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    <p>I will focus on pros and cons of the "<code>NotificationCenter</code>" solution as required by the question (though I'd call it <code>DataBus</code>, <code>Dispatcher</code> or <code>Publisher/Subscriber</code>, instead).</p> <p><strong>Pros</strong>:</p> <ul> <li>High decoupling of the classes</li> <li>The Subscriber of an event doesn't need to know the source of the event</li> <li>Concurrency management is simplified if you use an asynchronous <code>NotificationCenter</code> (you can avoid multithreading by using the <code>NotificationCenter</code> as a scheduler of cpu time).</li> <li>An event driven architecture is higly testable / observable</li> </ul> <p><strong>Cons</strong>:</p> <ul> <li>The interface is implicit (i.e. the interface of a class does not expose the events)</li> <li>The partitioning of the application is critical, in order to avoid duplication of the state in the classes</li> <li>Reusability in another application becomes harder (when you want to reuse a class in another application, it brings with itself the <code>NotificationCenter</code>)</li> <li>It's difficult to insert a new elaboration in the flow of events from input to output (i.e.: if class <code>A</code> emits event <code>E</code> and class <code>B</code> registers for notifications of <code>E</code>, you cannot insert a class <code>C</code> "between" <code>A</code> and <code>B</code> to change the content of <code>E</code> let's say to do some kind of elaboration on it)</li> <li>The <code>NotificationCenter</code> must manage client's errors (i.e. the <code>NotificationCenter</code> should provide an exception handler to catch the exeptions thrown in the event handlers).</li> </ul> <p>In addition, I'd like to point out that it's not indispensable that the NotificationCenter be a singleton. In fact you can have multiple instances of NotificationCenter, each managing different category of events (e.g. in an embedded system you can have a NotificationCenter for the low level hardware events and another for the high level logic).</p>
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    1. POPros and cons of a centralized event dispatch
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    1. USDaniele Pallastrelli
    1. USDaniele Pallastrelli
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    1. COFor "Concurrency management is simplified", actually i don't think so. Whatever you component and caller is multi-thread or only single thread, the notification center need to be thread safe (because it need to handle other component events), that require notification center implement need to be careful, it need release the lock as quickly as possible, else notification center may hold the lock when invoke slow event handler and block your component to fire event.
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      1. USZijingWu
    2. CO@ZijingWu In many cases you can avoid multithread by using the NotificationCenter as a scheduler. That's the best solution when you opt for an event-driven architecture.
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      1. USDaniele Pallastrelli
    3. COCan you provide an example how it can be used as scheduler. I actually doesn't understand how it can be used to avoid multi-thread.
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