A character has a scroll and wishes to use it if a certain event occurs in a combat round. Can that character take the Ready action and state "I will read the scroll if…?" Or must he begin reading the scroll to cast the spell and say "I will complete casting the spell if…", thus being forced to use the spell, but only cast it if the conditions are met?
Since (DMG p.200) states:
If the spell is on your class's spell list you can use an action to read the scroll and cast its spell without having to provide any of the spell's components.
It would seem to follow that it uses the Cast a Spell action and fall under that rules specific interaction with the Ready Action. [emphasis added]