use a small promise polyfill so task is not needed

README.md

@@ -789,8 +789,9 @@ Alternative syntax for async actions, similar to `flow` but more compatible with
 Typescript typings. Not to be confused with `asyncAction`, which is deprecated.
 
 `actionAsync` can be used either as a decorator or as a function.
-It takes an async function that internally must use `await task(promise)` rather than
-the standard `await promise`.
+
+It is _very_ important to await for any promises that are created _directly_ inside the async action,
+or else an exception will be thrown.
 
 When using the mobx devTools, an asyncAction will emit `action` events with names like:
 
@@ -811,15 +812,14 @@ The `step` number indicates the code block that is now being executed.
 ### Examples
 
 ```javascript
-import {actionAsync, task} from "mobx-utils"
+import {actionAsync} from "mobx-utils"
 
 let users = []
 
 const fetchUsers = actionAsync("fetchUsers", async (url) => {
   const start = Date.now()
-  // note the use of task when awaiting!
-  const data = await task(window.fetch(url))
-  users = await task(data.json())
+  const data = await window.fetch(url)
+  users = await data.json()
   return start - Date.now()
 })
 
@@ -828,7 +828,7 @@ console.log("Got users", users, "in ", time, "ms")
 ```
 
 ```javascript
-import {actionAsync, task} from "mobx-utils"
+import {actionAsync} from "mobx-utils"
 
 mobx.configure({ enforceActions: "observed" }) // don't allow state modifications outside actions
 
@@ -841,8 +841,7 @@ class Store {
     this.githubProjects = []
     this.state = "pending"
     try {
-      // note the use of task when awaiting!
-      const projects = await task(fetchGithubProjectsSomehow())
+      const projects = await fetchGithubProjectsSomehow()
       const filteredProjects = somePreprocessing(projects)
       // the asynchronous blocks will automatically be wrapped actions
       this.state = "done"

src/action-async.ts

@@ -1,6 +1,7 @@
 import { _startAction, _endAction, IActionRunInfo } from "mobx"
 import { invariant } from "./utils"
 import { decorateMethodOrField } from "./decorator-utils"
+import { fail } from "./utils"
 
 let runId = 0
 
@@ -17,14 +18,12 @@ const actionAsyncContextStack: IActionAsyncContext[] = []
 
 function getCurrentActionAsyncContext() {
     if (actionAsyncContextStack.length <= 0) {
-        fail(
-            "'actionAsync' context not present. did you await inside an 'actionAsync' without using 'task(promise)'?"
-        )
+        fail("'actionAsync' context not present")
     }
     return actionAsyncContextStack[actionAsyncContextStack.length - 1]!
 }
 
-export async function task<R>(promise: Promise<R>): Promise<R> {
+async function task<R>(promise: Promise<R>): Promise<R> {
     invariant(
         typeof promise === "object" && typeof promise.then === "function",
         "'task' expects a promise"
@@ -78,8 +77,9 @@ export function actionAsync<F extends (...args: any[]) => Promise<any>>(fn: F):
  * Typescript typings. Not to be confused with `asyncAction`, which is deprecated.
  * 
  * `actionAsync` can be used either as a decorator or as a function.
- * It takes an async function that internally must use `await task(promise)` rather than
- * the standard `await promise`.
+ * 
+ * It is *very* important to await for any promises that are created *directly* inside the async action,
+ * or else an exception will be thrown.
  * 
  * When using the mobx devTools, an asyncAction will emit `action` events with names like:
  * * `"fetchUsers - runid 6 - step 0"`
@@ -91,23 +91,22 @@ export function actionAsync<F extends (...args: any[]) => Promise<any>>(fn: F):
  * The `step` number indicates the code block that is now being executed.
  * 
  * @example
- * import {actionAsync, task} from "mobx-utils"
+ * import {actionAsync} from "mobx-utils"
  *
  * let users = []
  *
  * const fetchUsers = actionAsync("fetchUsers", async (url) => {
  *   const start = Date.now()
- *   // note the use of task when awaiting!
- *   const data = await task(window.fetch(url))
