fix actionAsync when using promises that resolve immediately (#225)

* fix actionAsync when using promises that resolve immediately

* change test name

CHANGELOG.md

@@ -1,3 +1,5 @@
+* Fix for `actionAsync` when awaiting promises that resolve immediately.
+
 # 5.5.1
 
 * Fix for `actionAsync` giving errors when it didn't await a task inside.

src/action-async.ts

@@ -16,6 +16,9 @@ interface IActionAsyncContext {
     args: IArguments
 }
 
+let taskOrderPromise = Promise.resolve()
+const emptyFunction = () => {}
+
 const actionAsyncContextStack: IActionAsyncContext[] = []
 
 export async function task<R>(value: R | PromiseLike<R>): Promise<R> {
@@ -34,7 +37,14 @@ export async function task<R>(value: R | PromiseLike<R>): Promise<R> {
     currentlyActiveIds.delete(runId)
 
     try {
-        return await value
+        const ret = await value
+
+        // we use this trick to force a proper order of execution
+        // even for immediately resolved promises
+        taskOrderPromise = taskOrderPromise.then(emptyFunction)
+        await taskOrderPromise
+
+        return ret
     } finally {
         // only restart if it not a dangling promise (the action is not yet finished)
         if (unfinishedIds.has(runId)) {

test/action-async.ts

@@ -534,3 +534,26 @@ test("complex case", async () => {
     expectNoActionsRunning()
     expect(values).toEqual([1, 2, 3])
 })
+
+test("immediately resolved promises", async () => {
+    mobx.configure({ enforceActions: "observed" })
+    const values = []
+    const x = mobx.observable({ a: 1 })
+    mobx.reaction(() => x.a, v => values.push(v), { fireImmediately: true })
+
+    const f1 = actionAsync("f1", async () => {
+        await task(Promise.resolve(""))
+        x.a = await task(Promise.resolve(3))
+    })
+
+    const f2 = actionAsync("f2", async () => {
+        const f1Promise = f1()
+        x.a = 2
+        x.a = await task(Promise.resolve(3))
+        await task(f1Promise)
+    })
+
+    await f2()
+    expect(values).toEqual([1, 2, 3])
+    expectNoActionsRunning()
+})