UINavigationController

viewControllers //所有处于栈中的控制器

topViewController //位于栈顶的控制器

visibleViewController //当前正在显?示的控制器

navigationBar //导航条 

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navigationcontroller直接控制viewcontrollers,navigationcontroller还包含navigationBar属性形成整个导航栏

navigationItem包含了bar视图的全部元素（如title,tileview,backBarButtonItem等），受当前viewcontroller管理, 每个navigationController页面的导航栏元素由所在页面的navigationItem管理。即设置当前页面的左右barbutton，用 self.navigationItem.leftBarButtonItem等

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    //导航条的颜色

    self.navigationController.navigationBar.barTintColor = [UIColor grayColor];

    //导航条的线条渲染颜色

    self.navigationController.navigationBar.tintColor = [UIColor blueColor];

    self.navigationController.navigationBar.translucent = NO;

    self.navigationController.navigationBar.hidden = NO;

    //导航条文字属性

    NSDictionary *dic = @{NSFontAttributeName:[UIFont systemFontOfSize:18], NSForegroundColorAttributeName:[UIColor redColor]};

    self.navigationController.navigationBar.titleTextAttributes = dic;

    [self.navigationController.navigationBar setBackgroundImage:[UIImage imageNamed:@"1"] forBarMetrics:UIBarMetricsDefault];

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    self.navigationItem.title = @"标题";

    UISegmentedControl *segmentC = [[UISegmentedControl alloc] initWithItems:@[@"郑州", @"新乡"]];

    self.navigationItem.titleView = segmentC;

    UIBarButtonItem *leftBtn = [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemCompose target:self action:nil];

    self.navigationItem.leftBarButtonItem = leftBtn;

//如果有多个btn,可以放在数组中,然后赋给barButtonItems

    UIBarButtonItem *rightBtn1 = [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemBookmarks target:self action:@selector(handleRightBtn:)];

    UIBarButtonItem *rightBtn2 = [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemCancel target:self action:@selector(handleRightBtn:)];

    UIBarButtonItem *rightBtn3 = [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemCompose target:self action:@selector(handleRightBtn:)];

    NSArray *arr = @[rightBtn1, rightBtn2, rightBtn3];

    self.navigationItem.rightBarButtonItems = arr;