Property Observers in Swift Guide

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Guide to Property Observers in Swift

Property observers in Swift allow you to monitor and respond to changes in a property's value. They can be exceptionally useful for tasks where you need to execute some code whenever a property changes.

Introduction

Swift provides two types of property observers: willSet and didSet. These observers are called automatically when the property is about to change (for willSet) or just after it changes (for didSet).

Using willSet Observer

The willSet observer gets called right before the property value is updated. You can use willSet to track the old value before it’s modified. The new value is passed as a parameter, often named newValue by default.

class ObservableProperty {
    var value: Int = 0 {
        willSet(newVal) {
            print("Value will change to \(newVal)")
        }
    }
}


In this example, every time the value property is about to change, the current and incoming values are printed to the console.

Using didSet Observer

The didSet observer gets called immediately after the property value changes. It is mainly used to perform actions in response to the new value of the property.

class ObservableProperty {
    var value: Int = 0 {
        didSet {
            print("Value has changed to \(value)")
        }
    }
}


In this example, after the value property is updated, the new value is printed to the console.

Combining willSet and didSet

You can use both observers together to monitor a property before and after its value changes, which can be helpful for debugging and tracking state changes.

class ObservableProperty {
    var value: Int = 0 {
        willSet {
            print("Value will change from \(value) to \(newValue)")
        }
        didSet {
            print("Value changed from \(oldValue) to \(value)")
        }
    }
}


Here, we observe both the upcoming change and the final new value, allowing us to see both states for comprehensive monitoring.

Conclusion

Property observers in Swift are a powerful feature to effectively monitor and react to property changes. Whether you need to validate data, trigger updates, or track state changes, willSet and didSet provide clean and concise ways to manage these tasks.

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