iOS Swift Gestures : How can I link two items with gestures

Question

I have 2 buttons in my view. I created 2 UIPanGestureRecognizer, one for each button. I can move them separately but I would like that when I slide one button to the right, the other one goes to the left. I really don't know how to do that. What are the best practices ?

@IBAction func button1(_ sender: Any) {}
@IBAction func button2(_ sender: Any) {}

@IBAction func bottomButtonGesture
    (recognizer:UIPanGestureRecognizer) {

    let translation = recognizer.translation(in: self.view)
    if let view = recognizer.view {
        view.center = CGPoint(x:view.center.x + translation.x,
                              y:view.center.y/* + translation.y*/)
    }
    recognizer.setTranslation(CGPoint.zero, in: self.view)
}

@IBAction func topButtonGesture
    (recognizer:UIPanGestureRecognizer) {

    let translation = recognizer.translation(in: self.view)
    if let view = recognizer.view {
        view.center = CGPoint(x:view.center.x - translation.x,
                              y:view.center.y/* + translation.y*/)
    }
    recognizer.setTranslation(CGPoint.zero, in: self.view)
}

Answer 1

In each pan gesture IBAction you want to set the button that is not being panned to have the opposite x position as the button being panned.

@IBAction func bottomButtonGesture
    (recognizer:UIPanGestureRecognizer) {

    let translation = recognizer.translation(in: self.view)
    if let view = recognizer.view {
        view.center = CGPoint(x: view.center.x + translation.x,
                              y: view.center.y)

        // Move the other button here too
        // Notice we are subtracting the translation.x value instead of adding
        button2.center = CGPoint(x: button2.center.x - translation.x,
                              y: button2.center.y)
    }
    recognizer.setTranslation(CGPoint.zero, in: self.view)
}

@IBAction func topButtonGesture
    (recognizer:UIPanGestureRecognizer) {

    let translation = recognizer.translation(in: self.view)
    if let view = recognizer.view {
        view.center = CGPoint(x: view.center.x + translation.x,
                              y: view.center.y)

        // Move the other button here too
        // Notice we are subtracting the translation.x value instead of adding
        button1.center = CGPoint(x: button1.center.x - translation.x,
                              y: button1.center.y)
    }
    recognizer.setTranslation(CGPoint.zero, in: self.view)
}

Here is a working Playground

import UIKit
import PlaygroundSupport

class ViewController: UIViewController {

    let button1 = UIButton(frame: CGRect(x: 160, y: 100, width: 40, height: 40))
    let button2 = UIButton(frame: CGRect(x: 160, y: 200, width: 40, height: 40))

    override func loadView() {
        super.loadView()

        // Tab Views
        button1.backgroundColor = .orange
        self.view.addSubview(button1)

        button2.backgroundColor = .blue
        self.view.addSubview(button2)

        let panGesture1 = UIPanGestureRecognizer(target: self, action: #selector(button1Pan(recognizer:)))
        button1.addGestureRecognizer(panGesture1)

        let panGesture2 = UIPanGestureRecognizer(target: self, action: #selector(button2Pan(recognizer:)))
        button2.addGestureRecognizer(panGesture2)
    }

    @objc func button1Pan(recognizer: UIPanGestureRecognizer) {
        let translation = recognizer.translation(in: self.view)
        if let view = recognizer.view {
            view.center = CGPoint(x:view.center.x + translation.x,
                                  y:view.center.y)

            // Move the other button here too
            button2.center = CGPoint(x: button2.center.x - translation.x,
                                     y: button2.center.y)
        }
        recognizer.setTranslation(CGPoint.zero, in: self.view)
    }

    @objc func button2Pan(recognizer: UIPanGestureRecognizer) {

            let translation = recognizer.translation(in: self.view)
            if let view = recognizer.view {
                view.center = CGPoint(x:view.center.x + translation.x,
                                      y:view.center.y)

                // Move the other button here too
                button1.center = CGPoint(x: button1.center.x - translation.x,
                                         y: button1.center.y)
            }
            recognizer.setTranslation(CGPoint.zero, in: self.view)
    }
}

PlaygroundPage.current.needsIndefiniteExecution = true
PlaygroundPage.current.liveView = ViewController()