SwiftUI获取子View的frame

可以通过PreferenceKey获取子View的Frame, 直接上货

用了backgroundPreferenceValue，是因为onPreferenceChange会丢失信息，

struct FramePreferenceKey: PreferenceKey {
    static var defaultValue: CGRect = .zero
    static func reduce(value: inout CGRect, nextValue: () -> CGRect) {}
}

extension View {
    func getFrame(onChange: @escaping (CGRect) -> Void) -> some View {
        background(
            GeometryReader { geo in
                let frame = geo.frame(in: .global)
                Color.clear.preference(key: FramePreferenceKey.self, value: frame)
            }
        )
            .backgroundPreferenceValue(FramePreferenceKey.self) { frame in
                return Color.clear.onAppear {
                    onChange(frame)
                }.id(frame.maxY)
            }
    }
}