Shuttle

Get To It Quick

• API Documentation
• Why Shuttle?
• How It Works
• Requirements
• Quick Start
• Usage
  • Transport with Intents
  • Transport with the Navigation Component
  • Pick Up Cargo at the Destination
  • Cargo States (LCE Pattern)
  • Using @ShuttleCargo
  • Cleaning Up
• Architecture
  • Context Diagram
  • Module Overview
  • Module Dependency Diagram
• Demo Apps
• Heads Up: Know the Tradeoffs
• Contributing
• License

🚨 Why Shuttle?

🗞️ Featured in Android Weekly #594 and Android Weekly #455, validated by the Android community twice.

You've seen this before. Maybe last week:

android.os.TransactionTooLargeException: data parcel size X bytes

It didn't show up in dev. It didn't show up in QA. It showed up at 2am, in production, for real users. Your Play Store rating took the hit before anyone on the team even knew.

So you triaged it. Filed the ticket. Wrote the fix. Reviewed the PR. Ran QA again. Cut the hotfix. And then you added it to the code review checklist, hoping the next engineer would catch it before it happened again.

They won't. Not reliably. You can't review your way out of a structural problem.

Shuttle provides a modern, guarded way to pass large Serializable objects with Intent objects or save them in Bundle objects to avoid app crashes. The crash class is structurally prevented, not governed against.

Why keep spending more time and money on governance through code reviews? Why not embrace the problem by providing a solution for it?

Shuttle reduces the high level of governance needed to catch TransactionTooLargeException inducing code by:

1. storing the Serializable and passing an identifier for the Serializable
2. using a small-sized Bundle for binder transactions
3. avoiding app crashes from TransactionTooLargeExceptions
4. enabling retrieval of the stored Serializable at the destination

Shuttle also excels by:

1. providing a solution with maven artifacts
2. providing Solution Building Blocks (SBBs) for building on
3. saving time by avoiding DB and table setup, especially when creating many tables for the content of different types of objects

When envisioning, designing, and creating the architecture, quality attributes and best practices were kept in mind. These attributes include usability, readability, recognizability, reusability, maintainability, and more.

⚙️ How It Works

The Shuttle framework takes its name from cargo transportation in the freight industry. Moving and storage companies experience scenarios where large moving trucks cannot transport cargo the entire way to the destination (warehouses, houses, et cetera). These scenarios might occur from road restrictions, trucks being overweight from large cargo, and more. As a result, companies use small Shuttle vans to transport smaller cargo groups on multiple trips to deliver the entire shipment.

After the delivery is complete, employees remove the cargo remnants from the shuttle vans and trucks. This clean-up task is one of the last steps for the job.

The Shuttle framework takes its roots in these scenarios:

• creating a smaller cargo bundle object to use in successfully delivering the data to the destination
• shuttling the corresponding large cargo to a warehouse and storing it for pickup
• linking the smaller cargo with the larger cargo by an identifier
• providing a single source of truth (Shuttle interface) to use for transporting cargo
• providing convenience functions to remove cargo (automatically or on-demand)

Shuttle applies this same logic to Android's binder transaction limit:

┌─────────────────────────────────────────────────────────────┐
│  Source Component                                           │
│  1. Large Serializable -> stored in Warehouse (Room/DB)     │
│  2. Small cargo ID     -> passed in Intent/Bundle           │
└──────────────────────────────┬──────────────────────────────┘
                               │ (tiny binder transaction)
┌──────────────────────────────▼──────────────────────────────┐
│  Destination Component                                      │
│  3. Cargo ID received -> retrieved from Warehouse           │
│  4. Large Serializable -> delivered via Kotlin Channel      │
│  5. Cleanup -> cargo removed from Warehouse automatically   │
└─────────────────────────────────────────────────────────────┘

Requirements

🚀 Quick Start

1. Add Dependencies

settings.gradle.kts — apply the Shuttle Gradle plugin so the @ShuttleCargo annotation processor is wired into your build:

@ShuttleCargo annotates the cargo class you want to transport between screens. At build time, the annotation processor generates the serialization code Shuttle needs to store and retrieve cargo, without any manual Serializable boilerplate.

pluginManagement {
    plugins {
        id("com.grarcht.shuttle.cargo") version "4.0.0"
    }
}

The Shuttle Cargo Gradle plugin configures KSP and registers the Shuttle compiler plugin automatically. Without it, @ShuttleCargo-annotated classes will not generate the required serialization glue code.

