Question

I am attempting to send a JSON representation of an object in an email link. The recipient will open the link and my app will respond via a url scheme. It must extract the JSON from the url and re-build the object.

I am serializing my object by building an NSDictionary and using:

return [NSJSONSerialization dataWithJSONObject:dictionary options:NSJSONWritingPrettyPrinted error:&error];

I’m not sure what comes next. Somehow I need to convert this NSData into a string so that I can prefix my url scheme and use it in a link.

On the receiving end, I then need to remove the prefix (which I can do) and turn the string back into an NSData.

What is the correct method for doing this? And how do I make sure that the contents of my data do not interfere with the JSON string encoding (e.g. if my object contains text including special characters)?

Answer 1

You need to do an additional encoding step, since there are characters in encoded JSON that also have significance when they are part of a URL. What we actually want to do is URL-encode the data so none of the characters in the resulting string conflict with what applications expect a URL to look like.

The first step is transforming our data into an NSString (this is basically just a memcpy since NSStrings are encoded in UTF-8 by default):

NSString *jsonString = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];

Now, there’s a function you might be tempted to use called -stringByAddingPercentEscapesUsingEncoding, but it doesn’t do a thorough enough job of escaping all the relevant characters, so we need to build our own.

I could repeat the code here, but since it’s been done many times already, just view this blog that shows how you can add a category to NSString to do proper encoding, after which you can append it and send it on its way. Writing the analogous decoding function with CFURLCreateStringByReplacingPercentEscapesUsingEncoding is an exercise for the reader of which many examples can be found floating around.

Make sure your payloads are quite small (on the order of a couple of kB), by the way, since there is probably an upper bound on how long URLs, even those used locally and with a custom scheme, can be.