int multiplier = 7;
int (^myBlock)(int) = ^(int num) {
    return num * multiplier;
};

typedef void (^BoolBlock)(BOOL);//一个只接受一个BOOL参数，没有返回值的block
typedef int (^IntBlock)(void);//一个没有参数，返回int的block
typedef BoolBlock (^HugeBlock)(IntBlock);//看看，这个HugeBlock的参数和返回值都是block

- (void)someMethod
{
    BoolBlock ablock = ^(BOOL bValue) {
        NSLog(@"Bool block!");
    };
    ablock(true);
}

typedef void (^BoolBlock)(BOOL);
- (BoolBlock)foo()
{
    BoolBlock ablock = ^(BOOL bValue) {
        NSLog(@"Bool block!");
    };
    return [[ablock copy] autorelease];//一定要copy，将其复制到堆上，更详细的原理，将在后续章节讲解
}

@interface OBJ1 : NSObject
@property (nonatomic, copy)BoolBlock block;//理由同上啊，同学们
@end
OBJ1 *obj1 = ...
obj1.block = ^(BOOL bValue) {
        NSLog(@"Bool block!");
    };

- (void)foo(BoolBlock block)
{
    if (block) {
        block();
    }
}

BoolBlock bBlock = ^(BOOL bV){if(Bv){/*do some thing*/}};
HugeBlock hBlock = ^(BoolBlock bB) {bB();};
hBolck(bBlock);

static int(^maxIntBlock)(int, int) = ^(int a, int b){return a>b?a:b;};
int main()
{
    printf("%d\n", maxIntBlock(2,10));  
    return 0;
}

int main()
{
    __block int i = 1024;
    BoolBlock bBlock = ^(BOOL bV) {
        if (bV) {
            i++;//如果没有__block标记，是无法通过编译的。
        }
    };
}

void foo()
{
    __block int i = 1024;
    int j = 1;
    void (^blk)(void);
    void (^blkInHeap)(void);
    blk = ^{ printf("%d, %d\n", i, j);};//blk在栈里
    blkInHeap = Block_copy(blk);//blkInHeap在堆里
}
- (void)fooBar
{
    _oi = 1;
    OBJ1* oj = self;
    void (^oblk)(void) = ^{ printf("%d\n", oj.oi);};
    void (^oblkInHeap)(void) = [oblk copy];//oblkInHeap在堆中
}

static int(^maxIntBlock)(int, int) = ^(int a, int b){return a>b?a:b;};
- (void)fooBar
{
     int(^maxIntBlockCopied)(int, int) =[maxIntBlock copy];
}
void foo()
{
     int(^maxIntBlockCopied)(int, int) = Block_copy(maxIntBlock);
}

static int gVar = 0;
//__block static int gMVar = 1;
void foo()
{
    static int stackVar = 0;
//    __block static int stackMVar = 0;
}

void foo()
{
    __block int i = 1024;//此时i在栈上
    int j = 1;//此时j在栈上
    void (^blk)(void);
    blk = ^{printf("%d, %d\n", i, j); };//此时，blk已经初始化，它会拷贝没有__block标记的常规变量自己所持有的一块内存区，这块内存区现在位于栈上，而对于具有__block标记的变量，其地址会被拷贝置前述的内存区中
    blk();//1024, 1
    void(^blkInHeap)(void) = Block_copy(blk);//复制block后，block所持有的内存区会被拷贝至堆上，此时，我们可以说，这个block现在位于堆上
    blkInHeap();//1024,1
    i++;
    j++;
    blk();//1025,1
    blkInHeap();//1025,1
}

struct __block_impl {
  void *isa;
  int Flags;
  int Reserved;
  void *FuncPtr;
};

blk();

((void (*)(__block_impl *))((__block_impl *)blk)->FuncPtr)((__block_impl *)blk);

- (void)of1
{
    OBJ1* oj = self;
    void (^oblk)(void) = ^{ printf("%d\n", oj.oi);};
    Block_copy(oblk);
}

struct __OBJ1__of1_block_impl_0 {
  struct __block_impl impl;
  struct __OBJ1__of1_block_desc_0* Desc;
  OBJ1 *oj;
  __OBJ1__of1_block_impl_0(void *fp, struct __OBJ1__of1_block_desc_0 *desc, OBJ1 *_oj, int flags=0) : oj(_oj) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};
static void __OBJ1__of1_block_func_0(struct __OBJ1__of1_block_impl_0 *__cself) {
  OBJ1 *oj = __cself->oj; // bound by copy
 printf("%d\n", ((int (*)(id, SEL))(void *)objc_msgSend)((id)oj, sel_registerName("oi")));}

