# 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](https://github.com/coil-kt/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.