int UIApplicationMain(int argc, char *argv[], NSString *principalClassName, NSString *delegateClassName);

argc、argv：直接传递给UIApplicationMain进行相关处理即可

int main(int argc, char * argv[]){ @autoreleasepool { 

/**
* argc: 系统或者用户传入的参数个数
* argv: 系统或者用户传入的实际参数 
* 1.根据传入的第三个参数创建UIApplication对象 
* 2.根据传入的第四个产生创建UIApplication对象的代理
* 3.设置刚刚创建出来的代理对象为UIApplication的代理 
* 4.开启一个事件循环 
**/ 
return UIApplicationMain(argc, argv, @"UIApplication", @"YYAppDelegate"); }}

NSString *string = @"LiMing";
    
NSString *copyString = [string copy];
    
NSString *mutableString = [string mutableCopy];
    
NSLog(@"string = %p",string);
    
NSLog(@"copyString = %p",copyString);
    
NSLog(@"mutableString = %p ",mutableString);

结论：
1.string 和 copyString 他们只是二个不同的指针，指向内存中的同一块地址，copy 只是指针复制
2.string 和 mutableString 打印出来的地址不同，是因为两个指针指向的地址本就不同，mutableCopy 是内容复制

注意：其他对象 NSArray 、NSMutableArray 、NSDictionary 、NSMutableDictionary 一样适用

GZQPerson *person = [[GZQPerson alloc] init];
person.age = 20;
person.name = @"GZQ";
GZQPerson *copyP = [person copy];  // 这里崩溃

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

#import "GZQPerson.h"

@interface GZQPerson ()<NSCopying,NSMutableCopying>

@end

@implementation GZQPerson

- (id)copyWithZone:(NSZone *)zone {

    GZQPerson *person = [[[self class] allocWithZone:zone] init];
    person.age = self.age;
    person.name = self.name;
    return person;
}

- (id)mutableCopyWithZone:(NSZone *)zone {

    GZQPerson *person = [[[self class] allocWithZone:zone] init];
    person.age = self.age;
    person.name = self.name;
    return person;
}

@end

#import "ViewController.h"
#import "GZQPerson.h"

@interface ViewController ()

@end

@implementation ViewController

- (void)viewDidLoad {
    
    [super viewDidLoad];

    GZQPerson *person = [[GZQPerson alloc] init];
    person.age = 20;
    person.name = @"GZQ";
    GZQPerson *copyP = [person copy];
    
    NSLog(@"copyP=%p",copyP);
    NSLog(@"person=%p",person);
    NSLog(@"person=%p",copyP.name);
    NSLog(@"person=%p",person.name);
    
}
@end

#import <Foundation/Foundation.h>

property copy 实际上就对name干了这个：

- (void)setName:(NSString *)name
{
    _name = [name copy];
}

@property (nonatomic, strong) NSString *name;

NSMutableString *string = [NSMutableString stringWithFormat:@"深拷贝-浅拷贝"];

GZQPerson *person = [[GZQPerson alloc] init];
person.name = string;

// 可以改变person.name的值，因为其内部没有生成新的对象
[string appendString:@"LALALA"];

NSLog(@"name = %@", person.name);

 NSArray *copyArray = [[NSArray alloc] initWithArray:array copyItems:YES];  // 完全复制

@property = ivar + getter + setter;

@property = getter + setter;

@interface Person : NSObject
@property NSString *firstName;
@property NSString *lastName;
@end

@interface Person : NSObject
- (NSString *)firstName;
- (void)setFirstName:(NSString *)firstName;
- (NSString *)lastName;
- (void)setLastName:(NSString *)lastName;
@end

typedef struct objc_property *objc_property_t;

struct property_t {
    const char *name;
    const char *attributes;
};

/// Defines a property attribute
typedef struct {
    const char *name;           /**< The name of the attribute */
    const char *value;          /**< The value of the attribute (usually empty) */
} objc_property_attribute_t;

@property (nonatomic, copy) NSString *string;

T@"NSString",C,N,V_string

@implementation Person
@synthesize firstName = _myFirstName;
@synthesize lastName = _myLastName;
@end

