How to detect Screen Size changes on iPad?

How can I detect the Screen Size change in my iPad application? An app on the iPad can be displayed in Portrait and Landscape modes, but also in Split View.

I'm thinking about observing the size with GeometryReader like:

GeometryReader { proxy in
...
}
.onChange(of: proxy.size) { newSize in
//Size changed, react accordingly
}

Or use PreferenceKey:

GeometryReader { proxy in
    Color.clear.preference(key: SizePreferenceKey.self, value: proxy.size)
}
.onPreferenceChange(SizePreferenceKey.self) { newValue in
//Size changed, react accordingly
}

Both options you provided can work for detecting screen size changes in your iPad application. The choice between them depends on your specific use case.
The first option using onChange(of:) is a simple and direct way to react to changes in size. It's a good choice if you want to trigger an action immediately when the screen size changes.
The second option using onPreferenceChange is useful if you want to collect information about the size changes for later use. By using a preference key, you can collect information from multiple sources in your app and then react to the changes all at once.
In either case, using GeometryReader is a good choice for detecting size changes, as it provides a view's size as a parameter to the closure. This means you can easily observe changes to the size of the view, and take action as needed.
However, keep in mind that GeometryReader provides the size of the view it's applied to, not the screen size. If you want to detect changes to the screen size specifically, you may want to consider using the UIScreen class instead.
Keywords: 
Detect screen size changes
iPad screen size
SwiftUI screen size
GeometryReader
Split View
iPad app development
iOS app development
User interface design
Responsive design
Adaptive design