Keep Data out of Code

Moving any resources, such as long strings, or tables, out of code and into external files will make the final download smaller, because external files compress more efficiently than when the data is compiled into a binary. See iOS App Store Specific Considerations for full technical background.

Compiler Options

Setting the Optimization Level build setting to Fastest, Smallest [-Os] can dramatically lower the size of your compiled binary. This setting is the default for the "Release" configuration in Xcode projects.

If you have a watchOS app written in Swift, use the procedure described in Inspecting an .ipa File to verify that only one copy of libSwiftCore.dylib is present in the subdirectories of the Watch folder. If you see multiple copies, ensure the Always Embed Swift Standard Libraries build setting is set to Yes for the watchOS app target, and set to No for the watchOS app extension target.

Sanity Check Your App

Use the procedure described in Inspecting an .ipa File to check what is actually shipping in your app bundle. Apps often contain extra files, like headers or a readme, that are never used.

Leverage Asset Catalogs

Use asset catalogs for images, textures, and data assets, tagging each asset with relevant metadata to indicate which devices the asset is for. Asset Catalogs help maximize the size reduction provided by App Slicing, which can be significant for apps with assets not needed by every device. For an introduction to App Slicing, watch the App Thinning WWDC Session.

Asset catalogs have configurable levels of compression for each asset type. In particular, apps using wide color images or using textures with Metal should consider the ASTC compression options. For more information on these compression options, watch the WWDC session Working With Wide Color.

Asset catalogs also provide the ability to define a resizable center area of an image with optional end caps, reducing the overall size of an image.

Use 8-bit Images Where Possible

Using an 8-bit PNG instead of a 32-bit PNG can decrease your image size by 4x, so use them whenever you can. However, 8-bit images can only have 256 different colors and photographs or other detailed color images might not be suitable in an 8-bit format. Greyscale images are a perfect candidate for 8-bit images, and images with a small set of colors may work well in an 8-bit format.

Use Compression for Images Where Possible

For 32 bit images, using Adobe Photoshop’s “Save For Web” feature can reduce the size of JPEG and PNG images considerably. By default, .png images not in an asset catalog will be compressed with pngcrush by Xcode automatically when the app is built.

Compress Audio

See the Audio Development for Games WWDC session for information on handling audio efficiently. As a general rule, you should compress audio using AAC or MP3, and experiment with a reduced bitrate. Quite often a 44.1khz sample is overkill and a lower-bitrate clip won't have a perceptible drop in quality.

Adopt On-Demand Resources

Analyze all of the assets your app uses, and determine which resources are used infrequently or only by some users, and group these resources in to asset packs that are delivered with On Demand Resources. See the On-Demand Resources Guide for more information.

Adopt Background Downloads

Apps with assets in the app bundle that could be downloaded from a web service instead should move those assets to the web service and download them with a background session provided by NSURLSession.