Can you perform a Ready action with the condition being that you complete your bonus action, then do your bonus action, and then complete your readied action?
For instance, say I have the Shield Master feat. I Ready the Attack action with the condition that the enemy is prone, then I take the bonus-action shove from Shield Master and knock him prone before I take my readied Attack action.
Is this a legal exploit to reverse the action order?
Best Answer
No, your Shield Master example won't work
Shield Master says:
If you take the Ready Action you are not taking the Attack action, even if you use it to attack.
Only once the trigger occurs and your attack completes are you counted as having taken the Attack action.
Since you don't take the Attack action, you do not get a bonus action shove. Since you don't get a bonus action shove, you won't be able to trigger your Readied action.
So in the end, attempting this only results in a wasted action. It is still a valid way to phrase the Ready action, it will just always result in no trigger happening (unless the enemy becomes prone and right next to you still for some other reason before your next turn).
In general, you can use the results of a bonus action as a trigger for a Ready action
Though the above example didn't work, there is no reason that you cannot use the results of a bonus action to trigger a Ready action. For example, you can have say after "I cast this bonus action spell, do X" or "after I take my bonus action attack, do X". As long as you are able to do the bonus action in the first place (unlike with the Shield Master bonus action) and you can perceive it, then you can use it as a trigger.
Note that you can't just set a trigger off of "when I use my bonus action" because bonus actions are not a thing in-universe and thus it is not perceivable. The results or actual actions done with the bonus action are of course valid as described above.