Class UIPinchGestureRecognizer


Detect a two-finger pinch or spread. 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 Mac option key as you drag on the simulator. To keep the distance between the fingers constant, hold down the shift key too.

The big white View has two subviews: a variable-size yellow UILabel, and a fixed-size bordered UIView. The init(coder:) method of the View centers these subviews in the big white View and gives them dimensions which are half of those of the big white View.

The dimensions of the big white View are 600 × 600 points when its init(coder:) method is called, but these dimensions will change before the View is displayed on the screen. They also change whenever the orientation of the device changes. When the dimensions change, the autoresizingMask property of each subview ensures that each subview will remain centered in the big white View, and ensures that the size of each subview will remain the same fraction of the size of the big white View.

Source code in Pinch.zip

  1. Class AppDelegate: unchanged.
  2. Class ViewController: unchanged.
  3. Class View: added methods init(coder:) and pinch(_:).
  4. Main.storyboard: the View is initially 600 × 600 points.
  5. Assets.xcassets, an asset catalog file. Can hold icons. Will hold the image file in the Assad exercise.
    1. AppIcon.appiconset
      1. Contents.json
  6. Info.plist

Create the project

Class View is a subclass of class UIView.

Things to try

  1. Comment out the assignments to the autoresizingMasks. Why are the subviews now exactly square? Then comment them back in.

  2. You can shorten
    NSTextAlignment.Center to .Center,
    UIViewAutoresizing.FlexibleLeftMargin to .FlexibleLeftMargin,
    UIGestureRecognizerState.Began to .Began,
    etc. See “a shorter dot syntax” in Enumeration Syntax.

  3. Pinch Bashar al-Assad. Save the file assad.jpg on your Desktop. To convert it from jpg to png, open it with the Preview application.
    File → Export…
    Export As: assad.png
    Format: PNG
    Save
    Then add it to the Assets.xcassets file of the project as we did in America.

    A UIImageView is a UIView specialized for displaying one image, just as a UILabel is a UIView specialized for displaying one line of text. The image displayed in a UIImageView is a UIImage object (see America), and the line of text displayed in a UILabel is a String object.

    Do not add the two existing subviews to the View. Add the following property to class View. 

    	let imageView: UIImageView = UIImageView(image: UIImage(named: "assad.png")!);

    In the init method of class View, you only have to do three things to the imageView: 

    		imageView.center = center;

		imageView.autoresizingMask = [
			UIViewAutoresizing.FlexibleLeftMargin,
			UIViewAutoresizing.FlexibleRightMargin,
			UIViewAutoresizing.FlexibleTopMargin,
			UIViewAutoresizing.FlexibleBottomMargin
		];

		addSubview(imageView);
    In the pinch(_:) method of class View, change label.transform to imageView.transform.