如何在集合视图中手动 select 下一个重点索引路径

Question

How can I manually decide the next focused index path for my collection view on tvOS?

My use case is that I have a collection view with dynamic cells that scrolls in all directions, and sometimes when a cell in one section spans across several cells in the next one, I want to be able to manually decide what cell gets focused next.

The default behaviour seems to be that the collection view will select the cell in the middle of the previous one.

For example, the green cell is currently focused. When I navigate down, the collection view wants to focus the red cell, but I want the blue cell to be focused next.

My initial idea was to implement the collectionView(_:shouldUpdateFocusIn:) delegate function, and return false + trigger a focus update, when the collection view selects the "wrong" index path.

However, for some reason, shouldUpdateFocusIn gets called several times when I return false from it and causes a visible lag.

func collectionView(_ collectionView: UICollectionView, shouldUpdateFocusIn context: UICollectionViewFocusUpdateContext) -> Bool {
    if let nextIndexPath = shouldFocusCell(context.focusHeading, (context.previouslyFocusedIndexPath, context.nextFocusedIndexPath)) {
        // The collection view did not select the index path of the cell we want to focus next.
        // Save the correct index path and trigger a focus update.
        lastFocusChange = (lastFocusChange.next, nextIndexPath)
        setNeedsFocusUpdate()
        // Using updateFocusIfNeeded() results in the following warning:
        // WARNING: Calling updateFocusIfNeeded while a focus update is in progress. This call will be ignored.
        return false
    }
    return true
}

Next idea was to do the same thing in collectionView(_:didUpdateFocusIn:with:) , but in this case we only update the focus after it has already moved to the "wrong" cell, so it becomes apparent to the user that the focus moves from the wrong cell to the correct one.

Not ideal either.

I'm using my own subclass of UICollectionViewLayout and UICollectionView , but I don't see anything I can override to be able to manually decide what index path to focus next when navigating up/down/left/right before shouldUpdateFocusIn is called.

Is there any way I can achieve this?

Answer 1

One possibility could be to use collectionView(_ collectionView:, canFocusItemAt:) to let your collectionView know if a given indexPath can receive the focus.

You can find here below a naive implementation of this concept. You will need to adjust the maths on it to your needs.

func collectionView(_ collectionView: UICollectionView, canFocusItemAt indexPath: IndexPath) -> Bool {
    guard let currentlyFocusedCellLayoutAttributes = collectionView.layoutAttributesForItem(at: focusedIndexPath) else { return false }
    guard let cellAtGivenIndexPathLayoutAttributes = collectionView.layoutAttributesForItem(at: indexPath) else { return false }
        
    let currentlyFocusedCellOriginX = currentlyFocusedCellLayoutAttributes.frame.origin.x
    let currentlyFocusedCellOriginY = currentlyFocusedCellLayoutAttributes.frame.origin.y
    let currentlyFocusedCellWidth = currentlyFocusedCellLayoutAttributes.frame.width
        
    let cellAtGivenIndexPathOriginX = cellAtGivenIndexPathLayoutAttributes.frame.origin.x
    let cellAtGivenIndexPathOriginY = cellAtGivenIndexPathLayoutAttributes.frame.origin.y
    let cellAtGivenIndexPathWidth = cellAtGivenIndexPathLayoutAttributes.frame.width
            
    let offsetX = collectionView.contentOffset.x

    // Scrolling horizontally is always allowed
    if currentlyFocusedCellOriginY == cellAtGivenIndexPathOriginY {
        return true
    }
    
    // Scrolling vertically is only allowed to the first cell (blue cell in the screenshot)
    if cellAtGivenIndexPathOriginX <= offsetX {
        return true
    }
    
    return false
}