# Why is NetStream.onMetaData a 'callback function' and not an Event?

**URL:** <https://forum.kirupa.com/t/why-is-netstream-onmetadata-a-callback-function-and-not-an-event/299464>\
**Category:** flash\
**Created:** [November 5, 2009, 11:50am UTC](https://forum.kirupa.com/t/why-is-netstream-onmetadata-a-callback-function-and-not-an-event/299464 "2009-11-05T11:50:17Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![IQAndreas](https://avatars.discourse-cdn.com/v4/letter/i/f08c70/32.png) [@IQAndreas](https://forum.kirupa.com/u/IQAndreas)\
**Post date:** [November 5, 2009, 11:50am UTC](https://forum.kirupa.com/t/why-is-netstream-onmetadata-a-callback-function-and-not-an-event/299464/1 "2009-11-05T11:50:17Z")

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I rarely use NetStream or any video in my projects, but someone I email with was wondering this, and I have no earthly idea.

Is there any reason why the NetStream’s “onMetaData” event cannot be listened to using addEventListener, but instead requires you to use a callback function?

Is it faster, avoiding the overhead of events? Is it to prevent more than one listener being attached?