Dragging on content placed above the trackedView resets scroll position

dudek-j commented 2 years ago

Description

My contentViewController consists of a UISearchBar placed above a UITableView. When dragging down the contentViewController while holding the search bar, upon release the contentOffset of the tracked tableView gets reset to the top.

If we instead use the grabber itself and carefully drag using that, the contentOffset persists correctly.

I was able to mitigate this issue somehow by increasing the surfaceView.grabberAreaOffset to match the height of the UISearchbar. This work if the user finger remains in the grabberArea. Unfortunately, if the users finger exists the grabberArea during the swipe (this happens if we swipe down quickly), the contentOffset is reset.

Expected behavior

I should be able to drag the floating panel using static child elements without affecting the scroll position of the tracked scrollView.

Actual behavior

If the contentViewController contains views other than the tracked scroll view, the offset of the scroll view is reset every time we drag using one of those views.

Steps to reproduce

Issue can be reproduced using the tiny code sample below or the map example in the FloatingPanel repository
Extend panel to full position
Drag up on the scrollView so it reaches the bottom
Drag down using the red are. You can drag just a tiny bit, the state of the panel does not have to change.
Release finger, contentOffset will reset to top.

Additional Example

Set fpc.surfaceView.grabberAreaOffset = 200 ( The height of the red area )
Place your finger within the red area, but at the very bottom.
Drag quickly down so your finger exits the red area
When releasing the finger, contentOffset will reset to top.

Code example that reproduces the issue

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, constant: 200),
            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
    }
}

How do you display panel(s)?

Add as child view controllers

How many panels do you displays?

1

Environment

Library version

Tested on 2.0.1 and 2.5.1 Issues occur on both

Installation method

CocoaPods
Swift Package Manager

iOS version(s) 15.2

Xcode version 13.2.1

dudek-j commented 2 years ago

After looking into this issue further it seems like setting the grabberAreaOffset is the intended solution to my issue. This should perhaps be added to the documentation and perhaps the maps example.

Unfortunately the second part of the bug is still there, if the pan interaction exits the grabber frame during the gesture the content offset is reset. This can fairly easily replicated in the simulator as shown in the video below.

https://user-images.githubusercontent.com/16480280/150013177-d1c883c5-5616-4472-a3bc-a589b3ce633b.mov

After poking around I noticed that the contentOffset reset occurs specifically in this code fragment.

It seems like using the current scrollView offset instead of the initial one solves the issue while still stopping the scrolling as intended

I am not sure if this has any potential side effects. I created a corresponding PR.

dudek-j commented 2 years ago

Resolved in 2.5.2 🎉

scenee / FloatingPanel