Open Radar

 
Area:
UIKit

Summary:
When calling `UICollectionView.performBatchUpdates` then soon after calling `UICollectionView.reloadData`, the `UICollectionView` is not properly reloaded. In this bad case, `UICollectionViewDataSource.collectionView(_:cellForItemAt:)` is never called for the `reloadData`.

This issue is not experience if `UICollectionView.reloadSections` is used in place of `reloadData`.

Here are some example method traces from the demo project:

Here we have `performBatchUpdates` followed by `reloadData` and we can see that no call is made to `UICollectionViewDataSource.collectionView(_:cellForItemAt:)` as expected with `reloadData`.
```
16:39:25.944 CollectionView.performBatchUpdates
16:39:25.945 ViewController.performBatchUpdates update closure
16:39:25.946 ViewController.collectionView(_:layout:sizeForItemAt: 0-0)
16:39:25.946 ViewController.collectionView(_:layout:sizeForItemAt: 0-1)
16:39:25.947 ViewController.collectionView(_:layout:sizeForItemAt: 0-2)
16:39:25.947 ViewController.collectionView(_:layout:sizeForItemAt: 0-3)
16:39:25.950 CollectionView.reloadData
16:39:25.951 ViewController.collectionView(_:layout:sizeForItemAt: 0-0)
16:39:26.000 ViewController.collectionView(_:layout:sizeForItemAt: 0-1)
16:39:26.000 ViewController.collectionView(_:layout:sizeForItemAt: 0-2)
16:39:26.000 ViewController.collectionView(_:layout:sizeForItemAt: 0-3)
16:39:26.001 ViewController.performBatchUpdates complete closure
16:39:26.002 CollectionView.layoutSubviews
```

Here we have `performBatchUpdates` followed by `reloadSections` and we can see that calls are properly made to `UICollectionViewDataSource.collectionView(_:cellForItemAt:)` as expected.
```
17:13:28.620 CollectionView.performBatchUpdates
17:13:28.621 ViewController.performBatchUpdates update closure
17:13:28.622 ViewController.collectionView(_:layout:sizeForItemAt: 0-0)
17:13:28.622 ViewController.collectionView(_:layout:sizeForItemAt: 0-1)
17:13:28.622 ViewController.collectionView(_:layout:sizeForItemAt: 0-2)
17:13:28.623 ViewController.collectionView(_:layout:sizeForItemAt: 0-3)
17:13:28.625 CollectionView.reloadSections
17:13:28.625 ViewController.collectionView(_:layout:sizeForItemAt: 0-0)
17:13:28.625 ViewController.collectionView(_:layout:sizeForItemAt: 0-1)
17:13:28.626 ViewController.collectionView(_:layout:sizeForItemAt: 0-2)
17:13:28.626 ViewController.collectionView(_:layout:sizeForItemAt: 0-3)
17:13:28.627 ViewController.collectionView(_:cellForItemAt: 0-0)
17:13:28.629 ViewController.collectionView(_:cellForItemAt: 0-1)
17:13:28.629 ViewController.collectionView(_:cellForItemAt: 0-2)
17:13:28.630 ViewController.collectionView(_:cellForItemAt: 0-3)
17:13:28.631 CollectionView.layoutSubviews
17:13:28.633 ViewController.performBatchUpdates complete closure
17:13:28.633 CollectionView.layoutSubviews
17:13:29.002 CollectionView.layoutSubviews
```

Steps to Reproduce:
1. Call `UICollectionView.performBatchUpdates(nil, completion: nil)`
2. Optionally, update the UICollectionViewDataSource (this will help visualize the issue)
3. Call `UICollectionView.reloadData()` immediately afterward (all of this in the same method is the easiest)

Expected Results:
All cells are reloaded by calling `UICollectionViewDataSource.collectionView(_:cellForItemAt:)`.

Observed Results:
`UICollectionViewDataSource.collectionView(_:cellForItemAt:)` is never called.

Version:
iOS 10.2.1 (14D27), iOS Simulator 10.2 (14C89), Xcode Version 8.2.1 (8C1002), iOS Simulator 10.3 (14E269), Xcode Version 8.3.2 (8E2002)

Notes:
For using the included demo project:

There are some notes at the top of ViewController.swift, the main file.

