https://stackoverflow.com/questions/39494058/behaviorsubject-vs-observable

BehaviorSubject is a type of subject, a subject is a special type of observable so you can subscribe to messages like any other observable.

BehaviorSubhect 是一種特殊的 Observable.

The unique features of BehaviorSubject are:

(1) It needs an initial value as it must always return a value on subscription even if it hasn’t received a next().

完整例子：

import { Component, OnInit } from '@angular/core'; import { BehaviorSubject } from 'rxjs/internal/BehaviorSubject';@Component({selector: 'app-behavior-subject',templateUrl: './behavior-subject.component.html' }) export class BehaviorSubjectComponent implements OnInit {ngOnInit(): void {const subject = new BehaviorSubject(123);// two new subscribers will get initial value => output: 123, 123subject.subscribe((data) => console.log('sub 1: ', data));subject.subscribe((data) => console.log('sub 2: ', data));// two subscribers will get new value => output: 456, 456console.log('subject emits 456!');subject.next(456);// new subscriber will get latest value (456) => output: 456subject.subscribe((data) => console.log('sub 3: ', data));// all three subscribers will get new value => output: 789, 789, 789console.log('subject emits 789!');subject.next(789);} }

立即打印初始值： 123 123

一旦 BehaviorSubject 被調用 subscribe 方法進行訂閱，訂閱函數會立即收到最新的數據。

Upon subscription, it returns the last value of the subject. A regular observable only triggers when it receives an onnext.

https://stackoverflow.com/questions/39494058/behaviorsubject-vs-observable

One very very important difference. Since Observable is just a function, it does not have any state, so for every new Observer, it executes the observable create code again and again. This results in:

The code is run for each observer . If its a HTTP call, it gets called for each observer

Observer 只是函數。

BehaviorSubject (or Subject ) stores observer details, runs the code only once and gives the result to all observers .

Observables

• They are cold: Code gets executed when they have at least a single observer.

• Creates copy of data: Observable creates copy of data for each observer.

• Uni-directional: Observer can not assign value to observable(origin/master).

Subject

• They are hot: code gets executed and value gets broadcast even if there is no observer.

• Shares data: Same data get shared between all observers.

• bi-directional: Observer can assign value to observable(origin/master).

• If are using using subject then you miss all the values that are broadcast before creation of observer. So here comes Replay Subject

ReplaySubject

• They are hot: code gets executed and value get broadcast even if there is no observer.

• Shares data: Same data get shared between all observers.

• bi-directional: Observer can assign value to observable(origin/master). plus

• Replay the message stream: No matter when you subscribe the replay subject you will receive all the broadcasted messages.

• In subject and replay subject you can not set the initial value to observable. So here comes Behavioral Subject

BehaviorSubject

• They are hot: code gets executed and value get broadcast even if there is no observer.

• Shares data: Same data get shared between all observers.

• bi-directional: Observer can assign value to observable(origin/master). plus

• Replay the message stream: No matter when you subscribe the replay subject you will receive all the broadcasted messages.

• You can set initial value: You can initialize the observable with default value.

The Observable object represents the object that sends notifications (the provider); the Observer object represents the class that receives them (the observer).

更多Jerry的原創文章，盡在：“汪子熙”:

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