Angular 8 - Angular Components and Templates

As we learned earlier, Components are building block of Angular application. The main job of Angular Component is to generate a section of web page called view. Every component will have an associated template and it will be used to generate views.
Let us learn the basic concept of component and template in this chapter.
Let us create a new component in our ExpenseManager application.
Open command prompt and go to ExpenseManager application.
cd /go/to/expense-manager

Create a new component using ng generate component command as specified below −
ng generate component expense-entry

The output is mentioned below −
CREATE src/app/expense-entry/expense-entry.component.html (28 bytes) 
CREATE src/app/expense-entry/expense-entry.component.spec.ts (671 bytes) 
CREATE src/app/expense-entry/expense-entry.component.ts (296 bytes) 
CREATE src/app/expense-entry/expense-entry.component.css (0 bytes) 
UPDATE src/app/app.module.ts (431 bytes)

Here,
ExpenseEntryComponent is created under src/app/expense-entry folder.
Component class, Template and stylesheet are created.
AppModule is updated with new component.
Add title property to ExpenseEntryComponent (src/app/expense-entry/expense-entry.component.ts) component.
import { Component, OnInit } from '@angular/core'; @Component({ 
   selector: 'app-expense-entry', 
   templateUrl: './expense-entry.component.html', styleUrls: ['./expense-entry.component.css'] 
}) 
export class ExpenseEntryComponent implements OnInit {
   title: string;
   constructor() { } 
   ngOnInit() { 
      this.title = "Expense Entry" 
   } 
}
Update template, src/app/expense-entry/expense-entry.component.htmlwith below content.
<p>{{ title }}</p>

Open src/app/app.component.html and include newly created component.
<h1>{{ title }}</h1>
<app-expense-entry></app-expense-entry>

Here,
app-expense-entry is the selector value and it can be used as regular HTML Tag.
Finally, the output of the application is as shown below −
We will update the content of the component during the course of learning more about templates.
The integral part of Angular component is Template. It is used to generate the HTML content. Templates are plain HTML with additional functionality.
Template can be attached to Angular component using @component decorator’s meta data. Angular provides two meta data to attach template to components.
templateUrl
We already know how to use templateUrl. It expects the relative path of the template file. For example, AppComponent set its template as app.component.html.
templateUrl: './app.component.html',

template
template enables to place the HTML string inside the component itself. If the template content is minimal, then it will be easy to have it Component class itself for easy tracking and maintenance purpose.
@Component({ 
   selector: 'app-root', 
   templateUrl: `<h1>{{ title }}</h1>`, 
   styleUrls: ['./app.component.css'] 
}) 
export class AppComponent implements OnInit { 
   title = 'Expense Manager'; 
   constructor(private debugService : DebugService) {} ngOnInit() { 
      this.debugService.info("Angular Application starts"); 
   } 
}
Angular Templates can use CSS styles similar to HTML. Template gets its style information from two sources, a) from its component b) from application configuration.
Component configuration
Component decorator provides two option, styles and styleUrls to provide CSS style information to its template.
Styles − styles option is used to place the CSS inside the component itself.
styles: ['h1 { color: '#ff0000'; }']

styleUrls − styleUrls is used to refer external CSS stylesheet. We can use multiple stylesheet as well.
styleUrls: ['./app.component.css', './custom_style.css']

Angular provides an option in project configuration (angular.json) to specify the CSS stylesheets. The styles specified in angular.json will be applicable for all templates. Let us check our angular.json as shown below −
{
"projects": { 
   "expense-manager": { 
      "architect": { 
         "build": { 
            "builder": "@angular-devkit/build-angular:browser", "options": { 
               "outputPath": "dist/expense-manager", 
               "index": "src/index.html", 
               "main": "src/main.ts", 
               "polyfills": "src/polyfills.ts", 
               "tsConfig": "tsconfig.app.json", 
               "aot": false, 
               "assets": [ 
                  "src/favicon.ico", 
                  "src/assets" 
               ], 
               "styles": [ 
                  "src/styles.css" 
               ], 
               "scripts": [] 
            }, 
         }, 
      } 
   }}, 
   "defaultProject": "expense-manager" 
}
Here,
styles option setssrc/styles.css as global CSS stylesheet. We can include any number of CSS stylesheets as it supports multiple values.
Let us include bootstrap into our ExpenseManager application using styles option and change the default template to use bootstrap components.
Open command prompt and go to ExpenseManager application.
cd /go/to/expense-manager

