NewMessageForm.js.diff

 export default class NewMessageForm extends Component {
+  state = { inputText: '' }
+
+  handleChangeText = (text) => {
+    this.setState({ inputText: text });
+  }
+
   render() {
+    const { inputText } = this.state;
     return (
       <View>
         <TextInput
+          value={inputText}
           testID="messageText"
+          onChangeText={this.handleChangeText}
         />