Make scroll handling contextual (#2200)

* Add an intermediate List component

* Fix type

* Add onScrolledDownChange

* Port pager to use onScrolledDownChange

* Fix on mobile

* Don't pass down onScroll (replacement TBD)

* Remove resetMainScroll

* Replace onMainScroll with MainScrollProvider

* Hook ScrollProvider to pager

* Fix the remaining special case

* Optimize a bit

* Enforce that onScroll cannot be passed

* Keep value updated even if no handler

* Also memo it

1 files changed, 97 insertions, 0 deletions

diff --git a/src/view/com/util/MainScrollProvider.tsx b/src/view/com/util/MainScrollProvider.tsx
new file mode 100644
index 000000000..31a4ef0c8
--- /dev/null
+++ b/src/view/com/util/MainScrollProvider.tsx
@@ -0,0 +1,97 @@
+import React, {useCallback} from 'react'
+import {ScrollProvider} from '#/lib/ScrollContext'
+import {NativeScrollEvent} from 'react-native'
+import {useSetMinimalShellMode, useMinimalShellMode} from '#/state/shell'
+import {useShellLayout} from '#/state/shell/shell-layout'
+import {isWeb} from 'platform/detection'
+import {useSharedValue, interpolate} from 'react-native-reanimated'
+
+function clamp(num: number, min: number, max: number) {
+  'worklet'
+  return Math.min(Math.max(num, min), max)
+}
+
+export function MainScrollProvider({children}: {children: React.ReactNode}) {
+  const {headerHeight} = useShellLayout()
+  const mode = useMinimalShellMode()
+  const setMode = useSetMinimalShellMode()
+  const startDragOffset = useSharedValue<number | null>(null)
+  const startMode = useSharedValue<number | null>(null)
+
+  const onBeginDrag = useCallback(
+    (e: NativeScrollEvent) => {
+      'worklet'
+      startDragOffset.value = e.contentOffset.y
+      startMode.value = mode.value
+    },
+    [mode, startDragOffset, startMode],
+  )
+
+  const onEndDrag = useCallback(
+    (e: NativeScrollEvent) => {
+      'worklet'
+      startDragOffset.value = null
+      startMode.value = null
+      if (e.contentOffset.y < headerHeight.value / 2) {
+        // If we're close to the top, show the shell.
+        setMode(false)
+      } else {
+        // Snap to whichever state is the closest.
+        setMode(Math.round(mode.value) === 1)
+      }
+    },
+    [startDragOffset, startMode, setMode, mode, headerHeight],
+  )
+
+  const onScroll = useCallback(
+    (e: NativeScrollEvent) => {
+      'worklet'
+      if (startDragOffset.value === null || startMode.value === null) {
+        if (mode.value !== 0 && e.contentOffset.y < headerHeight.value) {
+          // If we're close enough to the top, always show the shell.
+          // Even if we're not dragging.
+          setMode(false)
+          return
+        }
+        if (isWeb) {
+          // On the web, there is no concept of "starting" the drag.
+          // When we get the first scroll event, we consider that the start.
+          startDragOffset.value = e.contentOffset.y
+          startMode.value = mode.value
+        }
+        return
+      }
+
+      // The "mode" value is always between 0 and 1.
+      // Figure out how much to move it based on the current dragged distance.
+      const dy = e.contentOffset.y - startDragOffset.value
+      const dProgress = interpolate(
+        dy,
+        [-headerHeight.value, headerHeight.value],
+        [-1, 1],
+      )
+      const newValue = clamp(startMode.value + dProgress, 0, 1)
+      if (newValue !== mode.value) {
+        // Manually adjust the value. This won't be (and shouldn't be) animated.
+        mode.value = newValue
+      }
+      if (isWeb) {
+        // On the web, there is no concept of "starting" the drag,
+        // so we don't have any specific anchor point to calculate the distance.
+        // Instead, update it continuosly along the way and diff with the last event.
+        startDragOffset.value = e.contentOffset.y
+        startMode.value = mode.value
+      }
+    },
+    [headerHeight, mode, setMode, startDragOffset, startMode],
+  )
+
+  return (
+    <ScrollProvider
+      onBeginDrag={onBeginDrag}
+      onEndDrag={onEndDrag}
+      onScroll={onScroll}>
+      {children}
+    </ScrollProvider>
+  )
+}