- (void)pushViewController:(UIViewController *)viewController animated:(BOOL)animated {
    if (self.childViewControllers.count) {
        viewController.navigationItem.leftBarButtonItem = [[UIBarButtonItem alloc] initWithImage:[UIImage imageNamed:@"back"] style:UIBarButtonItemStylePlain target:self action:@selector(back)];
    }
    [super pushViewController:viewController animated:animated];
}

- (void)back {
    [self popViewControllerAnimated:YES];
}

+ (void)initialize {
	UIBarButtonItem *item = [UIBarButtonItem appearance];
	[item setBackButtonTitlePositionAdjustment:UIOffsetMake(NSIntegerMin, 0) forBarMetrics:UIBarMetricsDefault];
}

+ (void)initialize {
	UIBarButtonItem *item = [UIBarButtonItem appearance];
	NSMutableDictionary *attrs = [NSMutableDictionary dictionary];
	attrs[NSForegroundColorAttributeName] = [UIColor whiteColor]; // 文字颜色
	[item setTitleTextAttributes:attrs forState:UIControlStateNormal];
	item.tintColor = [UIColor whiteColor]; // 图片颜色
}

+ (void)initialize {
	UINavigationBar *bar = [UINavigationBar appearance];
	UIImage *image = [UIImage imageNamed:@"back"];
	bar.backIndicatorImage = image;
	bar.backIndicatorTransitionMaskImage = image;
}

+ (void)initialize {
    UINavigationBar *bar = [UINavigationBar appearance];
    [bar setBackgroundImage:[[UIImage alloc] init] forBarPosition:UIBarPositionAny barMetrics:UIBarMetricsDefault];
    [bar setShadowImage:[[UIImage alloc] init]];
}

@implementation UINavigationBar (VCHDropShadow)

- (void)dropShadowWithOffset:(CGSize)offset
                      radius:(CGFloat)radius
                       color:(UIColor *)color
                     opacity:(CGFloat)opacity {
    // 也可以不创建 path ，直接使用。
    CGMutablePathRef path = CGPathCreateMutable();
    CGPathAddRect(path, NULL, self.bounds);
    self.layer.shadowPath = path;
    CGPathCloseSubpath(path);
    CGPathRelease(path);
    
    self.layer.shadowColor = color.CGColor;
    self.layer.shadowOffset = offset;
    self.layer.shadowRadius = radius;
    self.layer.shadowOpacity = opacity;
 	
    self.clipsToBounds = NO;
}

@end

@try { 
	EOCSomeClass *object = [[EOCSomeClass alloc]init]; 
	[object doSomethingThatMayThrow]; 
	[object release]; 
} @catch (NSException *exception) { 
	NSLog(@"there was an error."); 
}
~~~  
事实上当 doSomethingThatMayThrow 发生异常时，就会直接跳出，不会再往下执行，所以 release 方法无法执行，也就出现内存泄漏了。  
使用 @finally 可以解决这个问题  

~~~ objc
EOCSomeClass *object = nil;
@try {
	object = [[EOCSomeClass alloc] init]; 
	[object doSomethingThatMayThrow];
} @catch (NSException *exception) {
	NSLog(@"there was an error.");
} @finally {
	[object release];
}
~~~  

在 ARC 环境下，也会出现这样的问题，由于 ARC 不能调用 release 方法。上面的代码同样会出问题    

~~~ objc
@try { 
	EOCSomeClass *object = [[EOCSomeClass alloc] init];
	[object doSomethingThatMayThrow];
} @catch (NSException *exception) {
	NSLog(@"there was an error.");
} @finally {
	
}

for(int i = 0; i < 100000; i++) {
	[self doSomethingWithInt:i]; // 方法内创建了大量的临时变量
}

for(int i = 0; i < 100000; i++) {
	@autoreleasepool {
		[self doSomethingWithInt:i];
	}
}

@protocol NSObject

- (instancetype)retain OBJC_ARC_UNAVAILABLE;
- (oneway void)release OBJC_ARC_UNAVAILABLE;
- (instancetype)autorelease OBJC_ARC_UNAVAILABLE;

@end

NSNumber *number = [[NSNumber alloc] initWithInt:1234];
[array addObject:number];
[number release];
NSLog(@"number = %@",number);

