import { Observable } from 'rxjs/Observable';
import { Pipe, PipeTransform, ChangeDetectorRef } from '@angular/core';
import { AsyncPipe } from '@angular/common';

@Pipe({
  name: 'timeSince',
  pure: false
})
export class TimeSincePipe extends AsyncPipe implements PipeTransform {
  value: Date;
  timer: Observable<number>;

  constructor(ref: ChangeDetectorRef) {
    super(ref);
  }

  transform(value: any): any {
    if (value instanceof Date) {
      this.value = value;

      if (!this.timer) {
        this.timer = this.getTimer();
      }

      return super.transform(this.timer);
    } else {
      throw new Error('TimeSincePipe: Value must be a date');
    }
  }

  private getTimer() {
    return Observable
      .interval(1000)
      .startWith(0)
      .map(() => {
        const now = new Date().getTime();
        const delta = (now - this.value.getTime()) / 1000;
        return delta;
      });
  }
}