Heron

Heron is a Jetpack Compose adaptive, reactive and offline-first bluesky client.

Screenshots

UI/UX and App Design

Heron uses Material design and motion and is heavily inspired by the Crane material study.

Libraries:

1. Geometric shapes are created with the Jetpack shapes graphics library.
2. UX patterns like pinch to zoom, drag to dismiss, collapsing headers, multipane layouts, and and so on are implemented with the Composables library.

Architecture

This is a multi-module Kotlin Multiplatform project targeting Android, iOS and Desktop that follows the Android architecture guide.

For more details about this kind of architecture, take a look at the Now in Android sample repository, this app follows the same architecture principles it does, and the architecture decisions are very similar.

There are 6 kinds of modules:

1. data-* is the data layer of the app containing models data and repository implementations for reading and writing that data. Data reads should never error, while writes are queued with a WriteQueue.
   • Jetpack Room is used for persisting data with SQLite.
     • Given the highly relational nature of the app, a class called a MultipleEntitySaver is used to save bluesky network models.
   • Jetpack DataStore is used for blob storage of arbitrary data with protobufs.
   • Ktor is used for network connections via the Ozone at-proto bindings.
2. domain-* is the domain layer where aggregated business logic lives. This is mostly domain-timeline where higher level abstractions timeline data manipulation exists.
3. feature-* contains navigation destinations or screens in the app. Multiple features can run side by side in app panes depending on the navigation configuration and device screen size.
4. ui-* contains standalone and reusable UI components and Jetpack Compose effects for the app ranging from basic layout to multimedia components for displaying photos and video playback.
5. scaffold contains the application state in the AppState class, and coordinates app level UI logic like pane display, drag to dismiss, back previews and so on. It is the entry point to the multiplatform application
6. /composeApp is app module that is the fully assembled app and depends on all other modules. It offers the entry point to the application for a platform. It contains several subfolders:
   • commonMain is for code that’s common for all targets.
   • Other folders are for Kotlin code that will be compiled for only the platform indicated in the folder name.
   • /iosApp is the entry point for the iOS app.

Dependency Injection

Dependency injection is implemented with the Kotlin Inject library which constructs the dependency graph at build time and therefore is compile time safe. Assisted injection is used for feature screens to pass navigation arguments information to the feature.

Navigation

Navigation uses the treenav experiment to implement Android adaptive navigation. Specifically it uses a ThreePane configuration, where up to 3 navigation panes may be shown, with one reserved for back previews and another for modals. Navigation state is also saved to disk and persisted across app restarts.

State production

• State production follows the Android guide to UI State Production.
• Each feature uses a single Jetpack ViewModel as the business logic state holder.
• State is produced in a lifecycle aware way using the Jetpack Lifecyle APIs.
  • The CoroutineScope for each ViewModel is obtained from the composition's LocalLifecycleOwner
• The specifics of producing state over time is implemented with the Mutator library.
  • Inputs to the state production pipeline are passed to the mutator in the inputs argument, or derived from an action in actionTransform.
  • Every coroutine launched is limited to running when the lifecycle of the component displaying it is resumed. When the lifecyle is paused, the coroutines are cancelled after 2 seconds: SharingStarted.WhileSubscribed(FeatureWhileSubscribed).
  • Each user Action is in a sealed hierarchy, and action parallelism is defined by the Action.key. Actions with different keys run in parallel while those in the same key are processed sequentially. Each distinct subtype of an Action hierarchy typically has it's own key unless sequential processing is required for example:
    • All subtypes of Action.Navigation typically share the same key.
    • All subtypes of pagination actions, also share the same key and are processed with the Tiling library.