What is meant by .delegate=self?

Question

Could anyone explain the meaning of someViewController.delegate = self and self.delegate? Where do they help us?

Answer 1

Delegate is used to pass/communicate data/message b/w two objects of classes. Here, tableView(Sender) sends data/message to viewController(Receiver). Consider example of implementing UITableView in custom viewController Here, UITableViewDataSource & UITableViewDelegate are actually protocols. Unfortunately, UIKit Framework is not open source. But I will assure this what internally happens after referring many articles.

Protocol is like basketball coach with some requirements in it. He/She tells players like class, struct, enum what to do? by using those requirements. But He/She doesn't knows how to do?by themself. So, the class or struct which conforms that protocol should provide implementation to those requirements while achieving to dunk the ball.

protocol UITableViewDelegate {
 func tableView(_ tableView: UITableView, didSelectRowAtIndexPath indexPath: NSIndexPath)
}

A Protocol is said to be DataSource protocol then it always contains required functions with "return type" as shown below.

protocol UITableViewDataSource {
 func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int
 func tableView(_ tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell 
}

Implementing UITableView inside custom viewController

class viewController: UIViewController, UITableViewDelegate, UITableViewDataSource {

    let tableView = UITableView()    

    override func viewDidLoad {
      tableView.delegate = self
      tableView.dataSource = self
    }

Here, tableView acts as Delegator(sender) & viewController object i.e (self) as Delegate(receiver).

In order to get UITableView in viewController.It should to conform to both the Protocols.

So, viewController class object has implemented all those required functions of both the protocols. Now self can be used either as UITableViewDelegate type or UITableViewDataSource type because Protocol can be used as type for an object of class which conforms to it. Now, both properties of tableView i.e delegate & dataSource are assigned to self because its having same respective protocol types.

The non-optional functions of both Protocols are implemented in viewController class object as below

Protocol UITableViewDelegate functions

func tableView(_ tableView: UITableView, didSelectRowAtIndexPath indexPath: NSIndexPath) { 
// Do further processes like pushing or poping another viewController
}

Protocol UITableViewDataSource functions

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

func tableView(_ tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
    return UITableViewCell(style: UITableViewCellStyle.Value1, reuseIdentifier: "Cell")
 }

1) When the user select a row in a section then tableview(Sender) i.e UItableView() calls the UITableViewDelegate func below shown by passing data to parameters tableView & indexPath which resides in viewController object(Receiver) through its delegate property. Now viewController uses those passed data to do further processes like pushing or poping to new custom viewController.

tableView.delegate?.tableView(UITableView, didSelectRowAtIndexPath indexPath: NSIndexPath)

2) Functions inside UITableViewDatasource protocol provides custom data to tableview(Sender). The tableview asks the viewController object by calling Datasource functions with passing data to parameters tableView & indexPath which resides in viewController object(Receiver) through its datasource property. Now viewController uses those passed data & returns custom data back tableview. Now tableview uses those data to create "10" cells in a section & kind of "cell" at indexpath

tableView.dataSource?.tableView(UITableView, numberOfRowsInSection section: Int) -> returns "10"

tableView.dataSource?.tableView(UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> returns "cell"

Finally, whole UIKit Framework uses delegate & datasource design patterns in all its classes such as UIApplication, UITableView, UICollectionView, UITextField & so on to communicate data. Unfortunately, UIKit Framework is not open source.