- (void)setFoo:(id)foo {
    [foo retain];
    [_foo release];
    _foo = foo;
}

- (NSString *)stringValue {
    NSString *str = [[NSString alloc] initWithFormat:@"I am this: %@",self];
    return str;
}

- (NSString *)stringValue {
    NSString *str = [[NSString alloc] initWithFormat:@"I am this: %@",self];
    return [str autorelease];
}

if ([self showLogMsg]) {
	NSString *str = [[NSString alloc] initWithFormat:@"I am this: %@",self];
	NSLog(@"%@",str);
}

if ([self showLogMsg]) {
	NSString *str = [[NSString alloc] initWithFormat:@"I am this: %@",self];
	NSLog(@"%@",str);
	[str release];
}

// 方法名以关键字 new 开头，ARC 不会加入 retain、release 或 autorelease 语句。
+ (EOCPerson *)newPerson {
	EOCPerson *person = [[EOCPerson alloc] init]; 
	return person; 
}

// 方法名不以关键字开头，ARC 会自动加上 autorelease 语句。
+ (EOCPerson *)somePerson {
	EOCPerson *person = [[EOCPerson alloc] init]; 
	return person; 
}

// ARC 会在函数末尾给 personOne 加上 release 语句。
- (void)doSomething {
	EOCPerson *personOne = [EOCPerson newPerson]; 
	EOCPerson *personTwo = [EOCPerson somePerson]; 
}

@protocol PersonDelegate <NSObject>
@required
- (NSDate *)whatTimeIsIt;

@optional
- (BOOL)isNiceDay;

@end

@interface Person : NSObject

@property (nonatomic, weak) id<PersonDelegate> personDelegate;

@end

@implementation Person

- (void)doWork {
    NSDate *date = [self.personDelegate whatTimeIsIt];
    NSLog(@"date = %@",date);
    if ([self.personDelegate respondsToSelector:@selector(isNiceDay)]) {
        BOOL isNiceDay = [self.personDelegate isNiceDay];
        NSLog(@"isNiceDay:%zd",isNiceDay);
    }
}

@end

@interface ViewController () <PersonDelegate>

@end

@implementation ViewController

Person *person = [[Person alloc] initWithEmail:@"123@163.com"];
person.personDelegate = self;
[person doWork];

@end

// log
// date = Thu May  3 19:43:05 2018
// isNiceDay:1

@interface NSString : NSObject <NSCopying, NSMutableCopying, NSSecureCoding>
// 0
@end

@interface NSString (NSStringExtensionMethods)
// 1
@end

@interface NSString (NSStringEncodingDetection)
// 2
@end

@interface NSString (NSItemProvider) <NSItemProviderReading, NSItemProviderWriting>
// 3
@end

@interface NSString (NSExtendedStringPropertyListParsing)
// 4
@end

@interface NSString (NSStringDeprecated)
// 5
@end

@interface UIWindow (VCHAnimalWindow)

- (void)vch_setRootViewController:(UIViewController *)rootViewController withOglFlipSubtype:(NSString *)subtype;
- (void)vch_setRootViewController:(UIViewController *)rootViewController animalType:(NSString *)type subtype:(NSString *)subtype duration:(CFTimeInterval)duration;

@end

// 在分类中定义一个属性
@interface Person (Special)

@property (nonatomic, weak) NSString *name;

@end

// Property 'name' requires method 'name' to be defined - use @dynamic or provide a method implementation in this category
// Property 'name' requires method 'setName:' to be defined - use @dynamic or provide a method implementation in this category

- (void)setName:(NSString *)name {
    objc_setAssociatedObject(self, "kPersonSpecial_name", name, OBJC_ASSOCIATION_COPY);
}

- (NSString *)name {
    return objc_getAssociatedObject(self, "kPersonSpecial_name");
}

@interface ViewController () <PersonDelegate>
{
    int _count;
}

@property (nonatomic, copy) Person *person;

@end

@property (nonatomic, weak) id <VCHDelegate> delegate;

@protocol EOCDatabaseConnection

- (void)connect;
- (void)disconnect;
- (BOOL)isConnected;
- (NSArray *)performQuery:(NSString *)query;

@end

@protocol EOCDatabaseConnection;  

@interface EOCDatabaseManger:NSObject  

+ (id)sharedInstance;  
- (id<EOCDatabaseConnection>) connectionWithIdentifier:(NSString *)identifier;  

