Custom validation property wrappers in Swift

Introduction

Custom validation property wrappers help you check and control the values stored in properties easily and clearly.

When you want to make sure a user's input meets certain rules before saving it.

When you want to keep your code clean by reusing validation logic for many properties.

When you want to automatically reject or fix wrong values in your data.

When you want to add clear error messages or feedback for invalid data.

When you want to separate validation rules from your main code for better organization.

Syntax

Swift

@propertyWrapper struct ValidationName {
    private var value: Type

    var wrappedValue: Type {
        get { value }
        set {
            // validation logic here
            if isValid(newValue) {
                value = newValue
            } else {
                // handle invalid value
            }
        }
    }

    init(wrappedValue initialValue: Type) {
        // validate initial value
        if isValid(initialValue) {
            self.value = initialValue
        } else {
            // handle invalid initial value
            self.value = defaultValue
        }
    }

    private func isValid(_ value: Type) -> Bool {
        // return true if value is valid
        return true
    }
}

The @propertyWrapper keyword defines a wrapper around a property.

The wrappedValue is the actual property value with validation logic inside the setter.

Examples

This wrapper makes sure a string is never empty. If empty, it sets a default string.

Swift

@propertyWrapper
struct NonEmpty {
    private var value: String = ""

    var wrappedValue: String {
        get { value }
        set { value = newValue.isEmpty ? "Default" : newValue }
    }

    init(wrappedValue: String) {
        self.wrappedValue = wrappedValue
    }
}

This wrapper ensures an integer is always positive. If a negative or zero is set, it resets to 1.

Swift

@propertyWrapper
struct Positive {
    private var value: Int = 1

    var wrappedValue: Int {
        get { value }
        set { value = newValue > 0 ? newValue : 1 }
    }

    init(wrappedValue: Int) {
        self.wrappedValue = wrappedValue
    }
}

Sample Program

This program uses a custom property wrapper @NonEmpty to make sure the user's name is never empty. If you try to set it empty, it changes to "Default" automatically.

Swift

import Foundation

@propertyWrapper
struct NonEmpty {
    private var value: String = ""

    var wrappedValue: String {
        get { value }
        set { value = newValue.isEmpty ? "Default" : newValue }
    }

    init(wrappedValue: String) {
        self.wrappedValue = wrappedValue
    }
}

struct User {
    @NonEmpty var name: String
}

var user = User(name: "")
print("User name is: \(user.name)")

user.name = "Alice"
print("User name is: \(user.name)")

user.name = ""
print("User name is: \(user.name)")

Important Notes

Property wrappers can add validation without repeating code everywhere.

Use init(wrappedValue:) to validate the initial value when the property is created.

Remember to handle invalid values gracefully to avoid crashes.

Summary

Custom validation property wrappers help keep your data clean and safe.

They let you reuse validation rules easily across your code.

They make your code easier to read by separating validation from main logic.

Practice

1. What is the main purpose of a custom validation property wrapper in Swift?

easy

A. To automatically check and enforce rules on property values

B. To change the type of a property at runtime

C. To make properties private and inaccessible

D. To store multiple values in a single property

Solution

1. Step 1: Understand property wrappers

   Property wrappers in Swift add extra behavior to properties, such as validation.

2. Step 2: Identify validation role

   Custom validation wrappers check if values meet certain rules before storing them.

3. Final Answer:

   To automatically check and enforce rules on property values -> Option A

4. Quick Check:

   Validation = enforce rules [OK]

Hint: Validation wrappers enforce rules on data automatically [OK]

Common Mistakes:

• Thinking wrappers change property type
• Confusing validation with access control
• Assuming wrappers store multiple values

2. Which of the following is the correct syntax to declare a custom validation property wrapper named NonEmpty for a String property in Swift?

easy

A. var name: @NonEmpty String

B. var @NonEmpty name: String

C. @NonEmpty var name: String

D. NonEmpty var name: String

Solution

1. Step 1: Recall property wrapper syntax

   In Swift, property wrappers are applied with @ before the wrapper name, placed before the variable declaration.

2. Step 2: Match correct syntax

   Correct syntax is @WrapperName var propertyName: Type, so @NonEmpty var name: String is correct.

