What is Dependency Injection | Dependency Injection in Flutter

Introduction

Dependency injection is one of the most useful design patterns that allow developers to write loosely coupled code. It also helps to keep our code testable.
Dependency injection is a concept that keeps popping up in the tech world. However, it is also a concept people usually don’t fully understand.
It can be hard to understand what it means or how it works across different frameworks.
In this guide, I will break down dependency injection and talk you through some common implementations using Dart and Flutter.
Buckle Up ......

What is Dependency Injection ??

First of all, let's understand what Dependency and Dependent Is.
When some class A needs class B to perform its operation then class A is called Dependent and class B is called Dependency
Simple explanation of DI (Dependency Injection) by John Munsch for 5 years olds :

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 don'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.
It means you don't have to go anywhere. You will get what you want.
In Technical Term:

DI is a design pattern in which a class requests dependencies from external sources rather than creating them. It means the dependent objects are injected or supplied to your class. The class doesn't have to do any initialization by itself. It will get what it wants by the injector.

Why we need Dependency Injection

DI helps you to make your code loose coupled rather than tightly coupled.
Loose coupling: When an object gets the object to be used from external sources, we call it loose coupling. In other words, loose coupling means that the objects are independent.
Tight coupling: When a group of classes are highly dependent on one another then it is called tight coupling.
By using DI pattern we can make the code more readable, reusable, and testable.
Let's see one example with and without DI.
Without DI
Let's make a Car class that has Engine as a dependency. It means Car is now dependent on Engine.

// Car.dart
import 'package:blog/Engine.dart';

class Car {
     Engine _engine = Engine();
     void start() {
       _engine.start();
     }
}

Engine class has one start method. Which simply prints a message.

// Engine.dart
class Engine {
     void start() {
       print("Engine Started");
     }
}

Now creating a Car object and starting its engine.

// main.dart
import 'package:blog/Car.dart';

void main() {
     Car car = Car();
     car.start();
}

Here our class Car is tightly coupled. It means the class Car is highly dependent on dependency Engine.
Now imagine the Car manufacturer wants to change their normal Engine to ElectricEngine. They should be able to do it right?
But if we look at the class Car there is no way to change the Engine.
We have hardcoded the Engine in the Car class.
We can solve this problem by using the Dependency Injection pattern. Dependency Injection is a technique that facilitates loosely coupled object-oriented software systems.
There are different ways to achieve dependency injection in programming :
Constructor Injection
Property Injection (aka setter injection)
Method Injection
With DI (Constructor Injection)
We can solve the above issue by passing the Engine object as a parameter of the Car constructor.

// Car.dart
import 'package:blog/Engine.dart';

Passing Engine object as a parameter

// Car.dart
class Car {
Engine? _engine;

Car(Engine engine) {
  this._engine = engine;
}

void start() {
  _engine!.start();
   }
}

// main.dart
import 'package:blog/Car.dart';
import 'package:blog/Engine.dart';

Creating Engine Object and passing it to the Car's constructor.

// main.dart
void main() {
Engine normalEngine = Engine();
Car car = Car(normalEngine);
car.start();
}

Now if we want to pass a different implementation of Engine for example ElectricEngine, we can simply create the class ElectricEngine which is an implementation of Engine class and pass it to the Car constructor.

class ElectricEngine implements Engine {
@override
void start() {
  print("Electric Engine Started");
}
}

// main.dart
void main() {
   // Engine normalEngine = Engine();
   Engine eletricEngine = ElectricEngine();
   Car car = Car(eletricEngine);
   car.start();
}

As we can see we are injecting the dependency from outside the implementation of the class.
That's It !! This is called Dependency Injection. It's easy, right? But constructor injection will also become an overhead when the application is large. That's why many languages provide their own libraries and frameworks to implement DI in a large project.
DI is very useful in Unit Testing .
It's because dependencies are injected from outside of the class. That's why our class is not any more dependent on other dependencies. And we can easily test our class in isolation.

