Initial work on UIKit style animation for header

src/components/Header/HeaderAnimated.tsx

@@ -29,23 +29,75 @@ type Props<T extends Route> = {
   style?: StyleProp<ViewStyle>;
 };
 
-const { interpolate, multiply } = Animated;
+const { interpolate, add } = Animated;
+
+const UIKitPreset: HeaderAnimationPreset = {
+  styleInterpolator: ({ current, next, layout }: InterpolationProps) => {
+    /**
+     * NOTE: this offset calculation is an approximation that gives us
+     * decent results in many cases, but it is ultimately a poor substitute
+     * for text measurement. See the comment on title for more information.
+     *
+     * - 70 is the width of the left button area.
+     * - 25 is the width of the left button icon (to account for label offset)
+     */
+    const buttonAreaSize = 70;
+    const buttonIconSize = 25;
+    const titleOffset = layout.width / 2 - buttonAreaSize + buttonIconSize;
+    const backTitleOffset = layout.width / 2 - buttonAreaSize - buttonIconSize;
+
+    const progress = add(current, next ? next : 0);
+
+    return {
+      leftButtonStyle: {
+        opacity: interpolate(progress, {
+          inputRange: [0, 1, 2],
+          outputRange: [0, 1, 0],
+        }),
+      },
+      backTitleStyle: {
+        opacity: interpolate(progress, {
+          inputRange: [0.7, 1, 1.3],
+          outputRange: [0, 1, 0],
+        }),
+        transform: [
+          {
+            translateX: interpolate(progress, {
+              inputRange: [0, 1, 2],
+              outputRange: [backTitleOffset, 0, -backTitleOffset],
+            }),
+          },
+        ],
+      },
+      titleStyle: {
+        opacity: interpolate(progress, {
+          inputRange: [0.5, 1, 1.7],
+          outputRange: [0, 1, 0],
+        }),
+        transform: [
+          {
+            translateX: interpolate(progress, {
+              inputRange: [0, 1, 2],
+              outputRange: [titleOffset, 0, -titleOffset],
+            }),
+          },
+        ],
+      },
+    };
+  },
+};
 
 const FadePreset: HeaderAnimationPreset = {
   styleInterpolator: ({ current, next }: InterpolationProps) => {
-    const progress = next
-      ? multiply(
-          current,
-          interpolate(next, {
-            inputRange: [0, 1],
-            outputRange: [1, 0],
-          })
-        )
-      : current;
+    const progress = add(current, next ? next : 0);
+    const opacity = interpolate(progress, {
+      inputRange: [0, 1, 2],
+      outputRange: [0, 1, 0],
+    });
 
     return {
-      leftButtonStyle: { opacity: progress },
-      titleStyle: { opacity: progress },
+      leftButtonStyle: { opacity },
+      titleStyle: { opacity },
     };
   },
 };
@@ -54,7 +106,7 @@ export default class HeaderAnimated<T extends Route> extends React.Component<
   Props<T>
 > {
   static defaultProps = {
-    preset: FadePreset,
+    preset: Platform.OS === 'ios' ? UIKitPreset : FadePreset,
   };
 
   render() {

src/components/Header/HeaderAnimatedItem.tsx

@@ -15,8 +15,9 @@ export type InterpolationProps = {
 export type StyleInterpolator = (
   props: InterpolationProps
 ) => {
-  leftButtonStyle: any;
-  titleStyle: any;
+  backTitleStyle?: any;
+  leftButtonStyle?: any;
+  titleStyle?: any;
 };
 
 export type HeaderAnimationPreset = {
@@ -62,7 +63,11 @@ export default class HeaderAnimatedItem<
       style,
     } = this.props;
 
-    const { titleStyle, leftButtonStyle } = this.getInterpolatedStyle(
+    const {
+      titleStyle,
+      leftButtonStyle,
+      backTitleStyle,
+    } = this.getInterpolatedStyle(
       preset.styleInterpolator,
       layout,
       scene.progress,
@@ -73,7 +78,11 @@ export default class HeaderAnimatedItem<
       <View style={[styles.content, style]}>
         {previous ? (
           <Animated.View style={[styles.left, leftButtonStyle]}>
-            <HeaderBackButton onPress={onGoBack} title={previous.title} />
+            <HeaderBackButton
+              onPress={onGoBack}
+              title={previous.title}
+              titleStyle={backTitleStyle}
+            />
           </Animated.View>
         ) : null}
         <HeaderTitle style={[previous ? styles.title : null, titleStyle]}>

