Angular6 implementation of drag function drag and drop example detailed explanation

import { Directive, ElementRef, OnInit, HostListener } from '@angular/core';
@Directive({
  selector: '[appDrag]'
})
export class DragDirective implements OnInit {
  constructor(public el: ElementRef) {
  }
  public isDown = false;
  public disX; // Record the location of the mouse click event X  public disY; // Record the location of the mouse click event Y  private totalOffsetX = 0; // Record the total offset X-axis  private totalOffsetY = 0; // Record the total offset Y-axis  // Click event  @HostListener('mousedown', ['$event']) onMousedown(event) {
     = true;
     = ;
     = ;
  }
  // Listen to document mobile event events  @HostListener('document:mousemove', ['$event']) onMousemove(event) {
    // Determine whether the element has been clicked.    if () {
       =  +  -  + 'px';
       =  +  -  + 'px';
    }
  }
  // Listen to document leaving event  @HostListener('document:mouseup', ['$event']) onMouseup(event) {
    // It will only trigger when the element has moved past and leaving the function body.    if () {
      ('fail');
       +=  - ;
       +=  - ;
       = false;
    }
  }
  ngOnInit() {
     = 'relative';
  }
}