An in-depth analysis of directives, pipes and services in Angular

What are instructions, pipes, and services in Angular? The following article will take you through the instructions, pipelines and services in Angular. I hope it will be helpful to you!

1. DirectiveDirective

Directive is the operation provided by AngularDOM path. Instructions are divided into attribute instructions and structure instructions. [Related tutorial recommendation: "angular tutorial"]

Attribute instructions: Modify the appearance or behavior of existing elements and use [] packages.

Structure instructions: add and delete DOM nodes to modify the layout, use * as the instruction prefix

##1.1 Built-in instructions

1.1.1 *ngIf

Rendering based on conditions

DOM Node or remove DOM Node

<div *ngIf="data.length == 0">没有更多数据</div>

<div *ngIf="data.length > 0; then dataList else noData"></div>
<ng-template #dataList>课程列表</ng-template>
<ng-template #noData>没有更多数据</ng-template>

1.1.2 [hidden]

Display according to conditions

DOM node or hiddenDOM node(display)

<div [hidden]="data.length === 0">没有更多数据</div>

1.1.3 *ngFor

Traverse data to generate HTML structure

interface List {
  id: number
  name: string
  age: number
}

list: List[] = [
  { id: 1, name: "张三", age: 20 },
  { id: 2, name: "李四", age: 30 }
]

<!--  
    i: 索引
    isEven: 是否为偶数行
    isOdd: 是否为奇数行
    isFirst: 是否是第一项
    isLast: 是否是最后一项
-->
<li
    *ngFor="
      let item of list; 
      let i = index;
      let isEven = even;
      let isOdd = odd;
      let isFirst = first;
      let isLast = last;
    "
></li>

<li *ngFor="let item of list; trackBy: identify"></li>

identify(index, item){
  return item.id; 
}

1.2 Custom instructions

Requirement: Set the default background color for the element when the mouse moves in The background color and the background color when moving out

<div [appHover]="{ bgColor: &#39;skyblue&#39; }">Hello Angular</div>

• Create custom instructions

$ ng g d appHover
# 全称 ng generate directive

import { AfterViewInit, Directive, ElementRef, HostListener, Input } from "@angular/core"

// 接收参的数类型
interface Options {
  bgColor?: string
}

@Directive({
  selector: "[appHover]"
})
export class HoverDirective implements AfterViewInit {
  // 接收参数
  @Input("appHover") appHover: Options = {}
  // 要操作的 DOM 节点
  element: HTMLElement
  // 获取要操作的 DOM 节点
  constructor(private elementRef: ElementRef) {
    this.element = this.elementRef.nativeElement
  }
  // 组件模板初始完成后设置元素的背景颜色
  ngAfterViewInit() {
    this.element.style.backgroundColor = this.appHover.bgColor || "skyblue"
  }
  // 为元素添加鼠标移入事件
  @HostListener("mouseenter") enter() {
    this.element.style.backgroundColor = "pink"
  }
  // 为元素添加鼠标移出事件
  @HostListener("mouseleave") leave() {
    this.element.style.backgroundColor = "skyblue"
  }
}

---

2. PipelinePipe

#The role of the pipe is to format component template data.

2.1 Built-in pipeline

• ##date

  Date formatting

• currency

  Currency formatting

• uppercase

  Convert to uppercase

• lowercase

  Convert to lower case

• json

  Formatjson Data

  <p>{{ date | date: "yyyy-MM-dd" }}</p>

2.2 Custom pipelineRequirement: The specified string cannot exceed the specified length

// summary.pipe.ts
import { Pipe, PipeTransform } from '@angular/core';

@Pipe({
   name: 'summary' 
});
export class SummaryPipe implements PipeTransform {
    transform (value: string, limit?: number) {
        if (!value) return null;
        let actualLimit = (limit) ? limit : 10;
        return value.substr(0, actualLimit) + '...';
    }
}

// app.module.ts
import { SummaryPipe } from './summary.pipe'
@NgModule({
    declarations: [SummaryPipe] 
});

3. Service

Service

3.1 Create service

$ ng g s services/TestService --skip-tests

import { Injectable } from '@angular/core';

@Injectable({
  providedIn: 'root'
})
export class TestService { }

export class AppComponent {
  constructor (private testService: TestService) {}
}

3.2 Scope of service

Using services can easily achieve cross-module Share data across components, depending on the scope of the service.

Register the service in the root injector, all modules use the same service instance object

• import { Injectable } from '@angular/core';
  
  @Injectable({
    providedIn: 'root'
  })
  
  export class CarListService {
  }

Register the service at the module level, in this module All components use the same service instance object

• import { Injectable } from '@angular/core';
  import { CarModule } from './car.module';
  
  @Injectable({
    providedIn: CarModule,
  })
  
  export class CarListService {
  }

  import { CarListService } from './car-list.service';
  
  @NgModule({
    providers: [CarListService],
  })
  export class CarModule {
  }

To register services at the component level, the component and its subcomponents use the same service instance object

• import { Component } from '@angular/core';
  import { CarListService } from '../car-list.service.ts'
  
  @Component({
    selector:    'app-car-list',
    templateUrl: './car-list.component.html',
    providers:  [ CarListService ]
  })

  Original address: https://juejin.cn/post/7008357218210807838

  
  Author: Qianyi_0810

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