(Swift) How to get rid of "Left side of mutating operator isn't mutable"

Question

complete error:

Left side of mutating operator isn't mutable: 'abc' is a 'let' constant

Happens because I am trying to change value of a variable sent by parameter to function.
Can I get rid of this, or find some other solution?
Code(My code is much complex, but in effect doing the same as this):

func generateABC() {
var abc = "this"
abc += "is"

let tapGesture = UITapGestureRecognizer(target: self, action: #selector(handleTap(abc)) )
tapGesture.delegate = self
webView.addGestureRecognizer(tapGesture)

abc += "function"
}

handleTap function :

@objc
func handleTap(_ someString: String) {   
     someString += "my"          
}

Why not directly access and modify the variable instead of passing it and modifying the parameter? Parameters of a function are 'constants' by default. If you really wish to modify the passed parameter, you could mark it as an `inout` parameter, but I would recommend against it for something like this... — Lokesh SN, Feb 17 '20 at 09:43
Hahaha sure I would do that directly, but as I said in my Question, my code is much more complex than this with ABC — faris97, Feb 17 '20 at 09:45
Okay, I get that. But, don't you think you need to provide more context to at least understand the complexity? The way it is right now, it is more likely to perceived as a simple question — Lokesh SN, Feb 17 '20 at 09:57

score 5 · Accepted Answer · answered Feb 17 '20 at 09:51

Short story: It's impossible to add custom parameters to (any) target/action

Either there is no parameter

@objc
func handleTap() { ...

or the affected recognizer is the parameter

@objc
func handleTap(_ recognizer : UITapGestureRecognizer) { ...

That's it. In both cases the corresponding declaration is

UITapGestureRecognizer(target: self, action: #selector(handleTap))

You have to use a temporary variable to handle the string.

Kishan Bhatiya · Answer 2 · 2020-02-17T13:21:08.417

1

For passing parameters using UITapGestureRecognizer, One approach would be to subClass UITapGestureRecognizer and then set a property as example below:

class SampleGesture: UITapGestureRecognizer {
    var someString = String()
}

class ViewController: UIViewController {

    let tapGesture = SampleGesture(target: self, action: #selector(self.handleTap))
    tapGesture.delegate = self
    webView.addGestureRecognizer(tapGesture)
    tapGesture.someString = //your text
}

And as for error others already said in answers that Parameters of a function are 'constants' by default

@objc func handleTap(sender: SampleGesture) {
     var newTitle: String = sender.someString   // you can declare as globally
     newTitle += "my"   
}

edited Feb 17 '20 at 13:21

answered Feb 17 '20 at 09:49

Kishan Bhatiya

2,175
8
14

In this line of your code : >let tapGesture = SampleGesture(target: self, action: #selector(handleTap(_:))) It throws an error: Use of unresolved identifier 'handleTap' – faris97 Feb 17 '20 at 10:26
You have to declare inside your viewcontroller not in subClass of `UITapGestureRecognizer ` – Kishan Bhatiya Feb 17 '20 at 10:31
I didnt understand you,sorry. Can you explain more? – faris97 Feb 17 '20 at 12:18
Where you are defining `handleTap` in your code? You have to add that func in viewcontroller – Kishan Bhatiya Feb 17 '20 at 12:31
handleTap is in VC – faris97 Feb 17 '20 at 13:09
still the same... You have a sender parameter in handleTap and call it like this: handleTap(_:)... Cannot work.. – faris97 Feb 17 '20 at 13:16
Ok now build is succeded but I dont see a way to concatenate my ABC string in handleTap... p.s. I mustn't make abc a global variable. – faris97 Feb 17 '20 at 13:39
Yes, you have to make `abc` variable globally – Kishan Bhatiya Feb 17 '20 at 13:44
You should have said that at the beginning. Thats not what I want because I need to create new abc variable every time the func is called. – faris97 Feb 17 '20 at 13:50

score 0 · Answer 3 · answered Feb 17 '20 at 09:40

0

You can not change a Passed variable's value within function. If you want to change value of someString then you have to store it into another variable and use it further like this.

@objc
func handleTap(_ someString: String) {  
     var newString: String = someString 
     newString += "my"          
}

answered Feb 17 '20 at 09:40

shraddha11

783
4
17

2

This action will never be called. – vadian Feb 17 '20 at 09:56

score -1 · Answer 4 · answered Feb 17 '20 at 09:38

-1

All parameters passed into a Swift function are constants, so you can’t change them. If you want, you can pass in one or more parameters as inout, which means they can be changed inside your function, and those changes reflect in the original value outside the function.

func doubleInPlace(number: inout Int) {
    number *= 2
}

Credit: Paul Hudson https://www.hackingwithswift.com/sixty/5/10/inout-parameters

Be careful if you are going with this approach, because you can end up with unexpected sideffects. You can also simply use a temp variable inside your function, and assign the parameter to it

@objc
func handleTap(_ someString: String)-> String {
     var tempString = someString   
     tempString += "Whatever" 
     return tempString         
}

answered Feb 17 '20 at 09:38

Simone

91
1
8

2

This action will never be called. – vadian Feb 17 '20 at 09:56
@vadian This is the solution to edit a parameter inside the body of a function. However, declaring the tapGesture variable, the error which is thrown is different, so the question is very misunderstanding. – Simone Feb 17 '20 at 10:59
Please see my answer. The syntax `handleTap(_ someString: String) -> String` is invalid. – vadian Feb 17 '20 at 11:31

(Swift) How to get rid of "Left side of mutating operator isn't mutable"

4 Answers4