Do you know Directives in Angular? Here are detailed explanations of the three directive instructions of angularjs

This article mainly introduces the three instruction types in angularjs. Which three types are they? Let this article tell you, and how to use these three types. It's all here. Now let's take a look

In Angular2 there are three types of directives (Directive), as follows:
1 . Attribute directives - directives that change the display and behavior of elements. For example: NgStyle …
2. Structural directives - directives that change the structure of the DOM by adding and removing DOM elements. For example: NgFor, NgIf …
3. Component — A directive that owns a template.

1. Attribute instructions (ngStyle, ngClass)

NgStyle
Bind an object with a shape such as CSS property name: value, The value is the specific css style, eg:

<p></p><p></p>

Note, in the description of ngStyle, we use single quotes for background-color, but not for color. Why is this? Because the parameter of ngStyle is a JavaScript object, and color is a legal key, no quotes are needed. But in background-color, hyphens are not allowed to appear in the key name of an object unless it is a string, so quotes are used. In general, try not to use quotes around object keys unless absolutely necessary.

//Dynamic use

<span>{{ color }} text</span>

//Judgment addition

<p></p><p></p>

NgClass
Use the NgClass command to add or remove multiple ones at the same time kind. NgClass binds an object with a shape such as CSS class name: value, where the value is a Boolean value. When the value is true, a template element of the corresponding type is added, otherwise it is removed.

//Basic usage

 <p>此时p不包含bordered 类名</p>
 <p>此时p含有bordered 类名</p>

//Judgment

 <i></i>

##2. Structural directives (ngIf, ngFor, ngSwitch)

NgIfSpecifies binding a Boolean expression , when the expression returns true, an element and its sub-elements can be added to the DOM tree node, otherwise they are removed.

If the result of the expression returns a false value, the element will be removed from the DOM.

Here are some examples:

<p></p> //不显示
<p> b"></p>//
<p></p> 
<p></p>

NgForThe NgFor instruction can perform certain operations repeatedly to display data. The NgFor directive supports an optional index.
The syntax is *ngFor="let item of items":
The let item syntax specifies a (template) variable that receives each element in the items array.
items is a collection of items from the component controller

this.cities = ['厦门', '福州', '漳州'];
<p>{{ c }}</p>

Get the index

When iterating through the array, we may also want to get the index of each item.
We can insert the syntax let idx = index into the value of the ngFor directive and separate it with a semicolon, so that we can get the index.

<p>{{ num+1 }} . {{ c }}</p>

The results are as follows:

1.Xiamen
2.Fuzhou
3.Zhangzhou

ngSwitchSometimes you need to follow a Render different elements given the conditions.
When encountering this situation, you may use ngIf multiple times like this:

<p>
    </p><p>Var is A</p>
    <p>Var is B</p>
    <p>Var is C</p>
    <p>Var is something else</p>

For this situation, Angular introduces the ngSwitch directive. (If you want to learn more, go to the PHP Chinese website

AngularJS Development Manual to learn)

NgSwitch: Bind to a value expression that returns a control condition

NgSwitchCase: Bind to a Returns the value expression that matches the condition
NgSwitchDefault: An attribute used to mark the default element, which is optional. If we don't use it, nothing will be rendered if myVar doesn't match any of the expected values ​​
.
Use the ngSwitch directive to rewrite the above example:

<p>
    </p><p>Var is A</p>
    <p>Var is B</p>
    <p>Var is C</p>
    <p>Var is something else</p>

3. Component

The creation of attribute directives requires at least one decorated with @Directive decorator controller class. The @Directive decorator specifies a selector name that indicates the name of the attribute associated with this directive. <p></p>Next, start creating a simple attribute-type directive. The function of this directive is to obtain the minimum height of .quotation-area when the user-quotation-view.component.html page is refreshed. <p></p>1. First, we confirm the command name, quotationArea<p></p>

<p></p>

Apply this command as an attribute to a DOM element, that is, we need to find a host element for this command. .

2. Then we create a quotationArea.directive.tss file with the following code structure:

import {Component, Directive, ElementRef, OnInit} from '@angular/core';
@Directive({ selector: '[quotationArea]'})
export class QuotationAreaDirective implements OnInit {

  el:ElementRef;
  constructor(el: ElementRef) {
    this.el = el;
  }
  ngOnInit() {
    const $el = $(this.el.nativeElement);
    const windowHeight = document.documentElement.clientHeight; //获取窗口高度
    const bar=document.getElementsByClassName('bar-nav')[0]
    const barHeight =bar.clientHeight;
    const heightValue=windowHeight - barHeight;
    $el.css('height',(heightValue) + 'px');

  }
}

3. Next, we need to explicitly declare our own defined directives in module.ts so that When Angularr parses the template, it can correctly identify an instruction we set ourselves. <p></p>

import {QuotationAreaDirective} from './user-quotation/user-quotation-view/quotationArea.directive';
    declarations: [QuotationAreaDirective]

The result is as shown:<p></p>

Okay, this article ends here (if you want to see more, go to the PHP Chinese website AngularJS User Manual to learn ), if you have any questions, you can leave a message below

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