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Serializing JavaScript Promise Execution

2013 Aug 10, 3:07
Occasionally I have need to run a set of unrelated promises in series, for instance an object dealing with a WinRT camera API that can only execute one async operation at a time, or an object handling postMessage message events and must resolve associated async operations in the same order it received the requests. The solution is very simply to keep track of the last promise and when adding a new promise in serial add a continuation of the last promise to execute the new promise and point the last promise at the result. I encapsulate the simple solution in a simple constructor:

    function PromiseExecutionSerializer() {
var lastPromise = WinJS.Promise.wrap(); // Start with an empty fulfilled promise.

this.addPromiseForSerializedExecution = function(promiseFunction) {
lastPromise = lastPromise.then(function () {
// Don't call directly so next promise doesn't get previous result parameter.
return promiseFunction();
});
}
}

The only thing to watch out for is to ensure you don't pass the result of a previous promise onto a subsequent promise that is unrelated.
PermalinkCommentsasync javascript promise technical

Considerate MessagePort Usage

2013 Aug 7, 7:14
Sharing by leezie5. Two squirrels sharing food hanging from a bird feeder. Used under Creative Commons license Attribution-NonCommercial-NoDerivs 2.0 Generic.When writing a JavaScript library that uses postMessage and the message event, I must be considerate of other JS code that will be running along side my library. I shouldn't assume I'm the only sender and receiver on a caller provided MessagePort object. This means obviously I should use addEventListener("message" rather than the onmessage property (see related What if two programs did this?). But considering the actual messages traveling over the message channel I have the issue of accidentally processing another libraries messages and having another library accidentally process my own message. I have a few options for playing nice in this regard:
Require a caller provided unique MessagePort
This solves the problem but puts a lot of work on the caller who may not notice nor follow this requirement.
Uniquely mark my messages
To ensure I'm acting upon my own messages and not messages that happen to have similar properties as my own, I place a 'type' property on my postMessage data with a value of a URN unique to me and my JS library. Usually because its easy I use a UUID URN. There's no way someone will coincidentally produce this same URN. With this I can be sure I'm not processing someone else's messages. Of course there's no way to modify my postMessage data to prevent another library from accidentally processing my messages as their own. I can only hope they take similar steps as this and see that my messages are not their own.
Use caller provided MessagePort only to upgrade to new unique MessagePort
I can also make my own unique MessagePort for which only my library will have the end points. This does still require the caller to provide an initial message channel over which I can communicate my new unique MessagePort which means I still have the problems above. However it clearly reduces the surface area of the problem since I only need once message to communicate the new MessagePort.
The best solution is likely all of the above.
Photo is Sharing by leezie5. Two squirrels sharing food hanging from a bird feeder. Used under Creative Commons license Attribution-NonCommercial-NoDerivs 2.0 Generic.
PermalinkCommentsDOM html javascript messagechannel postMessage programming technical

solarbird: Perfect.

2013 Jul 28, 1:46


solarbird:

Perfect.

PermalinkCommentshumor glasses cosplay

Subtleties of postMessage

2013 Jul 15, 1:00

In IE10 and other new browsers one may create MessageChannel objects that have two MessagePorts each connected (w3c spec calls it entangled) to one another such that postMessage on one port results in the message event firing on the other. You can pass an array of ports as the last parameter to postMessage and they show up in the ports property of the message event arg.

Origin

The postMessage here is like the worker postMessage and unlike the window and iframe postMessage in that it applies no origin checking:

  1. No origin postMessage in workers and MessagePorts: postMessage(messageData, ports)
  2. Origin postMessage in windows and iframes: postMessage(messageData, targetOrigin, ports)

Unfortunately the origin isn't an optional parameter at the end to make the two postMessages have the same signature.

On the event handler side, the event arg always has an origin property. But in the no origin case it is always the empty string.

Source

There is also a source property on the message event arg which if set is an object that has a postMessage property allowing you to post back to your caller. It is set for the origin case, however, in the no origin case this property is null. This is somewhat reasonable because in the case of MessagePort and Workers there are only two endpoints so you always know the source of a message implicitly. Unlike the origin case in which any iframe or window can be calling postMessage on any other iframe or window and the caller is unknown. So not unreasonable but it would be nice if the source property was always set for consistency.

