antd的Form validator无效，始终不能显示？代码如下。

usernameCheck=(rule,value,callback)=>{

    if (value.length>6) {
        callback('have value');
    } else {
        callback('have value less than 6');
    }
}

render(){

 const { getFieldDecorator } = this.props.form;
 return (
     <Modal visible={this.props.visible} closable={false}
     footer={[
         <Button key="back" onClick={this.clickCancel}>游客浏览</Button>
     ]}>
     <Tabs activeKey={this.props.SonSignInorSignUp}>
         <TabPane tab="Sign In" key="1" >
             <Form className="signin-form">
                 <FormItem label="UserName" ref="signin_username"  >
                     {getFieldDecorator('username', {
                         rules: [{
                             required: true, message: 'Please input your username!'
                            }, {
                             validator:this.usernameCheck,
                         }],
                     })
                     (<Input prefix={<Icon type="user" style={{ color: 'rgba(0,0,0,.25)' }}/>} placeholder="Username" />
                     )}
                 </FormItem>
                 ...
                 }