没有调用indexPathForPreferredFocusedView [英] indexPathForPreferredFocusedView is not being Called

问题描述

我需要指定哪个单元格应该获得焦点.

I need to specify which cell should receive focus.

根据Apple文档，如果remembersLastFocusedIndexPath属性为false，或者由于先前没有单元格而没有保存的索引路径，则应调用indexPathForPreferredFocusedView委托方法.

According to Apple Documentation the indexPathForPreferredFocusedView delegate method should be called if the remembersLastFocusedIndexPath property is false, or if there is no saved index path because no cell was previously focused.

就我而言，我在UIViewController中使用集合视图，并将remembersLastFocusedIndexPath设置为false，但未调用indexPathForPreferredFocusedView.

In my case I am using a collection view in a UIViewController and setting remembersLastFocusedIndexPath to false but indexPathForPreferredFocusedView is not being called.

如何解释这种行为?

推荐答案

函数indexPathForPreferredFocusedView是UICollectionViewDelegate的一部分，因此可能是未将委托分配给您的collectionView.

The function indexPathForPreferredFocusedView is part of UICollectionViewDelegate, so it might be that the delegate was not assigned to your collectionView.

或者，问题可能也出在焦点环境上，而不考虑您的UICollectionView.

Or, the problem might also be on the focus environment, not taking into account your UICollectionView.

作为参考，这里有一个具有5个单元格的简单collectionView的示例，默认情况下最初选择了一个位于中心的单元格

As a reference, here you have an example of a simple collectionView with 5 cells, having the one in the center initially selected by default

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)
    }
}

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