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's package structure is 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 they 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.

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
│  └──.excluded    # Excluded Directories 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
│  └──.pref        # Int preference add-on
├──.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.

• Create variables [Bindings, Adapters, etc]
• Set up the UI
• Set up ViewModel instances and LiveData observers

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 MutableLiveData or LiveData, 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.

DetailViewModel acts as the holder for the currently displaying items, along with having the navToItem LiveData that coordinates menu/playback navigation [Such as when a user presses "Go to artist"]

.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.

.music

This package contains all Music implementations, the music loading implementation, and the excluded directory system.

Key classes in this package include:

• MusicStore, which is the primary access point for music data.
• Indexer, which implements all of the MediaStore hacks to create a good metadata indexer for Auxio.

.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 UIs.
• 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:

• Customized views such as EdgeAppBarLayout, StyledImageView, and others, which provide extra styling and behavior not provided by default.
• Configuration models like DisplayMode and Sort, which are used in many places but aren't tied to a specific feature.
• newMenu and ActionMenu, which automates menu creation for most data types.
• The RecyclerView adapter framework described previously.
• The view-binding super classes described previously.
• BottomSheetLayout, a highly important layout that underpins Auxio's UI flow.
• Standard ViewHolder implementations that can be used for common datatypes.

.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 utilties 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 backported to older devices.