Custom validation property wrappers in Swift - Step-by-Step Execution

Concept Flow - Custom validation property wrappers

Define Property Wrapper

↓

Add Validation Logic

↓

Apply Wrapper to Property

↓

Set Property Value

↓

Validate Value Automatically

↓

Accept or Reject Value

This flow shows how a custom property wrapper is defined with validation logic, applied to a property, and automatically validates values when set.

Execution Sample

Swift

@propertyWrapper
struct NonEmpty {
  private var value: String = ""
  var wrappedValue: String {
    get { value }
    set {
      if newValue.isEmpty { print("Error: Empty string") } else { value = newValue }
    }
  }
}

struct User {
  @NonEmpty var name: String
}

var user = User()
user.name = "Alice"
user.name = ""

This code defines a property wrapper that validates a string is not empty, then applies it to a User's name property and tests setting valid and invalid values.

Execution Table

Step	Action	Value Set	Validation Check	Result	Output
1	Create User instance	-	-	name = ""	-
2	Set user.name = "Alice"	"Alice"	Is empty? No	Value accepted	-
3	Set user.name = ""	""	Is empty? Yes	Value rejected	Error: Empty string

💡 Execution stops after last assignment; invalid empty string rejected by validation.

Variable Tracker

Variable	Start	After Step 2	After Step 3
user.name	""	"Alice"	"Alice" (unchanged due to validation)

Key Moments - 2 Insights

Why does user.name not become empty after setting it to ""?

Because the validation in the property wrapper rejects empty strings (see execution_table step 3), so the value remains the previous valid string.

When is the validation logic triggered?

Validation runs every time the wrapped property is set (see execution_table steps 2 and 3), automatically inside the setter of wrappedValue.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of user.name after step 3?

A "" (empty string)

B "Alice"

C nil

D Error

💡 Hint

Check the 'Result' and 'Value Set' columns in step 3 showing rejection and unchanged value.

At which step does the validation detect an invalid value?

A Step 1

B Step 2

C Step 3

D No validation occurs

💡 Hint

Look at the 'Validation Check' column for when the empty string is detected.

If the validation allowed empty strings, what would change in variable_tracker after step 3?

A user.name would be ""

B user.name would be "Alice"

C user.name would be nil

D No change

💡 Hint

Refer to variable_tracker showing value remains "Alice" due to rejection.

Concept Snapshot

@propertyWrapper struct WrapperName {
  private var value: Type
  var wrappedValue: Type {
    get { value }
    set {
      // validation logic here
      if valid(newValue) { value = newValue }
    }
  }
}

Use @WrapperName on properties to auto-validate on set.

Full Transcript

This visual trace shows how to create a custom validation property wrapper in Swift. First, the wrapper struct is defined with a private stored value and a wrappedValue property that runs validation in its setter. When applied to a property, any assignment triggers the validation automatically. In the example, setting user.name to "Alice" passes validation and updates the value. Setting it to an empty string fails validation, prints an error, and keeps the old value. The variable tracker confirms the value remains unchanged after invalid input. This helps ensure properties always hold valid data without extra checks outside the wrapper.

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