iOSプロトコルの継承

今日pdfドキュメントを見て、プロトコルの継承を知りました.特に小さなdemoテストをしました.次はコードです.
OneDelegate.h

@protocol OneDelegate <NSObject>

@required

- (void)backStr:(NSString *)str;

@end

TwoDelegate.h

@protocol TwoDelegate <OneDelegate>

@optional

- (void)backInt:(int)i;

@end

ViewController.h

#import "ViewController2.h"

@interface ViewController : UIViewController <TwoDelegate>

@end

ViewController.m

- (void)viewDidLoad
{
    [super viewDidLoad];
	
    UIButton *button = [UIButton buttonWithType:UIButtonTypeRoundedRect];
    button.frame = CGRectMake(50, 50, 200, 100);
    button.backgroundColor = [UIColor yellowColor];
    [button setTitle:@"into VC2" forState:UIControlStateNormal];
    [button addTarget:self action:@selector(push) forControlEvents:UIControlEventTouchUpInside];
    [self.view addSubview:button];
    
}

- (void)push{
    ViewController2 *vc = [[ViewController2 alloc]init];
    vc.delegate = self;
    [self presentViewController:vc animated:YES completion:^{
    
    }];
}

//OneDelegate
- (void)backStr:(NSString *)str{
    NSLog(@"%@",str);
}

//TwoDelegate
- (void)backInt:(int)i{
    NSLog(@"%d",i);
}

ViewController2.h

#import <UIKit/UIKit.h>
#import "two.h"

@interface ViewController2 : UIViewController

@property(nonatomic ,unsafe_unretained)id delegate;

@end

ViewController2.m

- (void)viewDidLoad
{
    [super viewDidLoad];
    self.view.backgroundColor = [UIColor whiteColor];
    
	UIButton *button = [UIButton buttonWithType:UIButtonTypeRoundedRect];
    button.frame = CGRectMake(50, 50, 200, 100);
    button.backgroundColor = [UIColor yellowColor];
    [button addTarget:self action:@selector(backStrBtn) forControlEvents:UIControlEventTouchUpInside];
    [self.view addSubview:button];
    
    UIButton *button2 = [UIButton buttonWithType:UIButtonTypeRoundedRect];
    button2.frame = CGRectMake(50, 200, 200, 100);
    button2.backgroundColor = [UIColor blueColor];
    [button2 addTarget:self action:@selector(backIntBtn) forControlEvents:UIControlEventTouchUpInside];
    [self.view addSubview:button2];
}

- (void)backStrBtn{
    if (self.delegate && [self.delegate respondsToSelector:@selector(backStr:)]) {
        [self.delegate backStr:@" 1 "];
    }
}

- (void)backIntBtn{
    if (self.delegate && [self.delegate respondsToSelector:@selector(backInt:)]) {
        [self.delegate backInt:2];
        NSLog(@" ");
    }
}