Research signal-slot library features

[YarikTH]

observers from ureact have lots of things in common with a classic signal-slot Pattern implemented by boost/signals2, Qt and many other less known libraries (for e.g. KDAB/KDBindings and PG1003/observer).

Unlike ureact's events, signal-slots is not composable. But instead it provides one to many connection between one signal and many slots. Sometimes even backwards connection is possible in a form of collecting results of each slot in a container when signal is emitted. Also, signal-slots can have several args to pass instead of a single one in ureact.

Interesting features I have seen so far:

• if slot's functor receives less arguments than passed, then it is OK. It can be interesting not only for observer functions, but all other functions too
• connection can be temporary blocked. It is alive, bun don't call functor until unblocked
• lifetime tracking of slot's arguments See boost/signals2 tutorial. Looks complex, but it can allow more safe callback execution. It can be done manually inside observer callback though. Special case of such tracking is connection_owner.
• slots execution ordering. I'm not sure is it required, but it seems that unordered execution of observer callbacks could be a problem in some cases. I don't know should ordering be on per reactive value basis, or on context basis instead.