This is the overall explanation of What is DI and Why DI.
Now we are going to see how we can implement Dependency Injection in our Flutter Application.

Dependency Injection in Flutter

In Flutter we can implement Dependency Injection in different ways :
Using Constructor,
Using InheritedWidget,
Using Packages like get_it, provider etc.
Let's apply Dependency Injection by using these three approaches.
Constructor as Dependency Injection
Consider the below code where we have 4 Pages :

Now assume that the Page 4 requires the same Car object that is declared in the Page 1.
To give Page 4 that object, we have to manually pass that object of the Car to all the way down to the Page 4, like below
As you can see it's not the best way to pass any object like this.
It is because even if the Page 2 and Page 3 don't require the Car object, they still have to declare it inside their class. And that is not efficient.

`InheritedWidget` as Dependency Injection

To solve the above issue Flutter Provides an InheritedWidget.
It helps us to access any objects, methods, variables, etc which is defined in the InheritedWidget in the widget tree.
Example:
Here I've simply created an InheritedWidget.
Now wrap the MaterialApp inside the InheritedDI
Now we can access all the variables, methods, and objects, etc that are available inside the InheritedDI from anywhere inside the widget tree.
Let's see how we can access that car object inside Page 4 directly :
That's pretty neat, right? Now we can access the Car object inside any page without any prop drilling.
But there is one problem. See the below code :
```
InheritedDI.of(context)!.getCar
```
InheritedWidget needs context to access the object.
We can't inject this dependency inside the class which doesn't have context.
So that's a problem now. What if we want to access dependencies inside the classes like DB, Client, etc? There we won't have context, because they're just simple regular classes.

`get_it` as Dependency Injection

To solve the above issue we can use get_it package. get_it is a Service Locator. The purpose of the Service Locator pattern is to return the service instances on demand.
If you are not familiar with the concept of Service Locators, it's a way to decouple the interface (abstract base class) from a concrete implementation, and at the same time allows to access the concrete implementation from everywhere in your App over the interface.
Note: There are some differences between Dependency Injection and Service Locator. Check this answer for more detail.
Let's see how to use get_it as Dependency Injection.
First of all, install the get_it package inside pubspec.yaml
```
get_it: ^7.2.0
```
Now create serviceLocator.dart file. In this file first, create instance of GetIt.
At your start-up you register all the objects you want to access later like this:
After that call the setup() method before the app initialization i.e inside main() method
And now use it like this :
As we can see that get_it makes dependency injection very easy for us.
Because now we can access this dependency inside any type of class, whether that class has context or not.
get_it provides many different ways to register dependencies. Two of the common ways are:

registerFactory will give a new instance every time get() is called.

void registerFactory<T>(FactoryFunc<T> func) // returns an NEW instance of an implementation of T

registerSingleton will create only one instance of the Car at the initialization. This will not generate new instances every time like registerFactory.
```
void registerSingleton<T>(T instance) // returns a singleton instance of an implementation of T
```

Wrapping Up

Thank you for reading. Hope you liked it.
Comments and Feedbacks are welcomed 🙂

Master The Art of Dependency Injection

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Introduction

What is Dependency Injection ??

Why we need Dependency Injection

Without DI

With DI (Constructor Injection)

Dependency Injection in Flutter

Constructor as Dependency Injection

`InheritedWidget` as Dependency Injection

`get_it` as Dependency Injection

Wrapping Up

Command Palette

Comments (6)

Flutter

How to Publish Flutter App on Play Store

More from this blog

Introduction

What is Dependency Injection ??

Why we need Dependency Injection

Without DI

With DI (Constructor Injection)

Dependency Injection in Flutter

Constructor as Dependency Injection

InheritedWidget as Dependency Injection

get_it as Dependency Injection

Wrapping Up

`InheritedWidget` as Dependency Injection

`get_it` as Dependency Injection