sharing - Dave's Blog

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.

I had previously replaced my use of Delicious with Google Reader. Delicious had a number of issues during their switch over from Yahoo to the new owners and I was eventually fed up enough to remove it from daily use. I used Delicious to do the following things:

• Create a list of things to read later
• Save things to read again in the future
• Search through things I read and enjoyed (esp via tags)
• Annotate and share things on my blog

I realized that since I did most of my web browsing in Google Reader now anyway I may as well make use of its features. I star things to note I want to read it later or save to read again later. I can annotate with notes in Google Reader and I can share items to my web site by way of the shared items feed. Additionally for when I'm not in Google Reader there's a bookmarklet to add an arbitrary web site as a shared item in Google Reader.

Of course I wrote this and switched over about 1 week before Google removed the sharing feature from Google Reader. I'm irritated but in practice it forced me to find a different option which has worked out mostly better. New blog post coming soon about that...