IOS-UINavigationController详解

（一）UINavigationController及其相关控件之间的关系

@interface UINavigationController : UIViewController
@property(nonatomic,readonly) UINavigationBar *navigationBar;

@interface UIViewController (UINavigationControllerItem)
@property(nonatomic,readonly,strong) UINavigationItem *navigationItem;

@interface UINavigationBar : UIView
@property(nullable, nonatomic,readonly,strong) UINavigationItem *topItem;
@property(nullable, nonatomic,readonly,strong) UINavigationItem *backItem;
@property(nullable,nonatomic,copy) NSArray *items;

@interface UINavigationItem : NSObject
@property(nullable,nonatomic,strong) UIBarButtonItem *backBarButtonItem
@property(nullable,nonatomic,copy) NSArray *leftBarButtonItems;
@property(nullable,nonatomic,copy) NSArray *rightBarButtonItems;
@property(nullable, nonatomic,strong) UIBarButtonItem *leftBarButtonItem;
@property(nullable, nonatomic,strong) UIBarButtonItem *rightBarButtonItem;

@interface UIBarButtonItem : UIBarItem
@property(nullable, nonatomic)         SEL                  action; 

@interface UIBarItem : NSObject
@property(nullable, nonatomic,copy)             NSString    *title;
@property(nullable, nonatomic,strong)           UIImage     *image;

通过对上述几个类的属性的罗列，我们可以做个总结

基本介绍

• UIBarItem
  一个可以放置在Bar之上的所有小控件类的抽象类,可以设置标题，图片等
• UIBarButtonItem
  继承UIBarItem，增加了动作以及目标等button的属性。相当于放在UIToolBar或者UINavigationBar上的特殊的button。
• UINavigationItem
  包含了title,prompt,titleView,leftBarButtonItem,rightBarButtonItem,backBarButonItem等当前页面上所有的信息
• UINavigationBar
  NavigaitonBar就是导航栏 主要对UINavigationItem进行栈管理 展示导航栏的外观背景
• UINavigationController
  包含了viewcontrollers、navigationbar、toolbar

关系综述

• UINavigationController是一个容器类，对ViewController进行栈管理，包含navigationBar。
• UINavigationBar 即UINavigationController顶部的导航栏，主要负责外观背景的展示，并对navigationItem进行栈管理
• UINavigationItem是导航栏上显示的具体的元素的一个抽象类,UINavigationController 通过Category的方法为ViewController添加了一个navigationItem,把UINavigationItem交由ViewController管理

// Created on-demand so that a view controller may customize its navigation appearance.

这里引用叶落寒的一段介绍，更加的通俗易懂

通俗地说就是，UINavigationController是个容器，里面可以装很多UIViewController。装这么多UIViewController让用户怎么控制它们呢？总得有个工具吧，这个工具就是UINavigationBar。一个容器就这么一个bar，相当于控制台吧。但是管理那么多UIViewController，控制台上得按钮啊、标题啊，都千篇一律是不是看起来太无聊了。为了解决这个问题，UINavigationController为每个UIViewController生成一个UINavigationItem，通过这个UINavigationItem可以改变控制台“上面”的按钮和标题。如果你不自定义UINavigationItem，UINavigationController会使用默认的;

（二）UINavigationController及其相关控件的属性和方法

1. UIBarButtonItem

//初始化方法
- (instancetype)init;
- (nullable instancetype)initWithCoder:(NSCoder *)aDecoder;
- (instancetype)initWithImage:(nullable UIImage *)image landscapeImagePhone:(nullable     UIImage *)landscapeImagePhone style:(UIBarButtonItemStyle)style target:(nullable id)target action:(nullable SEL)action;
- (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;
- (instancetype)initWithCustomView:(UIView *)customView;

@property(nonatomic)         UIBarButtonItemStyle style;  //类型
@property(nonatomic)         CGFloat              width;
@property(nullable, nonatomic,copy)    NSSet   *possibleTitles;
@property(nullable, nonatomic,strong)  __kindof UIView     *customView;
@property(nullable, nonatomic)         SEL                  action;
@property(nullable, nonatomic,weak)    id                   target;
@property(nullable, nonatomic,strong) UIColor *tintColor

