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Object-C语言Block的实现方式

开场白
Block基本概念
中间态转换方法
Block编译后结果分析
Block运行时状态与编译状态对比
 
开场白
 
Object-C语言是对C语言的扩展，所以将OC源码进行编译的时候，会将OC源码会被转换成C\C++，所以想了解OC源码的实现细节，还是需要手动编译成中间状态进行观察。
命令1:
clang -rewrite-objc main.m

如果Xcode版本较高，可能会出现报错：

./block_VC.h::: fatal error: 'UIKit/UIKit.h' file not found

此时可尝试另一个命令：

clang -x objective-c -rewrite-objc -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator.sdk main.m
命令好长，每次输入这么长的指令很麻烦，此时可以考虑使用命令别名：alias

Block的基本概念

_NSConcreteGlobalBlock 全局的静态 block，不会访问任何外部变量。
_NSConcreteStackBlock 保存在栈中的 block，当函数返回时会被销毁。
_NSConcreteMallocBlock 保存在堆中的 block，当引用计数为 0 时会被销毁。
堆中的 block何时转换：
1.调用copy方法时
2.从一个函数返回时
3.将block传入dispatch等系统IPA参数block或者传递给带有usingBlock的Cocoa框架函数时。
4.将block赋给带有__strong修饰符的id类型或者Block类型时。
Block类型变换方式

1.没有捕获任何局部变量的block为_NSConcreteGlobalBlock，它以static函数的形式存储在代码区

2.捕获了局部变量的block为_NSConcreteStackBlock

3.当_NSConcreteStackBlock出现上面四种情况时，会变成_NSConcreteMallocBlock。（注意此时是调用了Block_Copy函数后）

Block编译后结果分析

基本类型变量

OC源码：

NSInteger age = ;

void(^completeBlock)(NSString *) = ^(NSString *name) {

    NSString *info = [NSString stringWithFormat:@"name:%@ - age:%d",name,age];

    NSLog(@"%@",info);

};

completeBlock(@"jack");

编译后的C\C++源码：

// __block_impl为block的基础struct

//1.*isa：当前block对象所属的类型（注意：在OC语言中带有isa指针的都是对象）

//2.Flags：标示位，当类型为NSConcreteMallocBlock时，表示需要ARC内存管理，其他默认不需要内存管理

//3.Reserved：预留标示位，暂时不管它

//4.block的实际static内部函数地址指针

struct __block_impl {

    void *isa;

    int Flags;

    int Reserved;

    void *FuncPtr;

};

// __main_block_impl_0为block的最终struct

//1.impl：为上面的__block_impl结构体实例

//2.Desc：为block的描述结构体__main_block_desc_0，里面包含了block结构体的信息描述，在下面讲解

//3.age：从外面补货的变量，block采用的方式是直接在block实例中定义一个变量，将捕获的值赋给它

//4.__main_block_impl_0：为block结构体的构造函数，参数中包含从外面捕获的变量值

struct __main_block_impl_0 {

    struct __block_impl impl;

    struct __main_block_desc_0* Desc;

    NSInteger age;

    __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, NSInteger _age, int flags=) : age(_age) {

        impl.isa = &_NSConcreteStackBlock;

        impl.Flags = flags;

        impl.FuncPtr = fp;

        Desc = desc;

    }

};

//__main_block_func_0为block具体实现的函数载体，具体的block的任务实现在此函数内

//参数：1是block的实例指针，2是block的外部传参name

static void __main_block_func_0(struct __main_block_impl_0 *__cself, NSString *name) {

    //从block实例中获取age变量的值

    NSInteger age = __cself->age; // bound by copy

    //生成字符串NSString *info = [NSString stringWithFormat:@"name:%@ - age:%d",name,age];

    NSString *info = ((NSString * _Nonnull (*)(id, SEL, NSString * _Nonnull, ...))(void *)objc_msgSend)((id)objc_getClass("NSString"), sel_registerName("stringWithFormat:"), (NSString *)&__NSConstantStringImpl__var_folders_4y_ks8945f50k51_0j95ytw7ss80000gn_T_main_83d55e_mi_0, (NSString *)name, (NSInteger)age);

    //打印NSLog(@"%@",info);

    NSLog((NSString *)&__NSConstantStringImpl__var_folders_4y_ks8945f50k51_0j95ytw7ss80000gn_T_main_83d55e_mi_1,info);

}

//reserved：保留字0

//Block_size:block实例在内存中占据内存大小

//__main_block_desc_0_DATA：默认desc结构体实例

static struct __main_block_desc_0 {

    size_t reserved;

    size_t Block_size;