- (void)of1
{
    __block OBJ1 *bSelf = self;
    ^{ printf("%d", bSelf.oi); };
}

//增加了如下行
struct __Block_byref_bSelf_0 {
  void *__isa;
__Block_byref_bSelf_0 *__forwarding;
 int __flags;
 int __size;
 void (*__Block_byref_id_object_copy)(void*, void*);
 void (*__Block_byref_id_object_dispose)(void*);
 OBJ1 *bSelf;
};
static void __Block_byref_id_object_copy_131(void *dst, void *src) {
 _Block_object_assign((char*)dst + 40, *(void * *) ((char*)src + 40), 131);
}
static void __Block_byref_id_object_dispose_131(void *src) {
 _Block_object_dispose(*(void * *) ((char*)src + 40), 131);
}
//声明处变为
    __block __Block_byref_bSelf_0 bSelf = {(void*)0,(__Block_byref_bSelf_0 *)&bSelf, 33554432, sizeof(__Block_byref_bSelf_0), __Block_byref_id_object_copy_131, __Block_byref_id_object_dispose_131, self};

/* the raw data space for runtime classes for blocks */
/* class+meta used for stack, malloc, and collectable based blocks */
BLOCK_EXPORT void * _NSConcreteStackBlock[32];
BLOCK_EXPORT void * _NSConcreteMallocBlock[32];
BLOCK_EXPORT void * _NSConcreteAutoBlock[32];
BLOCK_EXPORT void * _NSConcreteFinalizingBlock[32];
BLOCK_EXPORT void * _NSConcreteGlobalBlock[32];
BLOCK_EXPORT void * _NSConcreteWeakBlockVariable[32];

struct __OBJ1__of2_block_impl_0 {
  struct __block_impl impl;
  struct __OBJ1__of2_block_desc_0* Desc;
  OBJ1 *self;
  __OBJ1__of2_block_impl_0(void *fp, struct __OBJ1__of2_block_desc_0 *desc, OBJ1 *_self, int flags=0) : self(_self) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};

// Its a stack block.  Make a copy.
    if (!isGC) {
        struct Block_layout *result = malloc(aBlock->descriptor->size);
        ...
        result->isa = _NSConcreteMallocBlock;
        ...
        return result;
    }

BLOCK_EXPORT void * _NSConcreteAutoBlock[32];
BLOCK_EXPORT void * _NSConcreteFinalizingBlock[32];
BLOCK_EXPORT void * _NSConcreteWeakBlockVariable[32];

static void *_Block_copy_internal(const void *arg, const int flags) {
    ...
    aBlock = (struct Block_layout *)arg;
    ...
}

// reset refcount
        result->flags &= ~(BLOCK_REFCOUNT_MASK);    // XXX not needed
        result->flags |= BLOCK_NEEDS_FREE | 1;
        result->isa = _NSConcreteMallocBlock;
        if (result->flags & BLOCK_HAS_COPY_DISPOSE) {
            //printf("calling block copy helper %p(%p, %p)...\n", aBlock->descriptor->copy, result, aBlock);
            (*aBlock->descriptor->copy)(result, aBlock); // do fixup
        }

else if (aBlock->flags & BLOCK_IS_GLOBAL) {
        return aBlock;
    }

if (aBlock->flags & BLOCK_NEEDS_FREE) {
        // latches on high
        latching_incr_int(&aBlock->flags);
        return aBlock;
    }

struct Block_byref {
    void *isa;
    struct Block_byref *forwarding;
    int flags; /* refcount; */
    int size;
    void (*byref_keep)(struct Block_byref *dst, struct Block_byref *src);
    void (*byref_destroy)(struct Block_byref *);
    /* long shared[0]; */
};

struct __Block_byref_i_0 {
  void *__isa;
__Block_byref_i_0 *__forwarding;
 int __flags;
 int __size;
 int i;
};//所以我们知道，当此结构体被类型强转为Block_byref时，前四个成员是一致的，访问flags就相当于访问__flags，而内部实现就是这样使用的
...
__attribute__((__blocks__(byref))) __Block_byref_i_0 i = {(void*)0,(__Block_byref_i_0 *)&i, 0, sizeof(__Block_byref_i_0), 1024};//i初始化时__flags为0


