Dependency Injection

Why should I care?

• You'll be seeing it a lot in Sparkles
• It makes the code more testable
• Decreases coupling

In plain English

• Give me the stuff I need, instead of having me create it myself
• Passing dependencies (like objects) as a parameter

ELI5

• When you are hungry, you have a dependency on food. 
• Without DI, you would go to the kitchen and make yourself dinner
• With DI, you would call the restaurant and get the food delivered

Example

• Warrior class
• Has a sword created inside class
• What happens if we want to arm our warrior with a different weapon? 
• Tightly coupled

Example - DI

• Warrior class
• Has a weapon injected
• Now it is easy to give our warrior different weapons

Why is this useful?

• Testing!
• What if our weapon was actually a call to a database, or an API
• If we want to test our warrior class, every time we run the test we are hoping that the API is still online. 
• We also assume that the API does not change the return value

Testability

• Now we inject the API
• This allows us to mock and stub the API
• To hard code the return value
• It means we aren't testing against a depency
• Our unit test is truly testing a single unit