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paulrehkugler / xkcd

xkcd iPhone app
MIT License
44 stars 9 forks source link

App Won’t Update Past 1248 #74

Closed paulrehkugler closed 5 years ago

paulrehkugler commented 5 years ago

Some users cannot access comics more recent than 1248. As far as I can tell, https://xkcd.com/1248/info.0.json and https://xkcd.com/1249/info.0.json both have well-formed JSON, so this may be a local issue with the app.

azsn commented 5 years ago

I've been experiencing this issue since I got a new iPhone with iOS 12. I looked through the code of the app a bit a few months ago, but I wasn't able to find the problem. Unfortunately this lead me to switch to the xkcd: os app, but I don't like it -- I'd love to go back to your app if this issue gets fixed! I'll take a look at the code again tonight and see if I can find anything.

azsn commented 5 years ago

Okay, I actually ran it on the iOS emulator this time instead of just looking at the code. The high-level blame is this line:

https://github.com/paulrehkugler/xkcd/blob/2bad993731b0e3be74f9f4c20c0a1591f1ab9abd/Classes/FetchComicFromWeb.m#L74

Removing that function call (replacing it with NSData *fixedData = data; for example) fixes the problem and comics can load past 1248. Clearly something about the json for comic 1249 breaks that function's algorithm. I'll keep investigating.

EDIT:

I found the problem, and it's not a technically bug in that function. However, I don't understand the purpose of the function. It appears to be converting single-byte Unicode escape sequences (in the case of comic 1248, its the sequence \u00f6) and replacing them with their ASCII form. In this case it converts it to the Extended ASCII code 246, ö. However, JSON is defined to be encoded in UTF-8, and usage of the Extended ASCII codes are incompatible with UTF-8.

The function isn't technically breaking (at least not in this case) -- it successfully converts the escape sequence "\u00f6" to the single byte 0xF6 -- but it generates invalid UTF-8 which the the JSON parser errors on.

At least as of iOS 12, Apple's JSON library is capable of correctly parsing single-byte Unicode escape sequences. So this function dataByFixingFuckedUpUnicodeInJSON seems like it should be removed entirely.

paulrehkugler commented 5 years ago

Thanks so much for your help! This should be fixed with PR #75.