1. 在组件中定义自定义表单级验证器，并在控件初始化时将其添加到FormGroup中。

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

@Component({
  selector: 'my-app',
  template: `
    

      

        Name:
        
      
    
  `
})
export class AppComponent {
  formGroup: FormGroup;

  constructor() {
    const nameValidator: ValidatorFn = (control: AbstractControl): {[key: string]: any} | null => {
      const value = control.value;
      const invalid = value && value.length < 5;
      return invalid ? {'nameInvalid': {value}} : null;
    };

    this.formGroup = new FormGroup({
      name: new FormControl('', { validators: nameValidator, updateOn: 'blur' })
    });
  }
}

2. 在模板中绑定控件的错误提示信息。

  
Name must be at least 5 characters long