//为任意style的button设置背景图片
- (void)setBackgroundImage:(nullable UIImage *)backgroundImage forState:(UIControlState)state barMetrics:(UIBarMetrics)barMetrics
- (nullable UIImage *)backgroundImageForState:(UIControlState)state barMetrics:(UIBarMetrics)barMetrics

//为特定style的button设置背景图片
- (void)setBackgroundImage:(nullable UIImage *)backgroundImage forState:(UIControlState)state style:(UIBarButtonItemStyle)style barMetrics:(UIBarMetrics)barMetrics
- (nullable UIImage *)backgroundImageForState:(UIControlState)state style:(UIBarButtonItemStyle)style barMetrics:(UIBarMetrics)barMetrics


//设置背景图片垂直方向的偏移量
- (void)setBackgroundVerticalPositionAdjustment:(CGFloat)adjustment forBarMetrics:(UIBarMetrics)barMetrics
- (CGFloat)backgroundVerticalPositionAdjustmentForBarMetrics:(UIBarMetrics)barMetrics

//设置标题的偏移量
- (void)setTitlePositionAdjustment:(UIOffset)adjustment forBarMetrics:(UIBarMetrics)barMetrics
- (nullable UIImage *)backButtonBackgroundImageForState:(UIControlState)state barMetrics:(UIBarMetrics)barMetrics

//设置返回按钮标题偏移量
- (void)setBackButtonTitlePositionAdjustment:(UIOffset)adjustment forBarMetrics:(UIBarMetrics)barMetrics
- (UIOffset)backButtonTitlePositionAdjustmentForBarMetrics:(UIBarMetrics)barMetrics

//设置返回按钮背景图片在垂直方向上的偏移量
- (void)setBackButtonBackgroundVerticalPositionAdjustment:(CGFloat)adjustment forBarMetrics:(UIBarMetrics)barMetrics
- (CGFloat)backButtonBackgroundVerticalPositionAdjustmentForBarMetrics:(UIBarMetrics)barMetrics

typedef NS_ENUM(NSInteger, UIBarButtonSystemItem) {
    UIBarButtonSystemItemDone,//显示完成
    UIBarButtonSystemItemCancel,//显示取消
    UIBarButtonSystemItemEdit,  //显示编辑
    UIBarButtonSystemItemSave, //显示保存 
    UIBarButtonSystemItemAdd,//显示加号
    UIBarButtonSystemItemFlexibleSpace,//什么都不显示，占位一个空间位置
    UIBarButtonSystemItemFixedSpace,//和上一个类似
    UIBarButtonSystemItemCompose,//显示写入按钮
    UIBarButtonSystemItemReply,//显示循环按钮
    UIBarButtonSystemItemAction,//显示活动按钮
    UIBarButtonSystemItemOrganize,//显示组合按钮
    UIBarButtonSystemItemBookmarks,//显示图书按钮
    UIBarButtonSystemItemSearch,//显示查找按钮
    UIBarButtonSystemItemRefresh,//显示刷新按钮
    UIBarButtonSystemItemStop,//显示停止按钮
    UIBarButtonSystemItemCamera,//显示相机按钮
    UIBarButtonSystemItemTrash,//显示移除按钮
    UIBarButtonSystemItemPlay,//显示播放按钮
    UIBarButtonSystemItemPause,//显示暂停按钮
    UIBarButtonSystemItemRewind,//显示退后按钮
    UIBarButtonSystemItemFastForward,//显示前进按钮
    UIBarButtonSystemItemUndo,//显示消除按钮
    UIBarButtonSystemItemRedo ,//显示重做按钮
    UIBarButtonSystemItemPageCurl ,//在tool上有效
};

2.UINavigationItem

NS_CLASS_AVAILABLE_IOS(2_0) @interface UINavigationItem : NSObject 
//初始化
- (instancetype)initWithTitle:(NSString *)title NS_DESIGNATED_INITIALIZER;
- (nullable instancetype)initWithCoder:(NSCoder *)coder NS_DESIGNATED_INITIALIZER;

