feat: implement helper and error text support in TextInput

example/src/TextInputExample.js

@@ -1,8 +1,8 @@
 /* @flow */
 
 import * as React from 'react';
-import { ScrollView, StyleSheet } from 'react-native';
-import { TextInput, withTheme } from 'react-native-paper';
+import { ScrollView, StyleSheet, View } from 'react-native';
+import { TextInput, HelperText, withTheme } from 'react-native-paper';
 import type { Theme } from 'react-native-paper/types';
 
 type Props = {
@@ -11,21 +11,26 @@ type Props = {
 
 type State = {
   text: string,
+  name: string,
 };
 
 class TextInputExample extends React.Component<Props, State> {
   static title = 'TextInput';
 
   state = {
     text: '',
+    name: '',
   };
 
+  _isUsernameValid = () => /^[a-z]*$/.test(this.state.name);
+
   render() {
     const {
       theme: {
         colors: { background },
       },
     } = this.props;
+
     return (
       <ScrollView style={[styles.container, { backgroundColor: background }]}>
         <TextInput
@@ -40,6 +45,18 @@ class TextInputExample extends React.Component<Props, State> {
           style={styles.inputContainerStyle}
           label="Disabled Input"
         />
+        <View style={styles.inputContainerStyle}>
+          <TextInput
+            label="Input with helper text"
+            placeholder="Enter username, only letters"
+            value={this.state.name}
+            error={!this._isUsernameValid()}
+            onChangeText={name => this.setState({ name })}
+          />
+          <HelperText type="error" visible={!this._isUsernameValid()}>
+            Error: Only letters are allowed
+          </HelperText>
+        </View>
       </ScrollView>
     );
   }

src/components/HelperText.js

@@ -0,0 +1,162 @@
+/* @flow */
+
+import * as React from 'react';
+import color from 'color';
+import { Animated, StyleSheet } from 'react-native';
+import Text from './Typography/Text';
+import withTheme from '../core/withTheme';
+import type { Theme } from '../types';
+
+const AnimatedText = Animated.createAnimatedComponent(Text);
+
+type Props = {
+  /**
+   * Type of the helper text.
+   */
+  type: 'error' | 'info',
+  /**
+   * Whether to display the helper text.
+   */
+  visible?: boolean,
+  /**
+   * Text content of the HelperText.
+   */
+  children: React.Node,
+  style?: any,
+  /**
+   * @optional
+   */
+  theme: Theme,
+};
+
+type State = {
+  shown: Animated.Value,
+  textHeight: number,
+};
+
+/**
+ * Helper text is used in conjuction with input elements to provide additional hints for the user.
+ *
+ * <div class="screenshots">
+ *   <img class="medium" src="screenshots/helper-text.gif" />
+ * </div>
+ *
+ * ## Usage
+ * ```js
+ * import * as React from 'react';
+ * import { HelperText, TextInput } from 'react-native-paper';
+ *
+ * class MyComponent extends React.Component {
+ *   state = {
+ *     text: ''
+ *   };
+ *
+ *   render(){
+ *     return (
+ *       <View>
+ *         <TextInput
+ *           label="Email"
+ *           value={this.state.text}
+ *           onChangeText={text => this.setState({ text })}
+ *         />
+ *         <HelperText
+ *           type="error"
+ *           visible={!this.state.text.includes('@')}
+ *         >
+ *           Email address is invalid!
+ *         </HelperText>
+ *       </View>
+ *     );
+ *   }
+ * }
+ * ```
+ */
+class HelperText extends React.PureComponent<Props, State> {
+  static defaultProps = {
+    type: 'info',
+    visible: true,
+  };
+
+  state = {
+    shown: new Animated.Value(this.props.visible ? 1 : 0),
+    textHeight: 0,
+  };
+
+  componentDidUpdate(prevProps) {
+    if (prevProps.visible !== this.props.visible) {
+      if (this.props.visible) {
+        this._animateFocus();
+      } else {
+        this._animateBlur();
+      }
+    }
+  }
+
+  _animateFocus = () => {
+    Animated.timing(this.state.shown, {
+      toValue: 1,
+      duration: 150,
+    }).start();
+  };
+
+  _animateBlur = () => {
+    Animated.timing(this.state.shown, {
+      toValue: 0,
+      duration: 180,
+    }).start();
+  };
+
+  _handleTextLayout = e =>
+    this.setState({
+      textHeight: e.nativeEvent.layout.height,
+    });
+
+  render() {
+    const { style, type, visible, theme } = this.props;
+    const { colors, dark } = theme;
+
+    const textColor =
+      this.props.type === 'error'
+        ? colors.error
+        : color(colors.text)
+            .alpha(dark ? 0.7 : 0.54)
+            .rgb()
+            .string();
+
+    return (
+      <AnimatedText
+        onLayout={this._handleTextLayout}
+        style={[
+          styles.text,
+          {
+            color: textColor,
+            opacity: this.state.shown,
+            transform:
+              visible && type === 'error'
+                ? [
+                    {
+                      translateY: this.state.shown.interpolate({
+                        inputRange: [0, 1],
+                        outputRange: [-this.state.textHeight, 0],
+                      }),
+                    },
+                  ]
+                : [],
+          },
+          style,
+        ]}
+      >
+        {this.props.children}
+      </AnimatedText>
+    );
+  }
+}
+
+const styles = StyleSheet.create({
+  text: {
+    fontSize: 12,
+    paddingVertical: 4,
+  },
+});
+
+export default withTheme(HelperText);