[Angular] 笔记 11：可观察对象(Observable)

chatgpt:

在 Angular 中，Observables 是用于处理异步数据流的重要工具。它们被广泛用于处理从异步操作中获取的数据，比如通过 HTTP 请求获取数据、定时器、用户输入等。Observables 提供了一种机制来订阅这些数据流，并可以在数据到达时执行相应的操作。其优势在于能够处理异步操作、多个事件以及数据的转换和组合，使得对数据流的处理更为灵活和高效。在 Angular 中，Observables 常用于处理 HTTP 请求、与路由、表单和事件处理等方面。

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当涉及 Observable，20%的知识覆盖了80%的应用场景。

Observable在代码中被广泛应用于异步操作。

异步操作在前端确保 UI 界面不会冻结，在后端能够提高系统性能。

Observer 和 Observable 之间的关系：

接下来安装一个用于测试的后端： json-server.

vscode terminal 运行: npm i -g json-server，这里必须有 -g，否则后面的 json-server 命令会无法识别。
然后在项目的根目录创建文件 db.json，

db.json 文件内容：

{"pokemon": [{"id": 1,"name": "pikachu","type": "electric","isCool": false,"isStylish": true},{"id": 2,"name": "squirtle","isCool": true,"isStylish": true},{"id": 3,"name": "charmander","type": "fire","isCool": true,"isStylish": false}]
}

vscode terminal 运行命令:

json-server --watch db.json

PS D:\Angular\my-app> json-server --watch db.json\{^_^}/ hi!Loading db.jsonDoneResourceshttp://localhost:3000/pokemonHomehttp://localhost:3000

访问 http://localhost:3000/pokemon，可以看到用于测试的数据：

在 pokeman-base.module.ts 文件中 import HttpClientModule:

import { NgModule } from '@angular/core';
import { CommonModule } from '@angular/common';
import { PokemonListComponent } from './pokemon-list/pokemon-list.component';
import { PokemonDetailComponent } from './pokemon-detail/pokemon-detail.component';
import { PokemonService } from '../services/pokemon.service';
import { HttpClientModule } from '@angular/common/http';@NgModule({declarations: [PokemonListComponent, PokemonDetailComponent],imports: [CommonModule, HttpClientModule],  // Add HttpClientModule!!!exports: [PokemonListComponent, PokemonDetailComponent],providers: [PokemonService],
})
export class PokemonBaseModule {}

接下来访问 http client， pokemon.service.ts:

import { Injectable } from '@angular/core';
import { HttpClient } from '@angular/common/http'; // 新增 import
import { Pokemon } from '../models/pokemon';
import { Observable } from 'rxjs';// endpoint
const POKEMON_API = 'http://localhost:3000/pokemon';@Injectable({providedIn: 'root',
})
export class PokemonService {constructor(private http: HttpClient) {}getPokemons(): Observable<Pokemon[]> {return this.http.get<Pokemon[]>(POKEMON_API);}
}

pokeman-list.component.ts:

import { Component, OnInit } from '@angular/core';
import { Pokemon } from 'src/app/models/pokemon';
import { PokemonService } from 'src/app/services/pokemon.service';@Component({selector: 'app-pokemon-list',templateUrl: './pokemon-list.component.html',styleUrls: ['./pokemon-list.component.css'],
})
export class PokemonListComponent implements OnInit {pokemons!: Pokemon[];// 修改 constructorconstructor(private pokemonService: PokemonService) {}handleRemove(event: Pokemon) {this.pokemons = this.pokemons.filter((pokemon: Pokemon) => {return pokemon.id !== event.id;});}ngOnInit(): void {// 填充 pokemons 属性// this.pokemons = this.pokemonService.getPokemons();this.pokemonService.getPokemons().subscribe((data: Pokemon[])=>{console.log(data)this.pokemons = data;})}
}

运行 ng serve:

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