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CC++，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:9:9: 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

用法如下：https://www.cnblogs.com/zhou--fei/p/10224000.html

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 = 10;
void(^completeBlock)(NSString *) = ^(NSString *name) {
    NSString *info = [NSString stringWithFormat:@"name:%@ - age:%d",name,age];
    NSLog(@"%@",info);
};

completeBlock(@"jack");

编译后的CC++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=0) : 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 = { 0, sizeof(struct __main_block_impl_0)};

//main函数
int main(int argc, char * argv[]) {
    /* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool;

        NSInteger age = 10;
        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 = 10;
void(^completeBlock)(NSString *) = ^(NSString *name) {
    NSString *info = [NSString stringWithFormat:@"name:%@ - age:%d",name,age];
    NSLog(@"%@",info);
};

completeBlock(@"jack");

编译后的CC++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=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, 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, 8/*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, 8/*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 = { 0, 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*)0,(__Block_byref_age_0 *)&age, 0, sizeof(__Block_byref_age_0), 10};
        //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, 570425344));

        ((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CC++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=0) : 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, 3/*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, 3/*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 = { 0, 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, 570425344));

        ((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CC++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=0) : 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, 8/*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, 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, char * argv[]) {
    /* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool;

        __attribute__((__blocks__(byref))) __Block_byref_friends_0 friends = {(void*)0,(__Block_byref_friends_0 *)&friends, 33554432, 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, 570425344));

        ((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 5 -- completeBlock
(void (^)(NSString *)) $2 = 0x000000010a061130 {
  __isa = __NSMallocBlock__
  __flags = -1023410170
  __reserved = 0
  __FuncPtr = 0x000000010a061130 (iOS_KnowledgeStructure`__main_block_invoke at main.m:19) {}
}

利用clang转换后的CC++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=0) : 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;
// 1
if (!arg) return NULL;
// 2
aBlock = (struct Block_layout *)arg;
// 3
if (aBlock->flags & BLOCK_NEEDS_FREE) {
// latches on high
latching_incr_int(&aBlock->flags);
return aBlock;
}
// 4
else if (aBlock->flags & BLOCK_IS_GLOBAL) {
return aBlock;
}
// 5
struct Block_layout *result = malloc(aBlock->descriptor->size);
if (!result) return (void *)0;
// 6
memmove(result, aBlock, aBlock->descriptor->size); // bitcopy first
// 7
result->flags &= ~(BLOCK_REFCOUNT_MASK); // XXX not needed
result->flags |= BLOCK_NEEDS_FREE | 1;
// 8
result->isa = _NSConcreteMallocBlock;
// 9
if (result->flags & BLOCK_HAS_COPY_DISPOSE) {
(*aBlock->descriptor->copy)(result, aBlock); // do fixup
}
return result;
}