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Introduction

Last day, we started talking about touch events on the iPhone.
We defined the terms gesture, touch, tap, and event
We talked about the event responder chain.
We looked at the four gesture notification methods for iPhone.
Today, we will give some example code for handling touch on the iPhone and then discuss the analogous code for Android.

Touch Application

We will now create a simple app to experiment with touch events on the iPhone.
It will display a message saying what kind of touch event has occurred, and it will also say how many touches or taps there were.
The touch events are better demo'd on an actual device than in the simulator.
For this new project we use the View Based App template.

View Controller Header

We then modify the view controller header as follows:

#import <UIKit/UIKit.h>

@interface TouchAppViewController : UIViewController {
	IBOutlet UILabel *messageLabel;
	IBOutlet UILabel *tapsLabel;
	IBOutlet UILabel *touchesLabel;

}

@property (nonatomic, retain) UILabel *messageLabel;
@property (nonatomic, retain) UILabel *tapsLabel;
@property (nonatomic, retain) UILabel *touchesLabel;

-(void)updateLabelsFromTouches: (NSSet *)touches;
@end

Changes to TouchApp XIB file

We next modify the view controller's xib file by adding three labels.
We center the labels and change the text to Messages, Taps, Touches.
We control drag from the File's Owner to each of the labels and either select, the messagesLabel outlet, tapsLabel outlet, or touchesLabel outlet.
Finally, we click on the view in interface builder and click on the View Attributes tab.
We make sure that under the Interaction settings both User Interaction Enabled and Multiple Touch are checked.

View Controller Code

The view controller's code is then set up as follows:

#import "TouchAppViewController.h"

@implementation TouchAppViewController

@synthesize messageLabel;
@synthesize tapsLabel;
@synthesize touchesLabel;

-(void) updateLabelsFromTouches:(NSSet *)touches {
	NSUInteger numTaps = [[touches anyObject] tapCount];
	NSString *tapsMessage = [[NSString alloc]
           initWithFormat:@"%d taps detected", numTaps];
	tapsLabel.text = tapsMessage;
	[tapsMessage release];
	
	NSUInteger numTouches = [touches count];
	NSString *touchMsg = [[NSString alloc] initWithFormat:
           @"%d touches detected", numTouches];
	touchesLabel.text = touchMsg;
	[touchMsg release];
}


- (void)didReceiveMemoryWarning {
	// Releases the view if it doesn't have a superview.
    [super didReceiveMemoryWarning];
	
	// Release any cached data, images, etc that aren't in use.
}

- (void)viewDidUnload {
	// Release any retained subviews of the main view.
	// e.g. self.myOutlet = nil;
}


- (void)dealloc {
	[messageLabel release];
	[tapsLabel release];
	[touchesLabel release];
    [super dealloc];
}

#pragma mark -
- (void) touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
  messageLabel.text = @"Touches Began";
	[self updateLabelsFromTouches:touches];	
}

- (void) touchesCancelled:(NSSet *)touches withEvent:(UIEvent *)event {
	messageLabel.text = @"Touches Cancelled";
	[self updateLabelsFromTouches:touches];	
}

- (void) touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event {
	messageLabel.text = @"Touches Ended";
	[self updateLabelsFromTouches:touches];	
}

- (void) touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {
	messageLabel.text = @"Drag detected";
	[self updateLabelsFromTouches:touches];	
}

@end

Touch App Screenshot

Screenshot of the touch app running showing a message that a touch event just ended

Quiz

Which of the following statements is true:

In iPhone terminology a touch can involve more than one finger.
You can put your code to handle gestures in either a UIView object or in your UIViewController.
OpenGL 2.0 is backward compatible with OpenGL 1.1 .

Handling More Complicated Touch Events

To handle more complicated events on the iPhone such as swipes, pinches, and proper handling of multiple taps we use the basic structure we've already created and make a note of where things happen on the screen.
We say something is a swipe in some direction based on the coordinate where the finger first touched the screen and based on the coordinate on which it was released from the screen.
Similarly, for a pinch we check that two touches were registered on the screen and their starting coordinates are farther apart then their ending coordinates.

