Block Structure (1)

A block is actually an object with its own attributes. The structure is as follows:

• ISA pointer. All objects have this pointer for message transmission and other functions (IAS generally points to the parent class and metadata class)
• Flags, used to indicate additional block information by bit
• Reserved, reserved variable.
• Invoke, function pointer, pointing to the function call address implemented by the specific block.
• Descriptor, indicating the additional description of the block, mainly the size and pointer of the copy and dispose functions.
• Variables, a variable from capture, block can access its external local variables because these variables (or the address of the variable) are copied to the struct.

   

We use the clang-rewrite-objc command to decompile the. M file to obtain the. cpp file. M contains the following generation

-(void )test3{    NSString *_person2=@"person2";    NSLog(@"block init:%@,%p,%p",_person2,&_person2,_person2);    void (^myBlock)(int) = ^(int num) {        NSLog(@"block excuteing:%@,%p,%p",_person2,&_person2,_person2);    };    _person2=@"person22";    NSLog(@"block excutebefore:%@,%p,%p",_person2,&_person2,_person2);    myBlock(1);    NSLog(@"block excuteafter:%@,%p,%p",_person2,&_person2,_person2);}

 

Obtain some source code

// Here it should be a constant in the registration code, which will be used below

#define __OFFSETOFIVAR__(TYPE, MEMBER) ((long long) &((TYPE *)0)->MEMBER)static __NSConstantStringImpl __NSConstantStringImpl__var_folders_5l_2l25j3tn0wl_3zy1hpsq1rhc0000gp_T_KDBlockTest_51f509_mi_0 __attribute__ ((section ("__DATA, __cfstring"))) = {__CFConstantStringClassReference,0x000007c8,"KDBlockTest dealloc",19};static __NSConstantStringImpl __NSConstantStringImpl__var_folders_5l_2l25j3tn0wl_3zy1hpsq1rhc0000gp_T_KDBlockTest_51f509_mi_1 __attribute__ ((section ("__DATA, __cfstring"))) = {__CFConstantStringClassReference,0x000007c8,"person2",7};static __NSConstantStringImpl __NSConstantStringImpl__var_folders_5l_2l25j3tn0wl_3zy1hpsq1rhc0000gp_T_KDBlockTest_51f509_mi_2 __attribute__ ((section ("__DATA, __cfstring"))) = {__CFConstantStringClassReference,0x000007c8,"block init:%@,%p,%p",19};static __NSConstantStringImpl __NSConstantStringImpl__var_folders_5l_2l25j3tn0wl_3zy1hpsq1rhc0000gp_T_KDBlockTest_51f509_mi_3 __attribute__ ((section ("__DATA, __cfstring"))) = {__CFConstantStringClassReference,0x000007c8,"block excuteing:%@,%p,%p",24};static __NSConstantStringImpl __NSConstantStringImpl__var_folders_5l_2l25j3tn0wl_3zy1hpsq1rhc0000gp_T_KDBlockTest_51f509_mi_4 __attribute__ ((section ("__DATA, __cfstring"))) = {__CFConstantStringClassReference,0x000007c8,"person22",8};static __NSConstantStringImpl __NSConstantStringImpl__var_folders_5l_2l25j3tn0wl_3zy1hpsq1rhc0000gp_T_KDBlockTest_51f509_mi_5 __attribute__ ((section ("__DATA, __cfstring"))) = {__CFConstantStringClassReference,0x000007c8,"block excutebefore:%@,%p,%p",27};static __NSConstantStringImpl __NSConstantStringImpl__var_folders_5l_2l25j3tn0wl_3zy1hpsq1rhc0000gp_T_KDBlockTest_51f509_mi_6 __attribute__ ((section ("__DATA, __cfstring"))) = {__CFConstantStringClassReference,0x000007c8,"block excuteafter:%@,%p,%p",26};

// Here is the structure of the block object
// Imp: function pointer pointing to the function implemented by the specific block
// _ Person2: intercepted variable
// Isa, flags, funcptr, and desc will be mentioned later.

struct __KDBlockTest__test3_block_impl_0 {  struct __block_impl impl;  struct __KDBlockTest__test3_block_desc_0* Desc;  NSString *_person2;  __KDBlockTest__test3_block_impl_0(void *fp, struct __KDBlockTest__test3_block_desc_0 *desc, NSString *__person2, int flags=0) : _person2(__person2) {    impl.isa = &_NSConcreteStackBlock;    impl.Flags = flags;    impl.FuncPtr = fp;    Desc = desc;  }

// Functions implemented by block

static void __KDBlockTest__test3_block_func_0(struct __KDBlockTest__test3_block_impl_0 *__cself, int num) {  NSString *_person2 = __cself->_person2; // bound by copy        NSLog((NSString *)&__NSConstantStringImpl__var_folders_5l_2l25j3tn0wl_3zy1hpsq1rhc0000gp_T_KDBlockTest_51f509_mi_3,_person2,&_person2,_person2);    }

