Property observers (willSet, didSet) in Swift - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of willSet in Swift property observers?

willSet runs just before the property value changes. It lets you react to the new value before it is stored.

beginner

What does didSet do in Swift property observers?

didSet runs right after the property value changes. It lets you respond to the new value after it is stored.

intermediate

Can willSet and didSet be used together on the same property?

Yes, you can use both willSet and didSet on the same property to run code before and after the value changes.

intermediate

What is the default parameter name inside willSet and didSet closures?

In willSet, the new value is available as newValue. In didSet, the old value is available as oldValue.

intermediate

Can property observers be added to computed properties in Swift?

No, property observers willSet and didSet only work with stored properties, not computed properties.

When does willSet run in Swift property observers?

A Only when the property is first set

B Right after the property value changes

C Right before the property value changes

D Only when the property is read

willSet runs just before the property value changes, allowing you to react to the new value before it is stored.

What is the default name for the new value inside willSet?

A newValue

B nextValue

C value

D oldValue

The new value about to be set is available as newValue inside willSet.

Which property observer runs after the property value changes?

A willSet

B didSet

C init

D deinit

didSet runs immediately after the property value changes.

Can you use property observers on computed properties?

A No, never

B Yes, always

C Only if they have a setter

D Only if they are constants

Property observers cannot be added to computed properties, only to stored properties.

What is the default name for the old value inside didSet?

A lastValue

B newValue

C previous

D oldValue

The old value before the change is available as oldValue inside didSet.

Explain how willSet and didSet work in Swift property observers and give a simple example.

Think about when each observer runs relative to the property value change.

Describe the difference between stored and computed properties in Swift and why property observers only work with stored properties.

Consider how computed properties calculate values on the fly.