import UIKit

// The view controller will adopt this protocol (delegate)
// and thus must contain the keyWasTapped method
protocol KeyboardDelegate: class {
    func keyWasTapped(character: String)
}

class Keyboard: UIView {

    // This variable will be set as the view controller so that 
    // the keyboard can send messages to the view controller.
    weak var delegate: KeyboardDelegate?

    // MARK:- keyboard initialization

    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        initializeSubviews()
    }

    override init(frame: CGRect) {
        super.init(frame: frame)
        initializeSubviews()
    }

    func initializeSubviews() {
        let xibFileName = "Keyboard" // xib extention not included
        let view = NSBundle.mainBundle().loadNibNamed(xibFileName, owner: self, options: nil)[0] as! UIView
        self.addSubview(view)
        view.frame = self.bounds
    }

    // MARK:- Button actions from .xib file

    @IBAction func keyTapped(sender: UIButton) {
        // When a button is tapped, send that information to the 
        // delegate (ie, the view controller)
        self.delegate?.keyWasTapped(sender.titleLabel!.text!) // could alternatively send a tag value
    }

}

**Keyboard.h File**

 #import <UIKit/UIKit.h>

 // The view controller will adopt this protocol (delegate)
 // and thus must contain the keyWasTapped method
 @protocol KeyboardDelegate<NSObject>
 - (void)keyWasTapped:(NSString *)character;
 @end

 @interface Keyboard : UIView
 @property (nonatomic, weak) id<KeyboardDelegate> delegate;  
 @end

#import "Keyboard.h"

@implementation Keyboard

- (id)initWithCoder:(NSCoder *)aDecoder {
    self = [super initWithCoder:aDecoder];
    [self initializeSubviews];
    return self;
}

- (id)initWithFrame:(CGRect)frame {
    self = [super initWithFrame:frame];
    [self initializeSubviews];
    return self;
}

- (void)initializeSubviews {
    NSString *xibFileName = @"Keyboard"; // xib extention not included
    UIView *view = [[[NSBundle mainBundle] loadNibNamed:xibFileName owner:self options:nil] firstObject];
    [self addSubview:view];
    view.frame = self.bounds;
}

// MARK:- Button actions from .xib file

-(IBAction)keyTapped:(UIButton *)sender {
    // When a button is tapped, send that information to the
    // delegate (ie, the view controller)
    [self.delegate keyWasTapped:sender.titleLabel.text]; // could alternatively send a tag value
}

@end

import UIKit

class ViewController: UIViewController, KeyboardDelegate {

    @IBOutlet weak var textField: UITextField!

    override func viewDidLoad() {
        super.viewDidLoad()
    
        // initialize custom keyboard
        let keyboardView = Keyboard(frame: CGRect(x: 0, y: 0, width: 0, height: 300))
        keyboardView.delegate = self // the view controller will be notified by the keyboard whenever a key is tapped
    
        // replace system keyboard with custom keyboard
        textField.inputView = keyboardView
    }

    // required method for keyboard delegate protocol
    func keyWasTapped(character: String) {
        textField.insertText(character)
    }
}

#import <UIKit/UIKit.h>

 @interface ViewController : UIViewController

 @end

#import "ViewController.h"
#import "Keyboard.h"

@interface ViewController ()<KeyboardDelegate>

@property (nonatomic, weak) IBOutlet UITextField *textField;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.

    // initialize custom keyboard
    Keyboard *keyboardView = [[Keyboard alloc] initWithFrame:CGRectMake(0, 0, 0, 300)];
    keyboardView.delegate = self; // the view controller will be notified by the keyboard whenever a key is tapped

    // replace system keyboard with custom keyboard
    self.textField.inputView = keyboardView;
}

- (void)keyWasTapped:(NSString *)character {
    [self.textField insertText:character];
}

@end