CODEWITHSUNDEEP
iosswiftpositionuipangesturerecognizer
CizooDev
CizooDev

Reputation: 81

UIPanGestureRecognizer get the length between start and stop

This is my code :

override func viewDidLoad() {
    super.viewDidLoad()
    let panRecognizer = UIPanGestureRecognizer(target: self, action: #selector(PreviewViewController.respondsToPenGesture))
    self.view.addGestureRecognizer(panRecognizer)}

Here blow is the respondsToPenGesture Func:

func respondsToPenGesture(sender: UIPanGestureRecognizer) {
    let startLocation : CGPoint
    if (sender.state == UIGestureRecognizerState.Began) {
        startLocation = sender.locationInView(self.newEffectView)
    }
    else if (sender.state == UIGestureRecognizerState.Ended) {

        let stopLocation = sender.locationInView(self.newEffectView)

        let dx = stopLocation.x - startLocation.x;
        let dy = stopLocation.y - startLocation.y;
        let distance = sqrt(dx*dx + dy*dy );
        NSLog("Distance: %f", distance);

    }   
}

I want to use this length,and it shows error : Constant 'startLocation' used before being initialized.

And if I change it to :

func respondsToPenGesture(sender: UIPanGestureRecognizer) {
    //let startLocation : CGPoint
    if (sender.state == UIGestureRecognizerState.Began) {
        let startLocation = sender.locationInView(self.newEffectView)
    }
    else if (sender.state == UIGestureRecognizerState.Ended) {

        let stopLocation = sender.locationInView(self.newEffectView)

        let dx = stopLocation.x - startLocation.x;
        let dy = stopLocation.y - startLocation.y;
        let distance = sqrt(dx*dx + dy*dy );
        NSLog("Distance: %f", distance);
    }
}

It will show : Use of unresolved identifier 'startLocation'. (P.s: i already tried with use switch and case , it shows the same problem) Someone can help me with this problem ? Thx in advance .

Upvotes: 1

Views: 1296

Answers (2)

alexburtnik
alexburtnik

Reputation: 7741

The problem is that handlePanRecognizer() is called every time the gesture is updated, so when the gesture ends, you don't have the startLocation because it was set in another call of the handler method.

You should store last location as an optional property:

var startPanLocation: CGPoint?

Now set it in .began case and use it to calculate the distance in .ended case:

func handlePanRecognizer(panRecognizer: UIPanGestureRecognizer) {
    let currentLocation = panRecognizer.location(in: self.newEffectView)

    switch panRecognizer.state {
    case .began:
        startPanLocation = currentLocation
    case .ended:
        let dx = currentLocation.x - startPanLocation.x;
        let dy = currentLocation.y - startPanLocation.y;
        let distance = sqrt(dx*dx + dy*dy );
        startPanLocation = nil
    default: break
    }
}

Btw, you cannot use the variable if it was defined but not initialized: 

let startLocation : CGPoint      //defined
let startLocation = CGPoint.zero //initialized

Upvotes: 3

XmasRights
XmasRights

Reputation: 1509

Three issues here.

Firstly, let is used to define a const variable, or one that will not change once it is created. Your first bit of code fails because you are attempting to change the value of startLocation after it is created as an empty CGPoint, if you change that let into a var, you should be fine.

Secondly, the next block of code has an issue with scoping. Anytime you use { }, you create a new scope which has access to all the variables outside of the scope, and can create new variables within. Once that scope ends with the closing brace, everything that is created inside is destroyed, including your let startLocation, which is why the else if scope can't see it.

Finally, the scoping issue also applies to this function as a whole, every time the respondsToPanGesture(sender:) method is called, a brand new startLocation will be created, and all previous instances of it will be destroyed. If you want to maintain this value between calls, you'll need to create startLocation in the class itself, and update it within the function.

class MyClass
{
  var startLocation = CGPoint.zero

  func respondsToPenGesture(sender: UIPanGestureRecognizer) 
  {
    if (sender.state == UIGestureRecognizerState.Began) 
    {
      startLocation = sender.locationInView(self.newEffectView)
    }
    else if (sender.state == UIGestureRecognizerState.Ended) 
    {
      let stopLocation = sender.locationInView(self.newEffectView)

      let dx = stopLocation.x - startLocation.x;
      let dy = stopLocation.y - startLocation.y;
      let distance = sqrt(dx*dx + dy*dy );
      NSLog("Distance: %f", distance);
    }   
  }
}

Hope that helps :-)

Upvotes: 2

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