Class UIPinchGestureRecognizer

Detect a two-finger pinch or spread (or neither). Since we are using a UIPinchGestureRecognizer (a subclass of UIGestureRecognizer), we do not need to turn on the big white view’s multipleTouchEnabled property or the yellow label’s userInteractionEnabled property.

To simulate two fingers, press the option key as you drag on the simulator. To keep the distance between the fingers constant, hold down the shift key too.

Source code in Pinch.zip

1. main.m
2. Class PinchAppDelegate
3. Class View

Things to try

1. Pinch Bashar al-Assad. Convert the file assad.jpg to png and add it to the Supporting Files folder of the project. Change the yellow UILabel to a UIImageView. An UIImageView is a UIView that is specialized for displaying one image, just as a UILabel is a UIView that is specialized for displaying one line of text. The image displayed in a UIImageView is a UIImage (see America), and the line of text displayed in a UILabel is an NSString.

   In View.h, change the UILabel instance variable to the following.

   	UIImageView *imageView;
   

   In View.m, change the initialization of the label in initWithFrame: to the following.

   		UIImage *image = [UIImage imageNamed: @"assad.jpg"];
   		if (image == nil) {
   			NSLog(@"could not create image");
   			return nil;
   		}
   		imageView = [[UIImageView alloc] initWithImage: image];
   
   		//Keep the UIImageView the same size,
   		//but center it in the View.
   
   		CGRect b = self.bounds;
   
   		imageView.center = CGPointMake(
   			b.origin.x + b.size.width / 2,
   			b.origin.y + b.size.height/ 2
   		);
   
   		[self addSubview: imageView];
   

   In the pinch: method, resize the ImageView. The (0, 0) that we specify for the location of the upper left corner of the ImageView’s is overridden by the value we gave to the center property of the ImageView. The center of the ImageView remains at the center of the big white View.

   	CGSize i = imageView.image.size;
   	imageView.bounds = CGRectMake(0, 0, scale * i.width, scale * i.height);