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What's the relationship between a module's layout handle XML file and a theme's layout handle XML file? Which files have priority? When does one file replace another file? When does one file merge with another file? I haven't been able to find concrete information on this in the documentation, and I'm hoping for a clear answer before I spend too much time hunting this information down myself.

That is, in Magento 2 it's possible for a module to add a layout handle XML file

vendor//magento/module-theme/view/frontend/layout/default.xml

It's also possible for a theme to specify this exact same layout handle XML file

vendor//magento/theme-frontend-blank/Magento_Theme/layout/default.xml 

and for a theme that inherits from another theme to specify the exact same file.

vendor//magento/theme-frontend-luma/Magento_Theme/layout/default.xml 

What's not clear is the relationship between all these files. In Magento 1, creating a new layout update XML file in a theme would replace that original layout update XML file. Early versions of Magento 2 featured layout merging, where new layout files would be merged with the originals. It's not clear if merging made it into Magento 2. If it did, it's not clear what those merging rules are. It's also not clear if a theme replacing a module file behaves differently than a child theme file replacing a parent theme file.

Short of tracing the very abstract and complicated merging code myself, does anyone know the rules for this new layout merging?

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the order of layout files collecting is documented here under "Extend layouts" http://devdocs.magento.com/guides/v2.0/frontend-dev-guide/themes/theme-inherit.html

Extend layouts

The layouts processing mechanism does not involve fallback. The system collects layout files in the following order:

  1. Current theme layouts: /_/layout/
  2. Ancestor themes layouts, starting from the most distant ancestor, recursively until a theme with no parent is reached: /_/layout/
  3. Module layouts for the frontend area: /view/frontend/layout/
  4. Module layouts for the base area: /view/base/layout/

Unlike templates or images, layout can be not only overridden, but also extended. And the recommended way to customize layout is to extend it by creating theme extending layout files.

  • Is "extending" defined anywhere? i.e. are all layout rules from all files applied? Do the XML files merge with some nodes replacing others? etc. – Alan Storm Feb 5 '16 at 21:23
  • from my experience every node goes into the final layout. Each block / container must be defined once and then only referenced with referenceBlock / referenceContainer to change them (in this case if you change the arguments for example they are replaced if they already existed - like when you change the htmlClass of a container) – David Verholen Feb 5 '16 at 21:33

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