@end;  

#import <UIKit/UIKit.h>

@interface UIWindow (VCHAnimalWindow)

- (void)vch_setRootViewController:(UIViewController *)rootViewController withOglFlipSubtype:(NSString *)subtype;
- (void)vch_setRootViewController:(UIViewController *)rootViewController animalType:(NSString *)type subtype:(NSString *)subtype duration:(CFTimeInterval)duration;

@end

#import "UIWindow+VCHAnimalWindow.h"

@implementation UIWindow (VCHAnimalWindow)

- (void)vch_setRootViewController:(UIViewController *)rootViewController withOglFlipSubtype:(NSString *)subtype {
    [self vch_setRootViewController:rootViewController animalType:@"oglFlip" subtype:subtype duration:.5f];
}

- (void)vch_setRootViewController:(UIViewController *)rootViewController animalType:(NSString *)type subtype:(NSString *)subtype duration:(CFTimeInterval)duration {
    CATransition *animation = [CATransition animation];
    [animation setDuration:duration];
    [animation setType:type];
    [animation setSubtype:subtype];
    [self.layer addAnimation:animation forKey:@"VCHAnimalUIWindow"];
    [self setRootViewController:rootViewController];
}

@end

// [[UIApplication sharedApplication].keyWindow setRootViewController:ctrl];
[[UIApplication sharedApplication].keyWindow vch_setRootViewController:ctrl withOglFlipSubtype:kCATransitionFromRight];

#import <objc/runtime.h>

@implementation UIWindow (VCHAnimalWindow)

+ (void)load {
    Method originalMethod = class_getInstanceMethod([UIWindow class], @selector(setRootViewController:));
    Method swappedMethod = class_getInstanceMethod([UIWindow class], @selector(vch_setRootViewController:));
    method_exchangeImplementations(originalMethod, swappedMethod);
}

- (void)vch_setRootViewController:(UIViewController *)rootViewController {
    CATransition *animation = [CATransition animation];
    [animation setDuration:.5f];
    [animation setType:@"oglFlip"];
    [animation setSubtype:kCATransitionFromRight];
    [self.layer addAnimation:animation forKey:@"VCHAnimalUIWindow"];
    [self vch_setRootViewController:rootViewController];
}

@end

Person *p = [[Person alloc] initWithEmail:@"123@163.com"];
NSLog(@"%@",p);

// 输出
// <Person: 0x109ea6170>

- (NSString *)description {
    return [NSString stringWithFormat:@"<%@: %p, email = %@>", [self class], self, _email];
}

// 输出
// <Person: 0x12bd4f090, email = 123@163.com>

- (NSString *)debugDescription {
    return [NSString stringWithFormat:@"<%@: %p, email = %@>", [self class], self, _email];
}

// 输出
// <Person: 0x113dbff50, email = 123@163.com>
// (lldb) po p
// <Person: 0x113dbff50, email = 123@163.com>
// (lldb) 

@throw [NSException exceptionWithName:@"errorName" reason:@"errorReason" userInfo:@{@"key":@"value"}];

@protocol NSCopying

- (id)copyWithZone:(nullable NSZone *)zone;

@end

// .h
@interface Person : NSObject<NSCopying>

- (instancetype)initWithEmail:(NSString *)email;
@property (nonatomic, copy) NSString *email;
@property (nonatomic, copy) NSString *name;

@end

// .m
- (id)copyWithZone:(NSZone *)zone {
    Person *person = [[[self class] allocWithZone:zone] initWithEmail:_email];
    person.name = [_name copy];
    return person;
}

@protocol NSMutableCopying

- (id)mutableCopyWithZone:(nullable NSZone *)zone;

@end

- (instancetype)initWithSet:(NSSet<ObjectType> *)set copyItems:(BOOL)flag;
- (instancetype)initWithArray:(NSArray<ObjectType> *)array copyItems:(BOOL)flag;

// 以类方法调用时触发
+ (BOOL)resolveClassMethod:(SEL)sel
// 以实例方法调用时触发
+ (BOOL)resolveInstanceMethod:(SEL)sel

void run(id self, SEL _cmd) {
    NSLog(@"missRun -- run");
}

+ (BOOL)resolveInstanceMethod:(SEL)sel {
    if (sel == NSSelectorFromString(@"missRun")) {
        NSLog(@"sel == %@",NSStringFromSelector(sel));
        class_addMethod([self class], sel, (IMP)run, "v@:");
        return YES;
    }
    return [super resolveInstanceMethod:sel];
}

