export class singletonWorker {
  constructor(source) {
    this._worker = new Worker(source);
    this._isRunning = false;
    this._buffer = [];
  }
  
  delegate(...args) {
    return new Promise(async resolve => {
      if(this._isRunning) {
        //console.log(`The Web Worker is running, so this action is pending.`);
        this._buffer.forEach(pair => pair.resolve({state: 'cancelled'}));
        this._buffer = [{resolve, args}];
        return;
      }
      
      //console.log('Start running...');
      this._isRunning = true;
      resolve(await this._post(...args));
      if (this._buffer.length > 0) {
        this._buffer.forEach(async pair => await pair.resolve(await this._post(...pair.args)));
        this._buffer = [];
      }
      this._isRunning = false;
    });
  }
  
  async _post(...args) {
    this._worker.postMessage(...args);
    return new Promise(resolve => {
      const callback = e => {
        resolve({data: e.data, state: 'fullfilled'});
        this._worker.removeEventListener('message', callback);
      };
      this._worker.addEventListener('message', callback);
    });
  }
}