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One of the long-running debates while designing PSR-14 was how to handle events that were inherently mono-directional. Often, a library will trigger an Event that is, from its point of view, entirely informational. The Event object it fires has no mutator methods so Listeners have no way to interact with each other, which means that the order Listeners run in is irrelevant. It also means Listeners can pass no data back to the emitting library, which means the result of the Event can have no impact on the Emitter's further logic.

This special case opens up more options for how to execute the Listeners. Because there is guaranteed no communication from Listener to Listener or from Listener to Emitter, it's safe to run the Listeners concurrently. In fact, it's safe to run the Listeners concurrently with the Emitter. The Emitter doesn't even need to wait for the Listeners to fire before continuing.

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