title: this.state is undefined during onPress event in react native
body: <p>Hello i new in react native, my code is: </p>
  
  <p><div class="snippet" data-lang="js" data-hide="false">
  <div class="snippet-code">
  <pre class="snippet-code-html lang-html prettyprint-override"><code>import React, {
    View,
    Text,
    TextInput,
    Component
  } from 'react-native';
  
  import Style from './styles/signin';
  import Button from '../common/button';
  
  export default class SignIn extends Component {
    constructor(props) {
      super(props);
      this.state = {
        email: '',
        password: ''
      };
    }
  
    render(){
      return(
        &lt;View style={Style.container}&gt;
          &lt;Text style={Style.label}&gt;Email&lt;/Text&gt;
          &lt;TextInput
            style={Style.input}
            onChangeText={(text) =&gt; this.setState({email: text})}
            value={this.state.email}
          /&gt;
          &lt;Text style={Style.label}&gt;Password&lt;/Text&gt;
          &lt;TextInput
            style={Style.input}
            onChangeText={(text) =&gt; this.setState({password: text})}
            value={this.state.password}
            secureTextEntry={true}
          /&gt;
          &lt;Button text={'Sign in'} onPress={this.onPress}/&gt;
        &lt;/View&gt;
      );
    }
    onPress(){
      console.log(this.state.email);
    }
  }</code></pre>
  </div>
  </div>
  </p>
  
  <p>When i fill this form and press sign in, i have this error message: "Cannot read property 'email' of undefined". 
  Thank you for the help!</p>