react native get TextInput value

Question

I am stuck with a very simple problem. I have login form with username, password and button. In my button handler, I try to get the textinput value. But always get undefined value. Am I missing something?

render() {
        <ExScreen
          headerColor={this.state.headerColor}
          scrollEnabled={this.state.enableScroll}
          style={styles.container} >
          <View >
            <View  >
              <View style={[styles.inputContainer]} >
                <TextInput
                  ref= "username"
                  onChangeText={(text) => this.setState({text})}
                  value={this.state.username}
                />
              </View>
 <Button style={{color: 'white', marginTop: 30, borderWidth: 1, borderColor: 'white', marginLeft: 20*vw, marginRight: 20*vw, height: 40, padding: 10}} 
             onPress={this._handlePress.bind(this)}>
              Sign In
            </Button>   
...
 _handlePress(event) {
    var username=this.refs.username.value;

Answer 1

The quick and less optimized way to do this is by using arrow function inside your onChangeText callback, by passing username as your argument in your onChangeText callback.

<TextInput
    ref= {(el) => { this.username = el; }}
    onChangeText={(username) => this.setState({username})}
    value={this.state.username}
/>

then in your _handlePress method

_handlePress(event) {
    let username=this.state.username;
}

---

But this has several drawbacks!!!

1. On every render of this component a new arrow function is created.
2. If the child component is a PureComponent it will force re-renders unnecessarily, this causes huge performance issue especially when dealing with large lists, table, or component iterated over large numbers. More on this in React Docs

Best practice is to use a handler like handleInputChange and bind ```this`` in the constructor.

...
constructor(props) {
  super(props);
  this.handleChange= this.handleChange.bind(this);
}

...
handleChange(event = {}) {
  const name = event.target && event.target.name;
  const value = event.target && event.target.value;

  this.setState([name]: value);
}
...

render() {
  ...
  <TextInput
    name="username"
    onChangeText={this.handleChange}
    value={this.state.username}
  />
  ...
}

...

---

Or if you are using es6 class property shorthand which autobinds this. But this has drawbacks, when it comes to testing and performance. Read More Here

...
handleChange= (event = {}) => {
  const name = event.target && event.target.name;
  const value = event.target && event.target.value;

  this.setState([name]: value);
}
...

render() {
  ...
  <TextInput
    name="username"
    onChangeText={this.handleChange}
    value={this.state.username}
  />
  ...
}

...