Detailed explanation of Angular Component use cases

This time I will bring you a detailed explanation of the use cases of Angular Component. What are the precautions when using Angular Component? The following is a practical case, let's take a look.

Web Component

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

Definition

W3C proposes the standard of Web Component to unify the standard way of componentization.

Each component contains its own html, css, and js code.
Web Component standard includes the following four important concepts:
1.Custom Elements (custom tags): You can create custom HTML tags and elements;
2.HTML Templates (HTML templates): use < ;template> tag to predefine some content, but it does not load it into the page, but uses JS code to initialize it;
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 HTML documents, .

In summary, the ability to create custom tags to introduce components is the basis for front-end componentization. References to HTML files and HTML templates on the page are used to support writing component views and component resource management, while Shadow DOM It is to isolate the conflicts and impacts of code between components.

Example

Define hello-component

Use hello-component

As you can see from the above code, hello. html is a component defined by standards (named hello-component). This component has its own structure, style and logic. Then introduce the component file in index.html and use it like an ordinary tag.

Angular Component

Angular Component is a type of directive and can be understood as a directive with a template. The other two types are attribute directives and structural directives.

Basic composition

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

1. Component decorator: Each component class must be decorated with @component to become an Angular component.

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

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

4. Component template: Each component will be associated with a template, which will eventually be rendered on the page. The DOM element on the page is the host element of this component instance.

Component metadata

Self metadata attributes

Inherited from core/Directive

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：

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