Understanding Instantiation in Swift - Why not require in custom class - ios

Question

It was my understanding that in Swift you don't need to import your classes (only for some frameworks) but intatiation was required, but to my surprise this may not be true, I created a custom class (CustomCell.swift) for a custom cell I'm using in my UITableView and apparently there is no need to make an instance of the custom class before using it. Here is how I'm using it.
CustomCell Class
// CustomCell.swift
import UIKit
class CustomCell: UITableViewCell {
#IBOutlet weak var labelDisplayWattage: UILabel!
override func awakeFromNib() {
super.awakeFromNib()
// Initialization code
}
override func setSelected(selected: Bool, animated: Bool) {
super.setSelected(selected, animated: animated)
// Configure the view for the selected state
}
}
View Controller
// ViewController.swift
import UIKit
class ViewController: UIViewController, UITableViewDataSource {
// some code ...
func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
let cell = tableView.dequeueReusableCellWithIdentifier("reusableCell", forIndexPath: indexPath) as! CustomCell
cell.labelDisplayWattage.text = String(totalWattageList[indexPath.row])
return cell
}
}
As you can see there are no instances of the CustomCell class exept for the call on method cellForRowAtIndexPath (as! CustomCell).
Can someone tell me why there was no need to create an instance of class CustomCell before using it in method cellForRowAtIndexPath? I was expecting to see something like...
var myCustomCell = CustomCell()
Thanks

Answer 1

As per apple docs for Datasource method
func dequeueReusableCellWithIdentifier(_ identifier: String,forIndexPath indexPath: NSIndexPath) -> UITableViewCell
For performance reasons, a table view’s data source should generally
reuse UITableViewCell objects when it assigns cells to rows in its
tableView:cellForRowAtIndexPath: method. A table view maintains a
queue or list of UITableViewCell objects that the data source has
marked for reuse. Call this method from your data source object when
asked to provide a new cell for the table view. This method dequeues
an existing cell if one is available, or creates a new one based on
the class or nib file you previously registered, and adds it to the
table.
so basically it creates a new object for you if the queue is empty.
Source: https://developer.apple.com/library/ios/documentation/UIKit/Reference/UITableView_Class/#//apple_ref/occ/instm/UITableView/dequeueReusableCellWithIdentifier:forIndexPath:

Answer 2

There is a really important concept underneath the code you're seeing right now. If you're using storyboards, table views and collection views have the option of using a prototype cell. A prototype cell is a template of the cell that can/will be used with the table view or collection view. If your table/collection view is set to use a prototype cell, there is no need to create a new cell,UIKit takes care of creating one for you when you ask for a queued cell by calling tableView.dequeueReusableCellWithIdentifier.
Consider the following when NOT using prototype cells:
// ViewController.swift
import UIKit
class ViewController: UIViewController, UITableViewDataSource {
// some code ...
func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
var cell = tableView.dequeueReusableCellWithIdentifier("reusableCell", forIndexPath: indexPath) as? CustomCell
if nil == cell {
cell = CustomCell()
// more cell options can be set here
}
cell.labelDisplayWattage.text = String(totalWattageList[indexPath.row])
return cell
}
}

Answer 3

let cell = tableView.dequeueReusableCellWithIdentifier("reusableCell", forIndexPath: indexPath) as! CustomCell
Instantiation of the CustomCell is responsibility of tableview, we just fetch the cell, if the cell is not instantiated when we fetch it tableview will instantiate it before returning it to us.
This behavior is of architectural design, it is not specific to swift. It works in same way for objective-c as well.