Install bootstrap and JQuery library using below commands
npm install --save bootstrap@4.5.0 jquery@3.5.1

Here,
We have installed JQuery, because, bootstrap uses jquery extensively for advanced components.
Option angular.json and set bootstrap and jquery library path.
{ 
   "projects": { 
      "expense-manager": { 
         "architect": { 
            "build": {
               "builder":"@angular-devkit/build-angular:browser", "options": { 
                  "outputPath": "dist/expense-manager", 
                  "index": "src/index.html", 
                  "main": "src/main.ts", 
                  "polyfills": "src/polyfills.ts", 
                  "tsConfig": "tsconfig.app.json", 
                  "aot": false, 
                  "assets": [ 
                     "src/favicon.ico", 
                     "src/assets" 
                  ], 
                  "styles": [ 
                     "./node_modules/bootstrap/dist/css/bootstrap.css", "src/styles.css" 
                  ], 
                  "scripts": [ 
                     "./node_modules/jquery/dist/jquery.js", "./node_modules/bootstrap/dist/js/bootstrap.js" 
                  ] 
               }, 
            }, 
         } 
   }}, 
   "defaultProject": "expense-manager" 
}
Here,
scripts option is used to include JavaScript library. JavaScript registered through scripts will be available to all Angular components in the application.
Open app.component.html and change the content as specified below
<!-- Navigation --> 
<nav class="navbar navbar-expand-lg navbar-dark bg-dark static-top"> 
   <div class="container"> 
      <a class="navbar-brand" href="#">{{ title }}</a> <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarResponsive" aria-controls="navbarResponsive" aria-expanded="false" aria-label="Toggle navigation"> 
         <span class="navbar-toggler-icon">
         </span> 
      </button> 
      <div class="collapse navbar-collapse" id="navbarResponsive"> 
         <ul class="navbar-nav ml-auto"> 
            <li class="nav-item active"> 
            <a class="nav-link" href="#">Home
               <span class="sr-only">(current)
               </span>
            </a> 
            </li> 
            <li class="nav-item"> 
            <a class="nav-link" href="#">Report</a> 
            </li> 
            <li class="nav-item"> 
            <a class="nav-link" href="#">Add Expense</a> 
            </li> 
            <li class="nav-item"> 
            <a class="nav-link" href="#">About</a> 
            </li> 
         </ul> 
      </div> 
   </div> 
</nav> 
<app-expense-entry></app-expense-entry>
Here,
Used bootstrap navigation and containers.
Open src/app/expense-entry/expense-entry.component.html and place below content.
<!-- Page Content --> 
<div class="container"> 
   <div class="row"> 
      <div class="col-lg-12 text-center" style="padding-top: 20px;"> 
         <div class="container" style="padding-left: 0px; padding-right: 0px;"> 
            <div class="row"> 
            <div class="col-sm" style="text-align: left;"> {{ title }} 
            </div> 
            <div class="col-sm" style="text-align: right;"> 
               <button type="button" class="btn btn-primary">Edit</button> 
            </div> 
            </div> 
         </div> 
         <div class="container box" style="margin-top: 10px;"> 
         <div class="row"> 
         <div class="col-2" style="text-align: right;">  
            <strong><em>Item:</em></strong> 
         </div> 
         <div class="col" style="text-align: left;"> 
            Pizza 
         </div>
         </div> 
         <div class="row"> 
         <div class="col-2" style="text-align: right;">
            <strong><em>Amount:</em></strong> 
         </div> 
         <div class="col" style="text-align: left;"> 
            20 
         </div> 
         </div> 
         <div class="row"> 
         <div class="col-2" style="text-align: right;"> 
            <strong><em>Category:</em></strong> 
         </div> 
         <div class="col" style="text-align: left;"> 
            Food 
         </div> 
         </div> 
         <div class="row"> 
         <div class="col-2" style="text-align: right;"> 
            <strong><em>Location:</em></strong>
         </div> 
         <div class="col" style="text-align: left;"> 
            Zomato 
         </div> 
         </div> 
         <div class="row"> 
         <div class="col-2" style="text-align: right;"> 
            <strong><em>Spend On:</em></strong> 
         </div> 
         <div class="col" style="text-align: left;"> 
            June 20, 2020 
         </div> 
         </div> 
      </div> 
   </div> 
</div> 
</div>
Restart the application.
The output of the application is as follows −
We will improve the application to handle dynamic expense entry in next chapter.

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