Angular Development Practice (3): Analyzing Angular Component

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

   d477f9ce7bf77f53fbcf36bec1b69b7a tag to predefine some content but not load it into page, but use 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 an HTML document,

   8e02cc3535427c5f3bb11d561b7866f9.

In summary, you can create custom tags to introduce components, which is the basis of front-end componentization. References to HTML files and HTML templates on the page are used to support the writing of component views and component resources. Management, while Shadow DOM isolates code conflicts and impacts between components.

Example

Define hello-component

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        h1 {
            color: red;
        }
    531ac245ce3e4fe3d50054a55f265927
    4a249f0d628e2318394fd9b75b4636b1Hello Web Component!473f0a7621bec819994bb5020d29372a
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    // 指向导入文档，即本例的index.html
    var indexDoc = document;

    // 指向被导入文档，即当前文档hello.html
    var helloDoc = (indexDoc._currentScript || indexDoc.currentScript).ownerDocument;

    // 获得上面的模板
    var tmpl = helloDoc.querySelector('#hello-template');

    // 创建一个新元素的原型，继承自HTMLElement
    var HelloProto = Object.create(HTMLElement.prototype);

    // 设置 Shadow DOM 并将模板的内容克隆进去
    HelloProto.createdCallback = function() {
        var root = this.createShadowRoot();
        root.appendChild(indexDoc.importNode(tmpl.content, true));
    };

    // 注册新元素
    var hello = indexDoc.registerElement('hello-component', {
        prototype: HelloProto
    });
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Use hello-component

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    b2386ffb911b14667cb8f0f91ea547a7Web Component6e916e0f7d1e588d4f442bf645aedb2f
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    8540371773d08d87304e51f46eb14716
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As you can see from the above code, hello.html is defined according to the standard Component (named hello-component) has its own structure, style and logic. Then introduce the component file in index.html and use it like a normal 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 {}

• Component decorator: Each component class must be decorated with

  @component to 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

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

Component metadata

Self metadata attributes

NameTypeFunction##animationsAnimationEntryMetadata[]changeDetectionChangeDetectionStrategyencapsulation ViewEncapsulationentryComponentsany[] interpolation[string, string]{{}}moduleIdstringstyleUrlsstring[]stylesstring[ ]templatestringtemplateUrlstringviewProvidersProvider[]

从 core/Directive 继承

	Set the animation of the component
	Set the component's change detection strategy
	Set the component’s view packaging options
	Set the list of components that will be dynamically inserted into this component view
	The interpolation mark of the custom component, the default is double braces
	Set the module id of this component under the ES/CommonJS specification, which is used to resolve the relative path of template styles
	Set the external style file referenced by the component
	Set the inline style used by the component
	Setting Component’s inline template
	Set the path where the component template is located
	Sets the services available to the component and all its subcomponents (excluding ContentChildren)

名称	类型	作用
exportAs	string	设置组件实例在模板中的别名，使得可以在模板中调用
host	{[key: string]: string}	设置组件的事件、动作和属性等
inputs	string[]	设置组件的输入属性
outputs	string[]	设置组件的输出属性
providers	Provider[]	设置组件及其所有子组件（含ContentChildren）可用的服务（依赖注入）
queries	{[key: string]: any}	设置需要被注入到组件的查询
selector	string	设置用于在模板中识别该组件的css选择器（组件的自定义标签）

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 eventEmittereb5971c99090297ca98f84cac659caf1();
}

等价于：

@Component({
    selector: 'demo-component'
})
export class DemoComponent {
    @Output() ready = new eventEmittereb5971c99090297ca98f84cac659caf1();
}

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: `
        <input #theInput type=&#39;text&#39; />
        <p>Demo Component</p>
    `,
    queries: {
        theInput: new ViewChild(&#39;theInput&#39;)
    }
})
export class DemoComponent {
    theInput: ElementRef;
}

等价于：

@Component({
    selector: 'demo-component',
    template: `
        <input #theInput type=&#39;text&#39; />
        <p>Demo Component</p>
    `
})
export class DemoComponent {
    @ViewChild(&#39;theInput&#39;) theInput: ElementRef;
}

queries - 内容查询

<my-list>
    <li *ngFor="let item of items;">{{item}}</li>
</my-list>

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

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

等价于：

@Component({
    selector: 'my-list',
    template: `
        <ul>
            <ng-content></ng-content>
        </ul>
    `
})
export class MyListComponent {
    @ContentChild(ListItem) items: QueryList<ListItem>;
}

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：

生命周期钩子	调用时机
ngOnChanges	在ngOnInit之前调用，或者当组件输入数据（通过@Input装饰器显式指定的那些变量）变化时调用。
ngOnInit	第一次ngOnChanges之后调用。建议此时获取数据，不要在构造函数中获取。
ngDoCheck	每次变化监测发生时被调用。
ngAfterContentInit	使用d553bd28b5bbbbd4b6fb4990edbabbf0将外部内容嵌入到组件视图后被调用，第一次ngDoCheck之后调用且只执行一次（只适用组件）。
ngAfterContentChecked	ngAfterContentInit后被调用，或者每次变化监测发生时被调用（只适用组件）。
ngAfterViewInit	创建了组件的视图及其子视图之后被调用（只适用组件）。
ngAfterViewChecked	ngAfterViewInit，或者每次子组件变化监测时被调用（只适用组件）。
ngOnDestroy	销毁指令/组件之前触发。此时应将不会被垃圾回收器自动回收的资源（比如已订阅的观察者事件、绑定过的DOM事件、通过setTimeout或setInterval设置过的计时器等等）手动销毁掉。

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

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