Auxio/info/ARCHITECTURE.md

Architecture

This document is designed to provide an overview of Auxio's architecture and design decisions. It will be updated as Auxio changes.

Core Facets

Auxio has a couple of core systems or concepts that should be understood when working with the codebase.

Package Structure

Auxio is deliberately structured in a way that I call "Anti-CLEAN Architecture". There is one gradle project, with sub-packages that are strictly feature-oriented. For example, playback code is exclusively in the playback package, and detail code is exclusively in the detail package. Sub-packages can be related to the code it contains, such as detail.recycler for the detail UI adapters, or hey can be related to a sub-feature, like playback.queue for the queue UI.

The outliers here are .ui and .util, which are generic utility or component packages.

Sticking to a single project reduces compile times, makes it easier to add new features, and simply makes Auxio's code easier to reason about.

A full run-down of Auxio's current package structure as of the latest version is shown below.

org.oxycblt.auxio  # Main UIs
├──.detail         # Album/Artist/Genre detail UIs
│  └──.recycler    # RecyclerView components for detail UIs 
├──.home           # Home UI
│  ├──.fastscroll  # Fast scroller UI
│  ├──.list        # Home item lists
│  └──.tabs        # Home tab customization
├──.image          # Image loading components
├──.music          # Music data and loading
│  ├──.backend     # System-side music loading
│  └──.dirs        # Music Folders UI + Systems
├──.playback       # Playback UI + Systems
│  ├──.queue       # Queue UI
│  ├──.replaygain  # ReplayGain System + UIs
│  ├──.state       # Playback state backend
│  └──.system      # System-side playback [Services, ExoPlayer]
├──.search         # Search UI
├──.settings       # Settings UI + Systems
│  └──.ui          # Preference extensions
├──.ui             # Shared views and models
│  └──.accent      # Color Scheme UI + Systems
├──.util           # Shared utilities
└──.widgets        # AppWidgets

Each package is gone over in more detail later on.

UI Structure

Auxio only has one activity, MainActivity. Do not try to add more activities to the codebase. Instead, a new UI should be added as a new Fragment implementation and added to one of the two navigation graphs:

• nav_main: Navigation from MainFragment
• nav_explore: Navigation in MainFragment

Fragments themselves are based off a super class called ViewBindingFragment that takes a view-binding and then leverages it within the fragment lifecycle.

Generally:

• Most variables are kept as member variables, and cleared out when the view is destroyed.
• Observing data is done through the Fragment.launch extension, and always points to another function in order to reduce possible memory leaks.
• When possible (and readable), Fragment implementations inherit any listener interfaces they need, and simply clear them out when done.

findViewById is to only be used when interfacing with non-Auxio views. Otherwise, view-binding should be used in all cases. Code that involves retrieving the binding should be isolated into its own function, with the binding being obtained by calling requireBinding.

At times it may be more appropriate to use a View instead of a full blown fragment. This is okay as long as view-binding is still used.

Auxio uses RecyclerView for all list information. Due to the complexities of Auxio, the way one defines an adapter differs quite heavily from the normal library. Generally, start with MonoAdapter for a list with one type of data and MultiAdapter for lists with many types of data, then follow the documentation to see how to fully implement the class.

Object communication

Auxio's codebase is mostly centered around 4 different types of code that communicates with each-other.

• UIs: Fragments, RecyclerView items, and Activities are part of this class. All of them should have little data logic in them and should primarily focus on displaying information in their UIs.
• ViewModels: These usually contain data and values that a UI can display, along with doing data processing. The data often takes the form of MutableStateFlow or StateFlow, which can be observed.
• Shared Objects: These are the fundamental building blocks of Auxio, and exist at the process level. These are usually retrieved using getInstance or a similar function. Shared Objects should be avoided in UIs, as their volatility can cause problems. Its better to use a ViewModel and their exposed data instead.
• Utilities: These are largely found in the .util package, taking the form of standalone or extension functions that can be used anywhere.

Ideally, UIs should only be talking to ViewModels, ViewModels should only be talking to the Shared Objects, and Shared Objects should only be talking to other shared objects. All objects can use the utility functions where appropriate.

Data objects

Auxio represents data in multiple ways.

Item is the base class for most music and UI data in Auxio, with a single ID field meant to mark it as unique.

It has the following implementations:

• Music is a Item that represents music. It adds a name field that represents the raw name of the music (from MediaStore), and a resolveName method meant to resolve the name in context of the UI.
• MusicParent is a type of Music that contains children.
• Header corresponds to a simple header with a title and no interaction functionality. There are also the detail-specific DiscHeader and SortHeader, however these are largely unrelated to Header.

Other data types represent a specific UI configuration or state:

• Sealed classes like Sort contain an ascending state that can be modified immutably.
• Enums like DisplayMode and RepeatMode only contain static data, such as a string resource.

