UITapGestureRecognizer Experiment

Objective

I wanted to understand how UITapGestureRecognizer worked, so I built a simple iOS app that demonstrates how it works.

The app loads with a gray square in the middle of the screen. If you tap anywhere on that square, the label at the top of the screen will show the exact location (CGPoint) of where you tapped.

Key Learnings

1. You can add a UIGestureRecognizer to any UIView. In this example, funView is a UIView that was added as an IBOutlet in my ViewController.

// in ViewController \ viewDidLoad()
let gestureRecognizer = UITapGestureRecognizer(target: self, action: #selector(changeColor))
gestureRecognizer.numberOfTapsRequired = 3
funView.addGestureRecognizer(gestureRecognizer)

2. Define a function that does the actions you want after the user taps that view. Also, pass that function as a parameter to the initializer in step 1.

@objc func changeColor(_ sender: UITapGestureRecognizer) {
	funView.backgroundColor = UIColor.red
	
	let tappedView = sender.view!
	tappedView.layer.borderWidth = 5.0
	tappedView.layer.borderColor = UIColor.purple.cgColor
}

Other notes:

• One-to-one relationship between UIGestureRecognizer and UIView
• UITapGestureRecognizer has a property known as location which returns a CGPoint
• The location is relative to the top left corner the UIView.
• The top left corner of the UIView is the origin or (0.0, 0.0)
• What is the unit of measure of the CGPoint? The official answer is complicated.

A CGPoint, CGRect, or CGSize structure does not explicitly define the unit of measure for its member quantities. A point’s x- and y-coordinates or a size’s width and height are unitless quantities—whether such measurements are treated as pixels, scale-factor-independent points, texture elements (texels), or some other unit depends on the API using the measurement, and on the context in which that API is used.

Specs

• Swift 5
• Xcode 11.4

Mad Props

Jim Campagno's tutorial on UITapGestureRecognizer

UIGestureRecognizer - location

Truncate Float - StackOverflow