#import <UIKit/UIKit.h>      @ interface ViewController : UIViewController{         UIButton *btn1;         UIButton *btn2;                  UILabel *label1; }      -( void)testRespond1; -( void)notTest;      @end           #import "ViewController.h"      @implementation ViewController      - ( void)didReceiveMemoryWarning {         [super didReceiveMemoryWarning];          // Release any cached data, p_w_picpaths, etc that aren't in use. }      #pragma mark - View lifecycle      - ( void)viewDidLoad {         [super viewDidLoad];                  // Do any additional setup after loading the view, typically from a nib.                  btn1 = [UIButton buttonWithType:UIButtonTypeRoundedRect];         btn1.frame = CGRectMake(50, 100, 100, 50);         [btn1 setTitle: @"BTN1" forState:UIControlStateNormal];         [btn1 addTarget:self action:@selector(btn1Clicked:) forControlEvents:UIControlEventTouchUpInside];         [self.view addSubview:btn1];                  btn2 = [UIButton buttonWithType:UIButtonTypeRoundedRect];         btn2.frame = CGRectMake(200, 100, 100, 50);         [btn2 setTitle: @"BTN2" forState:UIControlStateNormal];         [btn2 addTarget:self action:@selector(btn2Clicked:) forControlEvents:UIControlEventTouchUpInside];         [self.view addSubview:btn2]; }      -( void)btn1Clicked:(id)sender {          if ([self respondsToSelector:@selector(testRespond1)])         {                 [self testRespond1];         } }      -( void)btn2Clicked:(id)sender {          if (![self respondsToSelector:@selector(notTest)])             {                 UIAlertView *alert = [[UIAlertView alloc] initWithTitle:nil message: @"notTest 没有被实现" delegate:nil cancelButtonTitle: @"OK" otherButtonTitles:nil, nil];                 [alert show];                 [alert release];         } }      -( void)testRespond1 {         label1 = [[UILabel alloc] initWithFrame:CGRectMake(50, 200, 220, 200)];         label1.text = @"testRespond1 被实现了";         [self.view addSubview:label1];         [label1 release]; }      - ( void)viewDidUnload {         [super viewDidUnload];          // Release any retained subviews of the main view.          // e.g. self.myOutlet = nil; }      -( void)dealloc {         [super dealloc]; }      @end