Angular FormBuilder的工作原理

Cart view里的form控件：

<form [formGroup]="checkoutForm" (ngSubmit)="onSubmit(checkoutForm.value)">
  <div>
    <label for="name">
      Name
    </label>
    <input id="name" type="text" formControlName="name">
  </div>

  <div>
    <label for="address">
      Address
    </label>
    <input id="address" type="text" formControlName="address">
  </div>
  <button class="button" type="submit">Purchase</button>

</form>

最后生成的html page：

如果Component的模板实现里，form的属性formGroup，即中括号属性后面的值checkoutForm2实际上不存在，那么在页面渲染之前会报错：ERROR Error: formGroup expects a FormGroup instance. Please pass one in.

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