} __main_block_desc_0_DATA = { , sizeof(struct __main_block_impl_0)};

//main函数

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

    /* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool;

        NSInteger age = ;

        void(*completeBlock)(NSString *) = ((void (*)(NSString *))&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, age));

        ((void (*)(__block_impl *, NSString *))((__block_impl *)completeBlock)->FuncPtr)((__block_impl *)completeBlock, (NSString *)&__NSConstantStringImpl__var_folders_4y_ks8945f50k51_0j95ytw7ss80000gn_T_main_83d55e_mi_2);

        return UIApplicationMain(argc, argv, __null, NSStringFromClass(((Class (*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("AppDelegate"), sel_registerName("class"))));

    }

}

block简单结构布局

struct __block_impl

struct __main_block_impl_0

static void __main_block_func_0

static struct __main_block_desc_0
详细解释见上面的中间编译状态。
block实例中定义了一个age变量，并将外部的age传了过去
 
__block基本类型变量
OC源码：
__block NSInteger age = ;

void(^completeBlock)(NSString *) = ^(NSString *name) {

    NSString *info = [NSString stringWithFormat:@"name:%@ - age:%d",name,age];

    NSLog(@"%@",info);

};

completeBlock(@"jack");
编译后的C\C++源码：
struct __block_impl {

    void *isa;

    int Flags;

    int Reserved;

    void *FuncPtr;

};

//age的block变量实例__Block_byref_age

//__isa：age的block变量实例所属的类型

//__forwarding：age的block变量实例的指针

//__flags：变量是否被内存管理标示

//__size:所占内存大小

//age:age变量值

struct __Block_byref_age_0 {

    void *__isa;

    __Block_byref_age_0 *__forwarding;

    int __flags;

    int __size;

    NSInteger age;

};

//原来的age变量值，变成了__Block_byref_age_0变量指针

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=) : 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, NSString *name) {

    //通过block实例获取变量的指针

    __Block_byref_age_0 *age = __cself->age; // bound by ref

    NSLog((NSString *)&__NSConstantStringImpl__var_folders_4y_ks8945f50k51_0j95ytw7ss80000gn_T_main_7acc63_mi_0,name,(age->__forwarding->age));

}

//当block从栈复制到堆上时，会对__Block_byref_age_0变量进行拷贝，也会从栈拷贝到堆上

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, /*BLOCK_FIELD_IS_BYREF*/);}

//__Block_byref_age_0变量的析构函数，当block实例的引用计数为0时，是否__Block_byref_age_0变量的内存空间

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

//desc结构体中新增了copy函数和dispose函数指针

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 = { , sizeof(struct __main_block_impl_0), __main_block_copy_0, __main_block_dispose_0};

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

    /* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool;

        //block变量实例

        __attribute__((__blocks__(byref))) __Block_byref_age_0 age = {(void*),(__Block_byref_age_0 *)&age, , sizeof(__Block_byref_age_0), };

        //block实例

        void(*completeBlock)(NSString *) = ((void (*)(NSString *))&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, (__Block_byref_age_0 *)&age, ));

        ((void (*)(__block_impl *, NSString *))((__block_impl *)completeBlock)->FuncPtr)((__block_impl *)completeBlock, (NSString *)&__NSConstantStringImpl__var_folders_4y_ks8945f50k51_0j95ytw7ss80000gn_T_main_7acc63_mi_1);

        return UIApplicationMain(argc, argv, __null, NSStringFromClass(((Class (*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("AppDelegate"), sel_registerName("class"))));

    }

}

简单结构布局

struct __block_impl

struct __main_block_impl_0

static void __main_block_func_0

static struct __main_block_desc_0

//新增

struct __Block_byref_age_0

static void __main_block_copy_0(

static void __main_block_dispose_0(

__block NSInteger age = 10;后，在简单布局结构中新增了3项

struct __Block_byref_age_0

static void __main_block_copy_0(

static void __main_block_dispose_0(
__Block_byref_age_0将基本变量转换成__block变量结构体实例指针
__main_block_copy_0和__main_block_dispose_0是对__block变量结构体实例的内存管理方法。
__block变量结构体实例从栈拷贝到堆时，调用方法：__main_block_copy_0
__block变量结构体实例引用计数为0时，调用方法：__main_block_dispose_0
 
指针类型变量

OC源码：

NSMutableArray *friends = [NSMutableArray array];;

void(^completeBlock)(NSString *) = ^(NSString *name) {

    NSLog(@"%@--%@",name,friends);

};

completeBlock(@"jack");

编译后的C\C++源码：

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;

    NSMutableArray *friends;

