iOS知道文件何时更改

Question

I am downloading a mp4 file with my iOS app, it works fine using

NSData dataWithContentsOfURL

but i need to updated only if the file has been updated,

can I check the file headers? or what is the best way to determine if the file has been updated so I downloaded it again?

Thanks

Answer 1

You can access the metadata using the http HEAD request, as explained in this SO answer . You'll need to create some sort of parser to pick out the information you need, though. Note that this might not work with every server, depending on how it has been set up.

If you have control of the server yourself, I'd recommend using a php script to output the date the file has been last changed, which you would call before downloading the file.

Personally, I prefer placing a manifest file (usually a plist) alongside the file in question, as it can hold even more data, for example metadata for several files, the number of entries in a database and the like. A backdraw of this approach is that you'll need to keep this file up to date, though. But often, that is worth the while.

Lastly, as rckoenes has mentioned, dataWithContentsOfURL is not a very good way to download files, espcially large ones. You really should be using some sort of datamanager class, which manages a NSURLConnection .