1. OBJC_IVAR_$类名$属性名称 ：该属性的“偏移量” (offset)，这个偏移量是“硬编码” (hardcode)，表示该变量距离存放对象的内存区域的起始地址有多远。
2. setter 与 getter 方法对应的实现函数
3. ivar_list ：成员变量列表
4. method_list ：方法列表
5. prop_list ：属性列表
也就是说我们每次在增加一个属性,系统都会在 ivar_list 中添加一个成员变量的描述,在 method_list 中增加 setter 与 getter 方法的描述,在属性列表中增加一个属性的描述,然后计算该属性在对象中的偏移量,然后给出 setter 与 getter 方法对应的实现,在 setter 方法中从偏移量的位置开始赋值,在 getter 方法中从偏移量开始取值,为了能够读取正确字节数,系统对象偏移量的指针类型进行了类型强转.

@property (nonatomic, getter=isOn) BOOL on;
     
(`setter=`这种不常用，也不推荐使用。故不在这里给出写法。）

@property(nonatomic, strong, getter=p_initBy, setter=setP_initBy:)NSString *initBy;

@property(nonatomic, readwrite, copy, null_resettable) NSString *initBy;

- (NSString *)initBy __attribute__((objc_method_family(none)));

1. 不常用的：nonnull,null_resettable,nullable

注意：很多人会认为如果属性具备 nonatomic 特质，则不使用 “同步锁”。其实在属性设置方法中使用的是自旋锁，自旋锁相关代码如下：

static inline void reallySetProperty(id self, SEL _cmd, id newValue, ptrdiff_t offset, bool atomic, bool copy, bool mutableCopy)
{
    if (offset == 0) 
    {
        object_setClass(self, newValue);
        return;
    }

    id oldValue;
    id *slot = (id*) ((char*)self + offset);

    if (copy) 
    {
        newValue = [newValue copyWithZone:nil];
    } 
    else if (mutableCopy) 
    {
        newValue = [newValue mutableCopyWithZone:nil];
    } 
    else 
    {
        if (*slot == newValue) return;
        newValue = objc_retain(newValue);
    }

    if (!atomic) 
    {
        oldValue = *slot;
        *slot = newValue;
    } 
    else 
    {
        spinlock_t& slotlock = PropertyLocks[slot];
        slotlock.lock();
        oldValue = *slot;
        *slot = newValue;        
        slotlock.unlock();
    }

    objc_release(oldValue);
}

void objc_setProperty(id self, SEL _cmd, ptrdiff_t offset, id newValue, BOOL atomic, signed char shouldCopy) 
{
    bool copy = (shouldCopy && shouldCopy != MUTABLE_COPY);
    bool mutableCopy = (shouldCopy == MUTABLE_COPY);
    reallySetProperty(self, _cmd, newValue, offset, atomic, copy, mutableCopy);
}

- (nullable id)valueForKey:(NSString *)key;                          //直接通过Key来取值
- (void)setValue:(nullable id)value forKey:(NSString *)key;          //通过Key来设值
- (nullable id)valueForKeyPath:(NSString *)keyPath;                  //通过KeyPath来取值
- (void)setValue:(nullable id)value forKeyPath:(NSString *)keyPath;  //通过KeyPath来设值

+ (BOOL)accessInstanceVariablesDirectly;
//默认返回YES，表示如果没有找到Set<Key>方法的话，会按照_key，_iskey，key，iskey的顺序搜索成员，设置成NO就不这样搜索
- (BOOL)validateValue:(inout id __nullable * __nonnull)ioValue forKey:(NSString *)inKey error:(out NSError **)outError;
//KVC提供属性值确认的API，它可以用来检查set的值是否正确、为不正确的值做一个替换值或者拒绝设置新值并返回错误原因。
- (NSMutableArray *)mutableArrayValueForKey:(NSString *)key;
//这是集合操作的API，里面还有一系列这样的API，如果属性是一个NSMutableArray，那么可以用这个方法来返回
- (nullable id)valueForUndefinedKey:(NSString *)key;
//如果Key不存在，且没有KVC无法搜索到任何和Key有关的字段或者属性，则会调用这个方法，默认是抛出异常
- (void)setValue:(nullable id)value forUndefinedKey:(NSString *)key;
//和上一个方法一样，只不过是设值。
- (void)setNilValueForKey:(NSString *)key;
//如果你在SetValue方法时面给Value传nil，则会调用这个方法
- (NSDictionary<NSString *, id> *)dictionaryWithValuesForKeys:(NSArray<NSString *> *)keys;
//输入一组key,返回该组key对应的Value，再转成字典返回，用于将Model转到字典。

