Objective-C block实现代码分析

block内部结构

我们先写一个block

void exampleBlock() {    // NSConcreteStackBlock    int a = 1;    __block int b = 2;    int(^blockTest0)(int c) = ^(int c){        return a + b + c;    };    int c = 3;    blockTest0(c);    // NSConcreteGlobalBlock    void(^blockTest2)(void) = ^(void){        ;    };    blockTest2();}

用clang转成c分析下

clang -rewrite-objc block.c

可以看到他们的定义是

struct __exampleBlock_block_impl_0 {  struct __block_impl impl;  struct __exampleBlock_block_desc_0* Desc;  int a;  __Block_byref_b_0 *b; // by ref  __exampleBlock_block_impl_0(void *fp, struct __exampleBlock_block_desc_0 *desc, int _a, __Block_byref_b_0 *_b, int flags=0) : a(_a), b(_b->__forwarding) {    impl.isa = &_NSConcreteStackBlock;    impl.Flags = flags;    impl.FuncPtr = fp;    Desc = desc;  }};// __block_implstruct __block_impl {  void *isa;  int Flags;  int Reserved;  void *FuncPtr;};// __exampleBlock_block_desc_0struct __exampleBlock_block_impl_0 {  struct __block_impl impl;  struct __exampleBlock_block_desc_0* Desc;  int a;  __Block_byref_b_0 *b; // by ref  __exampleBlock_block_impl_0(void *fp, struct __exampleBlock_block_desc_0 *desc, int _a, __Block_byref_b_0 *_b, int flags=0) : a(_a), b(_b->__forwarding) {    impl.isa = &_NSConcreteStackBlock;    impl.Flags = flags;    impl.FuncPtr = fp;    Desc = desc;  }};// __exampleBlock_block_impl_2struct __exampleBlock_block_impl_2 {  struct __block_impl impl;  struct __exampleBlock_block_desc_2* Desc;  __exampleBlock_block_impl_2(void *fp, struct __exampleBlock_block_desc_2 *desc, int flags=0) {    impl.isa = &_NSConcreteStackBlock;    impl.Flags = flags;    impl.FuncPtr = fp;    Desc = desc;  }};

初始化和执行代码

void exampleBlock() {    // blockTest0    int a = 1;    __attribute__((__blocks__(byref))) __Block_byref_b_0 b = {(void*)0,(__Block_byref_b_0 *)&b, 0, sizeof(__Block_byref_b_0), 2};    int(*blockTest0)(int c) = (int (*)(int))&__exampleBlock_block_impl_0((void *)__exampleBlock_block_func_0, &__exampleBlock_block_desc_0_DATA, a, (__Block_byref_b_0 *)&b, 570425344);    int c = 3;    ((int (*)(__block_impl *, int))((__block_impl *)blockTest0)->FuncPtr)((__block_impl *)blockTest0, c);    // blockTest2    void(*blockTest2)(void) = (void (*)())&__exampleBlock_block_impl_2((void *)__exampleBlock_block_func_2, &__exampleBlock_block_desc_2_DATA);    ((void (*)(__block_impl *))((__block_impl *)blockTest2)->FuncPtr)((__block_impl *)blockTest2);}

我们先看看blockTest2,它是由 结构体impl, 结构体Desc, 构造方法__exampleBlock_block_impl_2() 组成展开后是

• *isa 指向该实例对象(代码里是NSConcreteStackBlock,其实应该是NSConcreteGlobalBlock)
• Flags 用于按bit位表示一些block的附加信息
• reserved 保留变量
• *FuncPtr 函数指针,指向具体的block实现的函数调用地址(代码里是__exampleBlock_block_func_2)

static void __exampleBlock_block_func_2(struct __exampleBlock_block_impl_2 *__cself) {        ;}

• size_t reserved 这个传进来的是0
• Block_size 结构体的大小

static struct __exampleBlock_block_desc_2 {  size_t reserved;  size_t Block_size;} __exampleBlock_block_desc_2_DATA = { 0, sizeof(struct __exampleBlock_block_impl_2)};

---

然后我们在看blockTest,它比blockTest2多了2个变量a, b

• int a; 外部变量a,

• Block_byref_b_0 *b; 加了block修饰的b, 传的是 Block_byref_b_0

在生成的初始化代码中则多了3个传入值

    int(*blockTest0)(int c) = (int (*)(int))&__exampleBlock_block_impl_0((void *)__exampleBlock_block_func_0, &__exampleBlock_block_desc_0_DATA, a, (__Block_byref_b_0 *)&b, 570425344);

• a是这个是直接传值进去,然后被复制给 a

• b是传的地址, 是把 __Block_byref_b_0 赋值给 b

__Block_byref_b_0这个结构体是

struct __Block_byref_b_0 {  void *__isa;__Block_byref_b_0 *__forwarding; int __flags; int __size; int b;};

__forwarding 是一个指向自己的指针.

__Block_byref_b_0 的初始化代码如下:

__attribute__((__blocks__(byref))) __Block_byref_b_0 b = {(void*)0,(__Block_byref_b_0 *)&b, 0, sizeof(__Block_byref_b_0), 2};

我们可以看出a是直接复制进去的,b是被转到了一个结构体里,然后吧这个结构体的指针传进去,所以block不能修改a,能修改b.

• 570425344 这个应该是传给Flags

blockTest0的Desc和blockTest2也有所不同,展开后是

static struct __exampleBlock_block_desc_0 {  size_t reserved;  size_t Block_size;  void (*copy)(struct __exampleBlock_block_impl_0*, struct __exampleBlock_block_impl_0*);  void (*dispose)(struct __exampleBlock_block_impl_0*);} __exampleBlock_block_desc_0_DATA = { 0, sizeof(struct __exampleBlock_block_impl_0), __exampleBlock_block_copy_0, __exampleBlock_block_dispose_0};

多了2个函数指针copy, dispose,对于在调用前后修改相应变量的引用计数, 分别指向

static void __exampleBlock_block_copy_0(struct __exampleBlock_block_impl_0*dst, struct __exampleBlock_block_impl_0*src) {_Block_object_assign((void*)&dst->b, (void*)src->b, 8/*BLOCK_FIELD_IS_BYREF*/);}static void __exampleBlock_block_dispose_0(struct __exampleBlock_block_impl_0*src) {_Block_object_dispose((void*)src->b, 8/*BLOCK_FIELD_IS_BYREF*/);}

再来看下blockTest0的*FuncPtr

 static int __exampleBlock_block_func_0(struct __exampleBlock_block_impl_0 *__cself, int c) {  __Block_byref_b_0 *b = __cself->b; // bound by ref  int a = __cself->a; // bound by copy        return a + (b->__forwarding->b) + c;    }

可以看出a用的是拷贝进来的a, b用的是结构体里的b