swift - Assigning a value to an object of type AnyObject -

i have variable

var post :anyobject?

which object got parse api call. want

post["caption"]  = captiontextview.text

but following error message

cannot subscript value of type 'anyobject?' index of type 'string'

i figured out how value post variable. example, works fine extract value key "caption"

captiontextview.text = post!["caption"] as! string!

but don't know how change value. want change value of post variable , save can updated parse database. what's correct way this?

are sure don't want use pfobject class instead of anyobject?

var post :pfobject? post?["caption"]  = captiontextview.text

Macro