MessageChannel start

When a MessageChannel is created it has two MessagePorts, but until those ports are started they will queue up any messages they receive. Once started they will dispatch all queued messages. Ports don't have to be started to send messages.

A port may be started in two ways, either by explicitly calling the start method on the port, or by setting the onmessage callback property on the port. However, adding an event listener via addEventListener("message", does not start the port. It works this way in IE and Chrome and the spec states this as well.

The justification is that since you can have only one callback via onmessage that once set you must implicitly be ready to receive messages and its fine to start the port. As opposed to the addEventListener in which case the user agent cannot start implicitly because it doesn't know how many event listeners will be added.  I found Hixie stating this justification in geoloc meeting notes.

Links

W3C Spec

Opera introduction

PermalinkCommentsDOM html javascript postMessage technical web-worker worker

laughingsquid: Testing People’s Reaction Times With a Ruler in...

2013 Jul 5, 3:05


laughingsquid:

Testing People’s Reaction Times With a Ruler in Super Slow Motion

PermalinkCommentsbrain science video speed time humor

Words with Hints Windows 8 App Development Notes

2013 Jul 4, 1:00

My second completed app for the Windows Store was Words with Hints a companion to Words with Friends or other Scrabble like games that gives you *ahem* hints. You provide your tiles and optionally letters placed in a line on the board and Words with Hints gives you word options.

I wrote this the first time by building a regular expression to check against my dictionary of words which made for a slow app on the Surface. In subsequent release of the app I now spawn four web workers (one for each of the Surface's cores) each with its own fourth of my dictionary. Each fourth of the dictionary is a trie which makes it easy for me to discard whole chunks of possible combinations of Scrabble letters as I walk the tree of possibilities.

The dictionaries are large and takes a noticeable amount of time to load on the Surface. The best performing mechanism I found to load them is as JavaScript source files that simply define their portion of the dictionary on the global object and synchronously (only on the worker so not blocking the UI thread). Putting them into .js files means they take advantage of bytecode caching making them load faster. However because the data is mostly strings and not code there is a dramatic size increase when the app is installed. The total size of the four dictionary .js files is about 44Mb. The bytecode cache for the dictionary files is about double that 88Mb meaning the dictionary plus the bytecode cache is 132Mb.

To handle the bother of postMessage communication and web workers this was the first app in which I used my promise MessagePort project which I'll discuss more in the future.

This is the first app in which I used the Microsoft Ad SDK. It was difficult to find the install for the SDK and difficult to use their website, but once setup, the Ad SDK was easy to import into VS and easy to use in my app.

PermalinkCommentsdevelopment technical windows windows-store words-with-hints

STRIP SEARCH SPOILERS FOLLOW! BEWARE! jspowerblogger: WARNING:...

2013 Jun 19, 2:50


STRIP SEARCH SPOILERS FOLLOW! BEWARE!

jspowerblogger:

WARNING: THIS POST CONTAINS RIDICULOUS STRIP SEARCH SPOILERS

So I lost. Wah, boohoo, etc etc. It doesn’t mean I’m going to give up. I love The Last Halloween. If you also loved The Last Halloween, don’t worry, it’s happening. But first I have to Kickstart it! The Kickstarter goes up within the next few days, and I hope you guys will fund it, if you’ll have me.

I’ll do a much larger post when the Kickstarter kickstarts so you’ll all know my feelings and how great everything is and how much you will be into backing it. 

I’m one of these guys being sshhhhhsh’ed. Abby had the best comics on Strip Search and so for my continued entertainment I shall help kickstart!

PermalinkCommentskickstarter strip-search

laughingsquid: How Common Is Your Birthday, A Chart of Birth...

2013 Jun 10, 2:49


laughingsquid:

How Common Is Your Birthday, A Chart of Birth Date Frequencies

PermalinkCommentsbirth-day chart

laughingsquid: The Ultimate Spaceship Face-off, Interactive...

