程序执行流程

#import "AppDelegate.h"
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
    self.window.backgroundColor = [UIColor whiteColor];
    [self.window makeKeyAndVisible];
   
    //4、1、3、2
    
    //alloc]init]方法调用之后，会去调用 initWithNibName方法；之后，回到入口类继续向下执行 ；当window.rootViewController = vc 代码h执行之后，会去调用viewDidLoad方法；之后回到入口类继续执行
    FirstViewController * vc = [[FirstViewController alloc]init];
    NSLog(@"1");
    self.window.rootViewController = vc ;
    NSLog(@"2");
    
    return YES;
}

#import "FirstViewController.h"
- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil
{
    self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];
    if (self)
    {
        NSLog(@"4");
    }
    return self;
}
//先加载xib，再执行viewDidLoad ;注意视图的层次 
- (void)viewDidLoad
{
    [super viewDidLoad];
   
    NSLog(@"3");
}