Cannot assign a value of type ViewController to a value of type UITextFieldDelegate?

Question

Here's the error when I wrote the line self.MessageTextField.delegate = self:

/ChatApp/ViewController.swift:27:42: Cannot assign a value of type 'ViewController' to a value of type 'UITextFieldDelegate?'

Here's my Swift code (ViewerController.swift):

//
//  ViewController.swift
//  ChatApp
//
//  Created by David Chen on 15/4/12.
//  Copyright (c) 2015年 cwsoft. All rights reserved.
//

import UIKit
import Parse

class ViewController: UIViewController, UITableViewDelegate, UITableViewDataSource {

    var messagesArray:[String] = [String]()

    @IBOutlet weak var MessageTableView: UITableView!
    @IBOutlet weak var ButtonSend: UIButton!
    @IBOutlet weak var DockViewHeightConstraint: NSLayoutConstraint!
    @IBOutlet weak var MessageTextField: UITextField!

    override func viewDidLoad() {
        super.viewDidLoad()
        //
        self.MessageTableView.delegate = self
        self.MessageTableView.dataSource = self
        //Set delegate
        self.MessageTextField.delegate = self

        self.messagesArray.append("Test 1")
        self.messagesArray.append("Test 2")
        self.messagesArray.append("Test 3")
    }

    @IBAction func ButtonSendPressed(sender: UIButton) {
        self.view.layoutIfNeeded()
        UIView.animateWithDuration(0.5, animations: {
            self.DockViewHeightConstraint.constant = 400
            self.view.layoutIfNeeded()
            }, completion: nil)
    }

    //MARK : TextField Delegage Methods


    //MARK : Table View Delegate Methods

    func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
        let cell = self.MessageTableView.dequeueReusableCellWithIdentifier("MessageCell") as! UITableViewCell
        cell.textLabel?.text = self.messagesArray[indexPath.row]
        return cell
    }

    func tableView(tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        return messagesArray.count
    }
}

Answer 1

The line self.MessageTextField.delegate = self causes the error since you try to assign self as the delegate of a UITextField.
But your ViewController is not a UITextFieldDelegate. To make your class this kind of delegte, you need to adopt the UITextFieldDelegate protocol. This can be achieved by adding it to the list of protocols and classes your class inherits from / conforms to. In your case that is done by changing the line

class ViewController: UIViewController, UITableViewDelegate, UITableViewDataSource

to

class ViewController: UIViewController, UITableViewDelegate, UITableViewDataSource, UITextFieldDelegate