底層數(shù)據(jù)結(jié)構(gòu)

創(chuàng)建一個命令行項目幅虑，在main函數(shù)中定義一個Block并進行調(diào)用

#import <Foundation/Foundation.h>
int main(int argc, const char * argv[]) {
    @autoreleasepool {
        void (^block)(void) = ^{
            NSLog(@"Hello World !");
        };
        block();
    }
    return 0;
}

在控制臺中輸入：xcrun -sdk iphoneos clang -arch arm64 -rewrite-objc main.m 將OC代碼轉(zhuǎn)為C++代碼

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;
    //構(gòu)造函數(shù)（類似OC的init方法）。返回結(jié)構(gòu)體對象
  __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;
  }
};
//封裝了block執(zhí)行邏輯的函數(shù)
static void __main_block_func_0(struct __main_block_impl_0 *__cself) {

            NSLog((NSString *)&__NSConstantStringImpl__var_folders_ym_h2psj2ss52s_y619pxn42rfh0000gn_T_main_0a05f8_mi_0);
        }

//block的描述
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)};
int main(int argc, const char * argv[]) {
    /* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool;
        //定義block變量
//        void (*block)(void) = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA));
        //將一些強制轉(zhuǎn)換代碼刪除
        void (*block)(void) = &__main_block_impl_0(
                                                    __main_block_func_0,
                                                    &__main_block_desc_0_DATA
                                                   );
        //執(zhí)行block內(nèi)部的代碼
//        ((void (*)(__block_impl *))((__block_impl *)block)->FuncPtr)((__block_impl *)block);
        //將一些強制轉(zhuǎn)換代碼刪除
        block->FuncPtr(block);
    }
    return 0;
}

變量捕獲

1.基本數(shù)據(jù)類型auto變量
auto:自動變量顾犹，離開作用域就會銷毀

#import <Foundation/Foundation.h>
int main(int argc, const char * argv[]) {
    @autoreleasepool {
        int  age = 10;
        void (^block)(void) = ^{
            NSLog(@"Hello World ! %d",age);
        };
        age = 20;
        block();
    }
    return 0;
}

代碼運行打印的10翘单，下面我們就來分析一下底層代碼：

struct __main_block_impl_0 {
  struct __block_impl impl;
  struct __main_block_desc_0* Desc;
  int age;
//age(_age)    C++ 語法，將傳進來的_age賦值給結(jié)構(gòu)體的成員變量age
  __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, int _age, int flags=0) : age(_age) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};
static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
  int age = __cself->age; // bound by copy
            NSLog((NSString *)&__NSConstantStringImpl__var_folders_ym_h2psj2ss52s_y619pxn42rfh0000gn_T_main_5736dc_mi_0,age);
        }
int main(int argc, const char * argv[]) {
    /* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool; 
        int age = 10;
//定義block時將捕獲的age的值傳給main_block_impl_0這個函數(shù)蹦渣。
        void (*block)(void) = &__main_block_impl_0(__main_block_func_0, &__main_block_desc_0_DATA, age);
        age = 20;
        block)->FuncPtr(block);
    }
    return 0;
}

2.對象類型的auto變量

#import <Foundation/Foundation.h>
#import "Cat.h"

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        //沒有捕獲auto變量的block是Global
        Cat *cat = [[Cat alloc]init];
        cat.name = @"aaa";
        void (^block)(void) = ^{
            NSLog(@"Hello World ! %@",cat.name);
        };
        block();
        NSLog(@"%@",[block class]);
    }
    return 0;
}

在Cat類中重寫dealloc方法哄芜，打印Cat的釋放時間，運行上面代碼發(fā)現(xiàn)柬唯，Cat類不會被釋放认臊。
因為block會捕獲Cat類，強引用Cat類且block存放在堆上锄奢。

struct __main_block_impl_0 {
  struct __block_impl impl;
  struct __main_block_desc_0* Desc;
  Cat *cat;
  __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, Cat *_cat, int flags=0) : cat(_cat) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};
static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
  Cat *cat = __cself->cat; // bound by copy