    __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, NSMutableArray *_friends, int flags=) : friends(_friends) {

        impl.isa = &_NSConcreteStackBlock;

        impl.Flags = flags;

        impl.FuncPtr = fp;

        Desc = desc;

    }

};

static void __main_block_func_0(struct __main_block_impl_0 *__cself, NSString *name) {

    NSMutableArray *friends = __cself->friends; // bound by copy

    NSLog((NSString *)&__NSConstantStringImpl__var_folders_4y_ks8945f50k51_0j95ytw7ss80000gn_T_main_1b4a2f_mi_0,name,friends);

}

//block实例从栈复制到堆上时，将指针变量引用计数加一

static void __main_block_copy_0(struct __main_block_impl_0*dst, struct __main_block_impl_0*src) {_Block_object_assign((void*)&dst->friends, (void*)src->friends, /*BLOCK_FIELD_IS_OBJECT*/);}

//block实例引用计数为0时，析构指针变量对象

static void __main_block_dispose_0(struct __main_block_impl_0*src) {_Block_object_dispose((void*)src->friends, /*BLOCK_FIELD_IS_OBJECT*/);}

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 = { , sizeof(struct __main_block_impl_0), __main_block_copy_0, __main_block_dispose_0};

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

    /* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool;

        NSMutableArray *friends = ((NSMutableArray * _Nonnull (*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("NSMutableArray"), sel_registerName("array"));;

        void(*completeBlock)(NSString *) = ((void (*)(NSString *))&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, friends, ));

        ((void (*)(__block_impl *, NSString *))((__block_impl *)completeBlock)->FuncPtr)((__block_impl *)completeBlock, (NSString *)&__NSConstantStringImpl__var_folders_4y_ks8945f50k51_0j95ytw7ss80000gn_T_main_1b4a2f_mi_1);

        return UIApplicationMain(argc, argv, __null, NSStringFromClass(((Class (*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("AppDelegate"), sel_registerName("class"))));

    }

}

简单结构布局

struct __block_impl

struct __main_block_impl_0

static void __main_block_func_0

static struct __main_block_desc_0

static void __main_block_copy_0(

static void __main_block_dispose_0(
block实例中定义了一个NSMutableArray *friends;指针变量，并通过构造函数将指针传递给block实例
__main_block_copy_0：block实例从栈复制到堆上时，将指针变量引用计数加一
__main_block_dispose_0：block实例引用计数为0时，析构指针变量对象

__block指针类型变量

OC源码：

__block NSMutableArray *friends = [NSMutableArray array];;

void(^completeBlock)(NSString *) = ^(NSString *name) {

    NSLog(@"%@--%@",name,friends);

};

completeBlock(@"jack");

编译后的C\C++源码：

struct __block_impl {

    void *isa;

    int Flags;

    int Reserved;

    void *FuncPtr;

};

//__Block_byref_friends_0结构体实例中包含了friends指针

//并且多了__Block_byref_id_object_copy函数：对friends进行copy

//多了__Block_byref_id_object_dispose函数：对friends进行析构

struct __Block_byref_friends_0 {

    void *__isa;

    __Block_byref_friends_0 *__forwarding;

    int __flags;

    int __size;

    void (*__Block_byref_id_object_copy)(void*, void*);

    void (*__Block_byref_id_object_dispose)(void*);

    NSMutableArray *friends;

};

struct __main_block_impl_0 {

    struct __block_impl impl;

    struct __main_block_desc_0* Desc;

    __Block_byref_friends_0 *friends; // by ref

    __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, __Block_byref_friends_0 *_friends, int flags=) : friends(_friends->__forwarding) {

        impl.isa = &_NSConcreteStackBlock;

        impl.Flags = flags;

        impl.FuncPtr = fp;

        Desc = desc;

    }

};

static void __main_block_func_0(struct __main_block_impl_0 *__cself, NSString *name) {

    __Block_byref_friends_0 *friends = __cself->friends; // bound by ref

    NSLog((NSString *)&__NSConstantStringImpl__var_folders_4y_ks8945f50k51_0j95ytw7ss80000gn_T_main_252d0c_mi_0,name,(friends->__forwarding->friends));

}

//block实例从栈复制到堆上时，__block变量结构体实例将指针变量引用计数加一，同时__block变量结构体实例内的NSMutableArray *friends变量也递归加一

static void __main_block_copy_0(struct __main_block_impl_0*dst, struct __main_block_impl_0*src) {_Block_object_assign((void*)&dst->friends, (void*)src->friends, /*BLOCK_FIELD_IS_BYREF*/);}

