Dependency injection is a design pattern where dependencies or services are passed into dependent objects rather than dependent objects creating them. This loose coupling allows for flexible and modular code. Dagger is a dependency injection framework for Android that uses annotations like @Inject and @Provides to specify dependencies and instantiate them. It handles resolving dependencies and provides instances where needed.

1. Dependency Injection with
Dagger
by Lope Emano

2. What is Dependency Injection?

3. is a software design pattern in which one or more dependencies (or
services) are injected, or passed by reference, into a dependent object (or
client) and are made part of the client's state. The pattern separates the
creation of a client's dependencies from its own behavior, which allows
program designs to be loosely coupled and to follow the dependency
inversion and single responsibility principles.
-Wikipedia

5. - James Shore
Dependency Injection is a 25-dollar term for a 5-cent concept.
Dependency injection means giving an object its instance variables.

6. How to explain dependency injection to
a 5-year old?

7. When you go and get things out of the refrigerator for yourself, you can
cause problems. You might leave the door open, you might get something
Mommy or Daddy doesn't want you to have. You might even be looking for
something we don't even have or which has expired.
What you should be doing is stating a need, "I need something to drink
with lunch," and then we will make sure you have something when you sit
down to eat.

8. “Dependency injection isn’t an end
goal, it is a means to an end - a means
to achieving loosely coupled code”

9. Loose Coupling

10. What do you do when your laptop keyboard gets broken?

11. Tightly coupled code
1. Unscrew laptop
2. Remove cover
3. Remove broken keyboard
4. Position and install new keyboard to data cable
5. Close cover
6. Screw laptop

12. Loosely coupled code
1. Plug external keyboard to usb port

13. The usb port
1. Is agnostic to any device that connects to it as long as it
satisfies its specifications
2. USB 3.0 ports are USB 2.0 compatible
3. Today you use it for just about anything- mouse,
trackpad, coffee heaters, lighting, charging phones,
android development, etc..

14. Let’s see what this looks like in pseudocode! :)

15. interface USBPluggable{openInputStream(), deviceType()}
class Keyboard implements USBPluggable {}
class AndroidPhoneWire implements USBPluggable {}
class CoffeeCupHeater implements USBPluggable {}
class Keyboard implements USBPluggable {}

16. interface USBPluggable{openInputStream(), deviceType()}
class Keyboard implements USBPluggable {}
class AndroidPhoneWire implements USBPluggable {}
class CoffeeCupHeater implements USBPluggable {}
class Keyboard implements USBPluggable {}

17. Code depends upon abstractions instead of
implementations
class Computer(){
Computer(USBPluggable port1){
port1.openInputSream();
port1.deviceType();
}
}

18. Code that depends upon implementation
instead of abstraction
class Computer(){
Computer(Keyboard keyboardPort){
KeyboardInputStream ks = keyboardPort.openInputSream();
InputStream is = ComputerUtils.convertStream(ks);
}
}

19. Wait, so what does dagger have to do
with this?

20. The problem
We need to figure out the dependencies a class needs
We need to figure out how to instantiate them

21. A Solution : Dagger
We need to figure out the dependencies a class needs
We need to figure out how to instantiate them
Uses @Inject annotation to specify that this object needs
to be injected with a provided instance
Uses @Modules and @Provides annotations to
instantiate objects that are to be injected

22. That’s it! Now let’s have a look at documentation
http://square.github.io/dagger/

23. Non-demo code demo! :)

24. http://square.github.io/dagger/
https://www.youtube.com/watch?v=IKD2-MAkXyQ
http://www.manning.com/seemann/ - Dependency Injection with Dot Net by Mark Seeman
http://stackoverflow.com/questions/1638919/how-to-explain-dependency-injection-to-a-5-
year-old
References :