//设置导航栏中间的内容标题
@property(nullable, nonatomic,copy)   NSString        *title;         
//设置导航栏中间的内容视图    
@property(nullable, nonatomic,strong) UIView          *titleView;        
//提示
@property(nullable,nonatomic,copy)   NSString *prompt;      
//返回
@property(nullable,nonatomic,strong) UIBarButtonItem *backBarButtonItem; 
//是否隐藏返回Button
@property(nonatomic,assign) BOOL hidesBackButton;
- (void)setHidesBackButton:(BOOL)hidesBackButton animated:(BOOL)animated;

//左边数组Item
@property(nullable,nonatomic,copy) NSArray *leftBarButtonItems NS_AVAILABLE_IOS(5_0);
//右边数组Item
@property(nullable,nonatomic,copy) NSArray *rightBarButtonItems NS_AVAILABLE_IOS(5_0);
- (void)setLeftBarButtonItems:(nullable NSArray *)items animated:(BOOL)animated NS_AVAILABLE_IOS(5_0);
- (void)setRightBarButtonItems:(nullable NSArray *)items animated:(BOOL)animated NS_AVAILABLE_IOS(5_0);

//通过指定该属性为YES，可以让leftBarButtonItem和backBarButtonItem同时显示，其中leftBarButtonItem显示在backBarButtonItem的右边 默认值为NO
@property(nonatomic) BOOL leftItemsSupplementBackButton NS_AVAILABLE_IOS(5_0);

//左边Item
@property(nullable, nonatomic,strong) UIBarButtonItem *leftBarButtonItem;
//右边Item
@property(nullable, nonatomic,strong) UIBarButtonItem *rightBarButtonItem;
- (void)setLeftBarButtonItem:(nullable UIBarButtonItem *)item animated:(BOOL)animated;
- (void)setRightBarButtonItem:(nullable UIBarButtonItem *)item animated:(BOOL)animated;
@end

prompt 是一个NSString类型描述，注意添加该描述以后NavigationBar的高度会增加30，总的高度会变成74(不管当前方向是Portrait还是Landscape，此模式下navgationbar都使用高度44加上prompt30的方式进行显示)。

如：

self.navigationItem.prompt=@"这是什么？";
self.title=@"HAH";

prompt.png

3.UINavigationBar

NS_CLASS_AVAILABLE_IOS(2_0) @interface UINavigationBar : UIView  

//UIBarStyleDefault  灰色背景 白色文字 UIBarStyleBlack    纯黑色背景 白色文字
@property(nonatomic,assign) UIBarStyle barStyle;
@property(nullable,nonatomic,weak) id delegate;
//Translucent设置成透明度,设置成YES会有一种模糊效果
@property(nonatomic,assign,getter=isTranslucent) BOOL translucent NS_AVAILABLE_IOS(3_0) UI_APPEARANCE_SELECTOR; 
//UINavigationBar上面不只是简单的显示标题，它也将标题进行了堆栈的管理，每一个标题抽象为的对象在iOS系统中是UINavigationItem对象，我们可以通过push与pop操作管理item组。
//向栈中添加一个item，上一个item会被推向导航栏的左侧，变为pop按钮，会有一个动画效果
- (void)pushNavigationItem:(UINavigationItem *)item animated:(BOOL)animated;
//pop一个item
- (nullable UINavigationItem *)popNavigationItemAnimated:(BOOL)animated; 
//当前push到最上层的item
@property(nullable, nonatomic,readonly,strong) UINavigationItem *topItem;
//仅次于最上层的item，一般式被推向导航栏左侧的item
@property(nullable, nonatomic,readonly,strong) UINavigationItem *backItem;
//获取堆栈中所有item的数组
@property(nullable,nonatomic,copy) NSArray *items;
//设置一组item
- (void)setItems:(nullable NSArray *)items animated:(BOOL)animated; 

//系统类型按钮文字颜色
@property(null_resettable, nonatomic,strong) UIColor *tintColor;

//通过barTintColor来设置背景色
@property(nullable, nonatomic,strong) UIColor *barTintColor NS_AVAILABLE_IOS(7_0) UI_APPEARANCE_SELECTOR;  