1. Run the project on any device or simulator.
2. Tap the button labeled "Bad"
  - You should see that the cells change size but do not reload (they should reload to yellow). If you scroll around you will see that the cells do correctly configure when scrolled on to the screen.
3. Tap the button labeled "Reset"
  - This will reset the example to its original state
4. Tap the button labeled "Good"
  - You should see the proper reloading behavior since this uses `reloadSections`
5. Repeat 2-4 after changing `DISABLE_ANIMATIONS` at the top of the ViewController.swift file to `true`. This just demonstrates the same issue occurs when not using animations for the animated UICollectionView methods.

Configuration:
iPhone 7 Plus 128G AT&T, iPhone Simulator




Sample Project include (you only need the ViewController):
=====================================================================
//
//  ViewController.swift
//  CollectionViewBug
//
//  Created by Sam Miller on 3/22/17.
//  Copyright © 2017 Sam Miller. All rights reserved.
//
//  A view controller demonstrating a bug in UICollectionView reloadData.
//  To try, simply create an empty application in Xcode and create an instance of
//  ViewController as the rootViewController.
//
//  You will then see that the "Bad" button does not correctly reload the UICollectionView
//  whereas the "Good" button will correctly reload the cells. "Bad" incorrectly reloads
//  the cells by updating the layout but not reconfiguring the cells.
//
//  You can also try these methods both with animations left on, or with animations disabled. 
//  Just change the global variable at the top of this file, DISABLE_ANIMATIONS. This will enabled
//  or disable wrapping the UICollectionView reload methods with UIView.performWithoutUpdates.
//

import UIKit

let DISABLE_ANIMATIONS = false

class CollectionView: UICollectionView {
    override func layoutSubviews() {
        NSLog("CollectionView.layoutSubviews")
        super.layoutSubviews()
    }

    override func performBatchUpdates(_ updates: (() -> Void)?, completion: ((Bool) -> Void)? = nil) {
        NSLog("CollectionView.performBatchUpdates")
        super.performBatchUpdates(updates, completion: completion)
    }

    override func reloadData() {
        NSLog("CollectionView.reloadData")
        super.reloadData()
    }

    override func reloadSections(_ sections: IndexSet) {
        NSLog("CollectionView.reloadSections")
        super.reloadSections(sections)
    }
}

class DemoCell: UICollectionViewCell {

    let label = UILabel()

    public override init(frame: CGRect) {
        super.init(frame: frame)

        label.textAlignment = .center
        label.numberOfLines = 0
        contentView.addSubview(label)
    }

    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

    override func layoutSubviews() {
        super.layoutSubviews()

        label.frame = contentView.bounds
    }
}

struct CellData {
    let id: String
    let color: UIColor
    let height: CGFloat
}

class ViewController: UIViewController, UICollectionViewDelegateFlowLayout, UICollectionViewDataSource {

    static let defaultCells: [CellData] = [
        CellData(id: "0", color: .red,     height: 250),
        CellData(id: "1", color: .green,   height: 120),
        CellData(id: "2", color: .blue,    height: 200),
        CellData(id: "3", color: .purple,  height: 300),
        CellData(id: "4", color: .gray,    height: 250),
        CellData(id: "5", color: .orange,  height: 100),
        CellData(id: "5", color: .magenta, height: 50)
    ]

    let collectionViewLayout = UICollectionViewFlowLayout()
    let collectionView: UICollectionView
    let header = UIView()
    let button1 = UIButton(type: UIButtonType.custom)
    let button2 = UIButton(type: UIButtonType.custom)
    let button3 = UIButton(type: UIButtonType.custom)

    var cells: [CellData] = ViewController.defaultCells

    init() {
        collectionView = CollectionView(frame: .zero, collectionViewLayout: collectionViewLayout)
        super.init(nibName: nil, bundle: nil)
    }

