App Thinning: Ship Only What's Needed
App Thinning shrinks your app's download by delivering only the assets a specific device needs. The App Store uses Slicing for device-specific art and code, and you can use On-Demand Resources for assets downloaded after installation.
Why it exists
Large apps deter users from downloading, especially over cellular, and take up precious storage space. App Thinning was created to reduce the installed footprint of an app by ensuring that a user's device only downloads and stores the code and resources it actually needs.
The mental model
Think of your app archive as a master blueprint containing parts for every possible device. App Thinning is the factory process that looks at a customer's order (their device model) and builds a custom kit using only the parts from the blueprint that fit that specific model, shipping them a much smaller package.
How it works
App Thinning uses two main strategies. First, Slicing creates different app variants for different devices. The App Store delivers only the variant for a user's device, containing the correct executable architecture (like arm64) and resources (like @3x images for a Pro Max iPhone). Second, On-Demand Resources (ODR) allow you to host assets on the App Store that the app can download as needed after the initial install, perfect for game levels or tutorials. A third mechanism, Bitcode, which allowed Apple to recompile apps for new CPUs, is now deprecated and no longer used by the App Store as of Xcode 14.
When to use it
You should always leverage App Thinning. Slicing is largely automatic if you use asset catalogs correctly. Use On-Demand Resources for any content not required at first launch, such as in-app purchase content, tutorials, or later game levels. This significantly reduces the initial download size, which is critical for user acquisition.
When not to use it
There's no reason to avoid Slicing, as it's a core optimization. However, implementing On-Demand Resources adds complexity. Don't use ODR for essential, first-launch resources, as the network-dependent download would create a poor user experience. If your app is already very small and has no large, optional assets, the overhead of managing ODR might not be worthwhile.
One canonical example
A game developer includes 50 levels in their app. The initial download only contains the files for levels 1-3. When the player is about to complete level 3, the app uses the ODR API to request the assets for levels 4-6 from the App Store in the background. The user gets a small initial app and a seamless experience, while the developer avoids a massive upfront download that might lose them the user.
Interview question
An app includes a large, optional onboarding video. Which App Thinning strategy is best to minimize the initial download size while ensuring the video is available if needed?
- a.On-Demand Resources (ODR), allowing the video to be downloaded only when the user initiates it.Correct
- b.Bundling the video directly into the app, as App Thinning doesn't apply to large media files.
- c.Slicing, to deliver the video only to devices with sufficient storage.
- d.Bitcode, to optimize the video's encoding for various device processors.
Why? this is the answer
On-Demand Resources (ODR) are ideal for optional content not required at first launch, like tutorial videos, as they allow assets to be downloaded only when needed, significantly reducing the initial app download size. Slicing, in contrast, delivers device-specific variants of required assets and code, not optional content.
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