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I was an iOS engineer at Airbnb when we were battling binary size increases caused by Swift. I rewrote the watch app twice; once to migrate to watchOS 2 and onc

I was an iOS engineer at Airbnb when we were battling binary size increases caused by Swift. I rewrote the watch app twice; once to migrate to watchOS 2 and once to convert it all to ObjC (which saved ~9mb). While Swift does have its problems (and had a lot more back when we worked on this), there are many factors to why the Uber app (and Airbnb) is getting so large. Airbnb is 75% executables, 9.5% assets and 8% localizations. This is pretty different from Uber which is 60.7% executables, 26% localizations and only 3.8% assets. While the Uber executables are large, ~10mb more than Airbnb (note: this is only the Uber rider app, while Airbnb has host+guest in one app), their localizations seem to be driving a lot of the app’s 300+mb install size. Looking at how the localizations are laid out in the app bundle, there are thousands of *.strings files in separate bundles, seems to be one for each feature. Many of these files have comments to provide context for the strings. An example from the app is: "Various screens show this as a title in a snackbar message when there is no network connectivity available". Just stripping these comments out of the localization files would save about 17.6mb of the install size. Another side effect of splitting localizations into so many files is there are over 23k duplicated strings throughout these bundles. While the Swift code size is part of the problem with Uber’s app size, it's not the only way to free up space. There's a lot of work that goes into bringing a large scale iOS app’s size down, and not many tools to help. I left my job at Airbnb recently to work on making it easier to visualize, monitor and reduce app size, please email me (noah [at] emergetools.com) if you might be interested!

spoonjim 6y ago
How do you know all these facts about the Uber app? Can you inspect the binary?
- sond813 6y ago
  Parts of the binary on the App Store are encrypted so we can only get limited information from it, but the whole app bundle is available and the rest of the files are not encrypted. We’re building a startup to help monitor, analyze and reduce app size and I was able to quickly grab these statistics from the tool we’re working on, here’s a link to a quick demo: https://www.youtube.com/watch?v=3x-hLxxUNm8 https://www.youtube.com/watch?v=3x-hLxxUNm8
  - saagarjha 6y ago
    It is also quite easy to decrypt iOS applications using jailbroken devices.
StanTwinB 6y ago
Looking at the bundle now is not going to give you insight into the problems in the Uber app 4 years ago. Also you’re looking at the uncompressed install size, which was not the problem (over the air compressed download size was the limiting factor). Strings files are extremely compressible, encrypted binaries are not (Apple has done some work in subsequent years to make them more compressible but back then they were basically uncompressible).
- sond813 6y ago
  Absolutely right! I left out some details to try to keep the post concise. 4 years ago at Airbnb we had the watch app problem, but now a Swift watch app wouldn’t be an issue since the latest OSs don’t require embedded Swift standard libraries. The tweet that started this thread cited the 300+mb size of Uber’s app, which looks like the current install size (not thinned), that’s why I used it as the benchmark in my explanation. Lately Apple has been featuring size more prominently on the App Store, and it appears to be displaying the install size. This is also what users see when they look at which apps are taking up storage on their device in Settings. I think it’s important to reduce both, so users see a good number and downloads are fast/efficient. It is a very welcome change that binary encryption doesn’t negate all the gains from compression anymore. I had noticed it in the numbers but looked around for an official statement and didn’t find any, if you happen to know where this was announced I’d appreciate a link!
  - phink 6y ago
    Yes, installed size is also an issue. And on iOS tens of thousands of small files will waste a lot of space. One may say that compared to photos and videos that's small potatoes but I disagree. Efficient use of resources, memory, bandwidth, battery, etc. matters.

spoonjim 6y ago

How do you know all these facts about the Uber app? Can you inspect the binary?

sond813 6y ago
Parts of the binary on the App Store are encrypted so we can only get limited information from it, but the whole app bundle is available and the rest of the files are not encrypted. We’re building a startup to help monitor, analyze and reduce app size and I was able to quickly grab these statistics from the tool we’re working on, here’s a link to a quick demo: https://www.youtube.com/watch?v=3x-hLxxUNm8 https://www.youtube.com/watch?v=3x-hLxxUNm8
- saagarjha 6y ago
  It is also quite easy to decrypt iOS applications using jailbroken devices.

StanTwinB 6y ago

Looking at the bundle now is not going to give you insight into the problems in the Uber app 4 years ago. Also you’re looking at the uncompressed install size, which was not the problem (over the air compressed download size was the limiting factor). Strings files are extremely compressible, encrypted binaries are not (Apple has done some work in subsequent years to make them more compressible but back then they were basically uncompressible).

sond813 6y ago
Absolutely right! I left out some details to try to keep the post concise. 4 years ago at Airbnb we had the watch app problem, but now a Swift watch app wouldn’t be an issue since the latest OSs don’t require embedded Swift standard libraries. The tweet that started this thread cited the 300+mb size of Uber’s app, which looks like the current install size (not thinned), that’s why I used it as the benchmark in my explanation. Lately Apple has been featuring size more prominently on the App Store, and it appears to be displaying the install size. This is also what users see when they look at which apps are taking up storage on their device in Settings. I think it’s important to reduce both, so users see a good number and downloads are fast/efficient. It is a very welcome change that binary encryption doesn’t negate all the gains from compression anymore. I had noticed it in the numbers but looked around for an official statement and didn’t find any, if you happen to know where this was announced I’d appreciate a link!
- phink 6y ago
  Yes, installed size is also an issue. And on iOS tens of thousands of small files will waste a lot of space. One may say that compared to photos and videos that's small potatoes but I disagree. Efficient use of resources, memory, bandwidth, battery, etc. matters.