Mastering Text Field Constraints in Swift: Adding Character Counts and Blocking Unwanted Characters

Are you tired of dealing with text fields that allow users to enter whatever they want, whenever they want? Do you want to add an extra layer of control to your Swift application’s UI? Look no further! In this comprehensive guide, we’ll take you by the hand and walk you through the process of adding a maximum character count for multiple text fields and implementing a function to block certain characters from being entered in a text field.

Understanding the Problem

Before we dive into the solution, let’s talk about why this is important. Text fields can be a breeding ground for errors and inconsistencies if not properly constrained. Without limits, users can enter excessive amounts of data, leading to performance issues and data integrity problems. Moreover, allowing unrestricted input can lead to security vulnerabilities, like SQL injection attacks. By adding character counts and blocking unwanted characters, you can prevent these issues and ensure a smoother user experience.

Adding a Maximum Character Count for Multiple Text Fields

To add a maximum character count for multiple text fields, we’ll need to create a custom `UITextField` subclass and override the `shouldChangeCharactersIn` method. This method is called whenever the user types or pastes text into the text field. We’ll use this method to check the length of the input string and prevent the user from entering more characters once the maximum count is reached.

import UIKit

class CharacterCountTextField: UITextField {
    var maxCharacters: Int = 0

    override func shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
        guard let text = self.text else { return true }
        let newLength = text.count + string.count - range.length
        return newLength <= maxCharacters
    }
}

Here’s how the code works:

• The `maxCharacters` property is used to set the maximum allowed character count for the text field.
• The `shouldChangeCharactersIn` method is called whenever the user types or pastes text into the text field.
• We calculate the new length of the input string by adding the count of the replacement string to the current text length and subtracting the range length.
• We return `true` if the new length is less than or equal to the maximum allowed character count, and `false` otherwise.

Implementing a Function to Block Certain Characters from Being Entered

Now that we have a character count in place, let’s move on to blocking unwanted characters. We’ll create a function that takes a string as input and returns a boolean indicating whether the string contains any blocked characters.

func isValidInput(_ input: String) -> Bool {
    let blockedCharacters: CharacterSet = CharacterSet(charactersIn: "!@#$%^&*()_+=-{}[]:;'<>,./?~`")
    return input.rangeOfCharacter(from: blockedCharacters) == nil
}

Here’s how the code works:

• We define a `CharacterSet` instance that contains the blocked characters.
• The `rangeOfCharacter(from:)` method is used to search for the presence of any blocked characters in the input string.
• We return `true` if no blocked characters are found, and `false` otherwise.

Integrating the Character Count and Blocked Character Function

Now that we have our character count constraint and blocked character function, let’s integrate them into our `UITextField` subclass.

class CharacterCountTextField: UITextField {
    var maxCharacters: Int = 0

    override func shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
        guard let text = self.text else { return true }
        let newLength = text.count + string.count - range.length
        let isValid = isValidInput(string)

        if newLength <= maxCharacters && isValid {
            return true
        } else {
            return false
        }
    }

    func isValidInput(_ input: String) -> Bool {
        let blockedCharacters: CharacterSet = CharacterSet(charactersIn: "!@#$%^&*()_+=-{}[]:;'<>,./?~`")
        return input.rangeOfCharacter(from: blockedCharacters) == nil
    }
}

Here’s how the code works:

• We added the `isValidInput(_:)` function to the `CharacterCountTextField` class.
• In the `shouldChangeCharactersIn` method, we check if the new length is less than or equal to the maximum allowed character count and if the input string is valid according to the `isValidInput(_:)` function.
• We return `true` if both conditions are met, and `false` otherwise.

func isValidInput(_ input: String) -> Bool {
    let blockedCharacters: CharacterSet = CharacterSet(charactersIn: "!@#$%^&*()_+=-{}[]:;'<>,./?~`")
    let unicodeCharacters = input.unicodeScalars.filter { $0.properties.isEmojiPresentation }
    let blockedUnicodeCharacters = unicodeCharacters.filter { blockedCharacters.contains($0) }

    return blockedUnicodeCharacters.isEmpty
}