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UINavigaiongController的基本使用
UINavigaiongController实现控制器直接的切换
**层次结构 **

@interface UINavigationController : UIViewController 
@interface UIViewController (UINavigationControllerItem) 
@interface UINavigationItem : NSObject <NSCoding> 
@interface UISwitch : UIControl <NSCoding> 
@interface UIControl : UIView 
@interface UIBarButtonItem : UIBarItem <NSCoding>
@interface UIBarItem : NSObject <NSCoding, UIAppearance>

//UINavigationController的方法和属性 
@property(nonatomic,readonly,strong) UINavigationItem *navigationItem; 

//UINavigationItem的方法和属性 
@property(nullable, nonatomic,strong) UIView *titleView;
@property(nullable, nonatomic,strong) UIBarButtonItem *leftBarButtonItem;
@property(nullable,nonatomic,copy) 
  NSArray<UIBarButtonItem *> *leftBarButtonItems

// UIBarButtonItem 的方法和属性
- (instancetype)initWithTitle:(nullable NSString *)title style
  (UIBarButtonItemStyle)style target:
  (nullable id)target action:(nullable SEL)action;

- (instancetype)initWithBarButtonSystemItem
  (UIBarButtonSystemItem)systemItem target:
  (nullable id)target action:(nullable SEL)action;
//系统的按钮,枚举
typedef NS_ENUM(NSInteger, UIBarButtonSystemItem) {. . .} 

代码

- (void)viewDidLoad {
    [super viewDidLoad];
    //创建返回按钮
    //当'返回按钮'和 title 同时设置时,栈顶界面,只显示'返回按钮'
    UIBarButtonItem *backItem = [[UIBarButtonItem alloc]initWithTitle:
    @"返回按钮" style:UIBarButtonItemStylePlain target:nil action:nil];
    self.navigationItem.backBarButtonItem = backItem;
    
    //创建中间标题文字
    //self.navigationItem.title = @"这是个标题";
    
    //创建中间标题视图
    //self.navigationItem.titleView = [[UISwitch alloc]init];
    
    //创建左侧按钮
    UIBarButtonItem *lefItem2 = [[UIBarButtonItem alloc]initWithTitle:
    @"左侧按钮" style:UIBarButtonItemStylePlain target:nil action:nil];

    self.navigationItem.leftBarButtonItem = lefItem2;
    
    //创建系统的按钮
    //target - 目标
    //action - 事件
    UIBarButtonItem *lefItem = [[UIBarButtonItem alloc]
    initWithBarButtonSystemItem : UIBarButtonSystemItemDone target:self 
    action:@selector(hehe)];
    self.navigationItem.leftBarButtonItem = lefItem;
    
    //创建左侧,多个按钮
    self.navigationItem.leftBarButtonItems = @[lefItem,lefItem2];
    
    //创建右侧按钮
    //target: 目标
    //action: 事件
    UIBarButtonItem *rightItem = 
    [[UIBarButtonItem alloc]initWithTitle:@"右侧按钮"
    style:UIBarButtonItemStylePlain target:nil action:nil];
    self.navigationItem.rightBarButtonItem = rightItem;

    UIBarButtonItem *rightItem2 = 
    [[UIBarButtonItem alloc]initWithBarButtonSystemItem:
    UIBarButtonSystemItemSearch target:self action:@selector(hehe)];
    //创建右侧多个按钮
    self.navigationItem.rightBarButtonItems =@[rightItem,rightItem2];
}

效果图

返回按钮

中间标题内容

中间标题视图

左侧按钮

系统的按钮

注意点
导航栏的内容,由栈顶控制器的navigationItem属性决定!