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

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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.