如何使用Angular 2+绘制矩形来选择多个项目？

<canvas id="canvasId"></canvas>

import { Component, ViewChild, ElementRef, AfterViewInit } from '@angular/core';

@Component({
  selector: 'app-rectangle',
  templateUrl: './rectangle.component.html',
  styleUrls: ['./rectangle.component.css']
})
export class RectangleComponent implements AfterViewInit {
  @ViewChild('canvasId', { static: false }) canvas: ElementRef;
  private context: CanvasRenderingContext2D;

  ngAfterViewInit() {
    const canvasEl: HTMLCanvasElement = this.canvas.nativeElement;
    this.context = canvasEl.getContext('2d');
  }
}

import { Component, ViewChild, ElementRef, AfterViewInit } from '@angular/core';

@Component({
  selector: 'app-rectangle',
  templateUrl: './rectangle.component.html',
  styleUrls: ['./rectangle.component.css']
})
export class RectangleComponent implements AfterViewInit {
  @ViewChild('canvasId', { static: false }) canvas: ElementRef;
  private context: CanvasRenderingContext2D;
  private isDrawing: boolean = false;
  private startX: number;
  private startY: number;
  private currentX: number;
  private currentY: number;

  ngAfterViewInit() {
    const canvasEl: HTMLCanvasElement = this.canvas.nativeElement;
    this.context = canvasEl.getContext('2d');
    this.context.lineWidth = 2;
    this.context.strokeStyle = 'blue';
    this.context.fillStyle = 'rgba(0, 0, 255, 0.2)';
    this.context.fillRect(0, 0, canvasEl.width, canvasEl.height);
  }

  onMouseDown(event: MouseEvent) {
    this.isDrawing = true;
    this.startX = event.offsetX;
    this.startY = event.offsetY;
  }

  onMouseMove(event: MouseEvent) {
    if (!this.isDrawing) return;
    this.currentX = event.offsetX;
    this.currentY = event.offsetY;
    this.drawRectangle();
  }

  onMouseUp() {
    this.isDrawing = false;
  }

  drawRectangle() {
    this.context.clearRect(0, 0, this.context.canvas.width, this.context.canvas.height);
    this.context.fillRect(this.startX, this.startY, this.currentX - this.startX, this.currentY - this.startY);
  }
}

<canvas id="canvasId"
        (mousedown)="onMouseDown($event)"
        (mousemove)="onMouseMove($event)"
        (mouseup)="onMouseUp()"></canvas>