angular4 form表单验证

<!-- novalidate 清除浏览器默认的校验行为 -->
<form [formGroup]="formModel" (ngSubmit)="onSearch()" novalidate>
    <div class="form-group" [class.has-error]="formModel.hasError('minlength','title')">
        <label for="productTitle">商品名称:</label>
        <input formControlName="title" type="text"  placeholder="商品名称" class="form-control">
        <span class="help-block" [class.hidden]="!formModel.hasError('minlength','title')">
            请至少输入三个字
        </span>
    </div>
    <div class="form-group" [class.has-error]="formModel.hasError('positiveNumber','price')">
        <label for="productPrice">商品价格:</label>
        <input formControlName="price" type="number"  placeholder="商品价格" class="form-control">
        <span class="help-block" [class.hidden]="!formModel.hasError('positiveNumber','price')">
            请输入正数
        </span>
    </div>
    <div class="form-group">
        <label for="productCategory">商品类别:</label>
        <select formControlName="category"  class="form-control">
            <option value="-1">全部分类</option>
            <option *ngFor="let category of categories" [value]="category">{{category}}</option>
        </select>
    </div>
    <div class="form-group">
        <button type="submit" class="btn btn-primary btn-block">搜索</button>
    </div>
</form>

import { Component, OnInit } from '@angular/core';
import { FormGroup, FormBuilder, Validators,FormControl } from '@angular/forms';
import { ProductService } from '../share/product.service';

@Component({
  selector: 'app-search',
  templateUrl: './search.component.html',
  styleUrls: ['./search.component.scss']
})
export class SearchComponent implements OnInit {

    formModel:FormGroup;

    categories: string[];

  constructor(private productService:ProductService) {
    let fb = new FormBuilder();
    this.formModel = fb.group({
        title:['',Validators.minLength(3)], //最少不少于3个字符
        price:[null,this.positiveNumberValidator], //不能为负数
        category:['-1'] //默认值为-1 --全部分类
    })
  }

  ngOnInit() {
      this.categories = this.productService.getAllCategories();
  }

  positiveNumberValidator(control: FormControl):any{
      if(!control.value){//如果输入为空则返回空，相当于去空格
          return null;
      }
      let price = parseInt(control.value);

      if(price>0){ // 如果大于0不显示错误信息
          return null;
      }else{  // 不大于0则显示错误信息
          return {positiveNumber:true};
      }
  }
//如果验证通过的话就把表单对应的值打印到控制台
  onSearch(){
      if(this.formModel.valid){
          console.log(this.formModel.value);
      }
  }

}

productService.ts 在服务里声明返回类别方法

getAllCategories():string[]{
     return ["电子产品","硬件设备","图书"];
   }