indexPathForPreferredFocusedView未被调用

Question

我需要指定哪个单元格应该获得焦点.

根据Apple文档,indexPathForPreferredFocusedView如果remembersLastFocusedIndexPath属性是false,或者如果没有保存的索引路径,则应调用委托方法,因为之前没有单元格.

在我的情况下,我在a中使用集合视图UIViewController并设置remembersLastFocusedIndexPath为false但indexPathForPreferredFocusedView未被调用.

怎么解释这个行为？

Answer 1

You should set remembersLastFocusedIndexPath to true.

collectionView.remembersLastFocusedIndexPath = true

Answer 2

该函数indexPathForPreferredFocusedView是UICollectionViewDelegate的一部分,因此可能是委托未分配给collectionView.

或者,问题也可能出在焦点环境中,而不考虑您的UICollectionView.

作为参考,这里有一个简单的collectionView示例,其中包含5个单元格,默认情况下最初选中的单元格

import UIKit

class ViewController: UIViewController {

    private var collectionView: UICollectionView!
    private var items = ["One", "Two", "Three", "Four", "Five"]

    override var preferredFocusEnvironments: [UIFocusEnvironment] {
        return [collectionView]
    }

    override func viewDidLoad() {
        super.viewDidLoad()

        collectionView = UICollectionView(frame: view.bounds, collectionViewLayout: UICollectionViewFlowLayout())
        collectionView.remembersLastFocusedIndexPath = false
        MyCell.register(in: collectionView)
        view.addSubview(collectionView)

        collectionView.dataSource = self
        collectionView.delegate = self
    }
}

extension ViewController: UICollectionViewDataSource {

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

    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: MyCell.reuseIdentifier, for: indexPath) as! MyCell
        cell.titleLabel.text = items[indexPath.row]
        return cell
    }
}

extension ViewController: UICollectionViewDelegate {

    func indexPathForPreferredFocusedView(in collectionView: UICollectionView) -> IndexPath? {
            return IndexPath(row: 2, section: 0)
    }
}

class MyCell: UICollectionViewCell {

    static var reuseIdentifier: String { return String(describing: self) + "ReuseIdentifier" }

    var titleLabel: UILabel!

    public static func register(in collectionView: UICollectionView) {
        collectionView.register(MyCell.self, forCellWithReuseIdentifier: MyCell.reuseIdentifier)
    }

    override init(frame: CGRect) {
        super.init(frame: frame)
        titleLabel = UILabel(frame: bounds)
        backgroundColor = .blue
        contentView.addSubview(titleLabel)
    }

    override func didUpdateFocus(in context: UIFocusUpdateContext, with coordinator: UIFocusAnimationCoordinator) {
        backgroundColor = isFocused ? .red : .blue
    }

    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
    }
}