NEW: Start my new Ultimate Portfolio App course with a free Hacking with Swift+ trial! >>

When should you use willSet rather than didSet?

Paul Hudson    @twostraws   

Updated for Xcode 12.5

Both willSet and didSet let us attach observers to properties, meaning that Swift will run some code when they change so that we have a chance to respond to the change. The question is: do you want to know before the property changes, or after?

The simple answer is this: most of the time you will be using didSet, because we want to take action after the change has happened so we can update our user interface, save changes, or whatever. That doesn’t mean willSet isn’t useful, it’s just that in practice it is significantly less popular than its counterpart.

The most common time willSet is used is when you need to know the state of your program before a change is made. For example, SwiftUI uses willSet in some places to handle animations so that it can take a snapshot of the user interface before a change. When it has both the “before” and “after” snapshot, it can compare the two to see all the parts of the user interface that need to be updated.

Hacking with Swift is sponsored by Emerge

SPONSORED Emerge helps iOS devs write better, smaller apps by profiling binary size on each pull request and surfacing insights and suggestions. Companies using Emerge have reduced the size of their apps by up to 50% in just the first day. Built by a team with years of experience reducing app size at Airbnb.

Set up a demo!

Sponsor Hacking with Swift and reach the world's largest Swift community!

BUY OUR BOOKS
Buy Pro Swift Buy Swift Design Patterns Buy Testing Swift Buy Hacking with iOS Buy Swift Coding Challenges Buy Swift on Sundays Volume One Buy Server-Side Swift (Vapor Edition) Buy Advanced iOS Volume One Buy Advanced iOS Volume Two Buy Advanced iOS Volume Three Buy Hacking with watchOS Buy Hacking with tvOS Buy Hacking with macOS Buy Dive Into SpriteKit Buy Swift in Sixty Seconds Buy Objective-C for Swift Developers Buy Server-Side Swift (Kitura Edition) Buy Beyond Code

Was this page useful? Let us know!

Average rating: 4.3/5

 
Unknown user

You are not logged in

Log in or create account
 

Link copied to your pasteboard.