feature - add event filter operators like system.ignoreEvent, system.debounceOn

[beeth0ven]

We are proposal to add event filter operators. these operators can prevent unnecessary event dispatching. Here is initial imaging of these operators, it may change before we implementing them, but we issued the demand first:

• system.ignoreEvent - ignore candidate event when it met some condition
• system.debounceOn - apply debounce logic to some events

system.ignoreEvent

Ignore candidate event when it met some condition.

Usage:

  futureSystem
    ...
    .ignoreEvent(
      when: (state, event) => event is TriggerLoadData && state.loading // ignore trigger if already loading
    ) 
    ...

Above code shown if the system is already in loading status, then upcoming TriggerLoadData event will be ignored.
This operator will intercept event if some condition is met based on current state and the candidate event.

system.debounceOn

Apply debounce logic to some events

Usage:

  searchSystem
    ...
    .on<UpdateKeyword>(
      reduce: (state, event) => state.copyWith(keyword: event.keyword)
    )
    .debounceOn<UpdateKeyword>(duration: const Duration(seconds: 1))
    ...

Above code shown if UpdateKeyword event is dispatched with high frequency (quick typing), system will intercept these events to reduce unnecessary dispatching, it will pass (not inercept) event if 1 seconds has passed without dispatch another UpdateKeyword event.