//设置工具栏背景和阴影图案
- (void)setBackgroundImage:(nullable UIImage *)backgroundImage forBarPosition:(UIBarPosition)barPosition barMetrics:(UIBarMetrics)barMetrics NS_AVAILABLE_IOS(7_0) UI_APPEARANCE_SELECTOR;
- (nullable UIImage *)backgroundImageForBarPosition:(UIBarPosition)barPosition barMetrics:(UIBarMetrics)barMetrics NS_AVAILABLE_IOS(7_0) UI_APPEARANCE_SELECTOR;

//通过背景图片来设置导航栏的外观
- (void)setBackgroundImage:(nullable UIImage *)backgroundImage forBarMetrics:(UIBarMetrics)barMetrics NS_AVAILABLE_IOS(5_0) UI_APPEARANCE_SELECTOR;
- (nullable UIImage *)backgroundImageForBarMetrics:(UIBarMetrics)barMetrics NS_AVAILABLE_IOS(5_0) UI_APPEARANCE_SELECTOR;

//背景阴影图片 - 即分割线
@property(nullable, nonatomic,strong) UIImage *shadowImage NS_AVAILABLE_IOS(6_0) UI_APPEARANCE_SELECTOR;

//标题的富文本
@property(nullable,nonatomic,copy) NSDictionary *titleTextAttributes NS_AVAILABLE_IOS(5_0) UI_APPEARANCE_SELECTOR;

//标题垂直偏移
- (void)setTitleVerticalPositionAdjustment:(CGFloat)adjustment forBarMetrics:(UIBarMetrics)barMetrics NS_AVAILABLE_IOS(5_0) UI_APPEARANCE_SELECTOR;
- (CGFloat)titleVerticalPositionAdjustmentForBarMetrics:(UIBarMetrics)barMetrics NS_AVAILABLE_IOS(5_0) UI_APPEARANCE_SELECTOR;

//设置返回按钮的图片
@property(nullable,nonatomic,strong) UIImage *backIndicatorImage NS_AVAILABLE_IOS(7_0) UI_APPEARANCE_SELECTOR;
@property(nullable,nonatomic,strong) UIImage *backIndicatorTransitionMaskImage NS_AVAILABLE_IOS(7_0) UI_APPEARANCE_SELECTOR;
@end

4.UINavigationController

NS_CLASS_AVAILABLE_IOS(2_0) @interface UINavigationController : UIViewController

//UINavigationController初始化,自定义NavigationBar,自定义toolbar
- (instancetype)initWithNavigationBarClass:(nullable Class)navigationBarClass toolbarClass:(nullable Class)toolbarClass NS_AVAILABLE_IOS(5_0);

//UINavigationController初始化,导航控制器的根控制器
- (instancetype)initWithRootViewController:(UIViewController *)rootViewController;

//压栈：将目标控制器压入栈中
- (void)pushViewController:(UIViewController *)viewController animated:(BOOL)animated; 

//弹栈：将栈顶控制器从栈中弹出
- (nullable UIViewController *)popViewControllerAnimated:(BOOL)animated;

//弹栈：弹到指定的目标控制器
- (nullable NSArray<__kindof UIViewController *> *)popToViewController:(UIViewController *)viewController animated:(BOOL)animated; 

//弹栈：弹到根控制器
- (nullable NSArray<__kindof UIViewController *> *)popToRootViewControllerAnimated:(BOOL)animated;
//导航栈的栈顶视图 只读 就是某个导航栈的栈顶视图，和导航息息相关
@property(nullable, nonatomic,readonly,strong) UIViewController *topViewController; 
//当前显示的控制器 只读 visibleViewController和哪个导航栈没有关系，只是当前显示的控制器，也就是说任意一个导航的visibleViewController所返回的值应该是一样的
@property(nullable, nonatomic,readonly,strong) UIViewController *visibleViewController;

//栈里的视图控制器数组
@property(nonatomic,copy) NSArray<__kindof UIViewController *> *viewControllers;

//替换栈中的视图控制器数组
- (void)setViewControllers:(NSArray *)viewControllers animated:(BOOL)animated NS_AVAILABLE_IOS(3_0); 

//是否隐藏导航栏
@property(nonatomic,getter=isNavigationBarHidden) BOOL navigationBarHidden;

//设置导航栏隐藏 是否有动画
- (void)setNavigationBarHidden:(BOOL)hidden animated:(BOOL)animated; 