build.gradle.kts:

plugins {
    id("com.grarcht.shuttle.cargo")
}

dependencies {
    implementation(platform("com.grarcht.shuttle:shuttle-bom:4.0.0"))
    implementation("com.grarcht.shuttle:framework")
    implementation("com.grarcht.shuttle:framework-integrations-persistence")
    implementation("com.grarcht.shuttle:framework-integrations-extensions-room")
    implementation("com.grarcht.shuttle:framework-addons-navigation-component") // Optional

    // Annotation-based API — use @ShuttleCargo to own the serialization contract
    implementation("com.grarcht.shuttle:framework-annotations")
    ksp("com.grarcht.shuttle:framework-annotations-processor")
}

Version Catalog (libs.versions.toml):

[versions]
shuttle = "4.0.0"

[plugins]
shuttle-cargo = { id = "com.grarcht.shuttle.cargo", version.ref = "shuttle" }

[libraries]
shuttle-bom = { group = "com.grarcht.shuttle", name = "shuttle-bom", version.ref = "shuttle" }
shuttle-framework = { group = "com.grarcht.shuttle", name = "framework" }
shuttle-persistence = { group = "com.grarcht.shuttle", name = "framework-integrations-persistence" }
shuttle-room = { group = "com.grarcht.shuttle", name = "framework-integrations-extensions-room" }
shuttle-navigation = { group = "com.grarcht.shuttle", name = "framework-addons-navigation-component" }
shuttle-annotations = { group = "com.grarcht.shuttle", name = "framework-annotations" }
shuttle-annotations-processor = { group = "com.grarcht.shuttle", name = "framework-annotations-processor" }

2. Initialize Shuttle

Wire CargoShuttle once as a singleton. With Hilt:

@Provides @Singleton
fun provideShuttle(facade: ShuttleFacade, warehouse: ShuttleWarehouse): Shuttle =
    CargoShuttle(facade, warehouse)

@Provides @Singleton
fun provideShuttleFacade(
    @ApplicationContext context: Context,
    warehouse: ShuttleWarehouse
): ShuttleFacade = ShuttleCargoFacade(context as Application, warehouse)

@Provides @Singleton
fun provideShuttleWarehouse(
    dao: ShuttleDataAccessObject,
    factory: ShuttleDataModelFactory,
    @ApplicationContext context: Context,
    gateway: ShuttleFileSystemGateway
): ShuttleWarehouse = ShuttleRepository(dao, factory, context.filesDir.absolutePath, gateway)

The ShuttleDataAccessObject comes from ShuttleRoomDataDb.getInstance(ShuttleRoomDbConfig(context)).shuttleDataAccessObject. Inject Shuttle wherever you need to transport data.

3. Ship Your First Cargo

// Source: transport a large Serializable via Intent
shuttle.intentCargoWith(context, DestinationActivity::class.java)
    .transport(cargoId, myLargeSerializable)
    .cleanShuttleOnReturnTo(SourceFragment::class.java, DestinationActivity::class.java, cargoId)
    .deliver(context)

// Destination: pick up the cargo
lifecycleScope.launch {
    getShuttleChannel()
        .consumeAsFlow()
        .collectLatest { result ->
            when (result) {
                is ShuttlePickupCargoResult.Success<*> -> render(result.data as MyModel)
                is ShuttlePickupCargoResult.Error<*>   -> showError()
                ShuttlePickupCargoResult.Loading        -> showLoading()
            }
        }
}

That's it. No custom DB setup. No table management. No crash.

📦 Usage

The recommended entry point is the Shuttle interface with CargoShuttle as the implementation. It's a single source of truth for all cargo transport operations.

Transport with Intents

Source component:

val cargoId = ImageMessageType.ImageData.value

shuttle.intentCargoWith(context, MVCSecondControllerActivity::class.java)
    .transport(cargoId, imageModel)
    .cleanShuttleOnReturnTo(
        MVCFirstControllerFragment::class.java,
        MVCSecondControllerActivity::class.java,
        cargoId
    )
    .deliver(context)

ℹ️ cleanShuttleOnReturnTo is important. It ensures cargo is purged from the Warehouse when it's no longer needed.

