Use a pan gesture to select multiple cells in a CollectionView - swift

Question

I'm just a beginner in Swift but I was trying to implement this particular feature in a project of mine (word search puzzle): selecting multiple cells with a pan gesture. Each of my cells contains a label that holds a single character. I'd like to get all the letters the user wants to select through a pan gesture. Currently I'm doing this by calculating the distance from the center of each cell to the line passing though points a (touch began) and b (touch ended) and taking into consideration only the cells that lie close enough to that line. This works fine most of the time but it's still prone to errors and it is sure to lead to user dissatisfaction. Could you suggest a simpler way of doing this?
Thanks.
Some code:
func getGridInfo(){
for i in 0...numberOfCells-1{
let indPath = IndexPath(row:i, section:0);
let cell = collectionView?.cellForItem(at: indPath) as? CustomCell;
let frameRect = collectionView?.convert((cell?.frame)!, to: self.view);
coordinates[i] = (cell?.label.text, frameRect?.midX, frameRect?.midY) as? (String, CGFloat, CGFloat);
}
}
#objc func handlePan(touch: UIPanGestureRecognizer){
let touchPoint = touch.location(in: self.view)
switch touch.state{
case .began:
if let line = curLine{
line.removeFromSuperlayer();
}
firstPoint = touchPoint;
case .ended:
secondPoint = touchPoint;
guard let fP = firstPoint,
let sP = secondPoint else {
print("Something went wrong..");
return;
}
let word = getCloseLetters(start:fP, end:sP);
if (words.contains(word) || words.contains(String(word.reversed()))){
playSound(soundUrl: correctUrlSoundFx);
foundWordsLabel.text = word;
foundWordsLabel.textColor = UIColor.green;
// to do: update score
}else{
playSound(soundUrl: incorrectUrlSoundFx);
let attributedString = NSMutableAttributedString(string:word);
attributedString.addAttribute(NSAttributedStringKey.strikethroughStyle, value:2, range: NSMakeRange(0, attributedString.length));
foundWordsLabel.attributedText = attributedString;
foundWordsLabel.textColor = UIColor.red;
}
curLine = addLine(fromPoint: firstPoint!, toPoint: secondPoint!, width:20);
default:
return;
}
func getCloseLetters(start:CGPoint, end:CGPoint) -> String{
var letters = "";
for i in 0...numberOfCells-1{
let tuple = coordinates[i]!;
let x = tuple.1;
let y = tuple.2;
if (checkIfPointIsCloseOrOnTheLine(start: start, end: end, aPoint: CGPoint(x:x,y:y)) == true){
letters += tuple.0;
}
}
return letters;
}
func distanceFromPoint(p: CGPoint, v: CGPoint, w: CGPoint) -> CGFloat {
let pv_dx = p.x - v.x;
let pv_dy = p.y - v.y;
let wv_dx = w.x - v.x;
let wv_dy = w.y - v.y;
let dot = pv_dx * wv_dx + pv_dy * wv_dy;
let len_sq = wv_dx * wv_dx + wv_dy * wv_dy;
let param = dot / len_sq;
var int_x, int_y: CGFloat;
if param < 0 || (v.x == w.x && v.y == w.y) {
int_x = v.x;
int_y = v.y;
} else if param > 1 {
int_x = w.x;
int_y = w.y;
} else {
int_x = v.x + param * wv_dx;
int_y = v.y + param * wv_dy;
}
let dx = p.x - int_x;
let dy = p.y - int_y;
return sqrt(dx * dx + dy * dy);
}
func checkIfPointIsCloseOrOnTheLine(start:CGPoint, end:CGPoint, aPoint:CGPoint, tollerance:CGFloat = 15) -> Bool{
let barHeight = self.navigationController?.navigationBar.frame.size.height;
let newStartPoint = CGPoint(x:start.x, y:start.y - barHeight!);
let newEndPoint = CGPoint(x:end.x, y:end.y - barHeight!);
let distance = distanceFromPoint(p:aPoint, v:newStartPoint, w:newEndPoint);
if distance > tollerance{
return false;
}
return true;
}