Pass Parameters in Tap Gestures

Also known as stop casting / stop using tags

Before we start

Difficulty: Beginner | Easy | Normal | Challenging This article has been developed using Xcode 13.0, and Swift 5.3

Prerequisites

• It will be useful if you are familiar with subclassing in Swift

This project

The answer

If you've several UIView instances in a view and wish to easily identify which is in a UIViewController. So the end result look something like the following:

Isn't that nice! There are three different views here and we would like to differentiate between each one when the user taps them. Surely this is an easy task?

Now remember, there isn't any MVVM or great architecture here so I've crammed everything into the ViewController - this article is about the Gestures rather than anything else!

Now our multiple views using the same tap gesture recognizer

@objc func showAlert(_ sender: UITapGestureRecognizer){
    // Code to identify sender.view printed to the console
}

Can we reuse this? Well, since gesture recognizers can only be attached to a single view (a one-to-one relationship) so we will deal with this as we progress on this project.

Use the tag

UIView classes in Swift have a tag property, which can either be set through the Storyboard or programatically (which is done in this sample code).

let gesture = UITapGestureRecognizer(target: self, action: #selector(logToConsole))
firstView.addGestureRecognizer(gesture)
firstView.tag = 1

We can then retrieve using a logToConsole function and print the tag

@objc func logToConsole(_ sender: UITapGestureRecognizer){
    print("The sender is : \(sender.view?.tag)")
}

which is rather fantastic isn't it? Job done!

But you shouldn't be using tags to uniquely identify your views at all

So, there must be a better way! Luckily there is!

Subclass the view

class SubclassedView: UIView {
    var number: Int?
    // init from code
    override init(frame: CGRect) {
        super.init(frame: frame)
        setupView()
    }
    // init from xib or storyboard
    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        setupView()
    }
    // common setup code
    private func setupView() {
        backgroundColor = .green
    }
}

Of course the number can changed using the initialiser etc. but in this case we will reach in and use the property directly:

let gestureTwo = UITapGestureRecognizer(target: self, action: #selector(logToConsole))

secondView.addGestureRecognizer(gestureTwo)

(secondView as? SubclassedView)?.number = 2

Now I don't love the way that the Integer has been set here, but never mind. 

We can then declare logToConsole using the following code:

@objc func logToConsole(_ sender: UITapGestureRecognizer){
    if let numberSender = sender.view as? SubclassedView {
        print("The sender is: \(numberSender.number)")
    }
}

here if the sender is of SubclassedView type. 

But that Conditional cast if let numberSender = sender.view as? SubclassedView? I really don't like it. What if the type is not SubclassedView? This is all bad times.

Subclassed UITapGestureRecognizer

We can overcome this! We can subclass UITapGestureRecognizer to get the following:

class SubclassedTapGestureRecognizer: UITapGestureRecognizer {
    let number: Int
    init(target: AnyObject, action: Selector, number: Int) {
        self.number = number
        super.init(target: target, action: action)
    }
}

This gives us the option of creating the following code:

let gestureThree = SubclassedTapGestureRecognizer(target: self, action: #selector(logToConsoleCustom), number: 3)
thirdView.addGestureRecognizer(gestureThree)

which, the attentive reader will have noticed - calls logToConsoleCustom

@objc func logToConsoleCustom(_ sender: SubclassedTapGestureRecognizer){
    print(sender.number)
}

which, as you see enables us to use SubclassedTapGestureRecognizer which carries with the class an Integer. As this subclass has been initialized with the number 3, of course it will print 3 to the console.

Note that we've avoided the conditional casting madness, and therefore presented the best solution to this problem (at least from the solutions presented here).

Conclusion

Some might say that it is impossible to pass parameters. This article has presented solutions to the problem, and has presented this solution in situ in the attached repo. I hope this helps everyone who is reading this article!

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