1. 在组件中导入所需的模块和服务：

import { FormGroup, FormBuilder, Validators } from '@angular/forms';
import { MatSnackBar } from '@angular/material/snack-bar';

2. 声明表单和表单控件的验证规则：

myForm: FormGroup;

constructor(private fb: FormBuilder, private snackBar: MatSnackBar) {
  this.myForm = this.fb.group({
    username: ['', [Validators.required, Validators.minLength(4)]],
    email: ['', [Validators.required, Validators.email]],
    password: ['', [Validators.required, Validators.minLength(8)]],
    confirmPwd: ['', [Validators.required]]
  }, {validator: this.checkPasswords});
}

checkPasswords(group: FormGroup): any {
  let pass = group.controls.password.value;
  let confirmPass = group.controls.confirmPwd.value;
  return pass === confirmPass ? null : { notSame: true };
}

3. 在模板里绑定表单控件和相应的错误信息提示：

  
    Username
    
    Username is required
    Username should have at least 4 characters
  

  
    Email
    
    Email is required
    Email is invalid
  

  
    Password