//导航栏
@property(nonatomic,readonly) UINavigationBar *navigationBar; 

//toolbar是否隐藏
@property(nonatomic,getter=isToolbarHidden) BOOL toolbarHidden NS_AVAILABLE_IOS(3_0);

//toolbar是否隐藏 是否有动画
- (void)setToolbarHidden:(BOOL)hidden animated:(BOOL)animated NS_AVAILABLE_IOS(3_0); 

//toolbar对象
@property(null_resettable,nonatomic,readonly) UIToolbar *toolbar NS_AVAILABLE_IOS(3_0); 

//委托
@property(nullable, nonatomic, weak) id delegate;

//边缘侧滑返回手势
@property(nullable, nonatomic, readonly) UIGestureRecognizer *interactivePopGestureRecognizer NS_AVAILABLE_IOS(7_0);

//展示视图控制器
- (void)showViewController:(UIViewController *)vc sender:(nullable id)sender NS_AVAILABLE_IOS(8_0); // Interpreted as pushViewController:animated:

//输入键盘出现时将导航栏隐藏 IOS8特性
@property (nonatomic, readwrite, assign) BOOL hidesBarsWhenKeyboardAppears NS_AVAILABLE_IOS(8_0);

//滚动页面时隐藏Bar  IOS8特性
@property (nonatomic, readwrite, assign) BOOL hidesBarsOnSwipe NS_AVAILABLE_IOS(8_0);

//获取能够隐藏navigationBar的滑动手势 只读
@property (nonatomic, readonly, strong) UIPanGestureRecognizer *barHideOnSwipeGestureRecognizer NS_AVAILABLE_IOS(8_0);

//当设置为true时，横向方向时隐藏NavigationBar
@property (nonatomic, readwrite, assign) BOOL hidesBarsWhenVerticallyCompact NS_AVAILABLE_IOS(8_0);

//当设置为true时，如果有没处理的点击手势就会隐藏和现实navigationBar 
@property (nonatomic, readwrite, assign) BOOL hidesBarsOnTap NS_AVAILABLE_IOS(8_0);

//获取能够隐藏navigationBar的点击手势 只读
@property (nonatomic, readonly, assign) UITapGestureRecognizer *barHideOnTapGestureRecognizer NS_AVAILABLE_IOS(8_0);

@end

@interface UIViewController (UINavigationControllerItem)
//导航栏上面用户自定义视图
@property(nonatomic,readonly,strong) UINavigationItem *navigationItem; 
//推送时隐藏bottom
@property(nonatomic) BOOL hidesBottomBarWhenPushed;
//下级视图的导航控制器
@property(nullable, nonatomic,readonly,strong) UINavigationController *navigationController; 
@end

@interface UIViewController (UINavigationControllerContextualToolbarItems)
//属性设置工具条中包含的按钮
@property (nullable, nonatomic, strong) NSArray<__kindof UIBarButtonItem *> *toolbarItems NS_AVAILABLE_IOS(3_0);
- (void)setToolbarItems:(nullable NSArray *)toolbarItems animated:(BOOL)animated NS_AVAILABLE_IOS(3_0);
@end

（三）实际开发过程中的一些问题

1.UINavigationBar的背景颜色

-(void)changeNavigationBarBackgroundColor {
    //背景色
    self.navigationBar.barTintColor = [UIColor blueColor];

    //title字体
    //[self.navigationController.navigationBar setTitleTextAttributes:@{NSForegroundColorAttributeName:[UIColor lightGrayColor],NSFontAttributeName:[UIFont systemFontOfSize:17]}];

    //修改UIBarButtonItem 图片 title颜色
    self.navigationBar.tintColor = [UIColor greenColor];

    //是否半透明 当为YES时 设置的导航栏背景颜色会和实际rgb值有误差
    self.navigationBar.translucent = NO;

    //如果想要半透明效果 颜色没有色差 可以通过设置背景图片的方法 背景图片会覆盖barTintColor
    //- (void)setBackgroundImage:(nullable UIImage *)backgroundImage forBarMetrics:(UIBarMetrics)barMetrics
}

