Angular异步数据流编程-编程知识

1 目前常见的异步编程的几种方法

首先给出一个异步请求的实例：

import {Injectable} from '@angular/core';@Injectable({providedIn: 'root'
})
export class RequestServiceService {constructor() {}getData() {setTimeout(() => {let res = 'zhaoshuai-lc'return res}, 3000)}
}

import {Component, OnInit} from '@angular/core';
import {RequestServiceService} from "./services/request-service.service";@Component({selector: 'app-root',templateUrl: './app.component.html',styleUrls: ['./app.component.scss'],
})
export class AppComponent implements OnInit{constructor(public RequestServiceService: RequestServiceService) {}ngOnInit(): void {let data = this.RequestServiceService.getData()console.log(data)}
}

在这里插入图片描述

1.1 回调函数解决问题

import {Component, OnInit} from '@angular/core';
import {RequestServiceService} from "./services/request-service.service";@Component({selector: 'app-root',templateUrl: './app.component.html',styleUrls: ['./app.component.scss'],
})
export class AppComponent implements OnInit{constructor(public RequestServiceService: RequestServiceService) {}ngOnInit(): void {this.RequestServiceService.getData(data => {console.log(data)})}
}

import {Injectable} from '@angular/core';@Injectable({providedIn: 'root'
})
export class RequestServiceService {constructor() {}getData(callBack) {setTimeout(() => {let res = 'zhaoshuai-lc'callBack(res)}, 3000)}
}

在这里插入图片描述

1.2 Promise 处理异步

import {Injectable} from '@angular/core';@Injectable({providedIn: 'root'
})
export class RequestServiceService {constructor() {}getData() {return new Promise((resolve, reject) => {setTimeout(() => {let res = 'zhaoshuai-lc'resolve(res)}, 3000)})}
}

import {Component, OnInit} from '@angular/core';
import {RequestServiceService} from "./services/request-service.service";@Component({selector: 'app-root',templateUrl: './app.component.html',styleUrls: ['./app.component.scss'],
})
export class AppComponent implements OnInit {constructor(public RequestServiceService: RequestServiceService) {}ngOnInit(): void {let promiseData = this.RequestServiceService.getData()promiseData.then(data => {console.log(data)})}
}

1.3 RxJS 处理异步

import {Injectable} from '@angular/core';
import {Observable} from "rxjs";@Injectable({providedIn: 'root'
})
export class RequestServiceService {constructor() {}getData() {return new Observable(observer => {setTimeout(() => {let res = 'zhaoshuai-lc'observer.next(res)}, 3000)})}
}

import {Component, OnInit} from '@angular/core';
import {RequestServiceService} from "./services/request-service.service";@Component({selector: 'app-root',templateUrl: './app.component.html',styleUrls: ['./app.component.scss'],
})
export class AppComponent implements OnInit {constructor(public RequestServiceService: RequestServiceService) {}ngOnInit(): void {let observableData = this.RequestServiceService.getData()observableData.subscribe(data => {console.log(data)})}
}

从上面列子可以看到 RxJS 和 Promise 的基本用法非常类似，除了一些关键词不同。 Promise 里面用的是 then() 和 resolve()，而 RxJS 里面用的是 next() 和 subscribe()。

从上面例子我们感觉 Promise 和 RxJS 的用法基本相似。其实 Rxjs 相比 Promise 要强大很多。比如 Rxjs 中可以中途撤回、Rxjs 可以发射多个值、Rxjs 提供了多种工具函数等等。

2 Rxjs unsubscribe 取消订阅

Promise 的创建之后，动作是无法撤回的。Observable 不一样，动作可以通过 unsbscribe() 方法中途撤回，而且 Observable 在内部做了智能的处理。

import {Component, OnInit} from '@angular/core';
import {RequestServiceService} from "./services/request-service.service";@Component({selector: 'app-root',templateUrl: './app.component.html',styleUrls: ['./app.component.scss'],
})
export class AppComponent implements OnInit {constructor(public RequestServiceService: RequestServiceService) {}ngOnInit(): void {// 过一秒撤回let stream = this.RequestServiceService.getData()let res = stream.subscribe(data => {console.log(data)})setTimeout(() => {console.log('取消订阅了')res.unsubscribe()}, 1000)}
}

3 Rxjs 订阅后多次执行

如果我们想让异步里面的方法多次执行，这一点 Promise 是做不到的。对于 Promise 来说，最终结果要么 resolve（兑现）、要么 reject （拒绝），而且都只能触发一次。如果在同一个 Promise 对象上多次调用 resolve 方法，则会抛异常。而 Observable 不一样，它可以不断地触发下一个值，就像 next() 这个方法的名字所暗示的那样。

import {Injectable} from '@angular/core';
import {Observable} from "rxjs";@Injectable({providedIn: 'root'
})
export class RequestServiceService {constructor() {}getData() {return new Observable(observer => {setInterval(() => {let res = 'zhaoshuai-lc'observer.next(res)}, 1000)})}
}

import {Component, OnInit} from '@angular/core';
import {RequestServiceService} from "./services/request-service.service";@Component({selector: 'app-root',templateUrl: './app.component.html',styleUrls: ['./app.component.scss'],
})
export class AppComponent implements OnInit {constructor(public RequestServiceService: RequestServiceService) {}ngOnInit(): void {let stream = this.RequestServiceService.getData()let res = stream.subscribe(data => {console.log(data)})}
}

4 Angualr6.x 之后使用 Rxjs 的工具函数 map,filter

import {Injectable} from '@angular/core';
import {Observable} from "rxjs";@Injectable({providedIn: 'root'
})
export class RequestServiceService {constructor() {}getData() {return new Observable(observer => {let count = 0;setInterval(() => {count++observer.next(count)}, 1000)})}
}

import {Component, OnInit} from '@angular/core';
import {RequestServiceService} from "./services/request-service.service";
import {filter, map} from 'rxjs/operators';@Component({selector: 'app-root',templateUrl: './app.component.html',styleUrls: ['./app.component.scss'],
})
export class AppComponent implements OnInit {constructor(public RequestServiceService: RequestServiceService) {}ngOnInit(): void {let stream = this.RequestServiceService.getData()stream.pipe(filter(value => Number(value) % 2 == 0),map(value => Number(value) * Number(value))).subscribe(data => {console.log(data)})}
}