best way to store data locally and update from web from time to time ?

Question

Hi,

I have created an app which displays information in a organized manner about cultural places. The information is subject to changes, so I want it to be downloaded from the web. But not everytime. Only once in a while, because information doesn't change often.

What I want to do is, the first time the user opens the application, it downloads all data from the web. For the moment, I parse it from an xml (which is about 100Ko), and I get a NSMutableArray of "CulturalPlace" objects. but it is very slow. And what I would like to do is, to store this data locally (in case the user has an iPod touch an is not on a wifi, or if he is on EDGE and does not want to redownload all). So the user updates data only by clicking an "update button" on the top right of the screen. Otherwise it reads it from disk.

I really don't know what could be the best solution. I thought about Core Data, but I have several Tableview imbricated (Rootviewcontroller > ListofPlacesViewController > PlaceViewController) and I really cannot find good tutorial for a simple use like mine. (the iTunes "TopSongs" sample code seems too complex).

I thought also about not parsing the xml, but instead try an NSURLConnection and get a plist file. But I never managed to read anything from the local file.

So my main question is, should I keep the xml parsing method, or should I use another format to tranfert the data from the web? And what is the best way to store and read data like an NSMutableArray of custom Objects ?

Thanks in advance for your help, sorry for my approximate english.

Answer 1

You could use HTML5' localStorage. It's supported by Chrome and FF on the PC and Safari on Mac OS and iPhone (to the best of my knowledge). It acts like a local database. Bear in mind that if the user selects to clear all cookies (or "private settings"), your storage goes away.

Answer 2

You could opt to store the XML locally, and store in NSUserDefaults the date when last updated - then on app launch you can check to see if you have a new file.

ASIHTTPRequest makes it pretty easy to say "Save the contents of this URL to a file". So you'd always save the XML to a file, and always read from that file or fetch XML if it was not yet there.

Answer 3

In my experience XML is indeed much slower to parse than plist, even though they're technically the same thing. Fortunately, plist's are pretty easy to deal with and the API's take care of all of the archiving and de-archiving.

Once you have your data in memory, it probably wouldn't be too hard to convert it to the much faster plist representation, check out this doc for more info: http://developer.apple.com/iphone/library/documentation/Cocoa/Conceptual/Archiving/Archiving.html

If your app is divided into different pages, you might also consider splitting the file into separate files, and only parsing / de-archiving the information you need when you need it (if you did this on a separate thread and displayed a UIProgressView on the main thread, the delay would probably be barely an issue to the user).