*(for an intro to GStreamer, see [my previous article](https://archivedblog.jonobacon.com/?p=750))*

You know, one thing that gets people confused in GStreamer is the idea of *Dynamic Pads*. Some elements, most notably the `decodebin` and `gnlcomposition` only have certain pads at certain times. This is because they figure out what kind of content they are processing, and apply the relevant pads where needed. This is often told to new users, but then they wonder how exactly these dynamic pads are handled.

Here is how it works, using `decodebin` as an example:

* When the pipeline is run, the `decodebin` generates a signal that says a new pad has been added. In the case of `decodebin` this is the `new-decoded-pad` signal, in the case of `gnlcomposition` this is a `pad-added` signal.
* You hook this signal up to your own callback, such as `OnDecodedPad()`.
* The signal passes your callback the pad that was added and some other stuff. You then link this pad to the next element along in the pipeline. So if your pipeline is `filesrc ! decodebin ! audioconvert ! alsasink`, the pad passed to your `OnDecodeBin()` method would be linked to the `audioconvert`.

To outline this, I have written a sample script to show how it works:

#!/usr/bin/python

import pygst
pygst.require(“0.10”)
import gst
import pygtk
import gtk

class Main:
def __init__(self):
self.pipeline = gst.Pipeline(“mypipeline”)

self.filesrc = gst.element_factory_make(“filesrc”, “source”)
self.pipeline.add(self.filesrc)
self.filesrc.set_property(“location”, “/home/jono/Desktop/jonobacon-voicesoffreedom.ogg”)

self.decode = gst.element_factory_make(“decodebin”, “decode”)
self.decode.connect(“new-decoded-pad”, self.OnDynamicPad)
self.pipeline.add(self.decode)

self.filesrc.link(self.decode)

self.convert = gst.element_factory_make(“audioconvert”, “convert”)
self.pipeline.add(self.convert)

self.sink = gst.element_factory_make(“alsasink”, “sink”)
self.pipeline.add(self.sink)

self.convert.link(self.sink)

self.pipeline.set_state(gst.STATE_PLAYING)

def OnDynamicPad(self, dbin, pad, islast):
print “OnDynamicPad Called!”
pad.link(self.convert.get_pad(“sink”))

start=Main()
gtk.main()

You can grab the script from [here](https://archivedblog.jonobacon.com/files/decodebin.py). Hope this helps some of you. 🙂