====== Directive *ngIf ======
We can use ngIf directive to control an UI element to show or not. The syntax is:
<code><DOM_ELEMENT *ngIf="condition_expression"> ...THE CONTENT... </DOM_ELEMENT></code>
''DOM_ELEMENT'' can be like ''<div>'', ''<p>'', or anything HTML tag. The condition_expression could be a Boolean variable or a function that return Boolean result.

===== Example =====
In this example, we will let user to click a button. And if he clicked it, we will should "The button is clicked" on the top of it.
{{ :ng:ng_ngif.png?nolink&400 |}}
Assume our component name is ''servers''.

==== servers.component.html ====
Here we user the Boolean variable ''bIsClicked'' to check should the first <p>...</p> be shown. A method ''onButtonClicked()'' within the component core code will be called to change the value of ''bIsClicked''.
<code>
<p *ngIf="bIsClicked">The button is clicked</p>
<button (click)="onButtonClicked()">Click Me!</button>
</code>

==== servers.component.ts ====
Notice we have create a Boolean variable bIsClicked as boolean, and a function onButtonClicked(){...}. On the component init, we set the boolean to false, and if the function onButtonClicked() is called, we set the boolean to true.
<code>
import { Component, OnInit } from '@angular/core';

@Component({
  selector: 'app-servers',
  templateUrl: './servers.component.html',
  styleUrls: ['./servers.component.css']
})
export class ServersComponent implements OnInit {
  bIsClicked: boolean;

  constructor() { }

  ngOnInit() {
    this.bIsClicked = false;
  }

  onButtonClicked() {
    this.bIsClicked = true;
  }
}
</code>