3. Final Answer:

   @NonEmpty var name: String -> Option C

4. Quick Check:

   @WrapperName before var = correct syntax [OK]

Hint: Use @ before wrapper name, then var property [OK]

Common Mistakes:

• Placing @ after var
• Putting wrapper inside type
• Omitting @ symbol

3. Consider this Swift code using a custom validation property wrapper Positive that only allows positive integers:

 @propertyWrapper
 struct Positive {
     private var value: Int = 1
     var wrappedValue: Int {
         get { value }
         set { value = newValue > 0 ? newValue : 1 }
     }
 }

 struct Account {
     @Positive var balance: Int
 }

 let acc = Account()
 print(acc.balance)

What will be printed when this code runs?

medium

A. 1

B. 0

C. nil

D. Compilation error

Solution

1. Step 1: Understand default value in wrapper

   The wrapper's private value starts at 1, and sets new values only if positive; else resets to 1.

2. Step 2: Check initial property value

   Account's balance uses @Positive but no initial value is set, so it uses wrapper's default 1.

3. Final Answer:

   1 -> Option A

4. Quick Check:

   Default wrapper value = 1 [OK]

Hint: Default wrapper value used if no initial value given [OK]

Common Mistakes:

• Expecting 0 as default
• Assuming nil is possible
• Thinking code won't compile

4. This Swift code attempts to create a custom validation property wrapper NonNegative but has an error:

 @propertyWrapper
 struct NonNegative {
     var value: Int
     var wrappedValue: Int {
         get { value }
         set { if newValue >= 0 { value = newValue } }
     }
 }

 struct Score {
     @NonNegative var points: Int = -5
 }

What is the main problem causing this code to fail?

medium

A. wrappedValue setter does not handle negative values correctly

B. Property wrapper lacks an initializer to set initial value

C. Cannot assign default value to property with wrapper

D. Missing @ symbol before property wrapper

Solution

1. Step 1: Check property wrapper requirements

   Property wrappers must have an init(wrappedValue:) to accept initial values.

2. Step 2: Identify missing initializer

   NonNegative lacks init(wrappedValue:) so compiler cannot assign -5 to points.

3. Final Answer:

   Property wrapper lacks an initializer to set initial value -> Option B

4. Quick Check:

   Missing init(wrappedValue:) = error [OK]

Hint: Add init(wrappedValue:) to accept initial values [OK]

Common Mistakes:

• Ignoring missing initializer
• Blaming setter logic
• Thinking default value assignment is invalid

5. You want to create a custom validation property wrapper LimitedLength that limits a String property's length to a maximum number of characters. Which of the following implementations correctly enforces this limit and allows setting the max length when declaring the property?

 @propertyWrapper
 struct LimitedLength {
     private var value: String = ""
     private let maxLength: Int

     var wrappedValue: String {
         get { value }
         set {
             if newValue.count <= maxLength {
                 value = newValue
             } else {
                 value = String(newValue.prefix(maxLength))
             }
         }
     }

     // Which initializer is correct?
     ???
 }

 struct User {
     @LimitedLength(maxLength: 5) var username: String
 }

hard

A. init(maxLength: Int) { self.maxLength = maxLength }

B. init(wrappedValue: String) { self.value = wrappedValue; maxLength = 5 }

C. init() { maxLength = 5 }

D. init(wrappedValue: String, maxLength: Int) { self.maxLength = maxLength; self.value = wrappedValue }

Solution

1. Step 1: Understand property wrapper initialization with parameters

   When a wrapper has parameters and initial value, it needs init(wrappedValue:maxLength:) to accept both.

2. Step 2: Match initializer to usage

   Usage passes maxLength: 5 and no initial value, so init(wrappedValue:maxLength:) fits best to set both.

3. Final Answer:

   init(wrappedValue: String, maxLength: Int) { self.maxLength = maxLength; self.value = wrappedValue } -> Option D

4. Quick Check:

   Use init(wrappedValue:maxLength:) for parameters [OK]

Hint: Use init(wrappedValue:maxLength:) to accept value and limit [OK]

Common Mistakes:

• Using init(maxLength:) only misses initial value
• Using init() without parameters
• Using init(wrappedValue:) without maxLength