Скачать или смотреть Understanding Why @ StateObject Doesn't Update Views Like @ ObservedObject in SwiftUI

Explore the underlying issues of SwiftUI's `@ StateObject` and `@ ObservedObject`, and learn the best practices for updating your views.
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Why @ StateObject Doesn't Update Views Like @ ObservedObject in SwiftUI

Working with data in SwiftUI can sometimes lead to confusion, especially when dealing with property wrappers like @ StateObject and @ ObservedObject. Many developers encounter an issue where views using @ StateObject do not update when the underlying data changes, while those using @ ObservedObject do. In this guide, we will explore this problem in detail and provide a solution to ensure your views reflect data changes as expected.

The Problem: View Not Updating

When working in a SwiftUI application, you may notice that using @ StateObject results in your view not updating with new data, while using @ ObservedObject does. For example, if you have a DBViewModel that you're using to manage data from a Realm database, instantiating it as a @ StateObject in your view might lead to your view not updating as you would expect.

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In this setup, the view does not refresh automatically when modelData changes unless the entire view is rebuilt. This can be frustrating, especially if you're saving and loading data dynamically from the database.

Understanding the Mechanism

State and StateObject

When you use @ StateObject, you are creating a source of truth for your data that outlasts the view's body being invalidated and redrawn. However, the core function of @ StateObject is to create and manage the lifecycle of the instance of the object. This can lead to unintended behavior if you do not manage updates correctly.

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In certain situations, you'll need to manually send an update signal to your subscribers, which may not occur automatically with @ StateObject. As a result, your @ StateObject view might not re-render with the latest state changes unless it's explicitly instructed to do so.

ObservedObject vs StateObject

On the other hand, if you use @ ObservedObject, this property wrapper allows your view to respond to changes in an observable object it monitors. If the parent view redraws, the @ ObservedObject is recreated, hence the view reflects the changes immediately. Thus, when you switch to @ ObservedObject, your data updates are more responsive.

The Solution: Using EnvironmentObject Correctly

If your DBViewModel is already available as an @ EnvironmentObject, consider why you might be trying to recreate it within your view. Instead of using @ StateObject, leverage @ ObservedObject or @ EnvironmentObject as follows:

Remove @ StateObject: If the DBViewModel is already provided in the environment, it isn't necessary to instantiate it again in the view.

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Use ObservedObject: If you're passing the model directly without it being cached in the environment, use @ ObservedObject so that changes propagate properly without manual intervention.

Conclusion

By understanding the nuances between @ StateObject, @ ObservedObject, and @ EnvironmentObject in SwiftUI, you can effectively manage your data and ensure your views update as intended. Always aim to use the correct property wrapper based on the lifecycle you aim to maintain for the data model. This will save you many headaches and help to create smooth, interactive UI experiences in your iOS applications.

Make sure to incorporate these principles into your SwiftUI projects, and watch your views update effortlessly as data changes!