[minor] Merge pull request #1409 from nulladdict/flow-inference

[Typescript] Improving flow typings #1378

Author: mweststrate

packages/mobx-state-tree/__tests__/core/async.test.ts

@@ -329,7 +329,7 @@ test("flow typings", async () => {
         numberToNumber: flow(function*(val: number) {
             yield promise
             return val
-        }), // should be () => Promise<Promise<number>>
+        }), // should be () => Promise<number>
         voidToNumber: flow(function*() {
             yield promise
             return Promise.resolve(2)
@@ -345,4 +345,8 @@ test("flow typings", async () => {
     expect(b).toBe(4)
     const c: number = await m.voidToNumber()
     expect(c).toBe(2)
+    await m.voidToNumber().then(d => {
+        const _d: number = d
+        expect(_d).toBe(2)
+    })
 })

packages/mobx-state-tree/src/core/flow.ts

@@ -1,11 +1,16 @@
+/**
+ * @hidden
+ */
+export type FlowReturn<R> = R extends Promise<infer T> ? T : R
+
 /**
  * See [asynchronous actions](https://github.com/mobxjs/mobx-state-tree/blob/master/docs/async-actions.md).
  *
  * @returns The flow as a promise.
  */
 export function flow<R, Args extends any[]>(
-    generator: (...args: Args) => Generator<any, R, any>
-): (...args: Args) => Promise<R> {
+    generator: (...args: Args) => Generator<Promise<any>, R, any>
+): (...args: Args) => Promise<FlowReturn<R>> {
     return createFlowSpawner(generator.name, generator) as any
 }