2.UINavigationBar底部的shadowImage

Apple官方对shadowImage有这样的介绍：

/* Default is nil. When non-nil, a custom shadow image to show instead of the default shadow image. For a custom shadow to be shown, a custom background image must also be set with -setBackgroundImage:forBarMetrics: (if the default background image is used, the default shadow image will be used).
*/

设置shadowImage必须先setBackgroundImage，否则无法实现效果

-(void)changeNavigationBarBottonLine {
    //设置底部line颜色时需要同时设置backgroundImage即导航栏的背景图片 否则没有效果
    [self.navigationBar setBackgroundImage:[UIImage new] forBarMetrics:UIBarMetricsDefault];
    [self.navigationBar setShadowImage:[self imageWithColor:[UIColor redColor]]];
    //此处设置透明颜色的image,底部line即可隐藏，但此种方法隐藏，没有办法再显示 下面方法通过找到该view 控制其hidden属性
    //[self reducibilityHiddenNavogationBarLine];
}

---

找到该imageView设置起Hidden为YES

-(void)reducibilityHiddenNavogationBarLine {
    UIImageView * imageView = [self findLineImageViewUnder:self.navigationBar];
    if (imageView) {
       imageView.hidden = YES;
    }
}

---

找到该imageView

-(UIImageView *)findLineImageViewUnder:(UIView *)view {
    if ([view isKindOfClass:[UIImageView class]] && view.bounds.size.height <= 1.0) {
        return (UIImageView *)view;
    }
    for (UIView * subView in view.subviews) {
        UIImageView * imageView = [self findLineImageViewUnder:subView];
        if (imageView) {
            return imageView;
        }
    }
    return nil;
}

3.自定义导航栏的返回按钮

@property(nonatomic,readonly) UINavigationBar *navigationBar;// The navigation bar managed by the controller. Pushing, popping or setting navigation items on a managed navigation bar is not supported.

受controller管理的navigationBar 不支持对navigation items操作,所以此处对返回按钮的操作是在ViewController完成的，以下代码中self表示ViewController

Apple官方对setBackIndicatorImage和setBackIndicatorTransitionMaskImage做了如下解释，必须同时设置才能生效

/*
 The back indicator image is shown beside the back button.
 The back indicator transition mask image is used as a mask for content during push and pop transitions
 Note: These properties must both be set if you want to customize the back indicator image.
 */

1.自定义文字+图片

-(void)createCustomBackBarItem {
    
    //修改图片文字颜色
    self.navigationController.navigationBar.tintColor = [UIColor whiteColor];
    
    //替换图片
    [self.navigationController.navigationBar setBackIndicatorImage:[UIImage imageNamed:@"erwema"]];
    [self.navigationController.navigationBar setBackIndicatorTransitionMaskImage:[UIImage imageNamed:@"erwema"]];
    
    //设置文字
    UIBarButtonItem * backBarItem = [[UIBarButtonItem alloc]initWithTitle:@"返回" style:UIBarButtonItemStylePlain target:nil action:nil];
    self.navigationItem.backBarButtonItem = backBarItem;
}

注意：
对backBarButtonItem的修改是在当前viewController前一个页面完成的，在当前页面修改针对下一个viewController的navigationItem生效

2.不显示文字
设置Title在Y方向上的偏移量，使其移除屏幕，该方法在第一次进入时会有个文字移动的动画效果，效果不好，不推荐使用

[[UIBarButtonItem appearance] setBackButtonTitlePositionAdjustment:UIOffsetMake(0, -100) forBarMetrics:UIBarMetricsDefault];

3.使用leftBarButtonItem替代backBarButtonItem
使用这种方法，不能使用边缘滑动返回手势，且不能同时设置图片和标题

-(void)setLeftBarItemBack
{
    UIBarButtonItem *leftBarBtnItem = [[UIBarButtonItem alloc]initWithImage:[UIImage imageNamed:@"back"] style:UIBarButtonItemStylePlain target:self action:@selector(clickLeftBarBtnItem:)];
    [self.navigationItem setLeftBarButtonItem:leftBarBtnItem animated:YES];
    self.navigationItem.leftBarButtonItem.tintColor = NavigationLeftBackColor;
}

