如何在 Angular 单元测试中创建假 NgForm 对象？

我有一个带有如下模板的组件：

// Template
<form #f="ngForm" (ngSubmit)="onFormSubmit(f)">
  <!-- A bunch of form fields -->
</form>

我的组件有一个类似的方法：

onFormSubmit(form: NgForm) {
  this.save();
}

我想编写一个基本上如下所示的测试，测试提交表单时是否调用保存函数：

it('saves the widget when the form is submitted', () => {
  let testForm = new NgForm([], []);
  testForm.setValue({
    name: "Hello",
    category: "World"
  });
  component.onFormSubmit(testForm);

  // Run tests to check that the component data was updated
  expect(component.data.name).toEqual('Hello');
  expect(component.data.category).toEqual('World');
});

如何创建表单的模拟版本以传递给onFormSubmit()功能？我尝试执行上述操作，但收到错误："There are no form controls registered with this group yet. If you're using ngModel, you may want to check next tick (e.g. use setTimeout)."

这应该有效

const testForm = <NgForm>{
    value: {
        name: "Hello",
        category: "World"
    }
};