@discoveryjs/discovery/lib/core/observer.js

Frontend framework for rapid data (JSON) analysis, shareable serverless reports and dashboards

export class Observer {
  subscriber;
  value;
  constructor(initValue, shouldUpdate) {
    this.subscriber = null;
    this.value = initValue;
    this.shouldUpdate = typeof shouldUpdate === "function" ? shouldUpdate : this.shouldUpdate;
  }
  get readonly() {
    const host = this;
    return {
      // FIXME: TS should infer types for subscribe/subscribeSync/unsubscribe,
      // however it doesn't and produces `any` instead. Used `as Observer<T>[method]`
      // as a workaround.
      subscribe: this.subscribe.bind(this),
      subscribeSync: this.subscribeSync.bind(this),
      unsubscribe: this.unsubscribe.bind(this),
      get value() {
        return host.value;
      }
    };
  }
  subscribe(callback) {
    this.subscriber = {
      callback,
      subscriber: this.subscriber
    };
    return () => this.unsubscribe(callback);
  }
  subscribeSync(callback) {
    const unsubscribe = this.subscribe(callback);
    callback(this.value, unsubscribe);
    return unsubscribe;
  }
  unsubscribe(callback) {
    let prev = this;
    let cursor = this.subscriber;
    while (cursor !== null) {
      if (cursor.callback === callback) {
        cursor.callback = null;
        prev.subscriber = cursor.subscriber;
        break;
      }
      prev = cursor;
      cursor = cursor.subscriber;
    }
  }
  shouldUpdate(newValue, oldValue) {
    return newValue !== oldValue;
  }
  set(value) {
    return this.#setValue(value) !== false;
  }
  asyncSet(value) {
    const callbacks = this.#setValue(value);
    return callbacks === false ? Promise.resolve(false) : Promise.all(callbacks).then(() => true);
  }
  #setValue(value) {
    if (!this.shouldUpdate(value, this.value)) {
      return false;
    }
    const callbacks = [];
    let cursor = this.subscriber;
    this.value = value;
    while (cursor !== null) {
      const { callback } = cursor;
      if (callback !== null) {
        callbacks.push(callback(value, () => this.unsubscribe(callback)));
      }
      cursor = cursor.subscriber;
    }
    return callbacks;
  }
}