原文：
Callback (computer programming)
In computer programming, a callback is any executable code that is passed as an argument to other code, which is expected to call back (execute) the argument at a given time. This execution may be immediate as in a synchronous callback, or it might happen
at a later time as in an asynchronous callback. In all cases, the intention is to specify a function or subroutine as an entity[clarification needed] that is, depending on the language, more or less similar to a variable.
Programming languages support callbacks in different ways, often implementing them with subroutines, lambda expressions, blocks, or function pointers.