What does the "update" element do in Magento layout XML? - layout

Question

Right now I'm exploring the internals of the admin section of Magento and I stumbled on this piece of XML:
File: app/design/adminhtml/default/default/layout/catalog.xml, around line 55
50 <block type="core/template" template="catalog/wysiwyg/js.phtml"/>
51 </reference>
52 </adminhtml_catalog_product_new>
53
54 <adminhtml_catalog_product_edit>
55 <update handle="editor"/>
56 <reference name="content">
57 <block type="adminhtml/catalog_product_edit" name="product_edit"></block>
58 </reference>
What does the <update /> tag do?

Answer 1

The <update> basically pulls in another handle.
Assume you have this:
<layout>
<foo>
<reference name="header">
<block type="cms/block" name="some_block" as="someBlock">
<action method="setBlockId"><block_id>some_block</block_id></action>
</block>
</reference>
<reference name="left">
<block type="cms/block" name="some_totally_different_block" as="someTotallyDifferentBlock">
<action method="setBlockId"><block_id>some_totally_different_block</block_id></action>
</block>
</reference>
</foo>
<bar>
<update handle="foo" />
<reference name="header">
<block type="cms/block" name="some_other_block" as="someOtherBlock">
<action method="setBlockId"><block_id>some_other_block</block_id></action>
</block>
</reference>
</bar>
</layout>
The resulting XML for bar would be:
<layout>
<bar>
<reference name="header">
<!-- Start of part pulled in from foo -->
<block type="cms/block" name="some_block" as="someBlock">
<action method="setBlockId"><block_id>some_block</block_id></action>
</block>
<!-- End of part pulled in from foo -->
<block type="cms/block" name="some_other_block" as="someOtherBlock">
<action method="setBlockId"><block_id>some_other_block</block_id></action>
</block>
</reference>
<!-- Start of part pulled in from foo -->
<reference name="left">
<block type="cms/block" name="some_totally_different_block" as="someTotallyDifferentBlock">
<action method="setBlockId"><block_id>some_totally_different_block</block_id></action>
</block>
</reference>
<!-- End of part pulled in from foo -->
</bar>
</layout>
tl;dr: The update handle is basically a "merge this layout with my current layout".

Answer 2

This handle is used for merging existing layout handles to your current layout.
In your example <update handle="editor"/> will add to the <adminhtml_catalog_product_edit> following content:
<editor>
<reference name="head">
<action method="setCanLoadExtJs"><flag>1</flag></action>
<action method="addJs"><script>mage/adminhtml/variables.js</script></action>
<action method="addJs"><script>mage/adminhtml/wysiwyg/widget.js</script></action>
<action method="addJs"><script>lib/flex.js</script></action>
<action method="addJs"><script>lib/FABridge.js</script></action>
<action method="addJs"><script>mage/adminhtml/flexuploader.js</script></action>
<action method="addJs"><script>mage/adminhtml/browser.js</script></action>
<action method="addJs"><script>prototype/window.js</script></action>
<action method="addItem"><type>js_css</type><name>prototype/windows/themes/default.css</name></action>
<action method="addItem"><type>js_css</type><name>prototype/windows/themes/magento.css</name></action>
</reference>
</editor>
("editor" handle is defined in app/design/adminhtml/default/default/layout/main.xml)