What is Inversion of Control?

"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."

"Inversion of Control is what you get when your program callbacks, e.g. like a gui program.

For example, in an old school menu, you might have:

print ""enter your name""
read name
print ""enter your address""
read address
etc...
store in database

thereby controlling the flow of user interaction.

In a GUI program or somesuch, instead we say:

when the user types in field a, store it in NAME
when the user types in field b, store it in ADDRESS
when the user clicks the save button, call StoreInDatabase

So now control is inverted... instead of the computer accepting user input in a fixed order, the user controls the order in which the data is entered, and when the data is saved in the database.

Basically, anything with an event loop, callbacks, or execute triggers falls into this category."

What is Inversion of Control?