/**
 *  导航条leftBarBtn事件
 */
- (void)clickLeftBarBtnItem:(UIBarButtonItem *)sender {
    [self.navigationController popViewControllerAnimated:YES];
}

4.使用CustomView的方法
此方法不适于backBarButtonItem，只能用于leftBarButtonItem

注意：
1.如果B视图有一个自定义的左侧按钮（leftBarButtonItem），则会显示这个自定义按钮；
2.如果B没有自定义按钮，但是A视图的backBarButtonItem属性有自定义项，则显示这个自定义项；
3.如果前2条都没有，则默认显示一个后退按钮，后退按钮的标题是A视图的标题；

---

此处注意:
5.0中新增加了一个属性leftItemsSupplementBackButton，通过指定该属性为YES，可以让leftBarButtonItem和backBarButtonItem同时显示，其中leftBarButtonItem显示在backBarButtonItem的右边。

使用3、4方法，边缘返回会失效，此时加上这句代码依然可以实现边缘滑动返回

    self.navigationController.interactivePopGestureRecognizer.delegate = self;

这里推荐使用FDFullscreenPopGesture 全屏滑动手势返回，减少工作量。

4.navigationBar偶尔显示上一个页面的navigationBar

一般情况下都是正常的。但是在偶然情况下，会出现在进入新界面后，新界面的navigationBar会突然消失，出现的还是上一个界面的 navigationBar。从此以后，navigationBar 全乱了， kill 掉重新进，恢复正常。
原因：
一般我们会打点调用navigationBarHidden的属性来设置导航栏是否隐藏，这种方法是不带动画效果的。这样偶尔就会导致错乱，这应该是一个系统的bug,所以应尽量使用

[self.navigationController setNavigationBarHidden:YES animated:YES];

来设置navigationBar的Hidden属性。

5.修改系统navigationBar的高度

UINavigationBar * navigatioBar = self.navigationController.navigationBar;
CGFloat navBarHeight = 100.f;
CGRect frame = CGRectMake(0.0f, 0.0f, 320.0f, navBarHeight);
[navigatioBar setFrame:frame];

此种方法经测试只有在非rootViewController中才能生效，这样在第一次进入根视图的时候navigationBar并不会变高，不知有没有人知道该如何解决，欢迎赐教。

6.易混淆知识点

1.self.title、self.navigationItem.title、self.tabBarItem.title之间的关系

一目了然

self.navigationItem.title = @"my title"; //sets navigation bar title.
self.tabBarItem.title = @"my title"; //sets tab bar title.
self.title = @"my title"; //sets both of these.

1. 如果当前VC通过 self.navigationItem.titleView指定了自定义的titleView，系统将会显示指定的titleView,设置self.title、self.navigationItem.title不会改变导航栏的标题。

2. 如果当前VC没有指定titleView，系统则会根据当前VC的title或者当前VC的navigationItem.title的内容创建一个UILabel并显示。
3. self.title会重写navigationItem和tabBarItem的title。

2.self.navigationItem,self.navigationController.navigationItem的关系

navigationItem是UIViewController的一个属性，navigationController继承UIViewController，自然会继承viewControoler的navigationItem属性。此处self.navigationController.navigationItem是应该被忽视的。navigationItem直接由viewController管理。

3.UIBarMetrics和UIBarPosition

typedef NS_ENUM(NSInteger, UIBarMetrics) {
    UIBarMetricsDefault, //横屏
    UIBarMetricsCompact,//竖屏
    UIBarMetricsDefaultPrompt = 101, //横屏且设置了prompt属性  Applicable only in bars with the prompt property, such as UINavigationBar and UISearchBar
    UIBarMetricsCompactPrompt, //竖屏且设置了prompt属性
};

typedef NS_ENUM(NSInteger, UIBarPosition) {
    UIBarPositionAny = 0, //Bar在任何位置
    UIBarPositionBottom = 1, //Bar在底部
    UIBarPositionTop = 2, //Bar在顶部
    UIBarPositionTopAttached = 3, //Bar在顶部，且他的背景扩展到statusBar的区域
} NS_ENUM_AVAILABLE_IOS(7_0);

不正之处，还望多多指教！

参考文章
你真的了解UINavigationController吗?
UINavigationItem UINavigationBar 关系分析