This article introduces you to Angular development practice (3): Analyzing Angular Component. Interested friends can take a look at

Web Component

Before introducing Angular Component, Let’s first briefly understand W3C Web Components

Definition

• W3C is a unified component standard method and proposes the standard of Web Component.

• Each component contains its own html, css, and js code.

• Web Component standard includes the following four important concepts:

1. Custom Elements (custom tags): Yes Create custom HTML tags and elements;

2. ##HTML Templates: Use the

3. Shadow DOM (virtual DOM): You can create a DOM subtree that is completely independent from other elements;

4. HTML Imports: A method of introducing other HTML documents into an HTML document,

   .

@Component({ selector: 'demo-component', template: 'Demo Component' }) export class DemoComponent {}

Copy after login

• Component decorator: Each component class must be decorated with

  @componentto become an Angular component.

• Component metadata: Component metadata:

  selector,template, etc. The following will focus on the meaning of each metadata.

• Component class: Component is actually an ordinary class, and the logic of the component is defined and implemented in the component class.

• Component template: Each component will be associated with a template, which will eventually be rendered on the page. The

  DOMelement on the page is the host element of this component instance. .

从 core/Directive 继承

几种元数据详解

以下几种元数据的等价写法会比元数据设置更简洁易懂，所以一般推荐的是等价写法。

inputs

@Component({ selector: 'demo-component', inputs: ['param'] }) export class DemoComponent { param: any; }

等价于：

@Component({ selector: 'demo-component' }) export class DemoComponent { @Input() param: any; }

outputs

@Component({ selector: 'demo-component', outputs: ['ready'] }) export class DemoComponent { ready = new eventEmitter(); }

等价于：

@Component({ selector: 'demo-component' }) export class DemoComponent { @Output() ready = new eventEmitter(); }

host

@Component({ selector: 'demo-component', host: { '(click)': 'onClick($event.target)', // 事件 'role': 'nav', // 属性 '[class.pressed]': 'isPressed', // 类 } }) export class DemoComponent { isPressed: boolean = true; onClick(elem: HTMLElement) { console.log(elem); } }

等价于：

@Component({ selector: 'demo-component' }) export class DemoComponent { @HostBinding('attr.role') role = 'nav'; @HostBinding('class.pressed') isPressed: boolean = true; @HostListener('click', ['$event.target']) onClick(elem: HTMLElement) { console.log(elem); } }

queries - 视图查询

@Component({ selector: 'demo-component', template: `  
Demo Component
 `, queries: { theInput: new ViewChild('theInput') } }) export class DemoComponent { theInput: ElementRef; }

等价于：

@Component({ selector: 'demo-component', template: `  
Demo Component
 ` }) export class DemoComponent { @ViewChild('theInput') theInput: ElementRef; }

queries - 内容查询

 
{{item}}
 

@Directive({ selector: 'li' }) export class ListItem {}

@Component({ selector: 'my-list', template: ` 
  
 `, queries: { items: new ContentChild(ListItem) } }) export class MyListComponent { items: QueryList; }

等价于：

@Component({ selector: 'my-list', template: ` 
  
 ` }) export class MyListComponent { @ContentChild(ListItem) items: QueryList; }

styleUrls、styles

• styleUrls和styles允许同时指定。

• 优先级：模板内联样式 > styleUrls > styles。

• 建议：使用styleUrls引用外部样式表文件，这样代码结构相比styles更清晰、更易于管理。同理，模板推荐使用templateUrl引用模板文件。

changeDetection

• ChangeDetectionStrategy.Default：组件的每次变化监测都会检查其内部的所有数据（引用对象也会深度遍历），以此得到前后的数据变化。

• ChangeDetectionStrategy.OnPush：组件的变化监测只检查输入属性（即@Input修饰的变量）的值是否发生变化，当这个值为引用类型（Object，Array等）时，则只对比该值的引用。

• 显然，OnPush策略相比Default降低了变化监测的复杂度，很好地提升了变化监测的性能。如果组件的更新只依赖输入属性的值，那么在该组件上使用OnPush策略是一个很好的选择。

encapsulation

• ViewEncapsulation.None：无 Shadow DOM，并且也无样式包装。

• ViewEncapsulation.Emulated：无 Shadow DOM，但是通过Angular提供的样式包装机制来模拟组件的独立性，使得组件的样式不受外部影响，这是Angular的默认设置。

• ViewEncapsulation.Native：使用原生的 Shadow DOM 特性。

生命周期

当Angular使用构造函数新建组件后，就会按下面的顺序在特定时刻调用这些生命周期钩子方法：

相关推荐：

Angular开发实践（一）：环境准备及框架搭建

Angular开发实践（二）：HRM运行机制

The above is the detailed content of Angular Development Practice (3): Analyzing Angular Component. For more information, please follow other related articles on the PHP Chinese website!