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 是一种特殊的 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, 123 subject.subscribe((data) => console.log('sub 1: ', data)); subject.subscribe((data) => console.log('sub 2: ', data)); // two subscribers will get new value => output: 456, 456 console.log('subject emits 456!'); subject.next(456); // new subscriber will get latest value (456) => output: 456 subject.subscribe((data) => console.log('sub 3: ', data)); // all three subscribers will get new value => output: 789, 789, 789 console.log('subject emits 789!'); subject.next(789); } }
立即打印初始值： 123 123
一旦 BehaviorSubject 被调用 subscribe 方法进行订阅，订阅函数会立即收到最新的数据。
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 只是函数。
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).