// Copy function, which can be copied to the stack

static void __KDBlockTest__test3_block_copy_0(struct __KDBlockTest__test3_block_impl_0*dst, struct __KDBlockTest__test3_block_impl_0*src) {_Block_object_assign((void*)&dst->_person2, (void*)src->_person2, 3/*BLOCK_FIELD_IS_OBJECT*/);}

// Corresponding release function

static void __KDBlockTest__test3_block_dispose_0(struct __KDBlockTest__test3_block_impl_0*src) {_Block_object_dispose((void*)src->_person2, 3/*BLOCK_FIELD_IS_OBJECT*/);}

// Block Object Description (size, etc)

static struct __KDBlockTest__test3_block_desc_0 {  size_t reserved;  size_t Block_size;  void (*copy)(struct __KDBlockTest__test3_block_impl_0*, struct __KDBlockTest__test3_block_impl_0*);  void (*dispose)(struct __KDBlockTest__test3_block_impl_0*);} __KDBlockTest__test3_block_desc_0_DATA = { 0, sizeof(struct __KDBlockTest__test3_block_impl_0), __KDBlockTest__test3_block_copy_0, __KDBlockTest__test3_block_dispose_0};

// This is the objc test function.

static void _I_KDBlockTest_test3(KDBlockTest * self, SEL _cmd) {    NSString *_person2=(NSString *)&__NSConstantStringImpl__var_folders_5l_2l25j3tn0wl_3zy1hpsq1rhc0000gp_T_KDBlockTest_51f509_mi_1;    NSLog((NSString *)&__NSConstantStringImpl__var_folders_5l_2l25j3tn0wl_3zy1hpsq1rhc0000gp_T_KDBlockTest_51f509_mi_2,_person2,&_person2,_person2);    void (*myBlock)(int) = (void (*)(int))&__KDBlockTest__test3_block_impl_0((void *)__KDBlockTest__test3_block_func_0, &__KDBlockTest__test3_block_desc_0_DATA, _person2, 570425344);    _person2=(NSString *)&__NSConstantStringImpl__var_folders_5l_2l25j3tn0wl_3zy1hpsq1rhc0000gp_T_KDBlockTest_51f509_mi_4;    NSLog((NSString *)&__NSConstantStringImpl__var_folders_5l_2l25j3tn0wl_3zy1hpsq1rhc0000gp_T_KDBlockTest_51f509_mi_5,_person2,&_person2,_person2);    ((void (*)(__block_impl *, int))((__block_impl *)myBlock)->FuncPtr)((__block_impl *)myBlock, 1);    NSLog((NSString *)&__NSConstantStringImpl__var_folders_5l_2l25j3tn0wl_3zy1hpsq1rhc0000gp_T_KDBlockTest_51f509_mi_6,_person2,&_person2,_person2);}

 

Simple Analysis: 1) block initialization, combined with constructors:

__KDBlockTest__test3_block_impl_0(void *fp, struct __KDBlockTest__test3_block_desc_0 *desc, NSString *__person2, int flags=0) : _person2(__person2) {    impl.isa = &_NSConcreteStackBlock;    impl.Flags = flags;    impl.FuncPtr = fp;    Desc = desc;  }

void (*myBlock)(int) = (void (*)(int))&__KDBlockTest__test3_block_impl_0((void *)__KDBlockTest__test3_block_func_0, &__KDBlockTest__test3_block_desc_0_DATA, _person2, 570425344);

Passed in parameters: function pointer, block description, external variable _ person2. At this time, _ person2 was deeply copied in the block (in fact, I didn't see from where it was actually a deep copy, hope you can enlighten me) 2 ). execute Block

((void (*)(__block_impl *, int))((__block_impl *)myBlock)->FuncPtr)((__block_impl *)myBlock, 1);

In fact, the function _ kdblocktest _ test3_block_func_0 pointed to by the block function pointer is called, and the block itself is passed as a parameter.

static void __KDBlockTest__test3_block_func_0(struct __KDBlockTest__test3_block_impl_0 *__cself, int num) {  NSString *_person2 = __cself->_person2; // bound by copy        NSLog((NSString *)&__NSConstantStringImpl__var_folders_5l_2l25j3tn0wl_3zy1hpsq1rhc0000gp_T_KDBlockTest_51f509_mi_3,_person2,&_person2,_person2);    }

It is worth noting that we modify _ person2 before execution, but it does not affect the interior of the block, because when constructing the block, the block has already made a deep copy of the intercepted variables. When the block is executed, the function _ kdblocktest _ test3_block_func_0 is called directly. Inside the function, the _ person2 object copied by the block is referenced, I think this should be the feature of block interception variables.