When users added or deleted filters in the first iteration of my
Filter Chaining application, the updated array of user defined filters was passed into the
FiltersCollectionView instance where is simply invoked
reloadData() on its
UICollectionView. This worked fine, but there was no transition and the new filter suddenly jumped into view.
It would be better to animate the insert or delete and indicate to the user, through movement, how their collection of filters had changed. How to do this wasn’t immediately obvious and this post describes the technique I’ve used.
Inside the view controller, both the
deleteSelectedFilter() and
addNewFilter() methods update its
userDefinedFilters array:
// deleteSelectedFilter
userDefinedFilters = userDefinedFilters.filter({!($0 == previousFilter)})
// addNewFilter
userDefinedFilters.insert(newFilter, atIndex: userDefinedFilters.count - 1)
and this, in turn, sets the
userDefinedFilters property on the
FiltersCollectionView via a
didSet observer. When that property changes,
FiltersCollectionView’s own
didSet observer on
userDefinedFilters looks at the new version of the array to see whether an item has been added (i.e. the length of
oldValue is less that the new value) or deleted (i.e. the length of
oldValue is greater than the new value).
Selecting, adding and deleting items in a
UICollectionView uses
NSIndexPaths rather than, for example, integer indexes or items. Getting the
NSIndexPath for a specific item needs an intermediate step. My application always inserts new filters at the penultimate position in the collection view, so I need to find the
NSIndexPath for the last but one cell and invoke
insertItemsAtIndexPath(). To do this, I use on of the extensions that
UICollectionView provides to
NSIndexPath - a new
init() method:
init(forItem item: Int, inSection section: Int) -> NSIndexPath …and the implementation to get the required index path is and insert an item is: let insertIndexPath = NSIndexPath(forItem: oldValue.count - 1, inSection: 0) uiCollectionView.insertItemsAtIndexPaths([insertIndexPath])
When a user deletes a filter, the application assumes they are deleting the selected filter (a fuller implementation would compare oldValueand userDefinedFilters to figure out what’s been deleted). So, in this case, I need to invoke deleteItemsAtSelectedPaths() for the index paths of the selected items:
let deleteIndexPath = uiCollectionView.indexPathsForSelectedItems()[0] as NSIndexPath uiCollectionView.deleteItemsAtIndexPaths([deleteIndexPath])
As well as adding or removing filters, both of these set
FilterCollectonView’s
selectedFilter property. When a filter is added, the new filter is selected and when a filter is removed, the first filter is selected.
Inside its
didSet observer, I scroll the selected cell into view so that if the user selects a cell which is partially off-screen it becomes fully visible. In this case, I want to find the index of the selected cell inside
userDefinedFilters, then create an
NSIndexPath for that index and finally
selectItemAtIndexPath() on the new
NSIndexPath.
To that that, the
didSet observer on
selectedFilter looks like this:
var selectedFilter : UserDefinedFilter?
{
didSet
{
var selectedIndex : Int = -1
for (i: Int, filter: UserDefinedFilter) in enumerate(userDefinedFilters)
{
if filter == selectedFilter!
{
selectedIndex = i
}
}
if selectedIndex != -1
{
let selectedIndexPath = NSIndexPath(forItem: selectedIndex, inSection: 0)
uiCollectionView.selectItemAtIndexPath(selectedIndexPath, animated: true, scrollPosition: UICollectionViewScrollPosition.CenteredHorizontally)
}
sendActionsForControlEvents(.ValueChanged)
refresh()
}
}
There we have it: rather than cells jarringly appearing and disappearing, a little work with
NSIndexPath and properly using
UICollectionView’s own insert and delete support gives a smooth transition.
All of this code is available at my
GitHub repository here.
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