Question 68530 in IOS Swift Question posted in
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Xcode 13.3 warning: "'self' refers to the method '{object}.self', which may be unexpected – Ios swift

I just updated to Xcode 13.3 and I’m seeing several instances of a new warning that I’ve not seen with previous versions of Xcode. As an example, I have a simple table view cell named LabelAndSwitchTableViewCell that looks like this:

import UIKit

class LabelAndSwitchTableViewCell: UITableViewCell {

    private let label: UILabel = {
        let label = UILabel()
        label.translatesAutoresizingMaskIntoConstraints = false
        return label
    }()

    private let _switch: UISwitch = {
        let _switch = UISwitch()
        _switch.translatesAutoresizingMaskIntoConstraints = false
        _switch.addTarget(self, action: #selector(didToggleSwitch), for: .valueChanged)
        return _switch
    }()

    override init(style: UITableViewCell.CellStyle, reuseIdentifier: String?) {
        super.init(style: style, reuseIdentifier: reuseIdentifier)
        
        contentView.addSubview(label)
        contentView.addSubview(_switch)

        // layout constraints removed for brevity
    }
    
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
        
    @objc private func didToggleSwitch() {
        print("Switch was toggled...")
    }
}

As you can see, I’m adding a target to the switch that I want to be called when the value of the switches changes:

_switch.addTarget(self, action: #selector(didToggleSwitch), for: .valueChanged)

After updating to Xcode 13.3, I’m now seeing a new warning on this line:

'self' refers to the method 'LabelAndSwitchTableViewCell.self', which may be unexpected

Xcode’s suggestion to silence this warning is to replace:

_switch.addTarget(self, action: #selector(didToggleSwitch), for: .valueChanged)

…with…

_switch.addTarget(LabelAndSwitchTableViewCell.self, action: #selector(didToggleSwitch), for: .valueChanged)

Making this change does silence the warning but it also causes the app to crash (unrecognized selector) when I toggle the switch. Here’s the dump from that crash:

[app_mockup.LabelAndSwitchTableViewCell didToggleSwitch]: unrecognized selector sent to class 0x1043d86e8
*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '+[app_mockup.LabelAndSwitchTableViewCell didToggleSwitch]: unrecognized selector sent to class 0x1043d86e8'

Making the didToggleSwitch() method static will prevent the crash but I’m not sure why I’d want to do that. I can obviously revert the change (from LabelAndSwitchTableViewCell.self back to just self) but I’m wondering if there’s something else that I should be doing to address this?

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3

Answers


  1. 0

    You can fix by changing the lets to lazy var’s

    private lazy var _switch2: UISwitch = {
    let _switch = UISwitch()
    _switch.translatesAutoresizingMaskIntoConstraints = false
    _switch.addTarget(self, action: #selector(didToggleSwitch), for: .valueChanged)
    return _switch
}()

    The Xcode fix-it suggestion is just wrong.

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  2. 0

    The reason is self is not ready yet in phase 1 of object initialisation. Phase 1 is to set all stored properties, and only in phase 2, you can access to self.

    To fix your code, you can use lazy property, where the initialisation phase 1 is completed.

    Here is the reference:
    https://docs.swift.org/swift-book/LanguageGuide/Initialization.html
    enter image description here

    For this warning:

    1. If there is no super class, or the super class is not NSObject, self is an error when to use in a stored property directly.
    2. If the super class is NSObject, self gets compiled to an auto closure (ClassXXX) -> () -> ClassXXX. So in runtime, self will be used as current instance. But, since Swift 5.6 is warning us on that, I think the reference of self in stored property of a NSObject subclass may not be allowed in future swift versions.

    Example 1: error when there is no super class
    enter image description here

    Example 2: the compiled self in AST

    For this code:

    import Foundation
class MyTest: NSObject  {
    var myself = self
}

    Here is part of the compiled AST:
    enter image description here

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  3. 0

    LabelAndSwitchTableViewCell.self as suggested, will not work in most cases. Use nil and search the responder chain.

    addTarget(_:action:for:)

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