title: How to handle Form's onSubmit function in nativebase?
body: <p>I want to use redux-form in nativebase but it does not work quite as same as using in web. I don't think my <code>onSubmit</code> callback is not getting invoked and I am not sure why.</p>
  
  <pre><code>import React, { Component } from 'react';
  import { reduxForm, Field } from 'redux-form';
  import {
    Container,
    Content,
    Item,
    Input,
    Button,
    Form,
    Label,
    Text,
    Icon
  } from 'native-base';
  
  import * as actions from '../actions';
  
  class Signin extends Component {
    handleFormSubmit({ username, password }) {
      this.props.userSignIn({ username, password });
    }
  
    renderUsername() {
      return (
        &lt;Item floatingLabel&gt;
          &lt;Icon name="ios-person" /&gt;
          &lt;Label&gt;Username&lt;/Label&gt;
          &lt;Input autoCorrect={false} autoCapitalize="none"/&gt;
        &lt;/Item&gt;
      );
    }
  
    renderPassword() {
      return (
        &lt;Item floatingLabel&gt;
          &lt;Icon name="ios-lock" /&gt;
          &lt;Label&gt;Password&lt;/Label&gt;
          &lt;Input secureTextEntry={true} /&gt;
        &lt;/Item&gt;
      );
    }
  
    renderLoginBtn() {
      return (
        &lt;Container style={styles.button}&gt;
          &lt;Content&gt;
            &lt;Button primary action="submit"&gt;
              &lt;Text&gt;Login&lt;/Text&gt;
            &lt;/Button&gt;
          &lt;/Content&gt;
        &lt;/Container&gt;
      );
    }
  
    render() {
      const { handleSubmit, fields: { username, password } } = this.props;
      return (
        &lt;Container style={styles.container}&gt;
          &lt;Content&gt;
            &lt;Form onSubmit={handleSubmit(this.handleFormSubmit.bind(this))}&gt;
              &lt;Field name="username" component={this.renderUsername} /&gt;
              &lt;Field name="password" component={this.renderPassword} /&gt;
              {this.renderLoginBtn()}
            &lt;/Form&gt;
          &lt;/Content&gt;
        &lt;/Container&gt;
      );
    }
  }
  
  function mapStateToProps(state) {
    return {
      errorMsg: state.auth.error
    };
  }
  
  export default reduxForm({form: 'signin', fields: ['username', 'password']}, mapStateToProps, actions)(Signin);
  </code></pre>
  
  <p>I cannot find anywhere else how to use <code>onSubmit</code> callback with nativebase Form.</p>