Drag a CGRect using UIPanGestureRecognizer

Question

I am subclassing UIView, and am trying to "drag" a CGRect back and forth across the screen. Basically I want to move(redraw) the rectangle every time I drag my finger. So far, I have this code:

var rectangle: CGRect {
    get {
        return CGRect(x: 200,
                    y: 200,
                    width: frame.width / 6,
                    height: 15)
    }

    set {}
}

override func draw(_ rect: CGRect) {
    let gesture = UIPanGestureRecognizer(target: self, action: #selector(dragRectangle(recognizer:)))
    addGestureRecognizer(gesture)
    drawRectangle(rect)
}

func drawRectangle(_ rect: CGRect) {
    let path = UIBezierPath(rect: rectangle)
    UIColor.black.set()
    path.fill()
}

@objc func dragRectangle(recognizer: UIPanGestureRecognizer) {
    let translation = recognizer.translation(in: self)
    rectangle = CGRect(x: rectangle.midX + translation.x, y: rectangle.midY + translation.y, width: rectangle.width, height: rectangle.height)
    setNeedsDisplay()
    recognizer.setTranslation(CGPoint.zero, in: self)
}

This is my first time using UIPanGestureRecognizer, so I'm not sure of all the details that go into this. I have set breakpoints in drawRectangle and confirmed that this is being called. However, the rectangle on the screen does not move at all, no matter how many times I try to drag it. What's wrong?

Answer 1

Try like this (check comments through code):

@IBDesignable
class Rectangle: UIView {

    @IBInspectable var color: UIColor = .clear {
        didSet { backgroundColor = color }
    }
    // draw your view using the background color
    override func draw(_ rect: CGRect) {
        backgroundColor?.set()
        UIBezierPath(rect: rect).fill()
    }
    // add the gesture recognizer to your view
    override func didMoveToSuperview() {
        addGestureRecognizer(UIPanGestureRecognizer(target: self, action: #selector(pan)))
    }
    // your gesture selector
    @objc func pan(_ gesture: UIPanGestureRecognizer) {
        //  update your view frame origin
        frame.origin += gesture.translation(in: self)  
        // reset the gesture translation
        gesture.setTranslation(.zero, in: self)
    }
}

---

extension CGPoint {
    static func +=(lhs: inout CGPoint, rhs: CGPoint) {
        lhs.x += rhs.x
        lhs.y += rhs.y
    }
}

---

To draw rectangles on your view when panning you can do as follow:

import UIKit

class ViewController: UIViewController {
    var rectangles: [Rectangle] = []
    override func viewDidLoad() {
        super.viewDidLoad()
        view.addGestureRecognizer(UIPanGestureRecognizer(target: self, action: #selector(pan)))
    }
    @objc func pan(_ gesture: UIPanGestureRecognizer) {
        switch gesture.state {
        case .began:
            let rectangle = Rectangle(frame: .init(origin: gesture.location(in: view), size: .init(width: 0, height: 0)))
            rectangle.fillColor = .red
            rectangle.strokeColor = .white
            rectangle.lineWidth = 3
            view.addSubview(rectangle)
            rectangles.append(rectangle)
        case .changed:
            let distance = gesture.translation(in: view)
            let index = rectangles.index(before: rectangles.endIndex)
            let frame = rectangles[index].frame
            rectangles[index].frame = .init(origin: frame.origin, size: .init(width: frame.width + distance.x, height: frame.height + distance.y))
            rectangles[index].setNeedsDisplay()
            gesture.setTranslation(.zero, in: view)
        case .ended:
            break
        default:
            break
        }
    }
}

Sample Project