Transport with the Navigation Component

Source fragment:

val cargoId = ImageMessageType.ImageData.value

navController.navigateWithShuttle(shuttle, R.id.MVVMNavSecondViewActivity)
    ?.logTag(LOG_TAG)
    ?.transport(cargoId, imageModel as Serializable)
    ?.cleanShuttleOnReturnTo(
        MVVMNavFirstViewFragment::class.java,
        MVVMNavSecondViewActivity::class.java,
        cargoId
    )
    ?.deliver()

Pick Up Cargo at the Destination

In a Fragment/Activity:

lifecycleScope.launch {
    getShuttleChannel()
        .consumeAsFlow()
        .collectLatest { result ->
            when (result) {
                ShuttlePickupCargoResult.Loading        -> initLoadingView(view)
                is ShuttlePickupCargoResult.Success<*>  -> { showSuccessView(view, result.data as ImageModel); cancel() }
                is ShuttlePickupCargoResult.Error<*>    -> { showErrorView(view); cancel() }
            }
        }
}

In a ViewModel:

viewModelScope.launch {
    shuttle.pickupCargo<Serializable>(cargoId = cargoId)
        .consumeAsFlow()
        .collectLatest { result ->
            pickupCargoMutableStateFlow.value = result
            when (result) {
                is ShuttlePickupCargoResult.Success<*>,
                is ShuttlePickupCargoResult.Error<*> -> cancel()
                else -> { /* await */ }
            }
        }
}

Cargo States (LCE Pattern)

Shuttle returns sealed class results that promote the Loading-Content-Error (LCE) pattern, giving consumers full control over UI state, analytics, and error handling.

Using @ShuttleCargo

Annotate any data class you want to transport through Shuttle. The annotation processor generates the serialization code at build time so you never write it by hand:

@ShuttleCargo
data class ImageModel(
    val id: String,
    val title: String,
    val byteArray: ByteArray
)

That's the entire declaration. No Serializable implementation, no custom read/write methods. Then transport it the same way as any other object:

shuttle.intentCargoWith(context, DestinationActivity::class.java)
    .transport(cargoId, imageModel)
    .deliver(context)

Cleaning Up

Cargo is automatically removed when using cleanShuttleOnReturnTo. For manual control:

// Remove a specific cargo item
shuttle.removeCargoBy(cargoId)

// Remove all cargo
shuttle.removeAllCargo()

🏗️ Architecture

Shuttle is a layered Solution Building Block (SBB) framework. Each layer has a well-defined responsibility and no layer forces technology choices on consumers.

Module Overview

Context Diagram

Shuttle Context Diagram

Module Dependency Diagram

graph TD
    A[Your Application]
    A --> B[framework / core]
    A --> C[framework-addons-navigation-component]
    A --> D[your custom integration]
    B --> E[framework-integrations-persistence / abstraction layer]
    E --> F[framework-integrations-extensions-room / default implementation]

Why this layering matters: The persistence abstraction means you can swap Room for any other storage implementation without touching the framework or your application code. Bring your own persistence layer by implementing the integration interfaces.

Shuttle avoids bundling large reactive libraries. Asynchronous communication runs on Kotlin Coroutines and Channels only, which keeps the transitive dependency footprint lean.

🎬 Demo Apps

The demo apps show both the crash scenario and the Shuttle solution side-by-side, using image data transport. Image data is one of the most common real-world contributors to TransactionTooLargeException.

Two architecture patterns are covered:

MVVM: Activities/Fragments as View, ViewModel as state owner and liaison, Kotlin Channels for async notification.

Flow 1: Navigate with Shuttle ✅

Tap "Navigate using Shuttle" -> image loads successfully via warehouse pickup.

Main Menu	Cargo Unloaded
	image

Flow 2: Navigate Normally ❌

Tap "Navigate Normally" -> app crashes with TransactionTooLargeException.

Main Menu	After Crash

ℹ️ For image loading in production, use Glide or Coil. The demo uses raw image data intentionally to trigger the crash condition.

⚠️ Heads Up: Know the Tradeoffs

Other Parcelable objects in the same Intent can still crash your app. Shuttle protects the Serializable payload. It doesn't protect unrelated Parcelable data you're also passing.

Serializable is slower than Parcelable. Parcelable is optimized for IPC and faster to load, but it's unsafe for disk storage. Google recommends serialization for persistence, which is why Shuttle uses it. The LCE state pattern (loading state) gives your UI the hook it needs to handle the slightly longer load time gracefully.

These are documented tradeoffs, not bugs. Architecture is always about weighing options. This one is worth it.

🤝 Contributing

Pull requests are welcome. Check the Contributing Guide and Code of Conduct before opening one.

1. Fork the repo
2. Create a feature branch: git checkout -b feature/my-feature
3. Commit your changes: git commit -m 'Add: my feature'
4. Push the branch: git push origin feature/my-feature
5. Open a Pull Request

For bugs and feature requests, open an issue.

📄 License

MIT. See LICENSE.md for full terms.

Copyright © 2023 Craft & Graft LLC · GRARCHT ™ 2021