#import "AppDelegate.h"

 #import "FirstViewController.h"

 #import "SecondViewController.h"

 #import "ThirdViewController.h"

 @interface AppDelegate ()

 @end

 @implementation AppDelegate

 - (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {

     // Override point for customization after application launch.

     /**

      *  第一步：创建window对象

      */

     // 创建window

     self.window = [[UIWindow alloc] initWithFrame:[UIScreen mainScreen].bounds];

     self.window.backgroundColor = [UIColor whiteColor];

     [self.window makeKeyAndVisible];

     /**

      *  第二步：设置window的根视图控制器

      */

     // 2.1 创建UINavigationController控制器，并指定导航控制器的根视图控制器

     // 创建UITabBarController对象

     UITabBarController *mainTab = [[UITabBarController alloc] init];

     // 将mainTab作为导航控制器的根视图控制器

     UINavigationController *rootNav = [[UINavigationController alloc] initWithRootViewController:mainTab];

     // 设置控制器数组

     FirstViewController *firstVC = [[FirstViewController alloc] init];

     firstVC.tabBarItem = [[UITabBarItem alloc] initWithTabBarSystemItem:UITabBarSystemItemBookmarks tag:];

     SecondViewController *secondVC = [[SecondViewController alloc] init];

     secondVC.tabBarItem = [[UITabBarItem alloc] initWithTabBarSystemItem:UITabBarSystemItemDownloads tag:];

     ThirdViewController *thirdVC = [[ThirdViewController alloc] init];

     thirdVC.tabBarItem = [[UITabBarItem alloc] initWithTabBarSystemItem:UITabBarSystemItemHistory tag:];

     // 添加到控制器数组中

     mainTab.viewControllers = @[firstVC, secondVC, thirdVC];

     // 设置window的根控制器

     self.window.rootViewController = rootNav;

     // 一键设置

     [UINavigationBar appearance].barTintColor = [UIColor magentaColor];

     [UINavigationBar appearance].tintColor = [UIColor whiteColor];

     [UITabBar appearance].tintColor = [UIColor purpleColor];

     return YES;

 }

 @end

 #import "FirstViewController.h"

 @interface FirstViewController ()

 @end

 @implementation FirstViewController

 - (void)viewDidLoad {

     [super viewDidLoad];

     // Do any additional setup after loading the view.

 //    self.title = @"第一页";

 //    self.tabBarController.navigationItem.title = @"第一页";  // 只走一次

     NSLog(@"第一页已加载");

     // 在这里写，翻页的话，不会改变，永远留在这个位置，也就是添加统一的左按钮

     self.tabBarController.navigationItem.leftBarButtonItem = [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemCamera target:self action:@selector(click:)];

 }

 - (void)viewDidAppear:(BOOL)animated {

     NSLog(@"第一页出现");

     self.tabBarController.navigationItem.title = @"第一页";  // 每次点击这一页，都会加载一次

 }

 - (void)click:(UIBarButtonItem *)sender {

     NSLog(@"统一的左按钮被点击");

 }

 @end