Multi Touch Tutorial 3

In this tutorial I will show how to implement the pinch feature (zoom in/out).

• Touch Tutorial - Part One (touchesEnded Event)
• Touch Tutorial - Part Two (touchesBegan Event)
• Touch Tutorial - Part Three (Zoom In/Out)
• TouchTutorial - Part Four (pan left/right)

In this tutorial, I will show you how to use the touchesMoved event to zoom in and out the image on the UIImageView. Now, I have not exactly figured out a way to actually zoom in and out the actual image but here is the code on when to zoom in and zoom out. If anyone has figured out a way to zoom in/out the actual image, please share it everyone.

Before we can implement the touchesMoved event, we are going to declare some variables and methods in ImgViewController.h file. This is how the header file is going to look like

#import <UIKit/UIKit.h>


@interface ImgViewController : UIViewController {

IBOutlet UIImageView *imgView;
NSTimer *timer;
CGFloat initialDistance;

}

- (CGFloat)distanceBetweenTwoPoints:(CGPoint)fromPoint toPoint:(CGPoint)toPoint;
- (void) clearTouches;

@end

disanceBetweenTwoPoints is used to calculate as the method name implies, distance between two points. initialDistance is used to keep track of the distance when touchesBegan method is fired. To zoom in and zoom out, we first need to calculate the distance between the two fingers and store it in initialDistance variable. This is how the touchesBegan method will look like.

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {

NSSet *allTouches = [event allTouches];

switch ([allTouches count]) {
case 1: { //Single touch

//Get the first touch.
UITouch *touch = [[allTouches allObjects] objectAtIndex:0];

switch ([touch tapCount])
{
case 1: //Single Tap.
{
//Start a timer for 2 seconds.
timer = [NSTimer scheduledTimerWithTimeInterval:2 target:self
selector:@selector(showAlertView:) userInfo:nil repeats:NO];

[timer retain];
} break;
case 2: {//Double tap.

//Track the initial distance between two fingers.
UITouch *touch1 = [[allTouches allObjects] objectAtIndex:0];
UITouch *touch2 = [[allTouches allObjects] objectAtIndex:1];

initialDistance = [self distanceBetweenTwoPoints:[touch1 locationInView:[self view]]
toPoint:[touch2 locationInView:[self view]]];
} break;
}
} break;
case 2: { //Double Touch

} break;
default:
break;
}

}

We get the first touch objects at index 0 and 1 and then we calculate the initial distance between the two points. This is how the distanceBetweenTwoPoints method looks like

- (CGFloat)distanceBetweenTwoPoints:(CGPoint)fromPoint toPoint:(CGPoint)toPoint {

float x = toPoint.x - fromPoint.x;
float y = toPoint.y - fromPoint.y;

return sqrt(x * x + y * y);
}

In touchesMoved event, we find out if there are at least two touches on the screen. We then get the touch object at index 0 and 1, calculate the distance between the finalDistance and the initialDistance. If the initialDistance is greater then the finalDistance then we know that the image is being zoomed out else the image is being zoomed in. This is how the source code looks like

- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {

if([timer isValid])
[timer invalidate];

NSSet *allTouches = [event allTouches];

switch ([allTouches count])
{
case 1: {

} break;
case 2: {
//The image is being zoomed in or out.

UITouch *touch1 = [[allTouches allObjects] objectAtIndex:0];
UITouch *touch2 = [[allTouches allObjects] objectAtIndex:1];

//Calculate the distance between the two fingers.
CGFloat finalDistance = [self distanceBetweenTwoPoints:[touch1 locationInView:[self view]]
toPoint:[touch2 locationInView:[self view]]];

//Check if zoom in or zoom out.
if(initialDistance > finalDistance) {
NSLog(@"Zoom Out");
}
else {
NSLog(@"Zoom In");
}

} break;
}

}

NSLog tells us if what we are doing is correct or not. If someone knows how to zoom in/out an actual image, please let me know.

Since we keep track of the initialDistance, we need to clear that value when touches is canceled or when touches are ended. We do this in clearTouches method, which is called from touchesEnded event and touchesCanceled event. This is how the method looks like

- (void)clearTouches {

initialDistance = -1;
}

I hope you found this tutorial a little helpful without the actual zoom in/out functionality. You can download the source code here and please leave me your comments.

Happy Programming,
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