AssetDropKit
Built for Vocabloot, the photo vocabulary app for iOS and Android. Part of the Vocabloot open-source libraries.
Kotlin Multiplatform asset packs, downloaded on first use from the app stores themselves: Google Play Asset Delivery on Android, Apple-hosted Background Assets on iOS 26 and later, On-Demand Resources on iOS 17 and 18. The stores host the bytes. No server, no hosting bill.
Why
Games make an app big, and most players never open most of them. Shipping every game's art in the install costs downloads; hosting it yourself costs egress on every new device. Both stores already deliver extra content after install, for free, but each with its own API, states and packaging, and Apple has two systems depending on the iOS version. AssetDropKit gives Kotlin one API over all three, one pack layout, and one integrity check.
Features
- Store-hosted, zero cost. Play hosts on-demand asset packs; the App Store hosts Apple-hosted packs and On-Demand Resources tags. Nothing runs on your servers.
- One API, three systems.
AssetPacks.fetch, state, readBytes, remove, with the same PackState names on every store.
- Picks Apple's system by OS. Apple-hosted Background Assets on iOS 26 and later, On-Demand Resources on 17 and 18, from one Swift package.
- Never loads a half pack. Each pack is checked against its
assetdropkit.json (SHA-256 per file) once per version; a mismatch is removed, fetched once more, then reported as Failed(Corrupt). state() says Ready only after the check.
- Shared and cancellable. Two screens fetching one pack make one store request; cancelling the last caller cancels the download.
- Survives a relaunch. The first state comes from the store, so a download that was running shows as running.
- Offline-aware.
stallTimeoutMillis turns a download that stops moving into Failed(NoNetwork).
- No Objective-C. Every Apple call is Swift; Kotlin talks to it through a short adapter in your app.
AssetDropKit 101
// libs.versions.toml assetdropkit = { module = "com.vocabloot:assetdropkit", version = "0.1.3" }
// build.gradle.kts (shared) commonMain.dependencies { api(libs.assetdropkit) }
Package each pack once (packaging), then:
// Android (androidMain) // iOS (iosMain); store = the app's AssetDropAdapter
val source = PackSources.default(context, BuildConfig.DEBUG) val source = AppleStoreSource(store = store)
// common
val packs = AssetPacks(source = source, stallTimeoutMillis = 20_000)
when (val state = packs.fetch("postdelivery")) {
PackState.Ready -> {
val glb = packs.readBytes("postdelivery", "town/sedan.glb")
}
is PackState.Failed -> showFallback(state.reason) // NoNetwork, InsufficientStorage, Corrupt, ...
else -> askTheUser() // WaitingForWifi, NeedsPlayConfirmation (Android)
}
A more advanced example
Show progress while the pack arrives, and switch to the downloaded art only once it is verified:
@Composable
fun GameIntro(packs: AssetPacks) {
val state by remember { packs.state("postdelivery") }.collectAsState(initial = null)
LaunchedEffect(Unit) { packs.fetch("postdelivery") }
when (val s = state) {
is PackState.Downloading -> ProgressLine(s.bytes, s.totalBytes)
PackState.Ready -> PlayWithPackArt() // Ready means checked against the manifest
is PackState.Failed -> PlayWithBuiltInArt() // and fetch again next time
else -> Unit
}
}
On Android, register Play's prompts in onCreate (packs.registerPlayPrompts(this)) and call show() on WaitingForWifi or NeedsPlayConfirmation. On iOS, copy templates/ios/AssetDropAdapter.swift into the app. More in recipes.
This is the beginning of the README. Read the rest at the original. Read the full README ↗