    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

    override func viewDidLoad() {
        NSLog("ViewController.viewDidLoad")
        super.viewDidLoad()
        view.backgroundColor = .white
        collectionView.backgroundColor = .white
        view.addSubview(collectionView)

        collectionView.register(DemoCell.self, forCellWithReuseIdentifier: "cell")
        collectionView.dataSource = self
        collectionView.delegate = self
        setupHeader()
    }

    override func viewDidLayoutSubviews() {
        super.viewDidLayoutSubviews()

        collectionView.frame = view.bounds
        layoutHeader()
    }

    // MARK: CollectionView data

    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return cells.count
    }

    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        NSLog("ViewController.collectionView(_:cellForItemAt: \(indexPath.map({ "\(Int($0))" }).joined(separator: "-")))")
        let c = collectionView.dequeueReusableCell(withReuseIdentifier: "cell", for: indexPath)
        guard let cell = c as? DemoCell else { return c }

        let model = cells[indexPath.item]

        cell.label.text = "Cell \(model.id)"
        cell.backgroundColor = model.color

        return cell
    }

    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
        NSLog("ViewController.collectionView(_:layout:sizeForItemAt: \(indexPath.map({ "\(Int($0))" }).joined(separator: "-")))")
        return CGSize(width: view.bounds.width, height: cells[indexPath.item].height)
    }

    // MARK: Actions

    func badReload() {
        invalidateLayout()

        cells = yellowCells()

        UIView.performWithoutAnimation(DISABLE_ANIMATIONS) {
            collectionView.reloadData()
        }
    }

    func goodReload() {
        invalidateLayout()

        cells = yellowCells()

        UIView.performWithoutAnimation(DISABLE_ANIMATIONS) {
            collectionView.reloadSections(IndexSet(0..<collectionView.numberOfSections))
        }
    }

    func invalidateLayout() {
        UIView.performWithoutAnimation(DISABLE_ANIMATIONS) {
            collectionView.performBatchUpdates({
                NSLog("ViewController.performBatchUpdates update closure")
                }, completion: { (finished) in
                    NSLog("ViewController.performBatchUpdates complete closure")
            })
        }
    }

    func reset() {
        cells = ViewController.defaultCells

        collectionView.reloadData()
        collectionView.layoutIfNeeded()
    }

    // MARK: Buttons

    func handleButton1() {
        badReload()
    }

    func handleButton2() {
        goodReload()
    }

    func handleButton3() {
        reset()
    }

    func yellowCells() -> [CellData] {
        return [
            CellData(id: "A", color: .yellow, height: 100),
            CellData(id: "B", color: .yellow, height: 200),
            CellData(id: "C", color: .yellow, height: 300),
            CellData(id: "D", color: .yellow, height: 200),
            CellData(id: "E", color: .yellow, height: 100),
            CellData(id: "F", color: .yellow, height: 50),
            CellData(id: "G", color: .yellow, height: 400)
        ]
    }

    // MARK: Buttons Header

    func setupHeader() {
        collectionView.contentInset.bottom = 50

        header.backgroundColor = .lightGray
        view.addSubview(header)

        header.addSubview(button1)
        button1.setTitle("Bad", for: .normal)
        button1.addTarget(self, action: #selector(handleButton1), for: .touchUpInside)
        button1.backgroundColor = .darkGray

        header.addSubview(button2)
        button2.setTitle("Good", for: .normal)
        button2.addTarget(self, action: #selector(handleButton2), for: .touchUpInside)
        button2.backgroundColor = .darkGray

        header.addSubview(button3)
        button3.setTitle("Reset", for: .normal)
        button3.addTarget(self, action: #selector(handleButton3), for: .touchUpInside)
        button3.backgroundColor = .darkGray
    }

    func layoutHeader() {
        header.frame = view.bounds
        header.frame.origin.y = view.bounds.height - 50

        header.frame.size.height = 50

        let buttonWidth = (header.bounds.width - 5) / 3 - 5
        button1.frame = CGRect(x: (5 + buttonWidth) * 0 + 5, y: 5, width: buttonWidth, height: 40)
        button2.frame = CGRect(x: (5 + buttonWidth) * 1 + 5, y: 5, width: buttonWidth, height: 40)
        button3.frame = CGRect(x: (5 + buttonWidth) * 2 + 5, y: 5, width: buttonWidth, height: 40)
    }
}

extension UIView {
    open class func performWithoutAnimation(_ isWithoutAnimation: Bool, _ actionsWithoutAnimation: () -> Swift.Void) {
        if isWithoutAnimation {
            self.performWithoutAnimation(actionsWithoutAnimation)
        } else {
            actionsWithoutAnimation()
        }
    }
}