When we say Shehechiyanu in the morning by Kriyas HaMegila they announce in my Shul to have in mind the Mitzvos HaYom of Mishloach Manos, Matonos L'Evyonim, and Seudas Purim. Can you give Mishloach Manos or Matanos L'Evyonim prior to hearing the Megila in the morning with the Shehechiyanu?
The Magen Avraham says that if one did not have a m'gila one would not say birkas "shehechiyanu". This leads me to believe that if one had not read yet the halacha would be the same.
Rav Ya'akov Emden (in Mor Uk'tzi'a) challenges the premise, pointing out that we say the b'racha on Yom Kipur without an action necessarily associated with its recital, and the same should be possible on Purim. His logic is presumably based on the g'mara in Eruvin, even though he does not cite the reference for being able to say the b'racha in the shuk.