iOS版 hello，world版本2

//
//  main.m
//  Hello
//
//  Created by lishujun on 14-8-28.
//  Copyright (c) 2014年 lishujun. All rights reserved.
//

#import <UIKit/UIKit.h>

// 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 <UIApplicationDelegate>
{
    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");
    }
}