Avoid Flags

Whenever possible and convenient, avoid the usage of boolean flags. Prefer another mechanism to deal with your issue, for instance, a computed variable or a lazy property. Always think of the cost of having to calculate such variable each time it's accessed, sometimes it's just not worth it.

Example

A very common scenario:

Let's suppose you have a specific screen that can be accessed through multiple UI flows, and you need to know whether the navigation came from a specific place or from somewhere else.

You need to add some code in the corresponding UIViewController to know whether or not the navigation came from here or there, to perform different actions on each case.

Good

// In the UIViewController you need to distinguish where the navigation comes from

private var comesFromSignUpFlow: Bool {
    guard let navigationController = self.navigationController else {
        return false
    }
    return navigationController.containsViewControllerOfKind(SignUpViewController)
    }	// It's automatically calculated! You don't need to worry to maintain this property. Also, it's private.

Bad

// In the UIViewController you need to distinguish where the navigation comes from

var comesFromSignUpFlow = false	// Can't make it private. Bad.

// In SignUpViewController
yourOtherViewController.comesFromSignUpFlow = true	// You need to maintain it from the outside. Bad.

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Motivation

• Flags need to be maintained, which usually causes bugs due to the fact that soon or later you, or most likely other developer, will forget to set that flag to its proper value (true or false) somewhere in the codebase when adding new code or maintaining existent code.