You'll need to add an UITapGestureRecogniser and assign it to the view, and then call resign first responder on the textfield on it's selector.
The code:
In viewDidLoad
UITapGestureRecognizer*tap =[[UITapGestureRecognizer alloc]
initWithTarget:self
action:@selector(dismissKeyboard)];
[self.view addGestureRecognizer:tap];
In dismissKeyboard:
-(void)dismissKeyboard {[aTextField resignFirstResponder];}