This article introduces you to Angular development practice (3): Analyzing Angular Component. Interested friends can take a look at
Before introducing Angular Component, Let’s first briefly understand W3C Web Components
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:
Custom Elements (custom tags): Yes Create custom HTML tags and elements;
tag to predefine some content but not load it into page, but use JS code to initialize it;
.
<template id="hello-template"> <style> h1 { color: red; } </style> <h1>Hello Web Component!</h1> </template> <script> // 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 }); </script>
<!DOCTYPE html> <html lang="zh-cn"> <head> <meta charset="UTF-8"> <meta http-equiv="X-UA-COMPATIBLE" content="IE=edge"> <meta name="viewport" content="width=device-width, initial-scale=1"> <meta name="author" content="赖祥燃, laixiangran@163.com, http://www.laixiangran.cn"/> <title>Web Component</title> <!--导入自定义组件--> <link rel="import" href="hello.html"> </head> <body> <!--自定义标签--> <hello-component></hello-component> </body> </html>
@Component({ selector: 'demo-component', template: 'Demo Component' }) export class DemoComponent {}
@component to become an Angular component.
selector,
template, etc. The following will focus on the meaning of each metadata.
DOM element on the page is the host element of this component instance. .
Type | Function | |
---|---|---|
AnimationEntryMetadata[] |
Set the animation of the component |
|
ChangeDetectionStrategy |
Set the component's change detection strategy |
|
ViewEncapsulation |
Set the component’s view packaging options |
|
any[] |
Set the list of components that will be dynamically inserted into this component view |
|
[string, string] |
The interpolation mark of the custom component, the default is double braces | {{}}
|
string |
Set the module id of this component under the ES/CommonJS specification, which is used to resolve the relative path of template styles |
|
string[] |
Set the external style file referenced by the component |
|
string[ ] |
Set the inline style used by the component |
|
string |
Setting Component’s inline template |
|
string |
Set the path where the component template is located |
|
Provider[] |
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) |
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。
@Component({ selector: 'demo-component', inputs: ['param'] }) export class DemoComponent { param: any; }
等价于:
@Component({ selector: 'demo-component' }) export class DemoComponent { @Input() param: any; }
@Component({ selector: 'demo-component', outputs: ['ready'] }) export class DemoComponent { ready = new eventEmitter<false>(); }
等价于:
@Component({ selector: 'demo-component' }) export class DemoComponent { @Output() ready = new eventEmitter<false>(); }
@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); } }
@Component({ selector: 'demo-component', template: ` <input #theInput type='text' /> <p>Demo Component</p> `, queries: { theInput: new ViewChild('theInput') } }) export class DemoComponent { theInput: ElementRef; }
等价于:
@Component({ selector: 'demo-component', template: ` <input #theInput type='text' /> <p>Demo Component</p> ` }) export class DemoComponent { @ViewChild('theInput') theInput: ElementRef; }
<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。
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。
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。
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 |
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。 |
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