2013 May 22, 3:10


laughingsquid:

The Ultimate Spaceship Face-off, Interactive Guide For Comparing the Speeds of Famed Sci-Fi Ships

PermalinkCommentssci-fi scifi tardis doctor-who star-trek star-wars nerd

lohikaappaus: same Classic.

2013 May 17, 12:07


lohikaappaus:

same

Classic.

PermalinkCommentsxfiles scully humor x-files

laughingsquid: Arrested Development Season 4 Trailer Brings...

2013 May 13, 2:47


laughingsquid:

Arrested Development Season 4 Trailer Brings Back the Dysfunctional Bluth Family

Come on!

PermalinkCommentshumor trailer arrested-development awesome

wilwheaton: owlturdcomix: Thoughts I have instead of smart...

2013 May 13, 2:31




wilwheaton:

owlturdcomix:

Thoughts I have instead of smart thoughts.

http://i.imgur.com/b9h1rc5.png

Owl Turd is rapidly becoming one of my favourite webcomics.

This may have been a Serial Experiments Lain episode synopsis.

PermalinkCommentshumor map

robhuebel: Axe Cop coming this summer!   Nick Offerman, Tyler...

2013 May 13, 2:26


robhuebel:

Axe Cop coming this summer!  

Nick Offerman, Tyler the Creator, Me (Rob Huebel), Giancarlo Esposito and Vincent Kartheiser.  

PermalinkCommentsaxe-cop humor video tv

Will Arnett Explains the Origins of His Arrested Development Chicken Dance

2013 May 8, 11:26

thebluthcompany:

To decide what Gob’s bad impression of a chicken might be, Arnett consulted on set in 2003 with series executive producers Mitch Hurwitz and James Vallely. They all tried out different versions for each other. “Jimmy started doing a little bit, then Mitch got up and did some, and then I began trying things,” remembers Arnett. “Picture three grown men hopping around, working out what it would be … They were pitching this really taunting dance, but I wanted to give it this very sharp, almost roosterlike, chest-sticking-out mannerism, like a real macho bravado dance.” And how did clapping get introduced to the move? “Because I wanted it to be only sort of threatening.”

Read More | Vulture

PermalinkCommentshumor chicken chicken-dance arrested-development

laughingsquid: Time Travel in Movies Flowchart

2013 Apr 15, 3:41


laughingsquid:

Time Travel in Movies Flowchart

PermalinkCommentstime-travel movies chart movie bttf

eclecticmethod: Hackers

2013 Apr 13, 1:57

eclecticmethod:

You don’t use the same password over and over right? Let’s be honest this is the day and age of the hacker. Eclectic Method brings you “Hackers” , Hollywood’s celebration of basement dwelling 128 bit encryption masters. They’ll shut down before you can trace them, hack into the mainframe…

Hack the planet!

PermalinkCommentshack hackers movie video mix remix mashup eclectic-method

laughingsquid: Tetris Stackable LED Desk Lamp

2013 Apr 13, 12:57


laughingsquid:

Tetris Stackable LED Desk Lamp

PermalinkCommentstetris lamp game humor

wilwheaton: cameron-stewart: My contribution in full to the...

2013 Apr 4, 5:34








wilwheaton:

cameron-stewart:

My contribution in full to the #bartkira project. This was tons of fun to do.

Holy shit.

Everything’s coming up Milhouse

PermalinkCommentshumor comic art mashup simpsons akira

laughingsquid: An ‘American Psycho’ Parody Starring Huey Lewis...

2013 Apr 4, 5:33


laughingsquid:

An ‘American Psycho’ Parody Starring Huey Lewis & Weird Al

Hey its the high school band audition judge from Back to the Future.

PermalinkCommentshumor weird-al huey-lewis american-psycho video

laughingsquid: Two-Year-Old Picks the Lock to His Sister’s Room...

2013 Mar 28, 3:30


laughingsquid:

Two-Year-Old Picks the Lock to His Sister’s Room and Steals Her Toys

PermalinkCommentshumor child video
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