add read-size code snippet

SwiftUI-read-a-view-size/README.md

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+Code snippet from [How to read a view size in SwiftUI][fs].
+
+[fs]: https://fivestars.blog/articles/swiftui-share-layout-information/

SwiftUI-read-a-view-size/View+readSize.swift

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+// Original article here: https://fivestars.blog/articles/swiftui-share-layout-information/
+
+import SwiftUI
+
+/*
+
+Example:
+
+var body: some View {
+  childView
+    .readSize { newSize in
+      print("The new child size is: \(newSize)")
+    }
+}
+
+*/
+
+extension View {
+  func readSize(onChange: @escaping (CGSize) -> Void) -> some View {
+    background(
+      GeometryReader { geometryProxy in
+        Color.clear
+          .preference(key: SizePreferenceKey.self, value: geometryProxy.size)
+      }
+    )
+    .onPreferenceChange(SizePreferenceKey.self, perform: onChange)
+  }
+}
+
+private struct SizePreferenceKey: PreferenceKey {
+  static var defaultValue: CGSize = .zero
+  static func reduce(value: inout CGSize, nextValue: () -> CGSize) {}
+}