APP iA: Writer
So,let us firstly look at this app,yes through icon you can know this is a app for writing.
10 principles (Good design makes a product useful)
this app is really useful for those people who want wright on their iphone or ipad,you also can save or get your text through cloud or Dropbox ；this app also is convenient for you if you want print your text or email your text.
10 principles(Good design helps a product to be understood)
Of course when we open it we can understand this app quickly：for writing.Its structure clearly and have not got any extra design on it.
10principles(Good design is concerned with the environment)
This app consider what users will do after finish their text，so it has （print/email，or save for cloud or Dropbox） these options.