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HTML5 File API: First Impressions

Recently went to an HTML5 Hackathon at Google Kirkland. My group’s project was an in-browser IDE Chrome extension that zipped up a user-provided series of HTML/CSS/JS files into a package that could be uploaded to the Chrome Store. Issac Lewis came up with the idea after trying to develop chrome extensions on his chromebook and finding it basically impossible to do. Storing the files was a perfect use case for the FileSystem API, but I spent most of my time beating my head against the wall to get it working. Here are some of the things I wish I knew going in.

The FileSystem API is not LocalStorage.

LocalStorage is a key-value store, the FileSystem API really is an entire virtual file system, sandboxed on a user’s local file system. You write, read, and create files async. It’s also only implemented currently in Chrome. The documentation says 9+, but I hit errors until I switched from Chromium 12 to Chrome 13.

There’s no limit to the storage, currently.

Hell yeah, cache all your map data on the user’s local file system without needing an explicit download or local client built for it. That’s a big deal for conditions or places with little to no connectivity. Also a big deal for massive games with a ton of art assets. They go through some good use cases here.

Debugging is a pain.

You will hit the dreaded SECURITY_ERR or QUOTA_EXCEEDED_ERR at some point, and it will be because debugging locally (file://) doesn’t work well in my experience. The documentation suggests it’s possible by opening Chrome with the —unlimited-quota-for-files and —allow-file-access-from-files flags, but my problems were only resolved when I started debugging as an extension rather than as a local file. You also need to be careful about the flux the API is in. Throwing around BlobBuilder() and other pieces of the newer APIs can throw errors that can be difficult to track down. BlobBuilder didn’t work for me, I needed window.WebKitBlobBuilder. That webkit prefixing shows up elsewhere as well (like window.webkitRequestFileSystem).

Feel no guilt in lifting gratuitously from the sample docs when starting out. Async file access isn’t really any wierder than any other browser async work, but there is some boilerplate code that is worth snapping up. Example:

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//error handling 
function errorHandler(e) {
  var msg = '';


  switch (e.code) {
    case FileError.QUOTA_EXCEEDED_ERR:
      msg = 'QUOTA_EXCEEDED_ERR';
      break;
    case FileError.NOT_FOUND_ERR:
      msg = 'NOT_FOUND_ERR';
      break;
    case FileError.SECURITY_ERR:
      msg = 'SECURITY_ERR';
      break;
    case FileError.INVALID_MODIFICATION_ERR:
      msg = 'INVALID_MODIFICATION_ERR';
      break;
    case FileError.INVALID_STATE_ERR:
      msg = 'INVALID_STATE_ERR';
      break;
    default:
      msg = 'Unknown Error';
      break;
  };


  console.log('Error: ' + msg);
}
//file system instantiation
window.requestFileSystem(window.PERSISTENT, 5*1024*1024 /*5MB*/, FSCreatedSuccess, errorHandler);

This kind of thing is okay starting out, but you’ll want a lot more out of the error handling eventually. The message is fine, but the code tells you nothing about where the error occurred and in reference to what object or operation.

It’s not CRUD, mostly. Don’t look for an explicit create method somewhere, the default is get or create via [filesystem_obj].[directory].get[Directory|File]. All reading, writing, and updating is probably going to live in a closure that starts with that first get.

Don’t rush.

I made the mistake of looking at the limited time allocated and starting just throwing the example code in willy-nilly. This is not what you do with an unfamiliar and very new API. The typical help online is not there yet because it hasn’t been used yet in a widespread way, throwing those error messages into google is not going to help you (unless that is how you got to this page, naturally). Start with the example code, sure, but I would carefully read the entirety of the short intro before trying random things to get it to work.

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