            NSLog((NSString *)&__NSConstantStringImpl__var_folders_ym_h2psj2ss52s_y619pxn42rfh0000gn_T_main_b96010_mi_1,((NSString *(*)(id, SEL))(void *)objc_msgSend)((id)cat, sel_registerName("name")));
        }
//在ARC模式下失晴，系統(tǒng)會自動調(diào)用copy方法，_Block_object_assign會根據(jù)捕獲的auto變量的修飾符（__strong 拘央、__weak）做出強引用或弱引用涂屁。 相當(dāng)于retain
static void __main_block_copy_0(struct __main_block_impl_0*dst, struct __main_block_impl_0*src) {_Block_object_assign((void*)&dst->cat, (void*)src->cat, 3/*BLOCK_FIELD_IS_OBJECT*/);}

//_Block_object_dispose會釋放捕獲的auto變量，相當(dāng)于release
static void __main_block_dispose_0(struct __main_block_impl_0*src) {_Block_object_dispose((void*)src->cat, 3/*BLOCK_FIELD_IS_OBJECT*/);}

//當(dāng)block捕獲對象類型的auto變量灰伟，block底層的描述函數(shù)會多2個函數(shù)方法：copy  dispose 
static struct __main_block_desc_0 {
  size_t reserved;
  size_t Block_size;
  void (*copy)(struct __main_block_impl_0*, struct __main_block_impl_0*);
  void (*dispose)(struct __main_block_impl_0*);
} __main_block_desc_0_DATA = { 0, sizeof(struct __main_block_impl_0), __main_block_copy_0, __main_block_dispose_0};
int main(int argc, const char * argv[]) {
    /* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool; 

        Cat *cat = ((Cat *(*)(id, SEL))(void *)objc_msgSend)((id)((Cat *(*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("Cat"), sel_registerName("alloc")), sel_registerName("init"));
        ((void (*)(id, SEL, NSString * _Nonnull))(void *)objc_msgSend)((id)cat, sel_registerName("setName:"), (NSString *)&__NSConstantStringImpl__var_folders_ym_h2psj2ss52s_y619pxn42rfh0000gn_T_main_b96010_mi_0);
        void (*block)(void) = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, cat, 570425344));
        ((void (*)(__block_impl *))((__block_impl *)block)->FuncPtr)((__block_impl *)block);
        NSLog((NSString *)&__NSConstantStringImpl__var_folders_ym_h2psj2ss52s_y619pxn42rfh0000gn_T_main_b96010_mi_2,((Class (*)(id, SEL))(void *)objc_msgSend)((id)block, sel_registerName("class")));
    }
    return 0;
}

3.static變量

#import <Foundation/Foundation.h>

int main(int argc, const char * argv[]) {
    @autoreleasepool {
       static int  wide = 10;
        void (^block)(void) = ^{
            NSLog(@"Hello World ! %d",wide);
        };
        wide = 20;
        block();
    }
    return 0;
}

打印的值是20

struct __main_block_impl_0 {
  struct __block_impl impl;
  struct __main_block_desc_0* Desc;
  int *wide;
//wide(_wide)    C++ 語法拆又，將傳進來的__wide賦值給結(jié)構(gòu)體的成員變量*wide
  __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, int *_wide, int flags=0) : wide(_wide) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};
static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
  int *wide = __cself->wide; // bound by copy

            NSLog((NSString *)&__NSConstantStringImpl__var_folders_ym_h2psj2ss52s_y619pxn42rfh0000gn_T_main_d13ed3_mi_0,(*wide));
        }

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)};
int main(int argc, const char * argv[]) {
    /* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool; 
       static int wide = 10;
        //定義block時將捕獲的wide的地址傳給main_block_impl_0這個函數(shù)儒旬。
        void (*block)(void) = &__main_block_impl_0(__main_block_func_0, &__main_block_desc_0_DATA, &wide);
        wide = 20;
        block)->FuncPtr(block);
    }
    return 0;
}

4.全局變量
block不會捕獲全局變量，在block可以直接使用帖族。
self是局部變量栈源，block會捕獲self，每個OC方法都有2個隱藏參數(shù)竖般，self作為第一個參數(shù)

block類型

1.NSGlobalBlock

#import <Foundation/Foundation.h>
int main(int argc, const char * argv[]) {
    @autoreleasepool {
        //沒有捕獲auto變量的block是Global
        static int  age = 10;
        void (^block)(void) = ^{
            NSLog(@"Hello World ! %d",age);
        }; 
        NSLog(@"%@",[block class]);
    }
    return 0;
}
打印為：2021-09-17 16:50:01.955120+0800 Block的本質(zhì)[60317:3405819] __NSGlobalBlock__

