What is Inversion of Control?

Question

Inversion of Control (IoC) can be quite confusing when it is first encountered.

1. What is it?
2. Which problem does it solve?
3. When is it appropriate to use and when not?

Answer 1

The Inversion-of-Control (IoC) pattern, is about providing any kind of callback (which controls reaction), instead of acting ourself directly (in other words, inversion and/or redirecting control to external handler/controller). The Dependency-Injection (DI) pattern is a more specific version of IoC pattern, and is all about removing dependencies from your code.

Every DI implementation can be considered IoC, but one should not call it IoC, because implementing Dependency-Injection is harder than callback (Don't lower your product's worth by using general term "IoC" instead).

For DI example, say your application has a text-editor component, and you want to provide spell checking. Your standard code would look something like this:

public class TextEditor {      private SpellChecker checker;      public TextEditor() {         this.checker = new SpellChecker();     } } 

What we've done here creates a dependency between the TextEditor and the SpellChecker. In an IoC scenario we would instead do something like this:

public class TextEditor {      private IocSpellChecker checker;      public TextEditor(IocSpellChecker checker) {         this.checker = checker;     } } 

In the first code example we are instantiating SpellChecker (this.checker = new SpellChecker();), which means the TextEditor class directly depends on the SpellChecker class.

In the second code example we are creating an abstraction by having the SpellChecker dependency class in TextEditor's constructor signature (not initializing dependency in class). This allows us to call the dependency then pass it to the TextEditor class like so:

SpellChecker sc = new SpellChecker(); // dependency TextEditor textEditor = new TextEditor(sc); 

Now the client creating the TextEditor class has control over which SpellChecker implementation to use because we're injecting the dependency into the TextEditor signature.