Question

I am making an app that lets the user draw on the screen in different colors and brush sizes. I am storing the info about each drawn path in a JSON file once it has been drawn to keep it out of memory. Right now I have it parsing all existing paths, then adding the new one in and writing it all back out again. I want it to simply append the new data into the JSON file without having to read it in and parse it first, that will make it so only one path is ever in memory at a time.

I am using SBJSON, the JSONWriter has a few append functions but I think you need to have the JSON string to append it to first, not the file, meaning I would have to read in the file anyway. Is there a way to do this without reading in the file at all? I know exactly how the data is structured.

Answer 1

It’s possible, but you have to cheat a little. You can just create a stand-alone JSON document per path, and append that to the file. So you’ll have something like this in your file:

{"name":"path1", "from": [0,3], "to":[3, 9]}
{"name":"path2", "from": [0,3], "to":[3, 9]}
{"name":"path3", "from": [0,3], "to":[3, 9]}

Note that this is not ONE JSON document but THREE. Handily, however, SBJsonStreamParser supports reading multiple JSON documents in one go. Set the supportMultipleDocuments property and plug it into a SBJsonStreamParserAdapter, and off you go. This also has the benefit that if you have many, many paths in your file as you can start drawing before you’re finished reading the whole file. (Because you get a callback for each path.)

You can see some information on the use case here.