2.NSStackBlock
關(guān)閉Xcode的ARC甚垦，將Objective-C Automatic Reference Counting 的值設(shè)為NO，NSStackBlock存儲在棧上涣雕，當(dāng)block出了作用域艰亮，NSStackBlock就會被回收，再次調(diào)用時會出現(xiàn)數(shù)據(jù)錯亂挣郭。

截屏2021-09-17 下午4.57.04.png

#import <Foundation/Foundation.h>
int main(int argc, const char * argv[]) {
    @autoreleasepool {
        //捕獲auto變量的block是Stack
        int  age = 10;
        void (^block)(void) = ^{
            NSLog(@"Hello World ! %d",age);
        }; 
        NSLog(@"%@",[block class]);
    }
    return 0;
}
打印為：2021-09-17 16:55:35.195563+0800 Block的本質(zhì)[60355:3408920] __NSStackBlock__

3.NSMallocBlock
當(dāng)NSStackBlock調(diào)用copy方法后迄埃，就會變?yōu)镹SMallocBlock，存儲在堆上丈屹。
當(dāng)NSGlobalBlock調(diào)用copy方法不會改變block類型调俘，還是存儲在數(shù)據(jù)區(qū)。
當(dāng)NSMallocBlock調(diào)用copy方法旺垒，block對象的引用計數(shù)器會加一彩库。

在ARC環(huán)境下，編譯器會根據(jù)情況自動將棧上的block復(fù)制到堆上：
block作為函數(shù)返回值時
將block賦值給__strong指針時
block作為Cocoa API中方法名含有usingBlock的方法參數(shù)時
block作為GCD API中方法參數(shù)時

block修改變量的值

1.將auto局部變量加上static 修飾符 缺點：變量會一直存儲在內(nèi)存中先蒋，不會在超出作用域時釋放骇钦。
2.__block修飾 編譯器會將變量包裝成一個對象

//系統(tǒng)會將age 包裝給一個對象
struct __Block_byref_age_0 {
  void *__isa;
__Block_byref_age_0 *__forwarding;
 int __flags;
 int __size;
 int age;
};

struct __main_block_impl_0 {
  struct __block_impl impl;
  struct __main_block_desc_0* Desc;
  __Block_byref_age_0 *age; // by ref
  __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, __Block_byref_age_0 *_age, int flags=0) : age(_age->__forwarding) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};
static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
  __Block_byref_age_0 *age = __cself->age; // bound by ref

            NSLog((NSString *)&__NSConstantStringImpl__var_folders_ym_h2psj2ss52s_y619pxn42rfh0000gn_T_main_255acd_mi_0,(age->__forwarding->age));
        }
static void __main_block_copy_0(struct __main_block_impl_0*dst, struct __main_block_impl_0*src) {_Block_object_assign((void*)&dst->age, (void*)src->age, 8/*BLOCK_FIELD_IS_BYREF*/);}

static void __main_block_dispose_0(struct __main_block_impl_0*src) {_Block_object_dispose((void*)src->age, 8/*BLOCK_FIELD_IS_BYREF*/);}

static struct __main_block_desc_0 {
  size_t reserved;
  size_t Block_size;
  void (*copy)(struct __main_block_impl_0*, struct __main_block_impl_0*);
  void (*dispose)(struct __main_block_impl_0*);
} __main_block_desc_0_DATA = { 0, sizeof(struct __main_block_impl_0), __main_block_copy_0, __main_block_dispose_0};
int main(int argc, const char * argv[]) {
    /* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool; 



        __attribute__((__blocks__(byref))) __Block_byref_age_0 age = {(void*)0,(__Block_byref_age_0 *)&age, 0, sizeof(__Block_byref_age_0), 10};
        void (*block)(void) = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, (__Block_byref_age_0 *)&age, 570425344));
        ((void (*)(__block_impl *))((__block_impl *)block)->FuncPtr)((__block_impl *)block);
        NSLog((NSString *)&__NSConstantStringImpl__var_folders_ym_h2psj2ss52s_y619pxn42rfh0000gn_T_main_255acd_mi_1,((Class (*)(id, SEL))(void *)objc_msgSend)((id)block, sel_registerName("class")));
    }
    return 0;
}