[_tableView addObserver:self forKeyPath:@"contentOffset" options:NSKeyValueObservingOptionNew context:nil];

// 在dealloc中移除KVO监听：
[_tableView removeObserver:self forKeyPath:@"contentOffset" context:nil];

// 添加默认的响应回调方法：
- (void)observeValueForKeyPath:(NSString *)keyPath 
                      ofObject:(id)object
                        change:(NSDictionary *)change 
                       context:(void *)context
{
    [self doSomethingWhenContentOffsetChanges];
}

- (void)observeValueForKeyPath:(NSString *)keyPath 
                      ofObject:(id)object
                        change:(NSDictionary *)change 
                       context:(void *)context
{
    if (object == _tableView && [keyPath isEqualToString:@"contentOffset"]) 
    {
        [self doSomethingWhenContentOffsetChanges];
    }
}

- (void)observeValueForKeyPath:(NSString *)keyPath 
                      ofObject:(id)object
                        change:(NSDictionary *)change 
                       context:(void *)context
{
    if (object == _tableView && [keyPath isEqualToString:@"contentOffset"]) 
    {
        [self doSomethingWhenContentOffsetChanges];
    } 
    else 
    {
        [super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
    }
}

"应用程序包"
Documents
Library
    Caches
    Preferences
tmp

  NSString *path = [[NSBundle mainBundle] bundlePath];
  NSLog(@"%@", path);

  NSString *path = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES).firstObject;
  NSLog(@"%@", path);

  NSString *path = NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES).firstObject;
  NSLog(@"%@", path);

  NSString *path = NSTemporaryDirectory();
  NSLog(@"%@", path);

NSArray;
NSMutableArray;
NSDictionary;
NSMutableDictionary;
NSData;
NSMutableData;
NSString;
NSMutableString;
NSNumber;
NSDate;

NSString *path = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES).firstObject;
NSString *fileName = [path stringByAppendingPathComponent:@"123.plist"];

NSArray *array = @[@"123", @"456", @"789"];
[array writeToFile:fileName atomically:YES];

NSArray *result = [NSArray arrayWithContentsOfFile:fileName];
NSLog(@"%@", result);

// 只有以上列出的类型才能使用plist文件存储。
// 存储时使用writeToFile: atomically:方法。 其中atomically表示是否需要先写入一个辅助文件，再把辅助文件拷贝到目标文件地址。这是更安全的写入文件方法，一般都写YES。
// 读取时使用arrayWithContentsOfFile:方法。

//1.获得NSUserDefaults文件
NSUserDefaults *userDefaults = [NSUserDefaults standardUserDefaults];

//2.向文件中写入内容
[userDefaults setObject:@"AAA" forKey:@"a"];
[userDefaults setBool:YES forKey:@"sex"];
[userDefaults setInteger:21 forKey:@"age"];

//2.1立即同步
[userDefaults synchronize];

//3.读取文件
NSString *name = [userDefaults objectForKey:@"a"];
BOOL sex = [userDefaults boolForKey:@"sex"];
NSInteger age = [userDefaults integerForKey:@"age"];
NSLog(@"%@, %d, %ld", name, sex, age);

// 偏好设置是专门用来保存应用程序的配置信息的，一般不要在偏好设置中保存其他数据。
// 如果没有调用synchronize方法，系统会根据I/O情况不定时刻地保存到文件中。所以如果需要立即写入文件的就必须调用synchronize方法。
// 偏好设置会将所有数据保存到同一个文件中。即preference目录下的一个以此应用包名来命名的plist文件。

  //1.遵循NSCoding协议 
  @interface Person : NSObject   //2.设置属性
  @property (strong, nonatomic) UIImage *avatar;
  @property (copy, nonatomic) NSString *name;
  @property (assign, nonatomic) NSInteger age;
  @end

