Why can I not assign dataSource directly? - swift

Question

I would like to understand why to following Swift code does not work, but using the commented version does. I'm not sure if dataSources are typically wrapped into a separate class, but I don't think that should matter. I'm using Xcode 6.3.2, all up to date.
// MainViewController.swift
import UIKit
class MainViewController: UIViewController {
#IBOutlet weak var tableView: UITableView!
var dataSource:UITableViewDataSource?
override func viewDidLoad() {
super.viewDidLoad()
// dataSource = MainTableViewDataSource()
// tableView.dataSource = dataSource
tableView.dataSource = MainTableViewDataSource()
}
}
The MainTableViewDataSource is just a class which implements the UITableViewDataSource protocol and uses some dummy data.
// MainTableViewDataSource.swift
import UIKit
class MainTableViewDataSource : NSObject, UITableViewDataSource {
func numberOfSectionsInTableView(tableView: UITableView) -> Int {
return 100
}
func tableView(tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
return 1000
}
func tableView(tableView: UITableView, titleForHeaderInSection section: Int) -> String? {
return String(section + 1)
}
func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
var cell = UITableViewCell()
cell.textLabel?.text = "Joejoe"
return cell
}
}

Answer 1

According to Apple's documentation https://developer.apple.com/library/ios/documentation/UIKit/Reference/UITableView_Class/#//apple_ref/occ/instp/UITableView/dataSource
dataSource property of UITableView is unowned in Swift which is (assign) for Objective-C meaning that this property does not increase the reference count. So right after viewDidLoad function, when reference count of your MainTableViewDataSource becomes zero, it gets deallocated.
I recommend reading: https://developer.apple.com/library/ios/documentation/Cocoa/Conceptual/MemoryMgmt/Articles/MemoryMgmt.html
You'll run into strange outcomes--sometimes even inconsistent--if you don't do the memory management right.