Inversion of control

In software engineering, inversion of control (IoC) is a design pattern in which custom-written portions of a computer program receive the flow of control from a generic framework. The term "inversion" is historical: a software architecture with this design "inverts" control as compared to procedural programming. In procedural programming, a program's custom code calls reusable libraries to take care of generic tasks, but with inversion of control, it is the framework that calls the custom code.

Inversion of control has been widely used by application development frameworks since the rise of GUI environments and continues to be used both in GUI environments and in web server application frameworks. Inversion of control makes the framework extensible by the methods defined by the application programmer.

Event-driven programming is often implemented using IoC so that the custom code need only be concerned with the handling of events, while the event loop and dispatch of events/messages is handled by the framework or the runtime environment. In web server application frameworks, dispatch is usually called routing, and handlers may be called endpoints.

The phrase "inversion of control" has separately also come to be used in the community of Java programmers to refer specifically to the patterns of injecting objects' dependencies that occur with "IoC containers" in Java frameworks such as the Spring framework. In this different sense, "inversion of control" refers to granting the framework control over the implementations of dependencies that are used by application objects rather than to the original meaning of granting the framework control flow (control over the time of execution of application code e.g. callbacks).