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Development notes

In here are a few notes about how the code is organized, used concepts, etc.

The main code is all pure Python. It is highly modular. The main playing engine is implemented in C/C++ as a Python module (ffmpeg.c and related). It uses FFmpeg for decoding and PortAudio for output.

A basic principle is to keep the code as simple as possible so that it works. I really want to avoid to overcomplicate things.

The main entry point is main. It initializes all the modules. The list of modules is defined in State.modules. It contains for example queue, tracker, mediakeys, gui, etc.

Module

A module is controlled by the utils.Module class. It refers to a Python module (for example queue).

When you start a module (Module.start), it starts a new thread and executes the <modulename>Main function.

A module is supposed to be reloadable. There is the function Module.reload and State.reloadModules is supposed to reload all modules. This is mostly only used for/while debugging, though and is probably not stable and not well tested.

Multithreading and multiprocessing

The whole code makes heavy use of multithreading and multiprocessing. Every module already runs in its own thread. But some modules itself spawn also other threads. The GUI module spawns a new thread for most actions. Heavy calculations should be done in a seperate process so that the GUI and the playing engine (which run both in the main process) are always responsive. There is utils.AsyncTask and utils.asyncCall for an easy and stable way to do something in a seperate process.

Playing engine

This is all the Python native-C/C++ module.

The player module creates the player object as State.state.player. It setups the queue as queue.queue. State.state provides also some functions to control the player state (playPause, nextSong).

GUI

The basic idea is that Python objects are directly represented in the GUI. The main window corresponds to the State.state object. Attributes of an object which should be shown in the GUI are marked via the utils.UserAttrib decorator. There, you can specify some further information to specify more concretely how an attribute should be displayed.

The GUI has its own module gui. At the moment, only an OSX Cocoa interface (guiCocoa) is implemented but a PyQt implementation is planned. There is some special handling for this module as it needs to be run in the main thread in most cases. See main for further reference.

Database

This is the module songdb.

The database is intended to be an optional system which stores some extra data/statistics about a song and also caches some data which is heavy to calculate (e.g. the fingerprint).

It provides several ways to identify a song:

  • By the SHA1 of its path name (relative to the users home dir).
  • By the SHA1 of its file.
  • By the SHA1 of its AcoustId fingerprint.

This is so that the database stays robust in case the user moves a song file around or changes its metadata.

It uses SQLite as its backend. (As it is used mostly as a key/value store with optional external indexing, a complex SQL-like DB is not strictly needed. Earlier, I tried other DBs. For a history, see the comment in the source.)

It uses binstruct for the serialization.

Song attribute knowledge system

Some of the initial ideas are presented in attribs.txt. This is implemented now mostly for the Song class.

There are several sources where we can get some song attribute from:

  • The local song.__dict__.
  • The database.
  • The file metadata (e.g. artist, title, duration).
  • Calculate it from the file (e.g. duration, fingerprint, ReplayGain).
  • Look it up from some Internet service like MusicBrainz.

To have a generic attribute read interface which captures all different cases, there is the function:

Song.get(self, attrib, timeout, accuracy)

For each attrib, there might be functions:

  • Song._estimate_<attrib>, which is supposed to be fast. This is called no matter what the timeout is, in case we did not get it from the database.
  • Song._calc_<attrib>, which is supposed to return the exact value but is heavy to call. If this is needed, it will be executed in a seperate process.

See Song for further reference.

Playlist queue

The playlist queue is managed by the queue module. It has the logic to autofill the queue if there are too less songs in it. The algorithm to automatically select a new song uses the random file queue generator. This is a lazy directory unfolder and random picker, implemented in RandomFileQueue. Every time, it looks at a few songs and selects some song based on

  • the song rating,
  • the current recently played context (mostly the song genre / tag map).

Debugging

The module stdinconsole, when started with --shell, provides a handy IPython shell to the running application (in addition to the GUI which is still loaded). This is quite helpful to play around. In addition, as said earlier, all the modules are reloadable. I made this so I don't need to interrupt my music playing when playing with the code.

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