唯一标示符&Block原理

唯一标示符符

NSUUID

系统并没有存储

NSString *uuid = [[NSUUID UUID] UUIDString];
NSLog(@"%@", uuid);
//-> C7933B11-B8EE-49A8-A628-C285AEBDBC24

广告标示符（IDFA-identifierForIdentifier）

广告标示符是由系统存储着的

NSString *adUDID = [[[ASIdentifierManager sharedManager] advertisingIdentifier] UUIDString];
NSLog(@"%@",adUDID);
//-> D3B19AFD-A6B6-4551-8CF0-1070A3D1F756

Vindor标示符 (IDFV-identifierForVendor)vendor

非常简单：一个Vendor是CFBundleIdentifier（反转DNS格式）的前两部分。例如，com.doubleencore.app1 和 com.doubleencore.app2 得到的identifierForVendor是相同的，如果用户卸载了同一个vendor对应的所有程序，然后在重新安装同一个vendor提供的程序，此时identifierForVendor会被重置。

NSString *idfv = [[[UIDevice currentDevice] identifierForVendor] UUIDString];
NSLog(@"%@",idfv);
//-> B18BD9D8-D815-4774-813F-7135471BCFC7

iOS唯一标示符引导

Block

void (^ block)(void) = ^ {
    printf("honzon");
};
block();

经过clang -rewrite-objc main.m将main.m转成main.cpp文件

struct __block_impl {
  void *isa;
  int Flags;
  int Reserved;
  void *FuncPtr;
};


struct __main_block_impl_0 {
  struct __block_impl impl;
  struct __main_block_desc_0* Desc;
  __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, int flags=0) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};
static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
    printf("honzon");
 }
 
static struct __main_block_desc_0 {
  size_t reserved;
  size_t Block_size;
} __main_block_desc_0_DATA = { 0, sizeof(struct __main_block_impl_0)};

//声明
void (* block)(void) = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA));
//执行
((void (*)(__block_impl *))((__block_impl *)block)->FuncPtr)((__block_impl *)block);

__main_block_impl 结构体对应一个block的对象，包含一个block的所有信息

__main_block_func 函数实现具体的block需要执行的代码

__main_block_desc 内存管理

__block_impl 函数管理

引用外部变量

int integer_2 = 2;
void (^block_2)(void) = ^ {
    printf("integer %d", integer_2);
};
block_2();

struct __main_block_impl_1 {
  struct __block_impl impl;
  struct __main_block_desc_1* Desc;
  int integer_2;
  __main_block_impl_1(void *fp, struct __main_block_desc_1 *desc, int _integer_2, int flags=0) : integer_2(_integer_2) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};
static void __main_block_func_1(struct __main_block_impl_1 *__cself) {
  int integer_2 = __cself->integer_2; // bound by copy

            printf("integer %d", integer_2);
    }

static struct __main_block_desc_1 {
  size_t reserved;
  size_t Block_size;
} __main_block_desc_1_DATA = { 0, sizeof(struct __main_block_impl_1)};

可以看到，引用外部变量的block，最开始在__main_block_impl_1中添加了一个成员变量integer_2，然后在构造函数中对该变量进行赋值，最后在__main_block_func_1中取出该值使用。需要注意的是，在使用之前，进行了int integer_2 = __cself->integer_2;操作，来创建一个临时变量以进行后续操作。因为是值传递，所以并不会改变外部变量。

使用__Block修饰的外部变量

__block int integer_3 = 3;
void (^block_3)(void) = ^ {
    printf("integer %d", integer_3);
};
block_3();

struct __Block_byref_integer_3_0 {
  void *__isa;
__Block_byref_integer_3_0 *__forwarding;
 int __flags;
 int __size;
 int integer_3;
};


struct __main_block_impl_2 {
  struct __block_impl impl;
  struct __main_block_desc_2* Desc;
  __Block_byref_integer_3_0 *integer_3; // by ref
  __main_block_impl_2(void *fp, struct __main_block_desc_2 *desc, __Block_byref_integer_3_0 *_integer_3, int flags=0) : integer_3(_integer_3->__forwarding) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};
static void __main_block_func_2(struct __main_block_impl_2 *__cself) {
  __Block_byref_integer_3_0 *integer_3 = __cself->integer_3; // bound by ref

            printf("integer %d", (integer_3->__forwarding->integer_3));
        }
static void __main_block_copy_2(struct __main_block_impl_2*dst, struct __main_block_impl_2*src) {_Block_object_assign((void*)&dst->integer_3, (void*)src->integer_3, 8/*BLOCK_FIELD_IS_BYREF*/);}

static void __main_block_dispose_2(struct __main_block_impl_2*src) {_Block_object_dispose((void*)src->integer_3, 8/*BLOCK_FIELD_IS_BYREF*/);}

static struct __main_block_desc_2 {
  size_t reserved;
  size_t Block_size;
  void (*copy)(struct __main_block_impl_2*, struct __main_block_impl_2*);
  void (*dispose)(struct __main_block_impl_2*);
} __main_block_desc_2_DATA = { 0, sizeof(struct __main_block_impl_2), __main_block_copy_2, __main_block_dispose_2};


__attribute__((__blocks__(byref))) __Block_byref_integer_3_0 integer_3 = {(void*)0,(__Block_byref_integer_3_0 *)&integer_3, 0, sizeof(__Block_byref_integer_3_0), 3};

void (*block_3)(void) = ((void (*)())&__main_block_impl_2((void *)__main_block_func_2, &__main_block_desc_2_DATA, (__Block_byref_integer_3_0 *)&integer_3, 570425344));

((void (*)(__block_impl *))((__block_impl *)block_3)->FuncPtr)((__block_impl *)block_3);

大的改动相比不添加__block修饰并不多，主要是增加一个新的结构体__Block_byref_integer_3_0来保存变量的信息(主要是地址)，然后声明block_3的时候，将__block修饰的变量的地址传进去，然后保存在__Block_byref_integer_3_0中，最后在__main_block_func_2中通过地址去操作变量。

如果截获的自动变量是OC的对象,在ARC下，将会强引用这个对象一次从而保证了原对象不被销毁，但与此同时，也会导致循环引用问题.

而根据isa指针，block一共有3种类型的

block_NSConcreteGlobalBlock 全局静态

_NSConcreteStackBlock保存在栈中，出函数作用域就销毁

_NSConcreteMallocBlock 保存在堆中，retainCount == 0销毁

你真的理解__block修饰符的原理么？

Joshua Shen的回答