Block底層原理

底層數(shù)據(jù)結(jié)構(gòu)

創(chuàng)建一個(gè)命令行項(xiàng)目，在main函數(shù)中定義一個(gè)Block并進(jìn)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(tái)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(lèi)似OC的init方法）。返回結(jié)構(gòu)體對(duì)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(qiáng)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(qiáng)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(lèi)型auto變量
auto:自動(dòng)變量，離開(kāi)作用域就會(huì)銷(xiāo)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(yùn)行打印的10，下面我們就來(lái)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(yǔ)法，將傳進(jìn)來(lái)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(shí)將捕獲的age的值傳給main_block_impl_0這個(gè)函數(shù)。
        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(duì)象類(lèi)型的auto變量

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

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        //沒(méi)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(lèi)中重寫(xiě)dealloc方法，打印Cat的釋放時(shí)間，運(yùn)行上面代碼發(fā)現(xiàn)，Cat類(lèi)不會(huì)被釋放。
因?yàn)閎lock會(huì)捕獲Cat類(lèi)，強(qiáng)引用Cat類(lèi)且block存放在堆上。

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，系統(tǒng)會(huì)自動(dòng)調(diào)用copy方法，_Block_object_assign會(huì)根據(jù)捕獲的auto變量的修飾符（__strong 、__weak）做出強(qiáng)引用或弱引用。 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(huì)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(duì)象類(lèi)型的auto變量，block底層的描述函數(shù)會(huì)多2個(gè)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(yǔ)法，將傳進(jìn)來(lái)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(shí)將捕獲的wide的地址傳給main_block_impl_0這個(gè)函數(shù)。
        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(huì)捕獲全局變量，在block可以直接使用。
self是局部變量，block會(huì)捕獲self，每個(gè)OC方法都有2個(gè)隱藏參數(shù)，self作為第一個(gè)參數(shù)

block類(lèi)型

1.NSGlobalBlock

#import <Foundation/Foundation.h>
int main(int argc, const char * argv[]) {
    @autoreleasepool {
        //沒(méi)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，將Objective-C Automatic Reference Counting 的值設(shè)為NO，NSStackBlock存儲(chǔ)在棧上，當(dāng)block出了作用域，NSStackBlock就會(huì)被回收，再次調(diào)用時(shí)會(huì)出現(xiàn)數(shù)據(jù)錯(cuò)亂。

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，就會(huì)變?yōu)镹SMallocBlock，存儲(chǔ)在堆上。
當(dāng)NSGlobalBlock調(diào)用copy方法不會(huì)改變block類(lèi)型，還是存儲(chǔ)在數(shù)據(jù)區(qū)。
當(dāng)NSMallocBlock調(diào)用copy方法，block對(duì)象的引用計(jì)數(shù)器會(huì)加一。

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

block修改變量的值

1.將auto局部變量加上static 修飾符 缺點(diǎn)：變量會(huì)一直存儲(chǔ)在內(nèi)存中，不會(huì)在超出作用域時(shí)釋放。
2.__block修飾 編譯器會(huì)將變量包裝成一個(gè)對(duì)象

//系統(tǒng)會(huì)將age 包裝給一個(gè)對(duì)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;
}
最后編輯于 ：
?著作權(quán)歸作者所有,轉(zhuǎn)載或內(nèi)容合作請(qǐng)聯(lián)系作者
【社區(qū)內(nèi)容提示】社區(qū)部分內(nèi)容疑似由AI輔助生成，瀏覽時(shí)請(qǐng)結(jié)合常識(shí)與多方信息審慎甄別。
平臺(tái)聲明：文章內(nèi)容（如有圖片或視頻亦包括在內(nèi)）由作者上傳并發(fā)布，文章內(nèi)容僅代表作者本人觀點(diǎn)，簡(jiǎn)書(shū)系信息發(fā)布平臺(tái)，僅提供信息存儲(chǔ)服務(wù)。

相關(guān)閱讀更多精彩內(nèi)容

友情鏈接更多精彩內(nèi)容