feat(useAverage): new function (#1826)

Co-authored-by: Anthony Fu <anthonyfu117@hotmail.com>

packages/math/index.ts

@@ -3,6 +3,7 @@ export * from './createProjection'
 export * from './logicAnd'
 export * from './logicNot'
 export * from './logicOr'
+export * from './useAverage'
 export * from './useCeil'
 export * from './useClamp'
 export * from './useFloor'

packages/math/useAverage/index.md

@@ -0,0 +1,16 @@
+---
+category: '@Math'
+---
+
+# useAverage
+
+Get the average of an array reactively.
+
+## Usage
+
+```ts
+import { useAverage } from '@vueuse/math'
+
+const list = ref([1, 2, 3])
+const averageValue = useAverage(list)
+```

packages/math/useAverage/index.test.ts

@@ -0,0 +1,55 @@
+import { ref } from 'vue-demi'
+import { useAverage } from '.'
+
+describe('useAverage', () => {
+  it('should be defined', () => {
+    expect(useAverage).toBeDefined()
+  })
+
+  it('should be the average', () => {
+    const arr = ref([1, 2, 3])
+
+    const v = useAverage(arr)
+
+    expect(v.value).toBe(2)
+
+    arr.value = [4, 5, 6]
+    expect(v.value).toBe(5)
+  })
+
+  it('should be the average when some are ref', () => {
+    const a = ref(2)
+    const arr = ref([1, a, 9])
+
+    const v = useAverage(arr)
+
+    expect(v.value).toBe(4)
+
+    a.value = 8
+    expect(v.value).toBe(6)
+  })
+
+  it('should be the average when some items are getter', () => {
+    const a = ref(1)
+    const arr = ref([1, () => a.value + 1, 9])
+
+    const v = useAverage(arr)
+
+    expect(v.value).toBe(4)
+
+    a.value = 7
+    expect(v.value).toBe(6)
+  })
+
+  it('should be the average when the array is a getter', () => {
+    const arr = ref([1, 2, 3])
+    const last = ref(0)
+
+    const v = useAverage(() => arr.value.concat(last.value))
+
+    expect(v.value).toBe(1.5)
+
+    last.value = 10
+    expect(v.value).toBe(4)
+  })
+})

packages/math/useAverage/index.ts

@@ -0,0 +1,16 @@
+import type { ComputedRef } from 'vue-demi'
+import { computed } from 'vue-demi'
+import { resolveUnref } from '@vueuse/shared'
+import type { MaybeComputedRef } from '@vueuse/shared'
+
+/**
+ * Get the average of an array reactively
+ *
+ * @see https://vueuse.org/useAverage
+ */
+export function useAverage(array: MaybeComputedRef<MaybeComputedRef<number>[]>): ComputedRef<number> {
+  return computed(() => {
+    const arr = resolveUnref(array)
+    return arr.reduce<number>((sum, v) => sum += resolveUnref(v), 0) / arr.length
+  })
+}