you're doing, there's no problem dispatching a lambda which captures state from the calling thread. Since invoke blocks, any existing state in the thread's stack will be intact while running the function on the destination thread.

There is also an alternative fireAndForget function, which does not block, and hence does not run any function which returns a value. This can also be useful sometimes.