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Dependency injection

Dependency injection is a way to supply a new instance of a class with the fully-formed dependencies it requires. Most dependencies are services. Angular uses dependency injection to provide new components with the services they need.

Angular can tell which services a component needs by looking at the types of its constructor parameters. For example, the constructor of your HeroListComponent needs a HeroService:

Sample Code-

When Angular creates a component, it first asks an injector for the services that the component requires.

An injector maintains a container of service instances that it has previously created. If a requested service instance is not in the container, the injector makes one and adds it to the container before returning the service to Angular. When all requested services have been resolved and returned, Angular can call the component's constructor with those services as arguments. This is dependency injection.

The process of HeroService injection looks a bit like this:

If the injector doesn't have a HeroService, how does it know how to make one?

In brief, you must have previously registered a provider of the HeroService with the injector. A provider is something that can create or return a service, typically the service class itself.

You can register providers in modules or in components.

In general, add providers to the root module so that the same instance of a service is available everywhere.

Alternatively, register at a component level in the providers property of the @Component metadata:

  selector:    'hero-list',  templateUrl: './hero-list.component.html',  providers:  [ HeroService ]})

Registering at a component level means you get a new instance of the service with each new instance of that component.

Points to remember about dependency injection:

• Dependency injection is wired into the Angular framework and used everywhere.

• The injector is the main mechanism.

  • An injector maintains a container of service instances that it created.
  • An injector can create a new service instance from a provider.

• A provider is a recipe for creating a service.

• Register providers with injectors.

This algorithm reduces the definition of "an object is not needed anymore" to "an object is unreachable".

This algorithm assumes the knowledge of a set of objects called roots (In JavaScript, the root is the global object). Periodically, the garbage-collector will start from these roots, find all objects that are referenced from these roots, then all objects referenced from these, etc. Starting from the roots, the garbage collector will thus find all reachableobjects and collect all non-reachable objects.

This algorithm is better than the previous one since "an object has zero reference" leads to this object being unreachable. The opposite is not true as we have seen with cycles.

As of 2012, all modern browsers ship a mark-and-sweep garbage-collector. All improvements made in the field of JavaScript garbage collection (generational/incremental/concurrent/parallel garbage collection) over the last few years are implementation improvements of this algorithm, but not improvements over the garbage collection algorithm itself nor its reduction of the definition of when "an object is not needed anymore".

Jquery : - it is a fast and concise JavaScript Library that simplifies HTML document traversing, event handling, animating, and Ajax interactions for rapid web development

Advanctage of Jquery :-

• It helps to improve the performance of the application
• It helps to develop most browser compatible web page
• It helps to implement UI related critical functionality without writing hundreds of lines of codes
• It is fast
• It is extensible – jQuery can be extended to implement customized behavior
• The price is right. The jQuery library is free.
• It's light. Since jQuery pushes content to the client, it therefore reduces the wait time for server response. Plus, it's smaller than Flash, so it results in smoother playbacks and less errors.
• It works anywhere. HTML5 is cross-browser compatible-it will work on any browser, mobile phone or tablet, and even on Apple devices.
• There's a low learning curve. Since it's based on plain old JavaScript, developers can learn jQuery fast.
• Finally, jQuery is SEO friendly and CSS3 compliant.

I hope this lesson increase your knowledge about jquery basic .

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