Inversion of Control

In software engineering, inversion of control (IoC) is an object-oriented programming practice where the object coupling is bound at run time by an assembler object and is typically not known at compile time using static analysis.

In traditional programming, the flow of the business logic is determined by objects that are statically assigned to one another. With inversion of control, the flow depends on the object graph that is instantiated by the assembler and is made possible by object interactions being defined through abstractions. The binding process is achieved through dependency injection, although some argue that the use of a service locator also provides inversion of control.

In order for the assembler to bind objects to one another, the objects must possess compatible abstractions. For example, class A may delegate behavior to interface I which is implemented by class B; the assembler instantiates A and B then injects B to A.

In practice, inversion of control is a style of software construction where reusable code controls the execution of problem-specific code. It carries the strong connotation that the reusable code and the problem-specific code are developed independently, which often results in a single integrated application. Inversion of control as a design guideline serves the following purposes:

• There is a decoupling of the execution of a certain task from implementation.
• Every module can focus on what it is designed for.
• Modules make no assumptions about what other systems do but rely on their contracts.
• Replacing modules has no side effect on other modules.

Inversion of control is sometimes facetiously referred to as the "Hollywood Principle: Don't call us, we'll call you", because program logic runs against abstractions such as callbacks.