From 324ff521d64c7aea196d4911b4cdf0698f12930c Mon Sep 17 00:00:00 2001
From: Curt Grimes <curt@curtgrimes.com>
Date: Sun, 25 Sep 2022 20:39:24 -0500
Subject: [PATCH] feat(useScroll): support setting scroll position and toggling
 smooth scrolling (#1996)

---
 packages/core/useScroll/demo.vue | 39 ++++++++++++++++++--
 packages/core/useScroll/index.md | 41 ++++++++++++++++++++-
 packages/core/useScroll/index.ts | 63 +++++++++++++++++++++++++++-----
 3 files changed, 127 insertions(+), 16 deletions(-)

diff --git a/packages/core/useScroll/demo.vue b/packages/core/useScroll/demo.vue
index 680f30ad6b8..2c601e16b25 100644
--- a/packages/core/useScroll/demo.vue
+++ b/packages/core/useScroll/demo.vue
@@ -1,11 +1,32 @@
 <script setup lang="ts">
-import { ref, toRefs } from 'vue'
+import { computed, ref, toRefs } from 'vue'
 import { useScroll } from '@vueuse/core'
 
 const el = ref<HTMLElement | null>(null)
-const { x, y, isScrolling, arrivedState, directions } = useScroll(el)
+const smooth = ref(false)
+const behavior = computed(() => smooth.value ? 'smooth' : 'auto')
+const { x, y, isScrolling, arrivedState, directions } = useScroll(el, { behavior })
 const { left, right, top, bottom } = toRefs(arrivedState)
 const { left: toLeft, right: toRight, top: toTop, bottom: toBottom } = toRefs(directions)
+
+// Format the numbers with toFixed() to make them
+// nicer to displayœ
+const displayX = computed({
+  get() {
+    return x.value.toFixed(1)
+  },
+  set(val) {
+    x.value = parseFloat(val)
+  },
+})
+const displayY = computed({
+  get() {
+    return y.value.toFixed(1)
+  },
+  set(val) {
+    y.value = parseFloat(val)
+  },
+})
 </script>
 
 <template>
@@ -31,10 +52,20 @@ const { left: toLeft, right: toRight, top: toTop, bottom: toBottom } = toRefs(di
     </div>
     <div class="m-auto w-280px pl-4">
       <div class="px-6 py-4 rounded grid grid-cols-[120px_auto] gap-2 bg-gray-500/5">
-        <span text="right" opacity="75">Position</span>
+        <span text="right" opacity="75" class="py-4">X Position</span>
+        <div class="text-primary">
+          <div>
+            <input v-model="displayX" type="number" min="0" max="200" step="10" class="w-full !min-w-0">
+          </div>
+        </div>
+        <span text="right" opacity="75" class="py-4">Y Position</span>
         <div class="text-primary">
-          {{ x.toFixed(1) }}, {{ y.toFixed(1) }}
+          <div>
+            <input v-model="displayY" type="number" min="0" max="100" step="10" class="w-full !min-w-0">
+          </div>
         </div>
+        <label for="smooth-scrolling-option" text="right" opacity="75">Smooth scrolling</label>
+        <span><input id="smooth-scrolling-option" v-model="smooth" type="checkbox"></span>
         <span text="right" opacity="75">isScrolling</span>
         <BooleanDisplay :value="isScrolling" />
         <div text="right" opacity="75">
diff --git a/packages/core/useScroll/index.md b/packages/core/useScroll/index.md
index f7bbd34c6b4..879170b09bd 100644
--- a/packages/core/useScroll/index.md
+++ b/packages/core/useScroll/index.md
@@ -4,7 +4,7 @@ category: Sensors
 
 # useScroll
 
-Reactive scroll position and state
+Reactive scroll position and state.
 
 ## Usage
 
@@ -22,13 +22,50 @@ const { x, y, isScrolling, arrivedState, directions } = useScroll(el)
 </template>
 ```
 
+### With offsets
 ```js
-// With offsets
 const { x, y, isScrolling, arrivedState, directions } = useScroll(el, {
   offset: { top: 30, bottom: 30, right: 30, left: 30 },
 })
 ```
 
+### Setting scroll position
+
+Set the `x` and `y` values to make the element scroll to that position.
+```vue
+<script setup lang="ts">
+import { useScroll } from '@vueuse/core'
+
+const el = ref<HTMLElement | null>(null)
+const { x, y } = useScroll(el)
+</script>
+
+<template>
+  <div ref="el" />
+  <button @click="x += 10">
+    Scroll right 10px
+  </button>
+  <button @click="y += 10">
+    Scroll down 10px
+  </button>
+</template>
+```
+
+### Smooth scrolling
+
+Set `behavior: smooth` to enable smooth scrolling. The `behavior` option defaults to `auto`, which means no smooth scrolling. See the `behavior` option on [`window.scrollTo()`](https://developer.mozilla.org/en-US/docs/Web/API/Window/scrollTo) for more information.
+```ts
+import { useWindowScroll } from '@vueuse/core'
+
+const el = ref<HTMLElement | null>(null)
+const { x, y } = useScroll(el, { behavior: 'smooth' })
+
+// Or as a `ref`:
+const smooth = ref(false)
+const behavior = computed(() => smooth.value ? 'smooth' : 'auto')
+const { x, y } = useScroll(el, { behavior })
+```
+
 ## Directive Usage
 
