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Async methods that return void are intended for event handlers; in most cases where a synchronous method would return void, returning Task instead is recommended, as it allows for more intuitive exception handling. [18] Methods that make use of await must be declared with the async keyword.
The observer design pattern is a behavioural pattern listed among the 23 well-known "Gang of Four" design patterns that address recurring design challenges in order to design flexible and reusable object-oriented software, yielding objects that are easier to implement, change, test and reuse.
An Event Emitter does not know the consumers of the event, it does not even know if a consumer exists, and in case it exists, it does not know how the event is used or further processed. Sinks have the responsibility of applying a reaction as soon as an event is presented. The reaction might or might not be completely provided by the sink itself.
1. In this example, two text boxes are bound to observable variables on a data model. The "full name" display is bound to a dependent observable, whose value is computed in terms of the observables. When either text box is edited, the "full name" display is automatically updated, with no explicit event handling.
The actual logic is contained in event-handler routines. These routines handle the events to which the main program will respond. For example, a single left-button mouse-click on a command button in a GUI program may trigger a routine that will open another window, save data to a database or exit the application.
The event loop almost always operates asynchronously with the message originator. When the event loop forms the central control flow construct of a program, as it often does, it may be termed the main loop or main event loop. This title is appropriate, because such an event loop is at the highest level of control within the program.
These events can be handled synchronously, where the execution thread is blocked until the event handler completes its processing, or asynchronously, where the event is processed independently, often through an event loop. Even when synchronous handling appears to block execution, the underlying mechanism in many systems is still asynchronous ...
If many events come in asynchronously, they must be stored in a queue or dropped. In ReactiveX, an observer will never be called with an item out of order or (in a multi-threaded context) called before the callback has returned for the previous item. Asynchronous calls remain asynchronous and may be handled by returning an observable.