Add useAsync.

Author: ghengeveld

src/index.js

@@ -1,4 +1,5 @@
 import React from "react"
+export { default as useAsync } from "./useAsync"
 
 const isFunction = arg => typeof arg === "function"
 

src/useAsync.js

@@ -0,0 +1,96 @@
+import { useState, useEffect, useMemo } from "react"
+
+const useAsync = props => {
+  let counter = 0
+  let isMounted = false
+  let lastArgs = undefined
+
+  const initialError = props.initialValue instanceof Error ? props.initialValue : undefined
+  const initialData = initialError ? undefined : props.initialValue
+  const [data, setData] = useState(initialData)
+  const [error, setError] = useState(initialError)
+  const [startedAt, setStartedAt] = useState(props.promiseFn ? new Date() : undefined)
+  const [finishedAt, setFinishedAt] = useState(props.initialValue ? new Date() : undefined)
+
+  const cancel = () => {
+    counter++
+    setStartedAt(undefined)
+  }
+
+  const start = () => {
+    counter++
+    setStartedAt(new Date())
+    setFinishedAt(undefined)
+  }
+
+  const end = () => setFinishedAt(new Date())
+
+  const handleData = (data, callback = () => {}) => {
+    if (isMounted) {
+      end()
+      setData(data)
+      setError(undefined)
+      callback(data)
+    }
+    return data
+  }
+
+  const handleError = (error, callback = () => {}) => {
+    if (isMounted) {
+      end()
+      setError(error)
+      callback(error)
+    }
+    return error
+  }
+
+  const onResolve = count => data => count === counter && handleData(data, props.onResolve)
+  const onReject = count => error => count === counter && handleError(error, props.onReject)
+
+  const load = () => {
+    if (props.promiseFn) {
+      start()
+      props.promiseFn(props).then(onResolve(counter), onReject(counter))
+    }
+  }
+
+  const run = (...args) => {
+    if (props.deferFn) {
+      lastArgs = args
+      start()
+      return props.deferFn(...args, props).then(onResolve(counter), onReject(counter))
+    }
+  }
+
+  const reload = () => (lastArgs ? run(...lastArgs) : load())
+
+  useEffect(() => {
+    isMounted = true
+    return () => (isMounted = false)
+  }, [])
+
+  useEffect(load, [props.promiseFn, props.watch])
+
+  return useMemo(
+    () => ({
+      isLoading: startedAt && (!finishedAt || finishedAt < startedAt),
+      startedAt,
+      finishedAt,
+      data,
+      error,
+      cancel,
+      run,
+      reload,
+      setData: handleData,
+      setError: handleError,
+      initialValue: props.initialValue
+    }),
+    [data, error, startedAt, finishedAt]
+  )
+}
+
+const unsupported = () => {
+  throw new Error("useAsync requires [email protected] or later")
+}
+
+export default (useState ? useAsync : unsupported)