src/components/Header/HeaderBackButton.tsx

@@ -2,15 +2,14 @@ import * as React from 'react';
 import {
   I18nManager,
   Image,
-  Text,
   View,
   Platform,
   StyleSheet,
   LayoutChangeEvent,
-  StyleProp,
-  TextStyle,
+  Text,
+  MaskedViewIOS,
 } from 'react-native';
-
+import Animated from 'react-native-reanimated';
 import TouchableItem from '../TouchableItem';
 
 type Props = {
@@ -26,12 +25,12 @@ type Props = {
   truncatedTitle?: string | null;
   backTitleVisible?: boolean;
   allowFontScaling?: boolean;
-  titleStyle?: StyleProp<TextStyle>;
+  titleStyle?: React.ComponentProps<typeof Text>['style'];
   width?: number;
 };
 
 type State = {
-  initialTextWidth?: number;
+  initialTitleWidth?: number;
 };
 
 class HeaderBackButton extends React.Component<Props, State> {
@@ -48,11 +47,12 @@ class HeaderBackButton extends React.Component<Props, State> {
   state: State = {};
 
   private handleTextLayout = (e: LayoutChangeEvent) => {
-    if (this.state.initialTextWidth) {
+    if (this.state.initialTitleWidth) {
       return;
     }
+
     this.setState({
-      initialTextWidth: e.nativeEvent.layout.x + e.nativeEvent.layout.width,
+      initialTitleWidth: e.nativeEvent.layout.x + e.nativeEvent.layout.width,
     });
   };
 
@@ -83,7 +83,7 @@ class HeaderBackButton extends React.Component<Props, State> {
   private getTitleText = () => {
     const { width, title, truncatedTitle } = this.props;
 
-    let { initialTextWidth } = this.state;
+    let { initialTitleWidth: initialTextWidth } = this.state;
 
     if (title === null) {
       return null;
@@ -103,22 +103,41 @@ class HeaderBackButton extends React.Component<Props, State> {
       titleStyle,
       tintColor,
     } = this.props;
+    const { initialTitleWidth: titleWidth } = this.state;
+
     let backTitleText = this.getTitleText();
 
     if (!backTitleVisible || backTitleText === null) {
       return null;
     }
 
     return (
-      <Text
-        accessible={false}
-        onLayout={this.handleTextLayout}
-        style={[styles.title, !!tintColor && { color: tintColor }, titleStyle]}
-        numberOfLines={1}
-        allowFontScaling={!!allowFontScaling}
+      <MaskedViewIOS
+        maskElement={
+          <View style={styles.iconMaskContainer}>
+            <Image
+              source={require('../../assets/back-icon-mask.png')}
+              style={styles.iconMask}
+            />
+            <View style={styles.iconMaskFillerRect} />
+          </View>
+        }
       >
-        {this.getTitleText()}
-      </Text>
+        <Animated.Text
+          accessible={false}
+          onLayout={this.handleTextLayout}
+          style={[
+            styles.title,
+            tintColor ? { color: tintColor } : null,
+            titleWidth ? { paddingRight: titleWidth } : null,
+            titleStyle,
+          ]}
+          numberOfLines={1}
+          allowFontScaling={!!allowFontScaling}
+        >
+          {this.getTitleText()}
+        </Animated.Text>
+      </MaskedViewIOS>
     );
   }
 
@@ -173,6 +192,7 @@ const styles = StyleSheet.create({
   },
   title: {
     fontSize: 17,
+    letterSpacing: 0.25,
     paddingRight: 10,
   },
   icon: Platform.select({
@@ -200,6 +220,24 @@ const styles = StyleSheet.create({
           marginRight: 6,
         }
       : {},
+  iconMaskContainer: {
+    flex: 1,
+    flexDirection: 'row',
+    justifyContent: 'center',
+  },
+  iconMaskFillerRect: {
+    flex: 1,
+    backgroundColor: '#000',
+  },
+  iconMask: {
+    height: 21,
+    width: 13,
+    marginLeft: -14.5,
+    marginVertical: 12,
+    alignSelf: 'center',
+    resizeMode: 'contain',
+    transform: [{ scaleX: I18nManager.isRTL ? -1 : 1 }],
+  },
 });
 
 export default HeaderBackButton;