Things to keep in mind while working with music data:

• id is not derived from the MediaStore ID of the music data. It is actually a hash of the unique fields of the music data. Attempting to use it as a MediaStore ID will result in errors.
• Any field or method beginning with internal is off-limits. These fields are meant for use within MusicLoader and generally provide poor UX to the user. The only reason they are public is to make the loading process not have to rely on separate "Raw" objects.
• rawName is used when doing internal work, such as saving music data or diffing items
• sortName is used in the fast scroller indicators and sorting. Avoid it wherever else.
• resolveName() should be used when displaying any kind of music data to the user.
• For songs, individualArtistRawName and resolveIndividualArtistName should always be used when displaying the artist of a song, as it will always show collaborator information first before defaulting to the album artist.

Music Access

All music on a system is asynchronously loaded into the shared object MusicStore. More specifically, it is accessible within the Library construct. By the nature of music loading, Library may not be available at all times.

• ViewModels should try to await or gracefully exit the called method if Library is not available
• In the case that a ViewModel needs a Library instance to function, it can be asserted with requireNotNull. This should be done sparingly.
• Other shared objects that rely on MusicStore [like PlaybackStateManager] will no-op if music is not available.

If the loading status needs to be shown in a UI, MusicViewModel can be used to observe the current music loader response.

Playback System

Auxio's playback system is somewhat unorthodox, as it avoids much of the android-provided APIs in favor of a more controllable and sensible system. The diagram below highlights the overall structure and connections:

                   ┌──────────────────── PlaybackService ────────────────┐
                   │                             │                       │
PlaybackStateManager [Communicates with]         │                       │
                   │                             │ [Contains]            │ [Communicates with]
                   │                             │                       │
                   │                             ├ WidgetComponent       ┤
                   │                             ├ NotificationComponent ┤
                   │                             ├ MediaSessionComponent ┤
                   │                             └ Player                ┘
                   │
                   │
                   └──────────────────── PlaybackViewModel  ───────────────────── UIs
                                                             [Communicates with]

PlaybackStateManager is the shared object that contains the master copy of the playback state, doing all operations on it. This object should NEVER be used in a UI, as it does not sanitize input and can cause major problems if a Volatile UI interacts with it. It's callback system is also prone to memory leaks if not cleared when done. PlaybackViewModel should be used instead, as it exposes stable data and safe functions that UIs can use to interact with the playback state.

PlaybackService's job is to use the playback state to manage the ExoPlayer instance, the notification, the media session, the widget, and also modify the state depending on system events, such as when a button is pressed on a headset. It should never be bound to, mostly because there is no need given that PlaybackViewModel exposes the same data in a much safer fashion.

Data Integers

Integer representations of data/UI elements are used heavily in Auxio, primarily for efficiency. To prevent any strange bugs, all integer representations must be unique. To see a table of all current integers, see the C class within the project.

Some datatypes [like Tab and Sort] have even more fine-grained integer representations for other data. More information can be found in the documentation for those datatypes.

Package-by-package rundown

org.oxycblt.auxio

This is the root package and contains the application instance and the landing UIs. This should be kept sparse with most other code being placed into a package.

.detail

Contains all the detail UIs for some data types in Auxio. All detail user interfaces share the same base layout (A Single RecyclerView) and only change the adapter/data being used. The adapters display both the header with information and the child items of the item itself, usually with a data list similar to this:

Item being displayed | Header Item | Child Item | Child Item | Child Item...

Each adapter instance also handles the highlighting of the currently playing item in the detail menu.

.home

This package contains the components for the "home" UI in Auxio, or the UI that the user first sees when they open the app.

• The base package contains the top-level components that manage the FloatingActionButton, AppBar, and ViewPager instances.
• The fastscroll package contains the fast scroll component used in each list of music
• The list package contains the individual fragments for each list of music. These are all placed in the top-level ViewPager instance.
• The tabs package contains the data representation of an individual library tab and the UIs for editing them.

.image

Coil is the image loader used by Auxio. This package contains the components Auxio leverages to load images in a stable manner. Usually, you do not need to import this package elsewhere, but there are some important components:

• BitmapProvider, which allows external components (Such as in PlaybackService) to load a Bitmap in a way not prone to race conditions. This should not be used for UIs.
• BaseFetcher, which is effectively Auxio's image loading routine. Most changes to image loading should be done there, and not it's sub-classes like AlbumArtFetcher.

This package also contains the two UI components used for all covers in Auxio:

• StyledImageView, which adds extensions for dynamically loading covers, handles rounded corners, and a stable icon style.
• ImageGroup, an extension of StyledImageView that all of the previous features, alongside a playing indicator and one custom view.

.music

This package contains all Music implementations, the music loading implementation, and the music folder system. This is the second most complicated package in the app, as loading music in a sane way is horribly difficult.

