Handle storage of data "locally" on a Project better

[rufuspollock]

Why

Reasons why we need local data ...

• It means that stuff like the viz screw-up with house prices does not happen (we make a transformation in a script which adds a field, we use this field in a graphy, now reload and it breaks because trying to graph a field that is not there)
• It means that local file data loading works (i.e. we can load that project again later!)
• It means stuff does not break if data disappears
• It means you can work offline ...

  Implementation

Alternatives ...

• Use indexedb - http://caniuse.com/#feat=indexeddb
  • 46% and all 3 of the current big 3
• file storage: http://caniuse.com/#feat=filesystem
  • ONLY supported by chrome ...
• local storage - not possible due to size limitations
• just persist to gists (no local storage anymore)

  Plan

• We replace localStorage with indexedb. (Do we still use localStorage for basic config info and your list of projects ??)
• Just remove localStorage - only persist when we are logged in and do not save other people's projects #72

We will go for just removing localStorage

Notes on Indexed DB Libraries

Indexed DB itself has a fairly painful API. We therefore probably want to use a wrapper.

Two existing JS wrappers. Both do a good job of removing some of the pain of using IndexedDB and of working consistently cross-browser. These are my rough impressions:

• IDBWrapper by Jens Arp - https://github.com/jensarps/IDBWrapper
  • Comprehensive but somewhat inelegant API (IMO) (this is likely somewhat intentional as has declared purpose to expose full IDB functionality: "show how IDB works. The code is split up into short methods, so that it's easy to see what happens in what method")
  • Tutorials: Part i Part II
• db.js by Aaron Powell
  • https://github.com/aaronpowell/db.js
  • Promises style API and generally slightly cleaner API compared to IDBWrapper
  • When I checked it the Travis build was failing and it had not been updated for a few months (it may not need it)

[rufuspollock]

Added notes about IndexedDB wrapper libraries

[djw]

Local storage gone in #99. The plan is to persist data to gists in the short term, later reintroducing an IndexedDB-based local cache.

[djw]

There's a work-in-progress pull request here: #101

[rufuspollock]

Wonder if @maxogden's https://github.com/maxogden/level.js + levelup would be useful here ...