 ```html
diff --git a/packages/core/useScroll/index.ts b/packages/core/useScroll/index.ts
index 43aee8010fe..88ddc72f3c5 100644
--- a/packages/core/useScroll/index.ts
+++ b/packages/core/useScroll/index.ts
@@ -1,6 +1,6 @@
-import { reactive, ref } from 'vue-demi'
+import { computed, reactive, ref } from 'vue-demi'
 import type { MaybeComputedRef } from '@vueuse/shared'
-import { noop, useDebounceFn, useThrottleFn } from '@vueuse/shared'
+import { noop, resolveUnref, useDebounceFn, useThrottleFn } from '@vueuse/shared'
 import { useEventListener } from '../useEventListener'
 
 export interface UseScrollOptions {
@@ -48,6 +48,14 @@ export interface UseScrollOptions {
    * @default {capture: false, passive: true}
    */
   eventListenerOptions?: boolean | AddEventListenerOptions
+
+  /**
+   * Optionally specify a scroll behavior of `auto` (default, not smooth scrolling) or
+   * `smooth` (for smooth scrolling) which takes effect when changing the `x` or `y` refs.
+   *
+   * @default 'auto'
+   */
+  behavior?: MaybeComputedRef<ScrollBehavior>
 }
 
 /**
@@ -85,10 +93,45 @@ export function useScroll(
       capture: false,
       passive: true,
     },
+    behavior = 'auto',
   } = options
 
-  const x = ref(0)
-  const y = ref(0)
+  const internalX = ref(0)
+  const internalY = ref(0)
+
+  // Use a computed for x and y because we want to write the value to the refs
+  // during a `scrollTo()` without firing additional `scrollTo()`s in the process.
+  const x = computed({
+    get() {
+      return internalX.value
+    },
+    set(x) {
+      scrollTo(x, undefined)
+    },
+  })
+
+  const y = computed({
+    get() {
+      return internalY.value
+    },
+    set(y) {
+      scrollTo(undefined, y)
+    },
+  })
+
+  function scrollTo(_x: number | undefined, _y: number | undefined) {
+    const _element = resolveUnref(element)
+
+    if (!_element)
+      return
+
+    (_element instanceof Document ? document.body : _element)?.scrollTo({
+      top: resolveUnref(_y) ?? y.value,
+      left: resolveUnref(_x) ?? x.value,
+      behavior: resolveUnref(behavior),
+    })
+  }
+
   const isScrolling = ref(false)
   const arrivedState = reactive({
     left: true,
@@ -118,12 +161,12 @@ export function useScroll(
     ) as HTMLElement
 
     const scrollLeft = eventTarget.scrollLeft
-    directions.left = scrollLeft < x.value
-    directions.right = scrollLeft > x.value
+    directions.left = scrollLeft < internalX.value
+    directions.right = scrollLeft > internalY.value
     arrivedState.left = scrollLeft <= 0 + (offset.left || 0)
     arrivedState.right
       = scrollLeft + eventTarget.clientWidth >= eventTarget.scrollWidth - (offset.right || 0) - ARRIVED_STATE_THRESHOLD_PIXELS
-    x.value = scrollLeft
+    internalX.value = scrollLeft
 
     let scrollTop = eventTarget.scrollTop
 
@@ -131,12 +174,12 @@ export function useScroll(
     if (e.target === document && !scrollTop)
       scrollTop = document.body.scrollTop
 
-    directions.top = scrollTop < y.value
-    directions.bottom = scrollTop > y.value
+    directions.top = scrollTop < internalY.value
+    directions.bottom = scrollTop > internalY.value
     arrivedState.top = scrollTop <= 0 + (offset.top || 0)
     arrivedState.bottom
       = scrollTop + eventTarget.clientHeight >= eventTarget.scrollHeight - (offset.bottom || 0) - ARRIVED_STATE_THRESHOLD_PIXELS
-    y.value = scrollTop
+    internalY.value = scrollTop
 
     isScrolling.value = true
     onScrollEnd(e)