willSet

willSet is a Swift language feature called a property observer. It runs immediately before a stored property’s value is updated, allowing code to react to the impending change. willSet applies to stored properties declared with var and is not invoked for constant properties (let) or for computed properties. It is also not called during the initial assignment of a property’s initial value.

Syntax and parameters: You declare a willSet block inside the property declaration. The block can name the incoming value, for example willSet(newValue) { ... }, or you can omit the parameter list and refer to the upcoming value as newValue. Inside the block you can read the newValue, but you cannot modify the stored value from within willSet to cancel or alter the assignment. If you provide a name, you use that name inside the block; otherwise newValue is available implicitly.

Behavior: willSet runs before the actual storage takes place, while didSet runs after the new value has been stored. In didSet you can access oldValue to see the previous value. Neither observer is called during the property’s initialization when a default value is assigned.

Example:

var progress: Int = 0 {

willSet {

print("Progress will be set to \(newValue)")

}

didSet {

print("Progress changed from \(oldValue) to \(progress)")

}

}

Common uses: logging changes, validating or reacting to impending updates, updating related state or UI, or enforcing constraints before the new value is stored (though complex validation may require a computed property or a custom setter).