How to communicate between Classes in a hierarchy in Swift - ios

Question

With the inspiration coming from the idea that you can code anything, i tried my hand at a complicated CollectionView nested structure which goes as follows:
CustomCollectionViewController
--CustomCollectionViewCell
----CustomTableView
------CustomTableViewCell
--------CustomPickerView
In CustomCollectionViewController, the main data feed comes from property:
var cardFeed: [String: [Card]] = [:]
Card is my defined model and variable cardFeed is applied the usual way in UICollectionView Delegate & DataSource methods:
override func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "card", for: indexPath) as! CustomCollectionViewCell
cell.contentView.clipsToBounds = true
cell.delegate = self
cell.card = self.cardFeed[string]![indexPath.row]
cell.cardIndex = indexPath.row
}
From the delegate methods, cardFeed above sets a main property in CustomCollectionViewCell used to update the interface:
var card: Card! {
didSet{
setupCard()
}
}
The property card is also the data feed for UITableView Delegate & Datasource.
Everything works perfectly, everything shows up as it should. Except for the fact that when a user picks values from the CustomPickerView, the main data feed, namely cardFeed defined in CustomCollectionViewController (shown above) must update!
My solution is this:
(1) Given that there are three components, define array that records changes in CustomPickerView selected rows and a call back method to pass down the variable:
var selectedRow: [Int] = [1, 0, 0] {
didSet {
if updateRow != nil {
updateRow!(self.selectedRow)
}
}
}
var updateRow: ( ([Int]) -> () )?
(2) In CustomCollectionViewCell define another call back with an extra argument, to keep track of what cell actually sent the selected row array :
var passSelectedRow: (([Int], Int) -> ())?
which is called in tableViews cellForRowAtIndexPath method:
cell.updateRow = { selectedRow in
self.passSelectedRow!(selectedRow, indexPath.row)
}
(3) finally update cardFeed in CustomCollectionViewController cellForItemAtIndexPath:
cell.passSelectedRow = { selectedRow, forIndex in
if self.cardFeed[string]![indexPath.row].chosenFood[forIndex].selectedRow != selectedRow {
self.cardFeed[string]![indexPath.row].chosenFood[forIndex].selectedRow = selectedRow
}
}
But here is the problem, if i now add a didSet to cardFeed, it will create an infinite loop because cellForRowAtIndexPath will be called indefinitely. If i get the CustomCollectionViewCell reference anywhere other than cellForItemAtIndexPath, self.collectionView?.reload() does not work! Is there a way i can update my variable cardFeed in CustomCollectionViewController from the selected rows in CustomPickerView?

Answer 1

When communicating between objects it is bad practice to make the child object have a strong reference to its owner, that is how you end up with retain cycles and bugs.
Let's take a look at the two most common ways of communicating between objects: delegation and notification.
with delegation:
Create a protocol for communicating what you want, in your example:
protocol PickerFoodSelectedDelegate : class {
func selected(row : Int, forIndex : Int)
}
Add weak var selectionDelegate : PickerFoodSelectedDelegate as a variable in the picker class
In the tableView class, during cellForItemAtIndexPath, you assign self to picker.selectionDelegate
You then create a similar structure for communicating between the table and the collection view.
The key part is that delegate references be declared as weak, to avoid retain cycles and bugs.
With notifications you can use NotificationCenter.default to post a notification with any object you want, in this case you would:
Subscribe to a notification name you choose in the table view.
Post a notification from the picker view when an option is chosen.
When the table receives the notification, extract the object.
Do the same from the table to the collection view.
Hope this helps!