Task Management - Handle Promises "Then" or "Catch" callback case.

chrome/browser/resources/google_now/utility.js

Index: chrome/browser/resources/google_now/utility.js
diff --git a/chrome/browser/resources/google_now/utility.js b/chrome/browser/resources/google_now/utility.js
index f2236209fc498ffa15c0b2cc10c89d254e5a8c84..0a0dc0560dc565789258078a75c1db57cd55a2b6 100644
--- a/chrome/browser/resources/google_now/utility.js
+++ b/chrome/browser/resources/google_now/utility.js
@@ -437,26 +437,186 @@ wrapper.instrumentChromeApiFunction('identity.removeCachedAuthToken', 1);
 wrapper.instrumentChromeApiFunction('webstorePrivate.getBrowserLogin', 0);
 
 /**
- * Add task tracking support to Promise.then.
- * @override
+ * Promise adapter for all JS promises to the task manager.
  */
-Promise.prototype.then = function() {
+function registerPromiseAdapter() {
   var originalThen = Promise.prototype.then;
-  return function(callback) {
-    return originalThen.call(this, wrapper.wrapCallback(callback, false));
+  var originalCatch = Promise.prototype.catch;
+
+  /**
+   * Takes a promise and adds the callback tracker to it.
+   * @param {object} promise Promise that receives the callback tracker.
+   */
+  function instrumentPromise(promise) {
+    if (promise.__tracker === undefined) {
+      promise.__tracker = createPromiseCallbackTracker(promise);
+    }
   }
-}();
 
-/**
- * Add task tracking support to Promise.catch.
- * @override
- */
-Promise.prototype.catch = function() {
-  var originalCatch = Promise.prototype.catch;
-  return function(callback) {
-    return originalCatch.call(this, wrapper.wrapCallback(callback, false));
+  Promise.prototype.then = function(onResolved, onRejected) {
+    instrumentPromise(this);
+    return this.__tracker.handleThen(onResolved, onRejected);
   }
-}();
+
+  Promise.prototype.catch = function(onRejected) {
+    instrumentPromise(this);
+    return this.__tracker.handleCatch(onRejected);
+  }
+
+  /**
+   * Promise Callback Tracker.
+   * Handles coordination of 'then' and 'catch' callbacks in a task
+   * manager compatible way. For an individual promise, either the 'then'
+   * arguments or the 'catch' arguemnts will be processed, never both.

[rgustafson, 2014/02/26 18:31:04]

arguments

[robliao, 2014/02/26 18:46:17]

Done.

+   *
+   * Example:
+   *     var p = new Promise([Function]);
+   *     p.then([ThenA]);
+   *     p.then([ThenB]);
+   *     p.catch([CatchA]);
+   *     On resolution, [ThenA] and [ThenB] will be used. [CatchA] is discarded.
+   *     On rejection, vice versa.
+   *
+   * Clarification:
+   *     Chained promises create a new promise that is tracked separately from
+   *     the originaing promise, as the example below demonstrates:
+   *
+   *     var p = new Promise([Function]));
+   *     p.then([ThenA]).then([ThenB]).catch([CatchA]);
+   *         ^             ^             ^
+   *         |             |             + Returns a new promise.
+   *         |             + Returns a new promise.
+   *         + Returns a new promise.
+   *
+   *     Four promises exist in the above statement, each with its own
+   *     resolution and rejection state. However, by default, this state is
+   *     chained to the previous previous promise's resolution or rejection

[rgustafson, 2014/02/26 18:31:04]

only one previous

[robliao, 2014/02/26 18:46:17]

Done.

+   *     state.
+   *
+   *     If p resolves, then the then calls will execute until all the 'then'

[rgustafson, 2014/02/26 18:31:04]

use "'then' calls" to prevent confusion here

[robliao, 2014/02/26 18:46:17]

Done.

+   *     clauses are executed. If the result of either [ThenA] or [ThenB] is a
+   *     promise, then that execution state will guide the remaining chain.
+   *     Similarly, if [CatchA] returns a promise, it can also guide the
+   *     remaining chain. In this specific case, the chain ends, so there
+   *     is nothing left to do.
+   * @param {object} promise Promise being tracked.
+   * @return {object} A promise callback tracker.
+   */
+  function createPromiseCallbackTracker(promise) {
+    /**
+     * Callback Tracker. Holds an array of callbacks created for this promise.
+     * The indirection allows quick checks against the array and clearing the
+     * array without ugly splicing and copying.
+     * @typedef {{
+     *   callback: array.<Function>=
+     * }}
+     */
+    var CallbackTracker;
+
+    /** @type {CallbackTracker} */
+    var thenTracker = {callbacks: []};
+    /** @type {CallbackTracker} */
+    var catchTracker = {callbacks: []};
+
+    /**
+     * Returns true if the specified value is callable.
+     * @param {*} value Value to check.
+     * @return {boolean} True if the value is a callable.
+     */
+    function isCallable(value) {
+      return typeof value === 'function';
+    }
+
+    /**
+     * Takes a tracker and clears its callbacks in a manner consistent with
+     * the task manager. For the task manager, it also calls all callbacks
+     * by no-oping then first and then calling them.