  //解档
  - (id)initWithCoder:(NSCoder *)aDecoder {
      if ([super init]) {
          self.avatar = [aDecoder decodeObjectForKey:@"avatar"];
          self.name = [aDecoder decodeObjectForKey:@"name"];
          self.age = [aDecoder decodeIntegerForKey:@"age"];
      }
      return self;
  }
  
  //归档
  - (void)encodeWithCoder:(NSCoder *)aCoder {
      [aCoder encodeObject:self.avatar forKey:@"avatar"];
      [aCoder encodeObject:self.name forKey:@"name"];
      [aCoder encodeInteger:self.age forKey:@"age"];
  }

如果需要归档的类是某个自定义类的子类时，就需要在归档和解档之前先实现父类的归档和解档方法。即 [super encodeWithCoder:aCoder] 和 [super initWithCoder:aDecoder] 方法;

  NSString *file = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES).firstObject stringByAppendingPathComponent:@"person.data"];
  Person *person = [[Person alloc] init];
  person.avatar = self.avatarView.image;
  person.name = self.nameField.text;
  person.age = [self.ageField.text integerValue];
  [NSKeyedArchiver archiveRootObject:person toFile:file];

  NSString *file = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES).firstObject stringByAppendingPathComponent:@"person.data"];
  Person *person = [NSKeyedUnarchiver unarchiveObjectWithFile:file];
  if (person) 
  {
     self.avatarView.image = person.avatar;
     self.nameField.text = person.name;
     self.ageField.text = [NSString stringWithFormat:@"%ld", person.age];
  }

必须遵循并实现NSCoding协议
保存文件的扩展名可以任意指定
继承时必须先调用父类的归档解档方法

integer : 整数
real : 实数（浮点数）
text : 文本字符串
blob : 二进制数据，比如文件，图片之类的

//  打开数据库并创建一个表
- (void)openDatabase 
{
   //1.设置文件名
   NSString *filename = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES).firstObject stringByAppendingPathComponent:@"person.db"];
   
   //2.打开数据库文件，如果没有会自动创建一个文件
   NSInteger result = sqlite3_open(filename.UTF8String, &_sqlite3);
   
   if (result == SQLITE_OK) 
   {
       NSLog(@"打开数据库成功！");
       
       //3.创建一个数据库表
       char *errmsg = NULL;
       sqlite3_exec(_sqlite3, "CREATE TABLE IF NOT EXISTS t_person(id integer primary key autoincrement, name text, age integer)", NULL, NULL, &errmsg);
       
       if (errmsg) 
       {
           NSLog(@"错误：%s", errmsg);
       } 
       else 
       {
           NSLog(@"创表成功！");
       }
   } 
   else 
   {
       NSLog(@"打开数据库失败！");
   }
}

// 往表中插入1000条数据
- (void)insertData 
{
    NSString *nameStr;
    NSInteger age;
    
    for (NSInteger i = 0; i < 1000; i++) 
    {
      nameStr = [NSString stringWithFormat:@"Bourne-%d", arc4random_uniform(10000)];
      age = arc4random_uniform(80) + 20;
      NSString *sql = [NSString stringWithFormat:@"INSERT INTO t_person (name, age) VALUES('%@', '%ld')", nameStr, age];
      char *errmsg = NULL;
      sqlite3_exec(_sqlite3, sql.UTF8String, NULL, NULL, &errmsg);
      if (errmsg) 
      {
          NSLog(@"错误：%s", errmsg);
      }
    }
    NSLog(@"插入完毕！");   
}

// sqlite3_prepare_v2() : 检查sql的合法性
// sqlite3_step() : 逐行获取查询结果，不断重复，直到最后一条记录
// sqlite3_coloum_xxx() : 获取对应类型的内容，iCol对应的就是SQL语句中字段的顺序，从0开始。根据实际查询字段的属性，使用sqlite3_column_xxx取得对应的内容即可。
// sqlite3_finalize() : 释放stmt

