Feature request from mage2pro, posted on GitHub Oct 27, 2015
There is an internal field \Magento/Framework/View/Layout/Builder::$isBuilt
https://github.com/magento/magento2/blob/ffa02d82c6344c76ea6de8b50d9612b3200ea2c7/lib/internal/Magen...
Need a public method.
There is an event layout_generate_blocks_after
:
https://github.com/magento/magento2/blob/ffa02d82c6344c76ea6de8b50d9612b3200ea2c7/lib/internal/Magen...
But it is inconvenient to listen for the event while an extension just need to get the current layout state: built or not.