Maemo multimedia applications - Part II
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The article Maemo multimedia applications - Part I, describes how to use two important GStreamer tools (gst-inspect and gst-launch) when developing multimedia applications for the maemo platform. This article describes a simple maemo multimedia application (in Python).
The application is an MP3 player with a GTK+/Hildon interface. The application has 4 buttons (Play, Pause, Stop, and Open).
Contents |
The constructor
#!/usr/bin/python
import pygst
pygst.require("0.10")
import gst
import pygtk
import gtk
import hildon
class Player(hildon.Program):
def __init__(self):
hildon.Program.__init__(self)
self.window = hildon.Window()
self.window.connect("delete_event", gtk.main_quit)
self.window.set_title("Player")
self.add_window(self.window)
self.create_interface()
self.create_pipeline()
self.window.add(self.table)
self.window.show_all()
... [see the next source code] ...
#creates a player
start=Player()
gtk.main()
The code above is divided into two main methods: create_interface(), which creates the GUI elements of the application such as buttons, and the create_pipeline(), which creates the GStreamer elements and pipeline.
Creating GUI elements
The method create_interface() creates a layout (gtk.Table) and inserts 4 buttons. Then, a callback function is connected to each inserted button. For example, the callback function OnPlay() is connected to the button buttonPlay. The callback functions modify the state of pipeline (READY, PLAYING, PAUSED and NULL), according to the action. The callback function OnOpen also creates a Hildon dialog to choose the file media to be played.
#this method creates the GUI elements (layout and buttons)
def create_interface(self):
self.table = gtk.Table(2,2,True)
buttonPlay = gtk.Button("Play")
buttonPause = gtk.Button("Pause")
buttonStop = gtk.Button("Stop")
buttonOpen = gtk.Button("Open")
buttonPlay.connect("clicked",self.OnPlay)
buttonPause.connect("clicked",self.OnPause)
buttonStop.connect("clicked",self.OnStop)
buttonOpen.connect("clicked",self.OnOpen)
self.table.attach(buttonPlay,0,1,0,1)
self.table.attach(buttonPause,1,2,0,1)
self.table.attach(buttonStop,0,1,1,2)
self.table.attach(buttonOpen,1,2,1,2)
#sets GStreamer pipeline state to PLAYING
def OnPlay(self, widget):
self.pipeline.set_state(gst.STATE_PLAYING)
#sets GStreamer pipeline state to PAUSED
def OnPause(self, widget):
self.pipeline.set_state(gst.STATE_PAUSED)
#sets GStreamer pipeline state to NULL
def OnStop(self, widget):
if self.pipeline.get_state() != gst.STATE_PAUSED:
self.pipeline.set_state(gst.STATE_PAUSED)
if self.pipeline.get_state() != gst.STATE_NULL:
self.pipeline.set_state(gst.STATE_NULL)
#opens a dialog to choose the media file to play
def OnOpen(self, widget):
dialog = hildon.FileChooserDialog(self.window,gtk.FILE_CHOOSER_ACTION_OPEN)
response = dialog.run()
dialog.hide()
file_name = selector.get_filename()
if response == gtk.RESPONSE_OK:
self.audiosrc.set_property("location", file_name)
self.pipeline.set_state(gst.STATE_READY)
Creating GStreamer elements and pipeline
It is necessary to create a GStreamer pipeline to insert the elements that are created. Then, we have to create two elements: one from a gnomevfssrc, and the other from dspmp3sink. These elements are inserted on the pipeline so they can be "executed". In order to check if the elements exist on your machine, use the gst-inspect command. This code cannot be executed on the desktop because the element dspmp3sink exists only for the Internet Tablet OS. Therefore, if you try to execute the code, the system throws an error (gst.ElementNotFoundError: dspmp3sink). Finally, link the elements inserted in the pipeline by using the link() method.
#this method creates GStreamer pipeline and elements
def create_pipeline(self):
self.pipeline = gst.Pipeline("mypipeline")
self.audiosrc = gst.element_factory_make("gnomevfssrc","source")
self.pipeline.add(self.audiosrc)
self.audiosink = gst.element_factory_make("dspmp3sink", "sink")
self.pipeline.add(self.audiosink)
self.audiosrc.link(self.audiosink)
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