Observable.create() function 的参数是否必须明确定义观察者？

Question

I came across this code from here: https://medium.com/@mohandere/rxjs-5-in-5-minutes-1c3b4ed0d8cc

function multiplyByTen(input) {
  var output = Rx.Observable.create(function subscribe(observer) {
    input.subscribe({
      next: (v) => observer.next(10 * v),
      error: (err) => observer.error(err),
      complete: () => observer.complete()
    });
  });
  return output;
}

var input = Rx.Observable.from([1, 2, 3, 4]);
var output = multiplyByTen(input);
output.subscribe(x => console.log(x));
// Result In:
// 10 
// 20 
// 30 
// 40

I just do not see where the "observer" parameter for the subscribe function inside the multiplyByTen function comes from? Does it have to be explicitely defined or is it just some "default" object that gets passed to the create function in case it was not defined before? And why is it called function subscribe(observer) inside the function? Does it override the default.subscribe() function or could it also be an anonymous function?

Answer 1

It is just an RxJS construct to allow you to dispatch new values to the subscriber functions. Basically, this is how RxJS have designed the library to work. This allows you to control the dispatch of new values. One example of usage is when you have some asynchronous operation and you want to send a new value to the subscribers when it has resolved/completed.

You can read more about how it works here: https://www.learnrxjs.io/operators/creation/create.html

Also, check out this simple synchronous example in JsBin (taken from the docs link above)

/*
  Create an observable that emits 'Hello' and 'World' on  
  subscription.
*/
const hello = Rx.Observable.create(function(observer) {
  observer.next('Hello');
  observer.next('World');
});

const subscribe = hello.subscribe(val => console.log(val));
// prints.. the follwoing:
// Hello
// World

The asynchronous case I mentioned above is portrayed here: (from official RxJS jsBin examples - https://jsbin.com/lodilohate/1/edit?js,console ):

// RxJS v6+
import { Observable } from 'rxjs';

/*
  Increment value every 1s, emit even numbers.
*/
const evenNumbers = Observable.create(function(observer) {
  let value = 0;
  const interval = setInterval(() => {
    if (value % 2 === 0) {
      observer.next(value);
    }
    value++;
  }, 1000);

  return () => clearInterval(interval);
});
//output: 0...2...4...6...8
const subscribe = evenNumbers.subscribe(val => console.log(val));
//unsubscribe after 10 seconds
setTimeout(() => {
  subscribe.unsubscribe();
}, 10000);