To check if something is a double tap, but not a triple tap, you can use methods like:


case 2: //double tap
[NSObject cancelPreviousPerformRequestWithTarget:self
   selector:@selector(singleTap) object:nil];

[self performSelector:@selector(doubleTap) withObject:nil afterDelay: .4]
break;

case 3: //triple tap
[NSObject cancelPreviousPerformRequestWithTarget:self
   selector:@selector(doubleTap) object:nil];

[self performSelector:@selector(tripleTap) withObject:nil afterDelay: .4]
break;

in touchesBegan.

More complicated gestures like making a check-mark on the screen are similarly constructed out of the basic touch events
We conclude our discussion of iPhone Touch Events by giving an example of how to do swipes.

Swipe Example

To begin we create a new view-based application.
This application will display in a label whether a vertical or horizontal swipe was detected.

We modify the boiler-plate view controller header to look like:

#import <UIKit/UIKit.h>

#define kMinimumGestureLength  25
#define kMaximumVariance 5

@interface SwipeAppViewController : UIViewController {
	IBOutlet UILabel *label;
	
	CGPoint gestureStartPoint;
	
}

@property (nonatomic, retain) UILabel *label;
@property CGPoint gestureStartPoint;
-(void)eraseText;
@end

Next we edit the view controller's xib file and add a label and ctrl-click from the File's Owner to the label and connect it to our outlet.

Swipe Controller Code

#import "SwipeAppViewController.h"

@implementation SwipeAppViewController


@synthesize label;
@synthesize gestureStartPoint;

-(void)eraseText {
   label.text = @"";
}


- (void)didReceiveMemoryWarning {
	// Releases the view if it doesn't have a superview.
    [super didReceiveMemoryWarning];
	
	// Release any cached data, images, etc that aren't in use.
}

- (void)viewDidUnload {
	// Release any retained subviews of the main view.
	// e.g. self.myOutlet = nil;
}


- (void)dealloc {
	[label release];
    [super dealloc];
}

#pragma mark -
-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
	UITouch *touch = [touches anyObject];
	gestureStartPoint = [touch locationInView:self.view];
}

-(void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {
	UITouch *touch = [touches anyObject];
	CGPoint currentPosition = [touch locationInView:self.view];
	
	CGFloat deltaX = fabsf(gestureStartPoint.x - currentPosition.x);
	CGFloat deltaY = fabsf(gestureStartPoint.y - currentPosition.y);
	
	if(deltaX >= kMinimumGestureLength && deltaY <= kMaximumVariance) {
		label.text = @"Horizontal swipe detected";
		[self performSelector:@selector(eraseText) withObject:nil afterDelay: 2];
		
	} else 	if(deltaY >= kMinimumGestureLength && deltaX <= kMaximumVariance) {
		label.text = @"Vertical swipe detected";
		[self performSelector:@selector(eraseText) withObject:nil afterDelay: 2];
	}
}

@end

Android and Touch Events

We have already seen that in Android we can create a class which implements View.OnClickListener and then set use a View's setOnClickListener method to get this class to handle click events for that view.
Similarly, you can set a View's OnKeyListener with an object of a class implementing View.OnKeyListener to handle KeyEvents.
To implement this interface you have to define a method:
```
boolean onKey(View v, int keyCode, KeyEvent event)
```
the key code tells you what key was pressed.
KeyEvent's have predefined constants for the different keyCode's as well as methods to figure out keyboard state (was shift pressed? etc).
A View also can have a OnTouchListener which is an Object of a class implementing View.OnTouchListener.
To implement this class, you need to implement the method:
```
boolean onTouch(View v, MotionEvent event)
```
MotionEvent has a method getAction() which returns an int from a list of constant which says what kind of touch event it was.
The event actually holds an array of x, y coordinates associated with the event and has member functions getHistoricalX(int i), getHistoricalY(int i), getHistorySize() for accessing these.
The most recent points can be obtained with getX() and getY().

Finishing Touches

Outline