What is the relationship between the delegate class and the object where the delegate property is declared?

If you made ChristinesViewController conform to the UITableViewDelegate protocol, but did not set the delegate property of UITableView, the methods in ChristinesViewController would never get called. Sadly, this is a simple error I commonly make when developing! I forget to assign the delegates and then I sit there wondering why my methods aren't getting called!

The reason why you need to set the delegate property of the tableView is because you need to tell the tableView who its delegate it is so that it knows where to find the methods to manage it. If it doesn't know where to find the methods, they'll never get called. Remember that delegation is all about handing off responsibilities for another object to manage, so we have to make the original object aware of who its delegate is. The reason this whole thing works is because of protocols.

Remember that protocols are a blueprint of a class/structure. If a property/method is listed in a protocol, the object that conforms to the protocol must implement those methods! This is how the requirements are set up so that we know the delegate object can manage the original object. Let's give it some context:

If we take a look at the UITableViewDataSource protocol, we will see that it has 2 required methods.

1) tableView(_:numberOfRowsInSection:)

2) tableView(_:cellForRowAt:)

Now lets make our view controller conform to this protocol by adding these methods:

import UIKit

class ChristinesViewController: UITableViewDataSource {

    @IBOutlet weak var tableView: UITableView!

    override func viewDidLoad() {
        super.viewDidLoad()
        tableView.dataSource = self // ChristinesViewController
    }

    func tableView(tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
       return 10
    }

    func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
        return UITableViewCell()
    }
}

You can see in viewDidLoad we set the tableView's dataSource property to ChristinesViewController this time. This is because it conforms to the UITableViewDataSource protocol. Since that protocol has the required methods of numberOfRowsInSection and cellForRowAtIndexPath, and our view controller conforms to this protocol, it can act as our data source for the tableView. Now the tableView can be sure these methods exist in the ViewController and knows where to call them.

When does it call them? Thats all handled behind the scenes in the UIKit framework. Unfortunately, thats Apple's private property and we will never see the implementation of these methods. At some point determined by the tableView object, it will look for its delegate to populate the numberOfRows in the tableView. We just have to let it know where to find it!

Hope this helps.