Śledziłem tę Custom Collection View Layout tutorial z raywenderlich.com używając Xcode 8 i szybka 3.UICollectionViewLayout nie działa zgodnie z oczekiwaniami
Kiedy wpadłem aplikację po wdrożeniu wszystkich metod wymaganych w tutorialu, mam następujący błąd:
'no UICollectionViewLayoutAttributes instance for -layoutAttributesForItemAtIndexPath: {length = 2, path = 0 - 11}'
Więc dodałem następujące metody w mojej klasie CollectionViewFlowLayout:
override func layoutAttributesForItem(at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
return self.cache[indexPath.row]
}
To jest prawie pracy z wyjątkiem tego som Komórki e są nakładane na istniejące komórki podczas przewijania w dół, a następnie znikają. Jeśli przewijam w górę, wszystko działa idealnie.
Nie rozumiem pełnej logiki tego kodu, ale wielokrotnie sprawdzałem i testowałem go i nie mogę zrozumieć, która część kodu wywołuje to zachowanie.
Każdy pomysł?
import UIKit
/* The heights are declared as constants outside of the class so they can be easily referenced elsewhere */
struct UltravisualLayoutConstants {
struct Cell {
/* The height of the non-featured cell */
static let standardHeight: CGFloat = 100
/* The height of the first visible cell */
static let featuredHeight: CGFloat = 280
}
}
class UltravisualLayout: UICollectionViewLayout {
// MARK: Properties and Variables
/* The amount the user needs to scroll before the featured cell changes */
let dragOffset: CGFloat = 180.0
var cache = [UICollectionViewLayoutAttributes]()
/* Returns the item index of the currently featured cell */
var featuredItemIndex: Int {
get {
/* Use max to make sure the featureItemIndex is never < 0 */
return max(0, Int(collectionView!.contentOffset.y/dragOffset))
}
}
/* Returns a value between 0 and 1 that represents how close the next cell is to becoming the featured cell */
var nextItemPercentageOffset: CGFloat {
get {
return (collectionView!.contentOffset.y/dragOffset) - CGFloat(featuredItemIndex)
}
}
/* Returns the width of the collection view */
var width: CGFloat {
get {
return collectionView!.bounds.width
}
}
/* Returns the height of the collection view */
var height: CGFloat {
get {
return collectionView!.bounds.height
}
}
/* Returns the number of items in the collection view */
var numberOfItems: Int {
get {
return collectionView!.numberOfItems(inSection: 0)
}
}
// MARK: UICollectionViewLayout
/* Return the size of all the content in the collection view */
override var collectionViewContentSize : CGSize {
let contentHeight = (CGFloat(numberOfItems) * dragOffset) + (height - dragOffset)
return CGSize(width: width, height: contentHeight)
}
override func prepare() {
let standardHeight = UltravisualLayoutConstants.Cell.standardHeight
let featuredHeight = UltravisualLayoutConstants.Cell.featuredHeight
var frame = CGRect.zero
var y:CGFloat = 0
for item in 0 ... (numberOfItems - 1) {
let indexPath = NSIndexPath(item: item, section: 0)
let attributes = UICollectionViewLayoutAttributes(forCellWith: indexPath as IndexPath)
attributes.zIndex = item
var height = standardHeight
if indexPath.item == featuredItemIndex {
let yOffset = standardHeight * nextItemPercentageOffset
y = self.collectionView!.contentOffset.y - yOffset
height = featuredHeight
} else if item == (featuredItemIndex + 1) && item != numberOfItems {
let maxY = y + standardHeight
height = standardHeight + max((featuredHeight - standardHeight) * nextItemPercentageOffset, 0)
y = maxY - height
}
frame = CGRect(x: 0, y: y, width: width, height: height)
attributes.frame = frame
cache.append(attributes)
y = frame.maxY
}
}
/* Return all attributes in the cache whose frame intersects with the rect passed to the method */
override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
var layoutAttributes = [UICollectionViewLayoutAttributes]()
for attributes in cache {
if attributes.frame.intersects(rect) {
layoutAttributes.append(attributes)
}
}
return layoutAttributes
}
override func layoutAttributesForItem(at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
return self.cache[indexPath.row]
}
/* Return true so that the layout is continuously invalidated as the user scrolls */
override func shouldInvalidateLayout(forBoundsChange newBounds: CGRect) -> Bool {
return true
}
}