[rgustafson, 2014/02/26 18:31:04]

no-oping them

[robliao, 2014/02/26 18:46:17]

Done.

+     * @param {CallbackTracker} tracker Tracker to clear.
+     */
+    function clearTracker(tracker) {
+      if (tracker.callbacks) {
+        var callbacksToClear = tracker.callbacks;
+        // No-ops all callbacks of this type.
+        tracker.callbacks = undefined;
+        // Do not wrap the promise then argument!
+        // It will call wrapped callbacks.
+        originalThen.call(Promise.resolve(), function() {

[vadimt, 2014/02/26 01:03:58]

WOW!

+          for (var i = 0; i < callbacksToClear.length; i++) {
+            callbacksToClear[i]();
+          }
+        });
+      }
+    }
+
+    /**
+     * Takes the argument to a 'then' or 'catch' function and applies
+     * a wrapping to callables consistent to ECMA promises.
+     * @param {*} maybeCallback Argument to 'then' or 'catch'.
+     * @param {CallbackTracker} sameTracker Tracker for the call type.
+     *     Example: If the argument is from a 'then' call, use thenTracker.
+     * @param {CallbackTracker} otherTracker Tracker for the opposing call type.
+     *     Example: If the argument is from a 'then' call, use catchTracker.
+     * @return {*} Consumable argument with necessary wrapping applied.
+     */
+    function registerAndWrapMaybeCallback(
+          maybeCallback, sameTracker, otherTracker) {
+      // If sameTracker.callbacks is undefined, we've reached an ending state
+      // that means this callback will never be called back.
+      // We will still forward this call on to let the promise system
+      // handle further processing, but since this promise is in an ending state
+      // we can be confident it will never be called back.
+      if (isCallable(maybeCallback) && sameTracker.callbacks) {
+        var handler = wrapper.wrapCallback(function() {
+          if (sameTracker.callbacks) {

[rgustafson, 2014/02/26 18:31:04]

This check is already done.

[robliao, 2014/02/26 18:46:17]

This check needs to be checked again since it's in a function that could be
called later.

sameTracker.callbacks can change in the meantime.
On 2014/02/26 18:31:04, rgustafson wrote:

+            clearTracker(otherTracker);
+            maybeCallback.apply(null, arguments);
+          }
+        }, false);
+        sameTracker.callbacks.push(handler);
+        return handler;
+      } else {
+        return maybeCallback;
+      }
+    }
+
+    /**
+     * Tracks then calls equivalent to Promise.prototype.then.
+     * @param {*} onResolved Argument to use if the promise is resolved.
+     * @param {*} onRejected Argument to use if the promise is rejected.
+     * @return {object} Promise resulting from the 'then' call.
+     */
+    function handleThen(onResolved, onRejected) {
+      var resolutionHandler =
+          registerAndWrapMaybeCallback(onResolved, thenTracker, catchTracker);
+      var rejectionHandler =
+          registerAndWrapMaybeCallback(onRejected, catchTracker, thenTracker);
+      return originalThen.call(promise, resolutionHandler, rejectionHandler);
+    }
+
+    /**
+     * Tracks then calls equivalent to Promise.prototype.catch.
+     * @param {*} onRejected Argument to use if the promise is rejected.
+     * @return {object} Promise resulting from the 'catch' call.
+     */
+    function handleCatch(onRejected) {
+      var rejectionHandler =
+          registerAndWrapMaybeCallback(onRejected, catchTracker, thenTracker);
+      return originalCatch.call(promise, rejectionHandler);
+    }
+
+    // Seeds this promise with at least one 'then' and 'catch' so that we always
+    // know which set of callbacks will not occur.

[rgustafson, 2014/02/26 18:31:04]

I don't understand what this is saying at all. Change comment to make it more
clear.

[robliao, 2014/02/26 18:46:17]

Done.

+    handleThen(function() {});
+    handleCatch(function() {});
+
+    return {
+      handleThen: handleThen,
+      handleCatch: handleCatch
+    };
+  }
+}
+
+registerPromiseAdapter();
 
 /**
  * Builds the object to manage tasks (mutually exclusive chains of events).