static void __main_block_copy_0(struct __main_block_impl_0*dst, struct __main_block_impl_0*src) {_Block_object_assign((void*)&dst->i, (void*)src->i, 8/*BLOCK_FIELD_IS_BYREF*/);}

void _Block_object_assign(void *destAddr, const void *object, const int flags) {//此处flags为8，即BLOCK_FIELD_IS_BYREF
    ...
    if ((flags & BLOCK_FIELD_IS_BYREF) == BLOCK_FIELD_IS_BYREF)  {
        // copying a __block reference from the stack Block to the heap
        // flags will indicate if it holds a __weak reference and needs a special isa
        _Block_byref_assign_copy(destAddr, object, flags);
    }
    ...
}
static void _Block_byref_assign_copy(void *dest, const void *arg, const int flags) {//此处flags为8，即BLOCK_FIELD_IS_BYREF
    struct Block_byref **destp = (struct Block_byref **)dest;
    struct Block_byref *src = (struct Block_byref *)arg;
    ...
    else if ((src->forwarding->flags & BLOCK_REFCOUNT_MASK) == 0) {//当初次拷贝i时，flags为0，进入此分支会进行复制操作并改变flags值，置入BLOCK_NEEDS_FREE和初始的引用计数
       ...
    }
    // already copied to heap
    else if ((src->forwarding->flags & BLOCK_NEEDS_FREE) == BLOCK_NEEDS_FREE) {//当再次拷贝i时，则仅仅增加其引用计数
        latching_incr_int(&src->forwarding->flags);
    }
    // assign byref data block pointer into new Block
    _Block_assign(src->forwarding, (void **)destp);//这句仅仅是直接赋值，其函数实现只有一行赋值语句，查阅runtime.c可知
}

static void __OBJ1__of2_block_copy_0(struct __OBJ1__of2_block_impl_0*dst, struct __OBJ1__of2_block_impl_0*src) {_Block_object_assign((void*)&dst->self, (void*)src->self, 3/*BLOCK_FIELD_IS_OBJECT*/);}


void _Block_object_assign(void *destAddr, const void *object, const int flags) {
    ...
    else if ((flags & BLOCK_FIELD_IS_OBJECT) == BLOCK_FIELD_IS_OBJECT) {
        //printf("retaining object at %p\n", object);
        _Block_retain_object(object);//当我们没有开启arc时，这个函数会retian此object
        //printf("done retaining object at %p\n", object);
        _Block_assign((void *)object, destAddr);
    }
    ....
}

struct __Block_byref_bSelf_0 {
  void *__isa;
__Block_byref_bSelf_0 *__forwarding;
 int __flags;
 int __size;
 void (*__Block_byref_id_object_copy)(void*, void*);
 void (*__Block_byref_id_object_dispose)(void*);
 OBJ1 *bSelf;
};
static void __Block_byref_id_object_copy_131(void *dst, void *src) {
 _Block_object_assign((char*)dst + 40, *(void * *) ((char*)src + 40), 131);//131即为BLOCK_FIELD_IS_OBJECT|BLOCK_BYREF_CALLER
}
static void __Block_byref_id_object_dispose_131(void *src) {
 _Block_object_dispose(*(void * *) ((char*)src + 40), 131);
}
... //33554432即为BLOCK_HAS_COPY_DISPOSE
    __block __Block_byref_bSelf_0 bSelf = {(void*)0,(__Block_byref_bSelf_0 *)&bSelf, 33554432, sizeof(__Block_byref_bSelf_0), __Block_byref_id_object_copy_131, __Block_byref_id_object_dispose_131, self};

void _Block_object_assign(void *destAddr, const void *object, const int flags) {
    //printf("_Block_object_assign(*%p, %p, %x)\n", destAddr, object, flags);
    if ((flags & BLOCK_BYREF_CALLER) == BLOCK_BYREF_CALLER) {
        if ((flags & BLOCK_FIELD_IS_WEAK) == BLOCK_FIELD_IS_WEAK) {
            _Block_assign_weak(object, destAddr);
        }
        else {
            // do *not* retain or *copy* __block variables whatever they are
            _Block_assign((void *)object, destAddr);
        }
    }