Introduction
In SwiftUI, data generally flows downwards, meaning data is passed from parent views to child views. However, there are situations where you may want to pass data in the opposite direction—from a child view to a parent view.
SwiftUI’s PreferenceKey protocol is a powerful solution for these cases. PreferenceKey enables data to travel up the view hierarchy, allowing parent views to access values defined in their child views.
Creating a Custom PreferenceKey
To create a PreferenceKey, start by defining a new struct that conforms to the PreferenceKey protocol. This struct must have a defaultValue and a reduce function to aggregate multiple values.
struct MyPreferenceKey: PreferenceKey {
static var defaultValue: CGFloat = 0
static func reduce(value: inout CGFloat, nextValue: () -> CGFloat) {
value += nextValue()
}
}
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