问题
Description:
Answer for Objective-C and Swift2.0: How to center align the cells of a UICollectionView?
I usually would try to convert the Swift2.0 answer to the Swift3.0 solution, however the method:
func collectionView(collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, insetForSectionAtIndex section: Int) -> UIEdgeInsets {
let edgeInsets = (screenWight - (CGFloat(elements.count) * 50) - (CGFloat(elements.count) * 10)) / 2
return UIEdgeInsetsMake(0, edgeInsets, 0, 0);
}
doesn't seem to exist in Swift3.0,and the only other method I found that seems useful is:
func collectionView(_ collectionView: UICollectionView, transitionLayoutForOldLayout fromLayout: UICollectionViewLayout, newLayout toLayout: UICollectionViewLayout) -> UICollectionViewTransitionLayout {
<#code#>
}
but I am unsure how to implement it correctly.
Question:
How to center align the cells of a UICollectionView in Swift3.0?
(A simple and general solution for iOS and tvOS would be perfect)
回答1:
While rottenoats answer is great, it has an extra spacing bug and doesn't use much of the available swift 3 syntax. I've fixed that and reduced the number of dependencies to local variables.
func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, insetForSectionAt section: Int) -> UIEdgeInsets {
// Make sure that the number of items is worth the computing effort.
guard let flowLayout = collectionViewLayout as? UICollectionViewFlowLayout,
let dataSourceCount = collectionView.dataSource?.collectionView(collectionView, numberOfItemsInSection: section),
dataSourceCount > 0 else {
return .zero
}
let cellCount = CGFloat(dataSourceCount)
let itemSpacing = flowLayout.minimumInteritemSpacing
let cellWidth = flowLayout.itemSize.width + itemSpacing
var insets = flowLayout.sectionInset
// Make sure to remove the last item spacing or it will
// miscalculate the actual total width.
let totalCellWidth = (cellWidth * cellCount) - itemSpacing
let contentWidth = collectionView.frame.size.width - collectionView.contentInset.left - collectionView.contentInset.right
// If the number of cells that exist take up less room than the
// collection view width, then center the content with the appropriate insets.
// Otherwise return the default layout inset.
guard totalCellWidth < contentWidth else {
return insets
}
// Calculate the right amount of padding to center the cells.
let padding = (contentWidth - totalCellWidth) / 2.0
insets.left = padding
insets.right = padding
return insets
}
N.B.: This snippet only works for an horizontal scrolling but can easily be adjusted.
回答2:
This ended up being the solution I used. Read the code comments for a better understanding. Swift 3.0
func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, insetForSectionAt section: Int) -> UIEdgeInsets {
//Where elements_count is the count of all your items in that
//Collection view...
let cellCount = CGFloat(elements_count)
//If the cell count is zero, there is no point in calculating anything.
if cellCount > 0 {
let flowLayout = collectionViewLayout as! UICollectionViewFlowLayout
let cellWidth = flowLayout.itemSize.width + flowLayout.minimumInteritemSpacing
//20.00 was just extra spacing I wanted to add to my cell.
let totalCellWidth = cellWidth*cellCount + 20.00 * (cellCount-1)
let contentWidth = collectionView.frame.size.width - collectionView.contentInset.left - collectionView.contentInset.right
if (totalCellWidth < contentWidth) {
//If the number of cells that exists take up less room than the
//collection view width... then there is an actual point to centering them.
//Calculate the right amount of padding to center the cells.
let padding = (contentWidth - totalCellWidth) / 2.0
return UIEdgeInsetsMake(0, padding, 0, padding)
} else {
//Pretty much if the number of cells that exist take up
//more room than the actual collectionView width, there is no
// point in trying to center them. So we leave the default behavior.
return UIEdgeInsetsMake(0, 40, 0, 40)
}
}
return UIEdgeInsets.zero
}
回答3:
Slight adaptation of @rottenoats answer. This is more generic.
Most importantly remember to make your view controller conform to the UICollectionViewDelegateFlowLayout protocol.
func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, insetForSectionAt section: Int) -> UIEdgeInsets {
guard let flowLayout = collectionViewLayout as? UICollectionViewFlowLayout else {
return .zero
}
let cellCount = CGFloat(collectionView.numberOfItems(inSection: section))
if cellCount > 0 {
let cellWidth = flowLayout.itemSize.width + flowLayout.minimumInteritemSpacing
let totalCellWidth = cellWidth * cellCount
let contentWidth = collectionView.frame.size.width - collectionView.contentInset.left - collectionView.contentInset.right - flowLayout.headerReferenceSize.width - flowLayout.footerReferenceSize.width
if (totalCellWidth < contentWidth) {
let padding = (contentWidth - totalCellWidth + flowLayout.minimumInteritemSpacing) / 2.0
return UIEdgeInsetsMake(0, padding, 0, 0)
}
}
return .zero
}
回答4:
The method that you are looking for is present with a different signature in Swift 3. The new signature is this:
optional public func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, insetForSectionAt section: Int) -> UIEdgeInsets
P.S: This method is present in UICollectionViewDelegateFlowLayout protocol.
