From 2adc595f5d6cb152a47f0e8f405cce9cbd33ea57 Mon Sep 17 00:00:00 2001
From: Eric <eric@lugg.com>
Date: Wed, 8 Aug 2018 14:41:36 -0700
Subject: [PATCH] animate the position of headerBackground between scene
 transitions

---
 src/views/Header/Header.js                  | 37 ++++++++++++++++++---
 src/views/Header/HeaderStyleInterpolator.js | 22 ++++++++++++
 2 files changed, 54 insertions(+), 5 deletions(-)

diff --git a/src/views/Header/Header.js b/src/views/Header/Header.js
index 652700b..b3ddb22 100644
--- a/src/views/Header/Header.js
+++ b/src/views/Header/Header.js
@@ -80,6 +80,7 @@ class Header extends React.PureComponent {
     titleFromLeftInterpolator: HeaderStyleInterpolator.forCenterFromLeft,
     titleInterpolator: HeaderStyleInterpolator.forCenter,
     rightInterpolator: HeaderStyleInterpolator.forRight,
+    backgroundInterpolator: HeaderStyleInterpolator.forBackground,
   };
 
   static get HEIGHT() {
@@ -338,6 +339,29 @@ class Header extends React.PureComponent {
     );
   }
 
+  _renderBackground(props) {
+    const {
+      index,
+      key,
+      descriptor: { options },
+    } = props.scene;
+
+     const offset = this.props.navigation.state.index - index;
+
+     if (Math.abs(offset) > 2) {
+      // Scene is far away from the active scene. Hides it to avoid unnecessary
+      // rendering.
+      return null;
+    }
+
+    return this._renderSubView(
+      { ...props, style: StyleSheet.absoluteFill },
+      'background',
+      () => options.headerBackground,
+      this.props.backgroundInterpolator
+    );
+  }
+
   _renderModularSubView(
     props,
     name,
@@ -492,6 +516,7 @@ class Header extends React.PureComponent {
 
   render() {
     let appBar;
+    let background;
     const { mode, scene, isLandscape } = this.props;
 
     if (mode === 'float') {
@@ -505,12 +530,16 @@ class Header extends React.PureComponent {
         scene,
       }));
       appBar = scenesProps.map(this._renderHeader, this);
+      background = scenesProps.map(this._renderBackground, this);
     } else {
-      appBar = this._renderHeader({
+      const headerProps = {
         position: new Animated.Value(this.props.scene.index),
         progress: new Animated.Value(0),
         scene: this.props.scene,
-      });
+      };
+
+      appBar = this._renderHeader(headerProps);
+      background = this._renderBackground(headerProps);
     }
 
     const { options } = scene.descriptor;
@@ -590,9 +619,7 @@ class Header extends React.PureComponent {
         ]}
       >
         <SafeAreaView forceInset={forceInset} style={containerStyles}>
-          <View style={StyleSheet.absoluteFill}>
-            {options.headerBackground}
-          </View>
+          {background}
           <View style={styles.flexOne}>{appBar}</View>
         </SafeAreaView>
       </Animated.View>
diff --git a/src/views/Header/HeaderStyleInterpolator.js b/src/views/Header/HeaderStyleInterpolator.js
index 76d1bce..32445a0 100644
--- a/src/views/Header/HeaderStyleInterpolator.js
+++ b/src/views/Header/HeaderStyleInterpolator.js
@@ -325,6 +325,27 @@ function forCenterFromLeft(props) {
   };
 }
 
+const BACKGROUND_OFFSET = Dimensions.get('window').width;
+function forBackground(props) {
+  const { position, scene, scenes } = props;
+  const interpolate = getSceneIndicesForInterpolationInputRange(props);
+   if (!interpolate) return { opacity: 0 };
+   const { first, last } = interpolate;
+  const index = scene.index;
+  const offset = BACKGROUND_OFFSET;
+   const outputRange = [offset, 0, -offset];
+   return {
+    transform: [
+      {
+        translateX: position.interpolate({
+          inputRange: [first, index, last],
+          outputRange: I18nManager.isRTL ? outputRange.reverse() : outputRange,
+        }),
+      },
+    ],
+  };
+}
+
 export default {
   forLayout,
   forLeft,
@@ -333,4 +354,5 @@ export default {
   forCenterFromLeft,
   forCenter,
   forRight,
+  forBackground,
 };