Указанное значение отличается.
class LoginForm extends Component {
state = {text: ''}
...
<View style={ alignItems: "center",justifyContent: "center",flex: 1}>
<TextInput
value={state.text}
onChangeText={text => this.setState({text})}
placeholder={"Can you see this?"}
placeholderTextColor={"red"}
style={{height: 20, width: 100}}
/>
</View>
}
OR
class LoginForm extends Component {
constructor(props) {
super(props);
this.state = {
text: ''
};
}
...
<View style={ alignItems: "center",justifyContent: "center",flex: 1}>
<TextInput
value={this.state.text}
onChangeText={text => this.setState({text})}
style={{height: 20, width: 100}}
/>
<Button>
Log In
</Button>
</View>
}