Callback (computer programming)

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In computer programming, a callback is a programming pattern in which a function reference is passed from one context (consumer) to another (provider) such that the provider can call the function. If the function accesses state or functionality of the consumer, then the call is back to the consumer – backwards compared to the normal flow of control in which a consumer calls a provider. A function that accepts a callback parameter may be designed to call back before returning to its caller. But, more typically, a callback reference is stored by the provider so that it can call the function later (as deferred). If the provider invokes the callback on the same thread as the consumer, then the call is blocking, a.k.a. synchronous. If instead, the provider invokes the callback on a different thread, then the call is non-blocking, a.k.a. asynchronous. A callback can be likened to leaving instructions with a tailor for what to do when a suit is ready, such as calling a specific phone number or...

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