This is useful when the caller wants to be notified about the result of the asynchronous procedure but also when the asynchronous procedure fails. Moreover, the caller can register multiple callback procedures. In other words, the caller passes to the asynchronous procedure also “what should happen in the future when the result is available”. The caller can continue his work, and the callback procedure is called when the result is available. Instead, the caller registers a callback procedure that is called when the result is available. In contrast to a synchronous procedure call, where parameters are passed to the procedure, and the program flow doesn’t continue until the procedure returns the result, the caller doesn’t wait for the result in asynchronous communication. Such producer/consumer chaining then leads to reactive streams.Īs stated above, reactive systems make use of asynchronous communication. The consumer thread may also act as a producer by putting GUI update requests into a queue processed by a GUI update thread. A thread handling user input puts events into the queue, and a consumer thread takes events from the queue’s head and processes them. This approach is well known as the producer and consumer pattern. The decoupling is done by introducing an event queue and parallel event processing. The solution is to decouple the event handling from the GUI update. And waiting synchronously for an event that doesn’t come means that the whole application hangs and the GUI becomes unresponsive. But as the program was waiting synchronously (i.e., blocking) for the user input, nothing else could happen. It was an endless loop and was responsible for handling user input and updating the user interface. The heart of the application was a so-called event loop. In the first GUI applications, the user interface was updated only after some user action, like clicking a button. Asynchronous processing means that the processing of an event does not block the processing of other events. As we already pointed out in the introduction, reactive programming is based on the idea of asynchronous event processing.
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