From 60af1936be964b88ce958dd481f149836f8590f7 Mon Sep 17 00:00:00 2001
From: Captain Caius <241342+captaincaius@users.noreply.github.com>
Date: Sat, 13 Oct 2018 15:52:17 +0000
Subject: [PATCH] PoC for promises as first class async citizens with scheduled
 macrotasks until they're resolved

---
 lib/common/promise.ts | 56 +++++++++++++++++++++++++++++++++++++++----
 1 file changed, 52 insertions(+), 4 deletions(-)

diff --git a/lib/common/promise.ts b/lib/common/promise.ts
index 876d964bb..b11d39462 100644
--- a/lib/common/promise.ts
+++ b/lib/common/promise.ts
@@ -5,6 +5,13 @@
  * Use of this source code is governed by an MIT-style license that can be
  * found in the LICENSE file at https://angular.io/license
  */
+
+function scheduleMacroTaskWithCurrentZone(
+    source: string, callback: Function, data: TaskData, customSchedule: (task: Task) => void,
+    customCancel: (task: Task) => void): MacroTask {
+  return Zone.current.scheduleMacroTask(source, callback, data, customSchedule, customCancel);
+}
+
 interface Promise<T> {
   finally<U>(onFinally?: () => U | PromiseLike<U>): Promise<T>;
 }
@@ -94,6 +101,38 @@ Zone.__load_patch('ZoneAwarePromise', (global: any, Zone: ZoneType, api: _ZonePr
   const REJECTED = false;
   const REJECTED_NO_CATCH = 0;
 
+  const PROMISECONSTRUCT_SOURCE = 'Promise.constructor';
+
+  interface PromiseTaskData extends TaskData {
+    promise: ZoneAwarePromise<any>;
+  }
+
+  function placeholderCallback() {}
+
+  function scheduleTask(task: Task) {
+    return task;
+  }
+
+  function clearTask(task: Task) {
+  }
+
+  function makeTaskAwareResolver(promise: ZoneAwarePromise<any>, state: boolean): (value: any) => void {
+    return (v) => {
+      try {
+        resolvePromise(promise, state, v);
+      } catch (err) {
+        resolvePromise(promise, false, err);
+      }
+      // Do not return value or you will break the Promise spec.
+      const task: Task = (promise as any)['MACROTASK'];
+      if (task && task.state === 'scheduled') {
+        task.invoke();
+      } else if(task) {
+        task.cancelScheduleRequest();
+      }
+    };
+  }
+
   function makeResolver(promise: ZoneAwarePromise<any>, state: boolean): (value: any) => void {
     return (v) => {
       try {
@@ -339,10 +378,19 @@ Zone.__load_patch('ZoneAwarePromise', (global: any, Zone: ZoneType, api: _ZonePr
       }
       (promise as any)[symbolState] = UNRESOLVED;
       (promise as any)[symbolValue] = [];  // queue;
-      try {
-        executor && executor(makeResolver(promise, RESOLVED), makeResolver(promise, REJECTED));
-      } catch (error) {
-        resolvePromise(promise, false, error);
+
+      const options: PromiseTaskData = {promise: promise};
+
+      if(executor) {
+        const task = scheduleMacroTaskWithCurrentZone(PROMISECONSTRUCT_SOURCE, placeholderCallback, options, scheduleTask, clearTask);
+        (promise as any)['MACROTASK'] = task;
+
+        try {
+          executor(makeTaskAwareResolver(promise, RESOLVED), makeTaskAwareResolver(promise, REJECTED));
+        } catch (error) {
+          resolvePromise(promise, false, error);
+          task.cancelScheduleRequest();
+        }
       }
     }