A. Make a simple counter with rxjs

1. go to http://codesandbox.io then create a react one (mine is: https://codesandbox.io/s/rxjs-rewrite-behaviorsubject-wfh0n)
2. Create an observable object

   let valueObservable = new BehaviorSubject(0);
   

3. Create 2 separated components Result for showing the result and Panel for inc/dec value. They will communicate thru valueObservable

    function Result() {
      const [value, setValue] = useState(0);
      useEffect(() => {
        valueObservable.subscribe((it) => setValue(it));
      }, []);
      return <div> Result: {value} </div>;
    }

    function Panel() {
      return (
        <div>
          <button
            onClick={() => valueObservable.next(valueObservable.getValue() + 1)}
          > + </button>
          <button
            onClick={() => valueObservable.next(valueObservable.getValue() - 1)}
          > - </button>
        </div>
      );
    }

4. Then add them to the root App component and see the result

    export default function App() {
     return (
       <div className="App">
         <Result />
         <Panel />
       </div>
     );
   }

    B. Rewrite it without using rxjs

1. Create a class MyBehaviorSubject having same methods as rxjs BehaviorSubject

export default class MyBehaviorSubject {
  constructor(defaultValue) {
  }

  subscribe(fn) {
  }

  getValue() {
  }

  next(nextValue) {
  }
}

    Now go implementing each methods:

constructor(defaultValue) {
    this.value = defaultValue;
    this.subscribers = [];
  }

    The subscribe method will take a function as argument, then it should be saved to a list, then they can be trigged later when value changed. So in the constructor, a default value and an empty subscribers list should be created

subscribe(fn) {
    this.subscribers.push(fn);
    return () => {
      this.subscribers.pop();
    };
}

    Now the subscribe method will add each component update state function to that subscribers list. And you may wonder what is the purpose of that return function? It's for unmounted components. When a component unmounted, we should remove the callback out from the list, otherwise, it will be triggered every time value changed, even when it no longer appears on UI

getValue() {
    return this.value;
}

    This function just returns current value

next(nextValue) {
    this.value = nextValue;
    this.subscribers.forEach((fn) => fn(nextValue));
}

    This function update new value, and then loop thru subscribers and excecute them (note that every element of subscribers list is a component update state function. So the result of these executions is: all component will be synced with that new value)

    Then now, go to codesandbox and replace BehaviorSubject by MyBehaviorSubject. You should see the same result.

    C. Make sure we handle well unmounted component case.

    Now, implement something more to make sure that unmounted component callback will not be trigged anymore Add one more callback for sync number of subscribers that are active

constructor(defaultValue) {
    ......
    this.fn = () => {};
}

subscribe2(fn) {
    this.fn = fn;
}

    Trigger it every when a subscribe created or removed

subscribe(fn) {
    this.subscribers.push(fn);
    this.fn(this.subscribers.length);
    return () => {
      this.subscribers.pop();
      this.fn(this.subscribers.length);
    };
}

    And that's it. Result at https://codesandbox.io/s/rxjs-rewrite-behaviorsubject-wfh0n. Every time you show the Result Component, number of subscribers is 1. End when it hided, number of subscribers will be 0, means there is no callback execution when inc/dec button clicked.