Angular FormArray ValidatorFn不编译Typescript

import { Component } from '@angular/core';
import { FormArray, FormBuilder, FormControl, FormGroup, ValidatorFn, Validators } from '@angular/forms';

@Component({
  selector: 'app-form-array-validator',
  template: `
    <form [formGroup]="form">
      <div formArrayName="items">
        <div *ngFor="let item of items.controls; let i = index">
          <input [formControlName]="i" placeholder="Item {{i + 1}}">
        </div>
      </div>
      <button (click)="addItem()">Add Item</button>
      <button (click)="removeItem()">Remove Item</button>
      <div>{{ form.valid ? 'Form is valid' : 'Form is invalid' }}</div>
    </form>
  `,
})
export class FormArrayValidatorComponent {
  form: FormGroup;

  constructor(private formBuilder: FormBuilder) {
    this.form = this.formBuilder.group({
      items: this.formBuilder.array([this.createItem()], this.validateItems()),
    });
  }

  get items() {
    return this.form.get('items') as FormArray;
  }

  createItem(): FormControl {
    return this.formBuilder.control('', Validators.required);
  }

  addItem(): void {
    this.items.push(this.createItem());
  }

  removeItem(): void {
    this.items.removeAt(this.items.length - 1);
  }

  validateItems(): ValidatorFn {
    return (formArray: FormArray) => {
      const hasEmptyItem = formArray.controls.some(control => control.value === '');
      return hasEmptyItem ? { emptyItem: true } : null;
    };
  }
}