Angular 2 architecture

Module

The module consists of a piece of code that can be used to perform a simple task.

The Angular application is modular and has its own module system: NgModules .

Each Angular should have at least one module (root module), which can generally be named: AppModule .

The Angular module is a module with @NgModule the class of the decorator that receives a metadata object that describes the properties of the module.

Several important attributes are as follows:

• declarations (declaration)-the view class belongs to this module. Angular has three types of view classes: components, instructions, and pipes.

• exports -A subset of declaration that can be used for component templates in other modules.

• imports -modules in this module component template that need to be exported by other classes.

• providers -creator of the service. This module adds them to the global service table so that they can be accessed in any part of the application.

• bootstrap -the main view of the application, called the root component, is the host of all other application views. Only the root module needs to be set bootstrap property.

The simplest root module:

app/app.module.ts file:

import { NgModule }      from '@angular/core';
import { BrowserModule } from '@angular/platform-browser';
@NgModule({
  imports:      [ BrowserModule ],
  providers:    [ Logger ],
  declarations: [ AppComponent ],
  exports:      [ AppComponent ],
  bootstrap:    [ AppComponent ]
})
export class AppModule { }

Next we start the application by booting the root module, and the development process is usually in the main.ts to boot in the file AppModule the code is as follows:

app/main.ts file:

import { platformBrowserDynamic } from '@angular/platform-browser-dynamic';
import { AppModule } from './app.module';

platformBrowserDynamic().bootstrapModule(AppModule);

Components

A component is a control class of a template that handles the view portion of the application and logical pages.

Component is the foundation and core of Angular application, which can be used in the whole application.

The component knows how to render itself and configure dependency injection.

Components interact with the view through a number of API consisting of properties and methods.

There are three steps to create an Angular component:

• From @angular/core introduce into Component modifier

• Create a normal class and use the @Component Embellish it.

• In @Component , setting up selector custom tags, and template template

Templates

The default language for Angular templates is HTML.

We can tell Angular how to display the component by using a template to define the view of the component. Here is a simple example:

<div>
Website address : {{site}}
</div>

In Angular, double curly braces are used as the interpolation syntax by default, and the value in the middle of the braces is usually the variable name of a component property.

Metadata

The metadata tells Angular how to handle a class.

Consider the following situation where we have a component called Component is a class until we tell Angular , this is a component until now.

You can attach metadata to this class and tell Angular Component that it is a component.

In TypeScript, we use decorator to attach metadata.

@Component({
   selector : 'mylist',
   template : '<h2>Rookie Tutorial</h2>'
   directives : [ComponentDetails]
})
export class ListComponent{...}

Data binding

Data binding provides an application with a simple and consistent way to display data and data interaction, which is a mechanism for managing values in the application.

Through this mechanism, values can be taken and assigned from HTML, making it easier and faster to read and write data and to persist data.

As shown in the figure, there are four forms of data binding syntax. Each form has a direction-from DOM to DOM, in both directions, as indicated by the arrow in the figure.

• Interpolation: displays component values in the HTML tag.

<h3>
{{title}}
<img src="{{ImageUrl}}">
</h3>

• Attribute binding: sets the property of the element to the value of the property in the component.

<img [src]="userImageUrl">

• Event binding: triggered when the component method name is clicked.

<button (click)="onSave()">save</button>

• Two-way binding: usin Angular in NgModel instructions can be bound in both directions more easily.

<input [value]="currentUser.firstName"
       (input)="currentUser.firstName=$event.target.value" >

Directives

The Angular template is dynamic. When Angular renders them, it modifies the DOM according to the instructions.

An instruction is a class with instruction metadata. In TypeScript, you want to pass the @Directive the decorator attaches metadata to the class.

The following three types of instructions are included in Angular:

• Attribute instruction: an instruction used in the form of an attribute of an element.

• Structure instruction: used to change the structure of the DOM tree

• Component: as an important subclass of an instruction, a component can essentially be seen as an instruction with a template.

<li *ngFor="let site of sites"></li>
<site-detail *ngIf="selectedSite"></site-detail>

\*ngFor tell Angular to be sites each item in the list generates a <li> label.

\*ngIf indicates that the selected item is included only if it exists. SiteDetail module.

Service (Services)

A service in Angular2 is an independent module that encapsulates a specific function and can be injected into it for others to use.

There are many kinds of services, including values, functions, and features required by the application.

For example, when duplicate code occurs in multiple components, the duplicate code is extracted into the service for code reuse.

Here are several common services:

• Log service

• Data service

• Message bus

• Tax calculator

• Application configuration

The following example is a logging service that records logs to the browser’s console:

export class Logger {
  log(msg: any)   { console.log(msg); }
  error(msg: any) { console.error(msg); }
  warn(msg: any)  { console.warn(msg); }
}

Dependency injection

Inversion of Control, abbreviated as IoC, is a design principle in object-oriented programming that can be used to reduce the coupling between computer code. One of the most common methods is called Dependency Injection (DI), and another is called dependency lookup (Dependency Lookup).

Through control inversion, when an object is created, an external entity that regulates all objects in the system passes references to the objects onwhich it depends. It can also be said that dependencies are injected into the object.

In the traditional development model, the caller is responsible for managingthe dependencies of all objects, and circular dependency has always been a nightmare, while in the dependency injection model, this management is handed over to the Injector, which is responsible for the replacement of dependent objects at run time, not at compile time. This kind of control reversal, the characteristic of running injection is the essence of dependency injection.

Angular can see which services the component needs by looking at theparameter types of the constructor. For example, SiteListComponent theconstructor of the component requires a SiteService :

constructor(private service: SiteService) { }

When you Angular create a component, you first find an injector (Injector) for the services the component needs.

The injector is a container that maintains service instances and holds previously created instances.

If the requested service instance is not already in the container, the injector creates a service instance, adds it to the container, and returns the service to Angular.

When all services are parsed and returned, Angular invokes the component’s constructor with those services as parameters. This is dependency injection.