feat(useSum): reactively adds the values together

[LittleSound]

Description

import { useSum } from '@vueuse/math'

const value1 = ref(10)
const value2 = ref(20)

const sum = useSum([value1, value2])
// sum.value: 30

import { useSum } from '@vueuse/math'

const list = reactive([10, 20])

const sum = useSum(list)
// sum.value: 30

const sum = useSum([ref(10), ref(20)])
const sum = useSum(ref([10, 20]))
const sum = useSum(reactive([10, 20]))
// custom adder
const sum = useSum(ref([10, 20]), (sum, value) => sum * value)
// initialization value
const sum = useSum(ref([{ a: 1 }, { a: 2 }]), (sum, value) => sum + value.a, 0)

Additional context

issues #1812

---

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Before submitting the PR, please make sure you do the following

• Read the Contributing Guidelines.
• Read the Pull Request Guidelines.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.

[antfu]

Interesting, but I think this might be better to introduce a more general useArrayReduce in @vueuse/share first and reuse it.

[LittleSound]

Interesting, but I think this might be better to introduce a more general useArrayReduce in @vueuse/share first and reuse it.

I see that someone is already working on useArrayReduce and has a merged useSum method.

• Does this PR need to be turned off?
• Is there anything else I can do or contribute now?

[antfu]

Thanks for the contribution. I think #1874 is based on your implementation.