What is the dependency inversion principle and why is it important?

Question

What is the dependency inversion principle and why is it important?

Answer 1

Inversion of control (IoC) is a design pattern where an object gets handed its dependency by an outside framework, rather than asking a framework for its dependency.

Pseudocode example using traditional lookup:

class Service {
    Database database;
    init() {
        database = FrameworkSingleton.getService("database");
    }
}

Similar code using IoC:

class Service {
    Database database;
    init(database) {
        this.database = database;
    }
}

The benefits of IoC are:

• You have no dependency on a central framework, so this can be changed if desired.
• Since objects are created by injection, preferably using interfaces, it's easy to create unit tests that replace dependencies with mock versions.
• Decoupling off code.