Use scrollView.contentOffset when stopping ongoing scroll deceleration

[dudek-j]

Applies fixes as described in #526

[scenee]

Thank you for your PR. Unfortunately, I found a problem where the content offset of a tracking scroll view sometimes slips after I swipe a panel up from the half state to full state. I will upload a screen record later.

[dudek-j]

Please post your findings, I will try to further look into this issue.

[scenee]

Here is a screen record when I reproduced this issue.

content-slips.mp4

I guess we might need to modify a condition where initialScrollOffset is set to a content offset to fix #526 🤔

[dudek-j]

I started investigating how initalScrollOffset is set. I am having a bit of trouble figuring out what the purpose of the contentOffsetForPinning(of: scrollView) on line 804 is

FloatingPanel/Sources/Core.swift

Lines 801 to 806 in e39f634

	if grabberAreaFrame.contains(location) {
	initialScrollOffset = scrollView.contentOffset
	} else {
	initialScrollOffset = contentOffsetForPinning(of: scrollView)
	let offsetDiff = scrollView.contentOffset - contentOffsetForPinning(of: scrollView)
	switch layoutAdapter.position {

I noticed that replacing it with initialScrollOffset = scrollView.contentOffset fixes #526 without causing the problems you showed above. I updated the PR accordingly.

[dudek-j]

In general it seems as if either func floatingPanel(FloatingPanelController, contentOffsetForPinning: UIScrollView) -> CGPoint is broken, or I am not able to figure out the intended effect.

[scenee]

I confirmed your PR it's working well! And then thanks to your comments, I knew the root cause is Core.contentOffsetForPinning(of scrollView: UIScrollView) implementation. (Thank you so much 👍 )

So I pushed the commit of an alternative solution to update the behavior of Core.contentOffsetForPinning(of scrollView: UIScrollView).

This solution is a bit side effect but it works func floatingPanel(_ fpc: FloatingPanelController, contentOffsetForPinning trackingScrollView: UIScrollView) of FloatingPanelControllerDelegate is working well as before. As a result, a library consumer is able to fix the side effect using the delegate method.

[dudek-j]

Glad I could help!

Unfortunately I am getting some really strange behaviour after your changes to the implementation of Core.contentOffsetForPinning(of scrollView: UIScrollView) is changed. The scrollView of the panel is locked and no longer allows the user to get to the top. As shown in the example below, I am dragging on the tableView.

Simulator.Screen.Recording.-.iPhone.13.Pro.-.2022-01-31.at.12.30.18.mp4

[scenee]

Oh my gosh... Does the example code implement func floatingPanel(_ fpc: FloatingPanelController, contentOffsetForPinning trackingScrollView: UIScrollView) delegate method? If not, 5181ccd should be logically the same as de34fda 🤔

[dudek-j]

Create a new StoryBoard project and paste this into the ViewController file, the example is as basic as possible.

I think the issue with: 5181ccd is that Core.contentOffsetForPinning(of scrollView: UIScrollView) is used in other places than just line 804.

Meanwhile the change in de34fda is localised too that single place.

///
//  ViewController.swift
//  FloatingPanelLab
//
//  Created by Jakub Dudek on 2022-01-18.
//

import UIKit
import FloatingPanel

class ViewController: UIViewController {

    private lazy var floatingPanelController: FloatingPanelController = {
        let fpc = FloatingPanelController()
        fpc.set(contentViewController: tableViewController)
        fpc.track(scrollView: tableViewController.tableView)
        return fpc
    }()
    
    private lazy var tableViewController: TableViewController = {
        let vc = TableViewController()
        return vc
    }()
    
    override func viewDidLoad() {
        super.viewDidLoad()
        view.backgroundColor = .yellow
    
        view.addSubview(floatingPanelController.view)
        floatingPanelController.view.frame = view.bounds
        addChild(floatingPanelController)
        floatingPanelController.didMove(toParent: self)
        floatingPanelController.show()
    }
    
}

class TableViewController: UIViewController, UITableViewDelegate, UITableViewDataSource {
    
    let tableView = UITableView()
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        view.backgroundColor = .red
        tableView.translatesAutoresizingMaskIntoConstraints = false
        tableView.dataSource = self
        tableView.delegate = self
        
        view.addSubview(tableView)
         
        NSLayoutConstraint.activate([ 
            tableView.topAnchor.constraint(equalTo: view.topAnchor),
            tableView.leadingAnchor.constraint(equalTo: view.leadingAnchor),
            tableView.trailingAnchor.constraint(equalTo: view.trailingAnchor),
            tableView.bottomAnchor.constraint(equalTo: view.bottomAnchor)
        ])
        
        tableView.register(UITableViewCell.self, forCellReuseIdentifier: "Cell")
    }
    
    func numberOfSections(in tableView: UITableView) -> Int {
        return 1
    }    
    
    func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        return 20
    }
    
    func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
        let cell = tableView.dequeueReusableCell(withIdentifier: "Cell", for: indexPath)
        cell.textLabel?.text = "\(indexPath.row)"
        return cell
    }
    
} 

[dudek-j]

The issue is with Core.allowScrollPanGesture(for scrollView: UIScrollView).

    private func allowScrollPanGesture(for scrollView: UIScrollView) -> Bool {
        guard state == layoutAdapter.mostExpandedState else { return false }
        var offsetY: CGFloat = 0
        switch layoutAdapter.position {
        case .top, .left:
            offsetY = value(of: scrollView.fp_contentOffsetMax - scrollView.contentOffset)
        case .bottom, .right:
            offsetY = value(of: scrollView.contentOffset - contentOffsetForPinning(of: scrollView))
        }
        return offsetY <= -30.0 || offsetY > 0
    }

offsetY will always resolve to 0 with the current Core.contentOffsetForPinning(of scrollView: UIScrollView) implementation.

[scenee]

Thanks to your detail comments, I've realized I misunderstood that de34fda affected func floatingPanel(_ fpc: FloatingPanelController, contentOffsetForPinning trackingScrollView: UIScrollView)method. I tested it on a wrong condition. The commit still works for the delegate method! I'm really sorry. I will removed 5181ccd and merge this PR to the main branch 👍

[dudek-j]

No reason to apologise, glad to hear that things are working as intended. I really enjoy using your library, very happy that I could contribute a little.

[scenee]

Thank you for saying that. I'm so glad.