Outline

• Touch on the iPhone
• Quiz
• More touch stuff

Introduction

• Last week, we were covering the basics of how Core Graphics and OpenGL work on the iPhone.
• Today, we will look at how to handle various kinds of touch events on mobile phones.
• First, let's specify some common terms:
  • A gesture is any sequence of events that happens from the time you touch the screen with one or more fingers until you list your fingers off the screen.
  • An event is used to encapsulate information about one or more gestures and is generated when a user interacts with the screen.
  • A touch refers to a single finger being placed on the screen. The number of touches in a gesture is the number of fingers on the screen at the same time.
  • A tap is a gesture in which a single finger is touched to the screen and immediately lifted off without moving the finger around.
• You can also register events for multiple taps in quick succession. (double-tap, triple-tap, etc.)

Responder Chain

• Gesture get passed through the iPhone OS as events and get passed through the responder chain.
• Any class that has UIResponder as a super class is a responder. (For example, all UIView, UIController, UIViewController).
• If the first responder for an event doesn't handle a particular event, it passed that event up the responder chain.
• A responder can also both partially handle an event and still pass it along the chain, or it can choose to handle it and stop the percolation.
• The event chain proceeds up the view hierarchy for view that contained the events coordinates, until finally it reaches UIApplication.
• If this does not handle the event, then the event is discarded.

Forwarding Events

• Events aren't forwarded by default.
• Suppose your view responds to a swipe but not a tap.
• However, some enclosing view might responds to a tap.
• In order to forward, your event you might have code like:

  -(void)respondToMyEvent: (UIEvent *) event {
     if(condition) {
        [self handleEvent: event];
     } else {
        [self.nextResponder respondToMyEvent: event];
     }
  }
  

Quiz

Multitouch Architecture

• You can put your code to handle gestures in either a UIView object or in your UIViewController
• Examples of the former are things like UISwitch and UISlider.
• A switch might want to turn itself on or off based on a touch.
• However, if the gesture being processed affects more than the object being touched one typically puts the code in the view's controller class.

Gesture Notification

• There are four methods used to notify a responder about touches and gestures.
• When the user first touches the screen, the iPhone looks for a responder that has a method of signature touchesBegan:withEvent.
• Code for such a method might look like:

  - (void)touchesBegan: (NSSet *)touches withEvent:(UIEvent *)event {
    NSUInteger numTaps = [[touches anyObject] tapCount];
    NSUInteger numTouches = [touches count]; 
  }
  

• touches is a set of UITouch's, each represents one finger touching the screen.
• If there are more than one numTouches is more than 1 than numtaps == 1 as taps are defined as only involving one finger.
• If taps were 2 than it would correspond to a double tap.

More on Gesture Notification

• It is often useful to know about those touches which occurred only within the current view.
• You could find this out with code like:

  NSSet *myTouches = [event touchesForView:self.view];
  

• To find out where a touch occured, given a UITouch object we can do:

  CGPoint point = [touch locationInView:self];
  

• In addition to static touches one can get notified while a user is moving fingers across the screen by implementing, touchesMoved:withEvent:
• This method will actually get called mutliple times during a long drag event, and each time its is called you get another list of events which you can handle.
• UITouch objects also hold the previous location of a touch.

Still More on Gesture Notification

• You can find out when a finger is removed from the screen by having the method touchesEnded:withEvent:
• Finally, there is also a touchesCancelled:withEvent:
• This might be called if a user is touching the screen and it gets interuppted by something like an incoming phone call.