Why dependency injection

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In this video we will discuss why should we use dependency injection and the benefits it provide. Let us understand this with a very simple example.

We want to build a Computer. In reality to build a computer we need several objects like processor, ram, harddisk drive etc. To keep this example simple let's say we just need a processor object to build a computer.

Our Computer and Processor classes are as shown below. Notice at the moment, we are not using dependency injection. To build a Computer we need a Processor object and the Computer class is creating an instance of the Processor class it needs.

There are 3 fundamental problems with this code
1. This code is difficult to maintain over time
2. Instances of dependencies created by a class that needs those dependencies are local to the class and cannot share data and logic.
3. Hard to unit test

export class Computer {

private processor: Processor;

constructor() {
}
}

export class Processor {

constructor() {
}

}

Now let us understand why this code is difficult to maintain. Let us say, the Processor class needs to know the speed of the processor to be able to create an inatnce of it. One way to address this requirement is by passing the processor speed as a parameter to the constructor of the Processor class as shown below.

export class Processor {

constructor(speed: number) {
}

}

This change in the Processor class breaks the Computer class. So every time the Processor class changes, the Computer class also needs to be changed. At the moment, the Computer class has only one dependency. In reality it may have many dependencies and those dependencies inturn may have other dependencies. So when any of these dependencies change, the Computer class may also need to be changed. Hence this code is difficult to maintain.

The reason we have this problem is because the Computer class itself is creating the instance of the Processor class. Instead if an external source can create the processor instance and provide it to the computer class, then this problem can be very easily solved and that's exactly what dependency injection does. I have rewritten the above code using dependency injection, DI for short as shown below.

export class Computer {

private processor: Processor;

constructor(processor: Processor) {
}
}

export class Processor {

constructor(speed: number) {
}

}

Notice with DI, the Computer class is not creating the instance of the Processor class itself. Instead we have specified that the Computer class has a dependency on Processor class using the constructor. Now, when we create an instance of the Computer class, an external source i.e the Angular Injector will provide the instance of the Processor class to the Computer class. Since now the the Angular injector is creating the dependency instance, the Computer class need not change when the Processor class changes.

Now, let us understand the second problem - Instances of dependencies created by a class that needs those dependencies are local to the class and cannot share data and logic. The Processor class instance created in the Computer class is local to the Computer class and cannot be shared.

On the other hand if we use Dependency Injection (DI), the angular injector provides a Singleton i.e a single instance of the service so the data and logic can be shared very easily across all the components.

From unit testing standpoint, it is difficult to mock the processor object, so unit testing Computer class can get complex. In this example, the Computer class has just one dependency (i.e the dependency on the Processor object).

In a real world application, a given object may have a dependency on several other objects, and those dependencies inturn may have dependencies on other objects. Just imagine, how complicated unit testing can become with all these hierarchies of dependencies if we do not have the ability to mock the dependencies.

[mazdisoraja]

I watched like million videos on dependency injection, and i didn't get it still. They over complecated simple thing. You, just within few minutes made it so clear. Thank you teacher!

[bukunmi]

I code with java for android, this is the best explanation video i have seen on DI. 💯. Thanks!

[kreuza]

The only one who makes the things so easy to understand and makes me actually exited to learn and watch more

[vinothrajendran]

Thank God. Finally i understood the concept of Dependency Injection. Thanks a ton sir.

[ARSHABBIR]

This is the crux of the Angular....Dependency Injection is the most beautiful concept in the entire series.

[aymeninoubli]

Best and simlest explanation ever ... you are just awsome

[AlokKumar-huwm]

A perfect tutorial for learning dependency injection

[nammozhi]

You made it very easy to understand..Thanks a ton

[vijaykamble]

nice tutorial.... you are great hats off to you....

[nikhilumraskar]

how pass the speed while injecting processor into computer here 6:00

[kamdemkakengne]

Thank you Venkat for the good expanations!

[hemamalini]

Can you please explain this with c sharp code?

[naodagere]

Excellent explanation. Thanks as usual.

[ketan]

Hi Sir,
How the Angular Injector will know about the value of speed's parameter?

[kailasht]

we define the custom service in the "providers" section and its being injected by angular in the components. But when it comes to HttpClient, we are definiing the HttpClient module and then injecting it in the constructor. so just want to clarify if we define the module, then there its not required to define the service at the providers section

[vedvrat]

in this video u said that .If we use "new processor()" in computer class so it is difficult to "Mock" the processor .. can you please tell how to mock a processor class.

[maniadoc]

When a parameter is added, Processor(speed), how does DInjector knows what speed the Computer class needs?

[manchusandy]

where are we mentioning the provider property as you did in previous video?

[trustingod]

I don't understand why the processor instance within the computer class cannot share data and logic with other objects. Can't you create an instance of the computer class and access the data and logic within the processor class? Thanks !

[preethymitra]

Hello, Venkat!!

Thanks a lot for teaching us.

I just wanted to know that how many more videos would it take to complete Angular 2? This helps me with my schedule, that's why!