//
//  UIScrollView+Top.swift
//  Tusker
//
//  Created by Shadowfacts on 11/1/22.
//  Copyright © 2022 Shadowfacts. All rights reserved.
//

import UIKit

private let prevScrollOffsetBeforeScrollToTopKey = UnsafeMutableRawPointer.allocate(byteCount: 0, alignment: 0)

extension UIScrollView {
    private var prevScrollOffsetBeforeScrollToTop: CGFloat? {
        get {
            if let v = (objc_getAssociatedObject(self, prevScrollOffsetBeforeScrollToTopKey) as? NSNumber)?.doubleValue {
                return CGFloat(v)
            } else {
                return nil
            }
        }
        set {
            if let newValue {
                objc_setAssociatedObject(self, prevScrollOffsetBeforeScrollToTopKey, NSNumber(value: newValue), .OBJC_ASSOCIATION_COPY_NONATOMIC)
            } else {
                objc_setAssociatedObject(self, prevScrollOffsetBeforeScrollToTopKey, nil, .OBJC_ASSOCIATION_COPY_NONATOMIC)
            }
        }
    }
    
    func scrollToTop() {
        let top = -adjustedContentInset.top
        // +5 to add a little bit of wiggle room
        let isScrolledToTop = contentOffset.y < top + 5
        if isScrolledToTop {
            if let prevScrollOffsetBeforeScrollToTop {
                self.prevScrollOffsetBeforeScrollToTop = nil
                setContentOffset(CGPoint(x: 0, y: prevScrollOffsetBeforeScrollToTop), animated: true)
            }
        } else {
            prevScrollOffsetBeforeScrollToTop = contentOffset.y
            setContentOffset(CGPoint(x: 0, y: top), animated: true)
        }
    }
}