////  main.m//  Hello////  Created by lishujun on 14-8-28.//  Copyright (c) 2014年 lishujun. All rights reserved.//#import 
     
      // 视图控制器对象@interface HelloWorldViewController : UIViewController@end@implementation HelloWorldViewController-(void) loadView{    //创建视图对象    UIView *contentView = [[UIView alloc]initWithFrame:[[UIScreen mainScreen] applicationFrame]];    contentView.backgroundColor = [UIColor lightGrayColor];    self.view = contentView;        //创建label对象    UILabel *label = [[UILabel alloc]initWithFrame:CGRectMake(0.0, 0.0, 320.0, 30.0)];    label.text = @"Hello World";    label.center = contentView.center;             // 垂直居中    label.textAlignment = UITextAlignmentCenter;   // 水平居中    label.backgroundColor = [UIColor clearColor];    label.textColor = [UIColor redColor];        //在视图上添加label    [contentView addSubview:label];}@end// 委托对象@interface HelloWorldAppDelegate : NSObject 
      
       {    IBOutlet UIWindow *window;}@property (nonatomic, retain) UIWindow *window;@property (nonatomic, retain) HelloWorldViewController *viewController;//window 必须声明为属性,声明为局部变量则无法绘制视图，显示为黑屏//apple 官方文档把viewController也声明为属性了@end@implementation HelloWorldAppDelegate@synthesize window;@synthesize viewController;-(void) applicationDidFinishLaunching:(UIApplication *)application{    self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen]bounds]];    self.viewController = [[HelloWorldViewController alloc]init];    self.window.rootViewController = self.viewController;    [self.window makeKeyAndVisible];}@end// 程序入口int main(int argc, char * argv[]){    @autoreleasepool {        return UIApplicationMain(argc, argv, nil, @"HelloWorldAppDelegate");    }}