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When should you use willSet rather than didSet?

Paul Hudson    @twostraws   

Updated for Xcode 12.0

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.

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