manifest file download size limit

I am learning to use Manifest files and had this question. What if some website created a manifest file that will make me cache files for, say, 3MB. 3MB is a lot for my IPhone when I run on 3G. So some vicious website can create such manifest file and make me use up all my data allo开发者_开发百科wance?

Another question is that, as a responsible developer, is there anyway to detect if a user is using 3G or not so I can serve a smaller manifest file in the former case?

Thank you

Your question is tagged 'iPhone' so I am going to give an iOS-specific answer.

The offline storage, which holds the files linked to in your manifest, seems to be limited to 5MB. Evidence for this is all circumstantial, as the limit is not mentioned in Apple docs. However, some Apple engineers have conceded that there is a limit. The newest versions of Mobile Safari, in some circumstances, offer to allow the user to expand the storage, but you, as a developer, cannot force them to do so.

5MB is not really that much, and many users will certainly notice that something is going on, and if they don't want it to happen, simply close the tab. I understand that not every country has the same limits, but most users in the US are limited to 2GB/month. At this rate, an unsuspecting user would have to accidentally re-download your manifest, and the files it lists 100 times to use up just 25% of their bandwidth allotment. It's a fairly low limit, and the possibility of maliciously using up someone's data allotment is fairly remote.

As far as how to tell if a user is on 3G or WiFi, check out the Reachability example in the Apple Developer Center. It does exactly that. There is no way to do it in a web app (as opposed to a native app) that I am aware of.

1. Is there a size limit for HTML5 Manifest? No.
2. No, iOS does not implement W3C network information API as oppose to Andorid Webkit.

I'm not sure how you're getting this manifest file, but if you're using NSURLConnection, it's simply a matter of keeping a running total of how much data you've downloaded (best place would be connection:didReceiveData:) and cancelling the connection if it gets above 3MB. Remember to zero that number each connection though.

As for finding the device, the specifics seem trickier than they should be, but I've found a pretty simple example of how to do it (source code n' all): Determine If iPod is First or Second Generation