Tracking Position in a custom UIGestureRecognizer - ios

Question

I'm building my own UIGestureRecognizer subclass in order to combine the tap and swipe gestures and track the direction of their swiping motion. In this intended gesture, the user would touch the screen, let their finger bounce fully off the screen one or more times before returning their finger to the screen and dragging; that is a tap followed immediately by a swipe.
I have been able to get the actual gesture portion of this recognizer to work properly, but where I am finding issue is in storing the trail of points where the user's swipe has traveled.
First, I initialize an array of CGPoints as a class member of my custom recognizer class
class UITapSwipeGestureRecognizer: UIGestureRecognizer {
...
var trail: [CGPoint] = []
Then, in touchesBegan(_, with), I first clear the array if not empty, and feed the starting point into the array
override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent) {
...
if !trail.isEmpty { // EXC_BAD_ACCESS error thrown here
trail.removeAll(keepingCapacity: true)
}
trail.append(location(in: view?.window))
...
}
I would go on to discuss my usage in touchesMoved(_, with), but this is enough to trigger my problem. Upon the first attempt to access the array, at !trail.isEmpty, I get the following error:
Thread 1 EXC_BAD_ACCESS (code=1, address=0x10)
Do I need to make a thread safe version of this array? If so, how should I go about doing this? Or am I just going about this entirely the wrong way?

Answer 1

I have been able to replicate your error, by adding the Gesture Recognizer through the StoryBoard.
When I add it programatically, I don't get the error.
Here's the Gesture Recognizer class
import UIKit
import UIKit.UIGestureRecognizerSubclass
class MyGestureClass: UIGestureRecognizer
{
var trail: [CGPoint] = []
override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent) {
super.touchesBegan(touches, with: event)
state = .began
if !trail.isEmpty {
trail.removeAll(keepingCapacity: true)
}
trail.append(location(in: view?.window))
print(trail.count) // Just for debugging
}
override func touchesMoved(_ touches: Set<UITouch>, with event: UIEvent) {
super.touchesMoved(touches, with: event)
trail.append(location(in: view?.window))
print(trail.count) // Just for debugging
}
override func touchesEnded(_ touches: Set<UITouch>, with event: UIEvent) {
super.touchesEnded(touches , with: event)
state = .ended
print("Finished \(trail.count)") // Just for debugging
}
}
and here's how I access it from the View Controller
override func viewDidLoad()
{
super.viewDidLoad()
let myRecognizer = MyGestureClass(target: self, action: #selector(self.tap(_:)))
self.view.addGestureRecognizer(myRecognizer)
}
func tap(_ sender: UITapGestureRecognizer)
{
// No need to do anything here, but appears to be required
}