Unlike other apps, Auxio does not load music from MediaStore as it is shown in the UI. That is dumb and stupid and prevents any advanced features like Album Artists. Instead, we have a single loading process that constructs an entire in-memory music library, which does increase memory usage, but allows for very high-quality metadata.

The major classes are:

• MusicStore, which is the container for a Library instance. Any code wanting to access the library should use this.
• Indexer, which manages how music is loaded. This is only used by code that must reflect the music loading state.

Internally, there are several other major systems:

• IndexerService, which does the indexer work in the background.
• Indexer.Backend implementations, which actually talk to the media database and load music. As it stands, there are two classes of backend:
  • Version-specific MediaStoreBackend implementations, which transform the (often insane) music data from Android into a usable Song.
  • ExoPlayerBackend, which mutates audio with extracted ID3v2 and Vorbis tags. This enables some extra features and side-steps unfixable issues with MediaStore
• StorageFramework, which is a group of utilities that allows Auxio to be volume-aware and to work with both extension-based and format-based mime types.

The music loading process is roughly as follows:

1. Something triggers IndexerService to start indexing, either by the UI or by the service itself starting.
2. Indexer picks an appropriate Backend, and begins loading music. Indexer may periodically update it's state during this time with the current progress.
3. In the case that IndexerService is killed, Indexer falls back to a previous state (or null if there isn't one)
4. If the music loading process completes, Indexer will push a Response. IndexerService will read this, and in the case that the new Library differs, it will push it to MusicStore
5. MusicStore updates any Callback instances with the new Library.

.playback

This module not only contains the playback system described above, but also multiple other components:

• queue contains the Queue UI and it's fancy item system.
• replaygain contains the ReplayGain implementation and the UIs related to it. Auxio's ReplayGain implementation is somewhat different compared to other apps, as it leverages ExoPlayer's metadata and audio processing systems to not only parse ReplayGain tags, but also allow volume amplification above 100%.
• state contains the core playback state and persistence system.
• system contains the system-facing playback system, i.e PlaybackService

The base package contains the user-facing UIs representing the playback state, specifically the playback bar and the playback panel that it expands into. Note that while the playback UI does rely on BottomSheetLayout, the layout is designed to be at least somewhat re-usable, so it is in the generic .ui class.

.search

Package for Auxio's search functionality, SearchViewHolder handles the data results and filtering while SearchFragment/SearchAdapter handles the display of the results and user input.

.settings

The settings system is primarily based off of SettingsManager, a wrapper around SharedPreferences. This allows settings to be read/written in a much simpler/safer manner and without a context being needed. The Settings UI is largely contained in SettingsListFragment, which is a standard PreferenceFragment implementation wrapped by the more general SettingsFragment.

Internally, the settings package also leverages a couple custom preference implementations, notably IntListPreference, which enables a normal choice preference to be backed by the integer representations that Auxio uses.

.ui

Shared views and view configuration models. This contains:

• Important Fragment superclasses like ViewBindingFragment and MenuFragment
• Customized views such as EdgeAppBarLayout, and others, which just fix existing shortcomings with the views.
• Configuration models like DisplayMode and Sort, which are used in many places but aren't tied to a specific feature.
• The RecyclerView adapter framework described previously.
• BottomSheetLayout, a highly important layout that underpins Auxio's UI flow.
• Standard ViewHolder implementations that can be used for common datatypes.
• NavigationViewModel, which acts as an interface to control navigation to a particular item and navigation within MainFragment

.util

Shared utilities. This is primarily for QoL when developing Auxio. Documentation is provided on each method.

Utilities are separated into a few groups:

• Context utilities are extensions of Context and generally act as shortcuts for that class.
• Framework utilities extend a variety of view implementations to add new behavior or shortcuts.
• Primitive utilities operate on basic datatypes and are mostly shortcuts.
• Log utilities are a more light-weight logging framework that Auxio leverages instead of bloated and over-engineered libraries like Timber.

.widgets

This package contains Auxio's AppWidget implementation, which deviates from other AppWidget implementations by packing multiple different layouts into a single widget and then switching between them depending on the widget size. Note that since RemoteViews and the AppWidget API in general is incredibly outdated and limited, this package deviates from much of Auxio's normal UI conventions.

The playback service owns WidgetComponent, which listens to PlaybackStateManager for updates. During an update, it reloads all song metadata and playback state into a WidgetState, which is an immutable version of the playback state that negates some of the problems with using a volatile shared object.

WidgetProvider is the widget "implementation" exposed in the manifest. When WidgetComponent updates it, the class will create a series of layouts [e.g "Forms"] for a variety of "size buckets" that would adequately contain the widget. This is then used as the widget views, either with the native responsive behavior on Android 12 and above, or with the responsive behavior back-ported to older devices.