this.formControl.valueChanges.subscribe(value => {
      console.log(value);
})

<div *ngFor="let option of formItem.formItemOptions; index as i" class="form-check">
        <input  [formControl]="formControl" [value]="option.id"
                (click)="selectCheckBox(formControl.value, option)"
                 class="form-check-input"
                type="checkbox" >
        <label class="form-check-label mr-1" for="{{id}}_{{option.id}}">
          <span>{{option.content}}</span>
        </label>
      </div>

selectCheckBox(isCheck: boolean, option: FormItemOption): void {
    if (isCheck) {
      this.formItemValue.formItemOptions.push(option);
    } else {
      const index = this.formItemValue.formItemOptions.indexOf(option);
      this.formItemValue.formItemOptions.splice(index, 1);
    }
}

selectCheckBox(isCheck: boolean, option: FormItemOption): void {
    // 如果index值為-2，表示數(shù)組為定義，創(chuàng)造一個(gè)數(shù)組
    // 如果index值為-1，表示所選選項(xiàng)并未處于數(shù)組內(nèi)，添加之
    // 如果index值大于等于0，筆試所選選項(xiàng)處于數(shù)組內(nèi)，刪除之
    const index = Array.isArray(this.formItemValue.formItemOptions) ? this.formItemValue.formItemOptions.indexOf(option) : -2;
    if (index < -1) {
      this.formItemValue.formItemOptions = [option];
    } else if (index < 0) {
      this.formItemValue.formItemOptions.push(option);
    } else {
      this.formItemValue.formItemOptions.splice(index, 1);
    }
  }