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'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.

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

org.oxycblt.auxio  # Main UIs
├──.accent         # Color Scheme UI + Systems
├──.coil           # Image loading components
├──.detail         # Album/Artist/Genre detail UIs
│  └──.recycler    # RecyclerView components for detail UIs 
├──.excluded       # Excluded Directories UI + Systems
├──.home           # Home UI
│  ├──.fastscroll  # Fast scroller UI
│  ├──.list        # Home item lists
│  └──.tabs        # Home tab customization
├──.music          # Music data and loading
├──.playback       # Playback UI + Systems
│  ├──.queue       # Queue UI
│  ├──.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
├──.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 organized in order of lifecycle. So the first override would be onCreate, followed by onCreateView, and so on. onCreateView is where all view instantiation and configuration takes place, and is separated into three phases:

• 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, viewbinding should be used in all cases. If one needs to keep track of a viewbinding outside of onCreateView, then one can declare a binding by memberBinding(BindingClass::inflate) in order to have a binding that properly disposes itself on lifecycle events.

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

When creating a ViewHolder for a RecyclerView, one should use BaseViewHolder to standardize the binding process and automate some code shared across all ViewHolders. The only exceptions to this case are for ViewHolders that correspond to non-BaseModel data, in which a normal ViewHolder can be used instead.

Data is often bound using Binding Adapters, which are XML attributes assigned in layout files that can automatically display data, usually written as app:bindingAdapterName="@{data}". Its recommended to use these instead of applying the attributes directly unless absolutely necessary. Usually it's okay to apply data programmatically if you begin to duplicate layouts simply because they apply to different data objects, such as the detail UIs.

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 and .coil packages, 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.

BaseModel 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 BaseModel that represents music. It adds a name field that represents the raw name of the music (from MediaStore).
• MusicParent is a type of Music that contains children. It adds a resolveName field that converts the raw MediaStore name to a name that can be used in UIs.
• Header and ActionHeader are UI data objects that represent a header item. Header corresponds to a simple header with no action, while ActionHeader corresponds to an action with a dedicated icon, such as with sorting.

Other data types represent a specific UI configuration or state:

• Sealed classes like Sort contain data with them that can be modified.
• Enums like DisplayMode and LoopMode 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.
• Generally, name is used when saving music data to storage, while resolvedName is used when displaying music data to the user.
  • For Song instances in particular, prefer resolvedAlbumName and resolvedArtistName over album.resolvedName and album.artist.resolvedName
  • For Album instances in particular, prefer resolvedArtistName over artist.resolvedName

Music Access

All music on a system is asynchronously loaded into the shared object MusicStore. Because of this, MusicStore may not be available at all times.

• ViewModels should try to await or gracefully exit the called method if MusicStore is not available
• In the case that a ViewModel needs a MusicStore instance to function, an instance can be required. 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:

                                                                           [Requests update from]
                                                 ┌─────────────────────────────────────────────────────────────────────────────────┐
                                                 │                                                                                 │
                   ┌──────────────────── PlaybackService ──────────────────── WidgetController ──────────────────── WidgetProvider ┘
                   │                             │             [Contains]                           [Controls]
PlaybackStateManager [Communicates with]         │
                   │                             │ [Contains]
                   │                             │
                   │                             ├ Notification
                   │                             ├ MediaSession
                   │                             └ 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 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. A table of all current integers used are shown below:

0xA0XX | UI Integer Space [Required by android]

0xA000 | SongViewHolder
0xA001 | AlbumViewHolder
0xA002 | ArtistViewHolder
0xA003 | GenreViewHolder
0xA004 | HeaderViewHolder
0xA005 | ActionHeaderViewHolder

0xA006 | AlbumDetailViewHolder
0xA007 | AlbumSongViewHolder
0xA008 | ArtistDetailViewHolder
0xA009 | ArtistAlbumViewHolder
0xA00A | ArtistSongViewHolder
0xA00B | GenreDetailViewHolder
0xA00C | GenreSongViewHolder

0xA00D | QueueSongViewHolder

0xA0A0 | Auxio notification code
0xA0C0 | Auxio request code

0xA1XX | Data Integer Space [Stored for IO efficency]

0xA100 | LoopMode.NONE
0xA101 | LoopMode.ALL
0xA102 | LoopMode.TRACK

0xA103 | PlaybackMode.IN_GENRE
0xA104 | PlaybackMode.IN_ARTIST
0xA105 | PlaybackMode.IN_ALBUM
0xA106 | PlaybackMode.ALL_SONGS

0xA107 | Null DisplayMode [Filter Nothing]
0xA108 | DisplayMode.SHOW_GENRES
0xA109 | DisplayMode.SHOW_ARTISTS
0xA10A | DisplayMode.SHOW_ALBUMS
0xA10B | DisplayMode.SHOW_SONGS

0xA10C | Sort.Name
0xA10D | Sort.Artist
0xA10E | Sort.Album
0xA10F | Sort.Year

0xA110 | ReplayGainMode.OFF
0xA111 | ReplayGainMode.TRACK
0xA112 | ReplayGainMode.ALBUM
0xA113 | ReplayGainMode.DYNAMIC

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.

.accent

This package is responsible for Auxio's color schemes, internally known as accents due to legacy code. It contains an object that represents the attributes of an accent, but this should be avoided in favor of resolving color attributes directly, such as colorPrimary. This package also contains the UIs for picking an accent.

.coil

Coil is the image loader used by Auxio. All image loading is done through these four functions/binding adapters:

• app:albumArt: Binding Adapter that will load the cover art for a song or album
• app:artistImage: Binding Adapter that will load the artist image
• app:genreImage: Binding Adapter that will load the genre image
• loadBitmap: Function that will take a song and return a bitmap, this should not be used in anything UI related, that is what the binding adapters above are for.

This should be enough to cover most use cases in Auxio.

Internally, multiple fetchers are provided to transform Music instances into images. All of these fetchers inherit AuxioFetcher, which implements the necessary methods for loading album artwork and creating the mosaics shown in artist/genre images.

.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"]

.excluded

This package is responsible for the excluded directory system. It contains the database of excluded directories and the dialog that appears when editing them.

Note: Certain naming in this package might not line up with the current name of the package. This is because updating those names would break compatibility with previous versions of Auxio.

.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.

.music

This package contains all BaseModel implementations and the music loading implementation. This also includes Header/ActionHeader, as those data objects have to inherit BaseModel so that they can be placed alongside Music instances in RecyclerView instances.

.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 transitions
• state contains the core playback state and persistence system
• system contains the system-facing playback system

The most important part of this module is PlaybackLayout, which is a custom ViewGroup that implements the playback bar and it's ability to slide up into the full playback view. MainFragment controls this ViewGroup.

.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, while the pref sub-package contains IntListPreference, which allows Auxio's integer representations to be used with the preference UI. The about dialog also resides in this package.

.ui

Shared views and view configuration models. This contains:

• Customized views such as EdgeAppBarLayout and EdgeRecyclerView, which add some extra functionality 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
• memberBinding and MemberBinder, which allows for ViewBindings to be used as a member variable without memory leaks or nullability issues.

.util

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

.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.

When WidgetProvider creates a layout, it first turns the PlaybackStateManager instance into a WidgetState, which is an immutable version of the playback state that negates some of the problems with using a shared object here. It then picks a layout [e.g "Form"] depending on its current dimensions and applies the WidgetState object to that.

Note: The AppWidget implementation violates UI conventions by directly interfacing with coil and PlaybackStateManager. This is required due to RemoteView limitations.