<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> <HTML ><HEAD ><TITLE >Upstream events</TITLE ><META NAME="GENERATOR" CONTENT="Modular DocBook HTML Stylesheet Version 1.7"><LINK REL="HOME" TITLE="GStreamer Plugin Writer's Guide" HREF="index.html"><LINK REL="UP" TITLE="Events: Seeking, Navigation and More" HREF="chapter-advanced-events.html"><LINK REL="PREVIOUS" TITLE="Events: Seeking, Navigation and More" HREF="chapter-advanced-events.html"><LINK REL="NEXT" TITLE="All Events Together" HREF="section-events-definitions.html"></HEAD ><BODY CLASS="sect1" BGCOLOR="#FFFFFF" TEXT="#000000" LINK="#0000FF" VLINK="#840084" ALINK="#0000FF" ><DIV CLASS="NAVHEADER" ><TABLE SUMMARY="Header navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TH COLSPAN="3" ALIGN="center" ><SPAN CLASS="application" >GStreamer</SPAN > Plugin Writer's Guide</TH ></TR ><TR ><TD WIDTH="10%" ALIGN="left" VALIGN="bottom" ><A HREF="chapter-advanced-events.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="80%" ALIGN="center" VALIGN="bottom" >Chapter 20. Events: Seeking, Navigation and More</TD ><TD WIDTH="10%" ALIGN="right" VALIGN="bottom" ><A HREF="section-events-definitions.html" ACCESSKEY="N" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><DIV CLASS="sect1" ><H1 CLASS="sect1" ><A NAME="section-events-upstream" >20.2. Upstream events</A ></H1 ><P > Upstream events are generated by an element somewhere in the pipeline and sent using the <CODE CLASS="function" >gst_pad_send_event</CODE > function. This function simply realizes the pad and call the default event handler of that pad. The default event handler of pads is <CODE CLASS="function" >gst_pad_event_default</CODE > , it basically sends the event to the peer pad. So upstream events always arrive on the src pad of your element and are handled by the default event handler except if you override that handler to handle it yourself. There are some specific cases where you have to do that : </P ><P ></P ><UL ><LI STYLE="list-style-type: opencircle" ><P > If you have multiple sink pads in your element. In that case you will have to decide which one of the sink pads you will send the event to. </P ></LI ><LI STYLE="list-style-type: opencircle" ><P > If you need to handle that event locally. For example a navigation event that you will want to convert before sending it upstream. </P ></LI ></UL ><P > The processing you will do in that event handler does not really matter but there are important rules you have to absolutely respect because one broken element event handler is breaking the whole pipeline event handling. Here they are : </P ><P ></P ><UL ><LI STYLE="list-style-type: opencircle" ><P > Always forward events you won't handle upstream using the default <CODE CLASS="function" >gst_pad_event_default</CODE > method. </P ></LI ><LI STYLE="list-style-type: opencircle" ><P > If you are generating some new event based on the one you received don't forget to gst_event_unref the event you received. </P ></LI ><LI STYLE="list-style-type: opencircle" ><P > Event handler function are supposed to return TRUE or FALSE indicating if the event has been handled or not. Never simply return TRUE/FALSE in that handler except if you really know that you have handled that event. </P ></LI ></UL ><P > Here is an example of correct upstream event handling for a plugin that wants to modify navigation events. </P ><PRE CLASS="programlisting" > static gboolean gst_my_filter_handle_src_event (GstPad *pad, GstEvent *event) { GstMyFilter *filter = GST_MY_FILTER (gst_pad_get_parent (pad)); switch (GST_EVENT_TYPE (event)) { case GST_EVENT_NAVIGATION: GstEvent *new_event = gst_event_new (GST_EVENT_NAVIGATION);; /* Create a new event based on received one and then send it */ ... gst_event_unref (event); return gst_pad_event_default (pad, new_event); default: /* Falling back to default event handling for that pad */ return gst_pad_event_default (pad, event); } } </PRE ></DIV ><DIV CLASS="NAVFOOTER" ><HR ALIGN="LEFT" WIDTH="100%"><TABLE SUMMARY="Footer navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" ><A HREF="chapter-advanced-events.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="index.html" ACCESSKEY="H" >Home</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" ><A HREF="section-events-definitions.html" ACCESSKEY="N" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >Events: Seeking, Navigation and More</TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="chapter-advanced-events.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" >All Events Together</TD ></TR ></TABLE ></DIV ></BODY ></HTML >