// 从表中读取数据到数组中
- (void)readData 
{
   NSMutableArray *mArray = [NSMutableArray arrayWithCapacity:1000];
   char *sql = "select name, age from t_person;";
   sqlite3_stmt *stmt;
   NSInteger result = sqlite3_prepare_v2(_sqlite3, sql, -1, &stmt, NULL);
   
   if (result == SQLITE_OK) 
   {
       while (sqlite3_step(stmt) == SQLITE_ROW) 
       {
           char *name = (char *)sqlite3_column_text(stmt, 0);
           NSInteger age = sqlite3_column_int(stmt, 1);
           //创建对象
           Person *person = [Person personWithName:[NSString stringWithUTF8String:name] Age:age];
           [mArray addObject:person];
       }
       self.dataList = mArray;
   }
   sqlite3_finalize(stmt);
}

// FMDatabase
一个FMDatabase对象就代表一个单独的SQLite数据库，用来执行SQL语句

// FMResultSet
使用FMDatabase执行查询后的结果集

// FMDatabaseQueue
用于在多线程中执行多个查询或更新，它是线程安全的

NSString *path = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES).firstObject stringByAppendingPathComponent:@"person.db"];
FMDatabase *database = [FMDatabase databaseWithPath:path]; 
   
if (![database open]) 
{
    NSLog(@"数据库打开失败！");
}

// 具体文件路径，如果不存在会自动创建
// 空字符串@""，会在临时目录创建一个空的数据库，当FMDatabase连接关闭时，数据库文件也被删除
// nil，会创建一个内存中临时数据库，当FMDatabase连接关闭时，数据库会被销毁

//常用方法有以下 3 种：   
- (BOOL)executeUpdate:(NSString*)sql, ...

- (BOOL)executeUpdateWithFormat:(NSString*)format, ...

- (BOOL)executeUpdate:(NSString*)sql withArgumentsInArray:(NSArray *)arguments

//示例
[database executeUpdate:@"CREATE TABLE IF NOT EXISTS t_person(id integer primary key autoincrement, name text, age integer)"]; 
  
//或者  
[database executeUpdate:@"INSERT INTO t_person(name, age) VALUES(?, ?)", @"Bourne", [NSNumber numberWithInt:42]];

- (FMResultSet *)executeQuery:(NSString*)sql, ...
- (FMResultSet *)executeQueryWithFormat:(NSString*)format, ...
- (FMResultSet *)executeQuery:(NSString *)sql withArgumentsInArray:(NSArray *)arguments

//1.执行查询
FMResultSet *result = [database executeQuery:@"SELECT * FROM t_person"];

//2.遍历结果集
while ([result next]) 
{
    NSString *name = [result stringForColumn:@"name"];
    int age = [result intForColumn:@"age"];
}

FMDatabaseQueue *queue = [FMDatabaseQueue databaseQueueWithPath:aPath];

// 使用队列
[queue inDatabase:^(FMDatabase *database)
{    
          [database executeUpdate:@"INSERT INTO t_person(name, age) VALUES (?, ?)", @"Bourne_1", [NSNumber numberWithInt:1]];    
          [database executeUpdate:@"INSERT INTO t_person(name, age) VALUES (?, ?)", @"Bourne_2", [NSNumber numberWithInt:2]];    
          [database executeUpdate:@"INSERT INTO t_person(name, age) VALUES (?, ?)", @"Bourne_3", [NSNumber numberWithInt:3]];      
          FMResultSet *result = [database executeQuery:@"select * from t_person"];    
         while([result next]) {   
         }    
}];

// 而且可以轻松地把简单任务包装到事务里：
[queue inTransaction:^(FMDatabase *database, BOOL *rollback) {    
          [database executeUpdate:@"INSERT INTO t_person(name, age) VALUES (?, ?)", @"Bourne_1", [NSNumber numberWithInt:1]];    
          [database executeUpdate:@"INSERT INTO t_person(name, age) VALUES (?, ?)", @"Bourne_2", [NSNumber numberWithInt:2]];    
          [database executeUpdate:@"INSERT INTO t_person(name, age) VALUES (?, ?)", @"Bourne_3", [NSNumber numberWithInt:3]];      
          FMResultSet *result = [database executeQuery:@"select * from t_person"];    
             while([result next]) {   
             }   
           //回滚
           *rollback = YES;  
    }];