如何在angular 6单元测试中验证ngCopy方法调用

import { ComponentFixture, TestBed } from '@angular/core/testing';
import { MyComponent } from './my.component';
import { DebugElement } from '@angular/core';
import { By } from '@angular/platform-browser';

describe('MyComponent', () => {
  let component: MyComponent;
  let fixture: ComponentFixture<MyComponent>;
  let debugElement: DebugElement;

  beforeEach(async () => {
    await TestBed.configureTestingModule({
      declarations: [ MyComponent ]
    })
    .compileComponents();
  });

  beforeEach(() => {
    fixture = TestBed.createComponent(MyComponent);
    component = fixture.componentInstance;
    debugElement = fixture.debugElement;
    fixture.detectChanges();
  });

  it('should call ngCopy method', () => {
    spyOn(component, 'ngCopy');
    const button = debugElement.query(By.css('button'));
    button.triggerEventHandler('click', null);
    expect(component.ngCopy).toHaveBeenCalled();
  });
});