//block实例引用计数为0时，__block变量结构体实例被析构，同时__block变量结构体实例内的NSMutableArray *friends变量也递归析构

static void __main_block_dispose_0(struct __main_block_impl_0*src) {_Block_object_dispose((void*)src->friends, /*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 = { , sizeof(struct __main_block_impl_0), __main_block_copy_0, __main_block_dispose_0};

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

    /* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool;

        __attribute__((__blocks__(byref))) __Block_byref_friends_0 friends = {(void*),(__Block_byref_friends_0 *)&friends, , sizeof(__Block_byref_friends_0), __Block_byref_id_object_copy_131, __Block_byref_id_object_dispose_131, ((NSMutableArray * _Nonnull (*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("NSMutableArray"), sel_registerName("array"))};;

        void(*completeBlock)(NSString *) = ((void (*)(NSString *))&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, (__Block_byref_friends_0 *)&friends, ));

        ((void (*)(__block_impl *, NSString *))((__block_impl *)completeBlock)->FuncPtr)((__block_impl *)completeBlock, (NSString *)&__NSConstantStringImpl__var_folders_4y_ks8945f50k51_0j95ytw7ss80000gn_T_main_252d0c_mi_1);

        return UIApplicationMain(argc, argv, __null, NSStringFromClass(((Class (*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("AppDelegate"), sel_registerName("class"))));

    }

}

简单结构变量

struct __block_impl

struct __main_block_impl_0

static void __main_block_func_0

static struct __main_block_desc_0

static void __main_block_copy_0(

static void __main_block_dispose_0(

//新增

struct __Block_byref_age_0
和__block普通变量一样，出现了__Block_byref_age_0结构体
__Block_byref_friends_0结构体实例中包含了friends指针
并且多了__Block_byref_id_object_copy函数：对friends进行copy
多了__Block_byref_id_object_dispose函数：对friends进行析构
同时在函数__main_block_copy_0和__main_block_dispose_0中，出现的也是对__Block_byref_age_0结构体的调用。它是在将__Block_byref_age_0结构体析构之前，先析构friends对象。
 
Block编译中间态与运行时状态对比
利用lldb打印出来的结果
局部自由变量
//局部变量

__block NSMutableArray *friends = [NSMutableArray array];;

void(^completeBlock)(NSString *) = ^(NSString *name) {

    NSLog(@"%@--%@",name,friends);

};

(lldb) expression -P  -- completeBlock

(void (^)(NSString *)) $ = 0x000000010a061130 {

  __isa = __NSMallocBlock__

  __flags = -

  __reserved =

  __FuncPtr = 0x000000010a061130 (iOS_KnowledgeStructure`__main_block_invoke at main.m:) {}

}

利用clang转换后的C\C++源码

struct __main_block_impl_0 {

    struct __block_impl impl;

    struct __main_block_desc_0* Desc;

    __Block_byref_friends_0 *friends; // by ref

    __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, __Block_byref_friends_0 *_friends, int flags=) : friends(_friends->__forwarding) {

        impl.isa = &_NSConcreteStackBlock;

        impl.Flags = flags;

        impl.FuncPtr = fp;

        Desc = desc;

    }

};
证明clang编译器和llvm编译器编译后的结果有出入。发现isa指针不一致。
编译后的为：impl.isa = &_NSConcreteStackBlock;
运行时的为：__isa = __NSMallocBlock__
原因是block类型转换情况4:“将block赋给带有__strong修饰符的id类型或者Block类型时。”
 
注意：
_NSConcreteMallocBlock一般不会在源码中出现，它通常在block copy到堆时出现，变化的源码：
static void *_Block_copy_internal(const void *arg, const int flags) {

struct Block_layout *aBlock;

const bool wantsOne = (WANTS_ONE & flags) == WANTS_ONE;

//

if (!arg) return NULL;

//

aBlock = (struct Block_layout *)arg;

//

if (aBlock->flags & BLOCK_NEEDS_FREE) {

// latches on high

latching_incr_int(&aBlock->flags);

return aBlock;

}

//

else if (aBlock->flags & BLOCK_IS_GLOBAL) {

return aBlock;

}

//

struct Block_layout *result = malloc(aBlock->descriptor->size);

if (!result) return (void *);

//

memmove(result, aBlock, aBlock->descriptor->size); // bitcopy first

//

result->flags &= ~(BLOCK_REFCOUNT_MASK); // XXX not needed

result->flags |= BLOCK_NEEDS_FREE | ;

//

result->isa = _NSConcreteMallocBlock;

//

if (result->flags & BLOCK_HAS_COPY_DISPOSE) {

(*aBlock->descriptor->copy)(result, aBlock); // do fixup

}

return result;

}
 
05-06 23:46
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