Reacting to ScrollView offset

AppearanceObserver.swift

// Copyright (c) 2021-2025 Alexey Grigorev, Ivan Oparin
//
// Permission is hereby granted, free of charge, to any person obtaining a
// copy of this software and associated documentation files (the
// "Software"), to deal in the Software without restriction, including
// without limitation the rights to use, copy, modify, merge, publish,
// distribute, sublicense, and/or sell copies of the Software, and to
// permit persons to whom the Software is furnished to do so, subject to
// the following conditions:
//
// The above copyright notice and this permission notice shall be included
// in all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
// OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
// IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
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// SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

class AppearanceObserver: ObservableObject {
        private var pipelines: Set<AnyCancellable> = []
        private let timer = Timer.publish(every: 2, on: .main, in: .common)
        
        @Published var appearanceTransitionCompleted = false
        
        @Published var contentFrame: CGRect = .zero
        @Published var navBarBgOpacity: CGFloat = 0.0
        @Published var textColorProgress: CGFloat = 0.0
        let contentSpaceId = "ContentSpaceId"
        
        var willDisappear: Bool {
            appearanceTransitionCompleted
        }
        
        var willAppear: Bool {
            !appearanceTransitionCompleted
        }
        
        init() {
            initPipelines()
        }
        
        private func initPipelines() {
            timer
                .autoconnect()
                .map { date in
                    true
                }
                .removeDuplicates()
                .assign(to: &$appearanceTransitionCompleted)
            
            $contentFrame
                .map { frame in
                    print("DEBUG: contentFrame.origin.y = \(frame.origin.y)")
                    if frame.origin.y >= 0 {
                        print("DEBUG: Scrolling down or at top, textColorProgress = 0.0 (white)")
                        return 0.0
                    } else {
                        let normalized = min(abs(frame.origin.y) / 50, 1.0)
                        print("DEBUG: Pulling down, normalized = \(normalized)")
                        let baseProgress = pow(normalized, 4.0)
                        let progress = 0.2 + (0.8 * baseProgress)
                        print("DEBUG: textColorProgress = \(progress) (after pow and scaling)")
                        return progress
                    }
                }
                .removeDuplicates()
                .assign(to: &$textColorProgress)
        }