środa, 25 listopada 2015

Things I've learned while building an iOS SDK

There are times in programming when you think "I wish I knew that from the beginning". Building an SDK is quite different from making an app. You have more things to remember about and the distribution is more tricky. It's also a big responsibility, because if you screw something up, you can't just update right away - you might have a lot of users, and you don't control when they update your library and push to the store. This is a list of my i-wish-i-knews when building the Pulsate SDK

My SDK is too big!

I've started building the SDK with writing it from scratch, instead of refactoring the old code. One of my goals was to make the SDK smaller, as it was about 20 mb. When I finished an early version I was shocked to find out that the SDK still weighs the same. Only when I've educated myself about how does clang actually work - while writing this post - I realized that it's ok.

There's a big difference between a library and an executable. Library contains a lot of metadata that weighs a lot. Also, you want your static library to contain a lot of slices, including the simulator ones. The final executable strips all unneeded parts, making our demo executables with as much as 1 mb (with all the graphical assets etc.).

Also, Xcode 7 allows you to use bitcode which will reduce the executable on user's device even more. Remember to make sure you support bitcode in your library by using bitcode linker flags - if you don't, inform your users about that!

Watch out for symbol conflicts

Another big difference when building a library is that it's meant to be linked with someone else's code. Not only do you have to be careful to add prefixes to your classes to avoid symbol duplicates, but you have to make sure that you don't ship your dependencies (like AFNetworking) without making sure they're not going to clash with users using them too - It's not a problem if your code is open source and you're using CocoaPods, but it's a problem if you're shipping a compiled library. I've explained it in more detail in my other blog post.

Want to distribute compiled library through CocoaPods?

No problem! I didn't think it was possible, but it is and it's very easy. Our SDK is for commercial use only, so we couldn't open source it obviously. Check out our repo for the .podspec

Automate your process

It saves a lot of time to automate the building and distribution process. For a while I was using lipo to create a fat static library, commiting to GitHub and CocoaPods manually. I've written custom scripts that do that automatically now. It takes a couple of seconds to create a new release.

Here's my XCode script that builds both device and simulator, creating a fat library out of them and putting them in the release version folder.

Some things about the .bundle

If you need to ship your .framework with a .bundle (if you have some assets, or a CoreData model) there are some things you need to know.

The first thing that surprised me was that when we tried to publish an app with our SDK, it got rejected. Xcode automatically generates an executable and puts that executable info into the .plist file. You have to remove that executable, and .plist line from the .bundle in order for it to be accepted on iTunes. More here.

You can also distribute the .bundle file with CocoaPods.

Your users might not use some features. Create separate bundles, if you have a lot of assets, so users can choose which assets they need.

Some other tips

• Make sure you always distribute the release version
• Make sure you have a way to see what version of the SDK this is, just by looking into files. It's very useful
• Make sure you add the simulator slices
• If you're using a different server for testing, or any debug features and different for production use C macros and define a DEBUG identifier for debug builds so you make sure it won't get leaked into production like it just did for Vine.