// 注意这里 class_addMethod 的第一个参数是 [self superclass]
+ (BOOL)resolveClassMethod:(SEL)sel {
    if (sel == NSSelectorFromString(@"missRun")) {
        NSLog(@"sel == %@",NSStringFromSelector(sel));
        class_addMethod([self superclass], sel, (IMP)run, "v@:");
        return YES;
    }
    return [super resolveInstanceMethod:sel];
}

//Person *person = [[Person alloc] init];
//[person performSelector:NSSelectorFromString(@"missRun") withObject:nil];

[Person performSelector:NSSelectorFromString(@"missRun") withObject:nil];

- (id)forwardingTargetForSelector:(SEL)aSelector {
    Sutdent *student = [[Sutdent alloc] init];
    if ([student respondsToSelector:aSelector]) {
        return student;
    }
    return [super forwardingTargetForSelector:aSelector];
}

- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector {
    if (aSelector == NSSelectorFromString(@"missRun")) {
        return [NSMethodSignature signatureWithObjCTypes:"v@:"];
    }
    return [super methodSignatureForSelector:aSelector];
}

- (void)forwardInvocation:(NSInvocation *)anInvocation {
    if ([anInvocation selector] == NSSelectorFromString(@"missRun")) {
        Sutdent *student = [[Sutdent alloc] init];
        [anInvocation invokeWithTarget:student];
    }
}

+ (void)load {
    Method originalMethod = class_getInstanceMethod([NSString class], @selector(lowercaseString));
    Method swappedMethod = class_getInstanceMethod([NSString class], @selector(uppercaseString));
    method_exchangeImplementations(originalMethod, swappedMethod);
}

NSString *string = @"This is a String";
NSLog(@"lowercaseString = %@",string.lowercaseString);
NSLog(@"uppercaseString = %@",string.uppercaseString);

// 输出
// lowercaseString = THIS IS A STRING
// uppercaseString = this is a string

+ (void)load {
    Method originalMethod = class_getInstanceMethod([NSString class], @selector(lowercaseString));
    Method swappedMethod = class_getInstanceMethod([NSString class], @selector(vch_lowercaseString));
    method_exchangeImplementations(originalMethod, swappedMethod);
}

- (NSString *)vch_lowercaseString {
    NSString *string = [self vch_lowercaseString];
    NSLog(@"----%@",string);
    return string;
}

NSString *string = @"This is a String";
NSLog(@"lowercaseString = %@",string.lowercaseString);

// 输出
// ----this is a string
// lowercaseString = this is a string

NSString *string0 = @"this is a string";
id string1 = @"this is a string";

// objc.h

/// An opaque type that represents an Objective-C class.
typedef struct objc_class *Class;

/// Represents an instance of a class.
struct objc_object {
    Class _Nonnull isa  OBJC_ISA_AVAILABILITY;
};

/// A pointer to an instance of a class.
typedef struct objc_object *id;

// NSObject.h

@interface NSObject <NSObject> {
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wobjc-interface-ivars"
    Class isa  OBJC_ISA_AVAILABILITY;
#pragma clang diagnostic pop
}

// runtime.h

struct objc_class {
    Class _Nonnull isa  OBJC_ISA_AVAILABILITY;

#if !__OBJC2__
    Class _Nullable super_class                              OBJC2_UNAVAILABLE;
    const char * _Nonnull name                               OBJC2_UNAVAILABLE;
    long version                                             OBJC2_UNAVAILABLE;
    long info                                                OBJC2_UNAVAILABLE;
    long instance_size                                       OBJC2_UNAVAILABLE;
    struct objc_ivar_list * _Nullable ivars                  OBJC2_UNAVAILABLE;
    struct objc_method_list * _Nullable * _Nullable methodLists                    OBJC2_UNAVAILABLE;
    struct objc_cache * _Nonnull cache                       OBJC2_UNAVAILABLE;
    struct objc_protocol_list * _Nullable protocols          OBJC2_UNAVAILABLE;
#endif

} OBJC2_UNAVAILABLE;