Custom validation property wrappers in Swift - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a property wrapper in Swift?

A property wrapper is a Swift feature that lets you add extra behavior to properties, like validation or formatting, by wrapping the property’s value with custom code.

intermediate

How do you define a custom validation property wrapper?

You create a struct or class with the @propertyWrapper attribute, include a wrappedValue property, and add validation logic inside the setter of wrappedValue.

intermediate

What happens if a value fails validation in a custom property wrapper?

You can handle it by rejecting the value, setting a default, throwing an error, or logging a message, depending on how you design the wrapper.

beginner

Show a simple example of a custom validation property wrapper that ensures a string is not empty.

@propertyWrapper struct NonEmpty { private var value: String init(wrappedValue: String) { self.value = wrappedValue.isEmpty ? "Default" : wrappedValue } var wrappedValue: String { get { value } set { value = newValue.isEmpty ? "Default" : newValue } } } struct User { @NonEmpty var name: String }

beginner

Why use custom validation property wrappers instead of validating manually?

They keep your code clean by centralizing validation logic, reduce repeated code, and make properties safer and easier to maintain.

What keyword do you use to mark a custom property wrapper in Swift?

A @validate

B @wrapper

C @custom

D @propertyWrapper

The correct keyword to define a property wrapper is @propertyWrapper.

Where do you put the validation logic inside a custom property wrapper?

A Inside the init method only

B Inside the wrappedValue setter

C Outside the property wrapper

D Inside the wrappedValue getter

Validation logic is usually placed inside the setter of wrappedValue to check or modify new values.

What does the wrappedValue property represent in a property wrapper?

A The actual value being wrapped

B The name of the property

C The type of the property

D The validation function

wrappedValue holds the actual value that the property wrapper manages.

If you want to provide a default value when validation fails, where do you do it?

A In the wrappedValue setter

B In the property declaration only

C In the getter of wrappedValue

D In the main struct using the property wrapper

You provide default values or fallback logic inside the setter of wrappedValue when validation fails.

Which of these is NOT a benefit of using custom validation property wrappers?

A Centralizing validation logic

B Reducing repeated code

C Automatically fixing all bugs

D Making properties safer

Property wrappers help with validation but do not automatically fix all bugs.

Explain how to create a custom validation property wrapper in Swift and why it is useful.

Think about how you can check or change values when they are set.

Describe a simple example of a custom validation property wrapper that ensures a string is never empty.

Imagine you want to avoid empty names in a user profile.