Hope this would help.
回答5:
Assuming that you are conforming to UICollectionViewDelegateFlowLayout, it should be:
func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, insetForSectionAt section: Int) -> UIEdgeInsets {
let edgeInsets = (screenWight - (CGFloat(elements.count) * 50) - (CGFloat(elements.count) * 10)) / 2
return UIEdgeInsets(top: 0, left: edgeInsets, bottom: 0, right: 0)
}
回答6:
func collectionView(collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, insetForSectionAtIndex section: Int) -> UIEdgeInsets {
let allCellWidth = KcellWidth * numberOfCell
let cellSpacingWidth = CellSpacing * (numberOfCell - 1)
let leftInset = (collectionViewWidth - CGFloat(allCellWidth + cellSpacingWidth)) / 2
let rightInset = leftInset
return UIEdgeInsetsMake(0, leftInset, 0, rightInset)
}
回答7:
Here's a recent solution, works for Swift 5.0
extension YourViewController: UICollectionViewDelegateFlowLayout {
func collectionView(_ collectionView: UICollectionView,
layout collectionViewLayout: UICollectionViewLayout,
insetForSectionAt section: Int) -> UIEdgeInsets {
let itemWidth = 80
let spacingWidth = 4
let numberOfItems = collectionView.numberOfItems(inSection: section)
let cellSpacingWidth = numberOfItem * spacingWidth
let totalCellWidth = numberOfItems * itemWidth + cellSpacingWidth
let inset = (collectionView.layer.frame.size.width - CGFloat(totalCellWidth)) / 2
return UIEdgeInsets(top: 5, left: inset, bottom: 5, right: inset)
}
}
回答8:
You need to conform to UICollectionViewDelegateFlowLayout protocol to find the collectionView(_:layout:insetForSectionAt:) method, it is defined there. the following implementation works fine in swift 4 for a horizontal collectionView
extension YourViewController: UICollectionViewDelegateFlowLayout {
func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, insetForSectionAt section: Int) -> UIEdgeInsets {
let cellWidth = CGFloat(integerLiteral: YourCellsWidth)
let count = self.YourCollectionViewDataSource.count
let top = CGFloat(integerLiteral: 0) // I don't need to set top and bottom spacing
let cellsAccumulatedWidth = CGFloat(integerLiteral: count) * cellWidth
let cellsSpacingAccumulatedWidth = CGFloat(integerLiteral: (count - 1) * 5)
let left = (self.collectionView.frame.size.width - cellsAccumulatedWidth - cellsSpacingAccumulatedWidth) / 2
let bottom = top
let right = left
return UIEdgeInsetsMake(top, left, bottom, right);
}
}
回答9:
This code should center horizontally any type of collection view even with extra items in Swift 4.0:
func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, insetForSectionAt section: Int) -> UIEdgeInsets {
let flowLayout = collectionViewLayout as! UICollectionViewFlowLayout
let cellWidth: CGFloat = flowLayout.itemSize.width
let cellSpacing: CGFloat = flowLayout.minimumInteritemSpacing
let cellCount = CGFloat(collectionView.numberOfItems(inSection: section))
var collectionWidth = collectionView.frame.size.width
if #available(iOS 11.0, *) {
collectionWidth -= collectionView.safeAreaInsets.left + collectionView.safeAreaInsets.right
}
let totalWidth = cellWidth * cellCount + cellSpacing * (cellCount - 1)
if totalWidth <= collectionWidth {
let edgeInset = (collectionWidth - totalWidth) / 2
return UIEdgeInsetsMake(flowLayout.sectionInset.top, edgeInset, flowLayout.sectionInset.bottom, edgeInset)
} else {
return flowLayout.sectionInset
}
}
Don't forget if you are not subclassing UICollectionViewController, make sure your class conforms to UICollectionViewDelegateFlowLayout protocol
来源：https://stackoverflow.com/questions/40585822/how-to-center-align-the-cells-of-a-uicollectionview-in-swift3-0