Difference between @StateObject and @ObservedObject on SwiftUI

 Difference between @StateObject and @ObservedObject on SwiftUI

Both @StateObject and @ObservedObject are SwiftUI property wrappers used to manage state for reference types.

They allow your SwiftUI views to react and re-render when changes occur in the observed object's published properties(@Published)

However, their fundamental difference lies in their ownership and lifecycle management within the SwiftUI view hierarchy.

@StateObject

Purpose: 

To create and own an instance of an ObservableObject within a view.

Lifecycle:

- When a view declares a @StateObject, SwiftUI takes responsibility for creating that instance and keeping it alive for the entire lifetime of the view, regardless of whether the view itself is redrawn or recreated.

- The object is instantiated only once when the view is first created.

- It's ideal for data models that are unique to a single view or are the source of truth for a portion of your app's state

Use Cases:

- When a view needs to manage its own complex state that can be encapsulated in an ObservableObject.

- A view that manages a data store for its subviews.

- A view that fetches data from a network or database and holds that data.

- When an object needs to persist its state even if the view's body is recomputed multiple times.

@ObservedObject

Purpose: 

To observe an instance of an ObservableObject that is created and owned elsewhere.

Lifecycle:

- When a view declares an @ObservedObject, it expects to receive an existing instance of the ObservableObject from its environment or as a parameter.

- SwiftUI does not manage the lifecycle of the object itself. If the view is recreated, the @ObservedObject will point to the new instance, potentially losing state from the previous one.

- If the object changes to a different instance of the same type, SwiftUI will treat it as a new object, and the view will observe the new instance.

Use Cases:

- When passing an ObservableObject down the view hierarchy to a child view.

- When a child view needs to observe changes to an object that's managed by its parent or a shared data store.

- To share an ObservableObject that's already owned by a @StateObject or @EnvironmentObject in an ancestor view.