Move the feed selector to the header (#352)

* Reorg pager components into their own folder

* Move the feed selector to the header

* Fix lint

src/view/com/pager/TabBar.tsx

@@ -0,0 +1,161 @@
+import React, {createRef, useState, useMemo} from 'react'
+import {
+  Animated,
+  StyleSheet,
+  TouchableWithoutFeedback,
+  View,
+} from 'react-native'
+import {Text} from '../util/text/Text'
+import {usePalette} from 'lib/hooks/usePalette'
+import {isDesktopWeb} from 'platform/detection'
+
+interface Layout {
+  x: number
+  width: number
+}
+
+export interface TabBarProps {
+  selectedPage: number
+  items: string[]
+  position: Animated.Value
+  offset: Animated.Value
+  indicatorPosition?: 'top' | 'bottom'
+  indicatorColor?: string
+  onSelect?: (index: number) => void
+  onPressSelected?: () => void
+}
+
+export function TabBar({
+  selectedPage,
+  items,
+  position,
+  offset,
+  indicatorPosition = 'bottom',
+  indicatorColor,
+  onSelect,
+  onPressSelected,
+}: TabBarProps) {
+  const pal = usePalette('default')
+  const [itemLayouts, setItemLayouts] = useState<Layout[]>(
+    items.map(() => ({x: 0, width: 0})),
+  )
+  const itemRefs = useMemo(
+    () => Array.from({length: items.length}).map(() => createRef<View>()),
+    [items.length],
+  )
+  const panX = Animated.add(position, offset)
+
+  const indicatorStyle = {
+    backgroundColor: indicatorColor || pal.colors.link,
+    bottom:
+      indicatorPosition === 'bottom' ? (isDesktopWeb ? 0 : -1) : undefined,
+    top: indicatorPosition === 'top' ? (isDesktopWeb ? 0 : -1) : undefined,
+    transform: [
+      {
+        translateX: panX.interpolate({
+          inputRange: items.map((_item, i) => i),
+          outputRange: itemLayouts.map(l => l.x + l.width / 2),
+        }),
+      },
+      {
+        scaleX: panX.interpolate({
+          inputRange: items.map((_item, i) => i),
+          outputRange: itemLayouts.map(l => l.width),
+        }),
+      },
+    ],
+  }
+
+  const onLayout = () => {
+    const promises = []
+    for (let i = 0; i < items.length; i++) {
+      promises.push(
+        new Promise<Layout>(resolve => {
+          itemRefs[i].current?.measure(
+            (x: number, _y: number, width: number) => {
+              resolve({x, width})
+            },
+          )
+        }),
+      )
+    }
+    Promise.all(promises).then((layouts: Layout[]) => {
+      setItemLayouts(layouts)
+    })
+  }
+
+  const onPressItem = (index: number) => {
+    onSelect?.(index)
+    if (index === selectedPage) {
+      onPressSelected?.()
+    }
+  }
+
+  return (
+    <View style={[pal.view, styles.outer]} onLayout={onLayout}>
+      <Animated.View style={[styles.indicator, indicatorStyle]} />
+      {items.map((item, i) => {
+        const selected = i === selectedPage
+        return (
+          <TouchableWithoutFeedback key={i} onPress={() => onPressItem(i)}>
+            <View
+              style={
+                indicatorPosition === 'top' ? styles.itemTop : styles.itemBottom
+              }
+              ref={itemRefs[i]}>
+              <Text type="xl-bold" style={selected ? pal.text : pal.textLight}>
+                {item}
+              </Text>
+            </View>
+          </TouchableWithoutFeedback>
+        )
+      })}
+    </View>
+  )
+}
+
+const styles = isDesktopWeb
+  ? StyleSheet.create({
+      outer: {
+        flexDirection: 'row',
+        paddingHorizontal: 18,
+      },
+      itemTop: {
+        paddingTop: 16,
+        paddingBottom: 14,
+        marginRight: 24,
+      },
+      itemBottom: {
+        paddingTop: 14,
+        paddingBottom: 16,
+        marginRight: 24,
+      },
+      indicator: {
+        position: 'absolute',
+        left: 0,
+        width: 1,
+        height: 3,
+      },
+    })
+  : StyleSheet.create({
+      outer: {
+        flexDirection: 'row',
+        paddingHorizontal: 14,
+      },
+      itemTop: {
+        paddingTop: 10,
+        paddingBottom: 10,
+        marginRight: 24,
+      },
+      itemBottom: {
+        paddingTop: 8,
+        paddingBottom: 12,
+        marginRight: 24,
+      },
+      indicator: {
+        position: 'absolute',
+        left: 0,
+        width: 1,
+        height: 3,
+      },
+    })