- *   users = await task(data.json())
+ *   const data = await window.fetch(url)
+ *   users = await data.json()
  *   return start - Date.now()
  * })
  *
  * const time = await fetchUsers("http://users.com")
  * console.log("Got users", users, "in ", time, "ms")
  *
  * @example
- * import {actionAsync, task} from "mobx-utils"
+ * import {actionAsync} from "mobx-utils"
  *
  * mobx.configure({ enforceActions: "observed" }) // don't allow state modifications outside actions
  *
@@ -120,8 +119,7 @@ export function actionAsync<F extends (...args: any[]) => Promise<any>>(fn: F):
  *     this.githubProjects = []
  *     this.state = "pending"
  *     try {
- *       // note the use of task when awaiting!
- *       const projects = await task(fetchGithubProjectsSomehow())
+ *       const projects = await fetchGithubProjectsSomehow()
  *       const filteredProjects = somePreprocessing(projects)
  *       // the asynchronous blocks will automatically be wrapped actions
  *       this.state = "done"
@@ -187,7 +185,7 @@ function actionAsyncFn(actionName: string, fn: Function): Function {
             const ctx = actionAsyncContextStack.pop()
             if (!ctx || ctx.runId !== nextRunId) {
                 fail(
-                    "'actionAsync' context not present or invalid. did you await inside an 'actionAsync' without using 'task(promise)'?"
+                    "'actionAsync' context not present or invalid. did you forget to await a promise directly created inside the async action?"
                 )
             }
 
@@ -200,3 +198,39 @@ function actionAsyncFn(actionName: string, fn: Function): Function {
 function getActionAsyncName(actionName: string, runId: number, step: number) {
     return `${actionName} - runid ${runId} - step ${step}`
 }
+
+let promisePolyfilled = false
+
+function polyfillPromise() {
+    if (promisePolyfilled) {
+        return
+    }
+    promisePolyfilled = true
+
+    const OrigPromise: any = Promise
+
+    const MobxPromise = function Promise(this: any, ...args: any[]) {
+        const p = new OrigPromise(...args)
+
+        if (actionAsyncContextStack.length > 0) {
+            // inside an async action
+            return task(p)
+        } else {
+            return p
+        }
+    }
+
+    // hoist statics
+    for (const pname of Object.getOwnPropertyNames(OrigPromise)) {
+        const desc = Object.getOwnPropertyDescriptor(OrigPromise, pname)
+        Object.defineProperty(MobxPromise, pname, desc)
+    }
+    for (const pname of Object.getOwnPropertySymbols(OrigPromise)) {
+        const desc = Object.getOwnPropertyDescriptor(OrigPromise, pname)
+        Object.defineProperty(MobxPromise, pname, desc)
+    }
+
+    Promise = MobxPromise as any
+}
+
+polyfillPromise()

src/mobx-utils.ts

@@ -16,4 +16,4 @@ export * from "./expr"
 export * from "./create-transformer"
 export * from "./deepObserve"
 export { computedFn } from "./computedFn"
-export { actionAsync, task } from "./action-async"
+export { actionAsync } from "./action-async"

src/utils.ts

@@ -4,8 +4,12 @@ export const NOOP = () => {}
 
 export const IDENTITY = (_: any) => _
 
+export function fail(message: string): never {
+    throw new Error("[mobx-utils] " + message)
+}
+
 export function invariant(cond: boolean, message = "Illegal state") {
-    if (!cond) throw new Error("[mobx-utils] " + message)
+    if (!cond) fail(message)
 }
 
 const deprecatedMessages: string[] = []

test/__snapshots__/action-async.ts.snap

@@ -35,6 +35,14 @@ Array [
   Object {
     "spyReportEnd": true,
   },
+  Object {
+    "arguments": Array [
+      1,
+    ],
+    "name": "f - runid 7 - step 1",
+    "spyReportStart": true,
+    "type": "action",
+  },
   Object {
     "arguments": Array [
       2,
@@ -68,14 +76,6 @@ Array [
   Object {
     "spyReportEnd": true,
   },
-  Object {
-    "arguments": Array [
-      1,
-    ],
-    "name": "f - runid 7 - step 1",
-    "spyReportStart": true,
-    "type": "action",
-  },
   Object {
     "key": "a",
     "name": "ObservableObject@16",