概述
在iOS开发中Block使用比较广泛，对于使用以及一些常规的技术点这里不再赘述，主要利用C++角度分析Block内部数据底层实现，解开开发中为什么这样编写代码解决问题。
Block底层结构窥探
Block本质也是一个OC对象，内部也有isa指针，最终继承NSObject。它是封装了函数调用以及函数调用环境的OC对象。
接下来编写一个Block，利用clang编译器指令可以将我们编写的OC代码转换成C++代码，更好的看清Block底层结构。
#import <Foundation/Foundation.h>
int main(int argc, const char * argv[]) {
@autoreleasepool {
// 定义block
void(^block)(void) = ^{
NSLog(@"-----------");
};
// 执行block
block();
}
return 0;
}
执行命令
xcrun -sdk iphoneos clang -arch arm64 -rewrite-objc main.m -o main.cpp
上面编写OC代码转换成C++代码main.cpp
#pragma clang assume_nonnull end
struct __block_impl {
void *isa;
int Flags;
int Reserved;
void *FuncPtr;
};
// Block底层的结构
struct __main_block_impl_0 {
void *isa;
int Flags;
int Reserved;
void *FuncPtr; // 保存内部代码块执行的函数地址
// struct __block_impl impl;
struct __main_block_desc_0* Desc; // Block的信息
// 构造方法
__main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, int flags=0) {
impl.isa = &_NSConcreteStackBlock;
impl.Flags = flags;
impl.FuncPtr = fp;
Desc = desc;
}
};
// Block内部执行代码块封装的函数
static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
NSLog((NSString *)&__NSConstantStringImpl__var_folders_zz_1js09xx15fz7j7qyrylx4xm80000gn_T_main_3df4b0_mi_0);
}
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)/*block自己结构的内存大小*/};
int main(int argc, const char * argv[]) {
/* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool;
// 定义block
// block真实类型 struct __main_block_impl_0 *
void(*block)(void) = &__main_block_impl_0(__main_block_func_0, &__main_block_desc_0_DATA);
// 执行block
block->FuncPtr(block);
}
return 0;
}
Block底层数据结构是__main_block_impl_0类型，定义的block变量是指向__main_block_impl_0类型的对象类型指针。__main_block_impl_0构造方法的需要两个参数，第一个参数：__main_block_func_0函数是对Block内部代码块的封装。第二个参数类型是__main_block_desc_0_DATA类型指针，内部主要对Block的信息封装，比如包含block的占用内部空间大小。
在__main_block_impl_0 构造方法中将封装block代码块的__main_block_func_0函数地址传入__main_block_impl_0 对象的FuncPtr成员保存。
当执行block，直接通过指针访问到FuncPtr成员的函数地址进行调用。
Block变量的捕获
在开发中block内部会访问外部的变量，为了保证内部能正常访问外部的变量，block有个变量捕获的机制。
编写代码验证
#import <Foundation/Foundation.h>
// 全局变量
int weight = 120;
int main(int argc, const char * argv[]) {
@autoreleasepool {
// 定义block
int age = 10;
static int height = 178;
void(^block)(void) = ^{
NSLog(@"age is %d", age);
NSLog(@"height is %d", height);
NSLog(@"weight is %d", weight);
};
// 执行block
block();
}
return 0;
}
clang转成对应的C++代码
#pragma clang assume_nonnull end
int weight = 120;
struct __main_block_impl_0 {
struct __block_impl impl;
struct __main_block_desc_0* Desc;
int age; // 局部变量age的捕获
int *height; // static修饰的局部变量的捕获
__main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, int _age, int *_height, int flags=0) : age(_age), height(_height) {
impl.isa = &_NSConcreteStackBlock;
impl.Flags = flags;
impl.FuncPtr = fp;
Desc = desc;
}
};
// 执行block内部代码块函数
static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
int age = __cself->age; // 取出内部捕获的age
int *height = __cself->height; // 取出内部捕获的height
// 打印age
NSLog((NSString *)&__NSConstantStringImpl__var_folders_zz_1js09xx15fz7j7qyrylx4xm80000gn_T_main_848c45_mi_0, age);
// 打印height
NSLog((NSString *)&__NSConstantStringImpl__var_folders_zz_1js09xx15fz7j7qyrylx4xm80000gn_T_main_848c45_mi_1, (*height));
// 打印weight
NSLog((NSString *)&__NSConstantStringImpl__var_folders_zz_1js09xx15fz7j7qyrylx4xm80000gn_T_main_848c45_mi_2, weight);
}
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)};
int main(int argc, const char * argv[]) {
/* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool;
int age = 10;
static int height = 178;
void(*block)(void) = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, age, &height));
((void (*)(__block_impl *))((__block_impl *)block)->FuncPtr)((__block_impl *)block);
}
return 0;
}
默认定义一个局部变量都为auto自动变量(离开大括号作用域自动销毁)，block内部的代码块不确定什么时候执行，固然不能通过指针捕获。static修饰变量在数据段，全局有效，因此可以通过内存地址捕获。
全局变量本身在整个程序全局都可以访问，生命周期整个程序。所以无需捕获，在block执行时直接访问即可。
Block的类型
block有3中类型，可以通过调用class方法或者isa指针查看具体类型，最终都是继承自NSBlock类型
为了更好的研究block，先将Xcode的Automatic Reference Counting关闭。
没有访问auto变量为__NSGlobalBlock__类型
访问了auto变量为__NSStackBlock__ 类型
对__NSStackBlock__ 类型block进行copy操作，会将栈中空间的block拷贝到堆中引用计数器+1且类型__NSMallocBlock__。注意必须是对__NSStackBlock__ 类型block进行copy操作才会拷贝到堆中。
#import <Foundation/Foundation.h>
int main(int argc, const char * argv[]) {
@autoreleasepool {
int age = 12;
void(^block)(void) = [^{
NSLog(@"%d", age);
} copy];
// 释放block
[block release];
}
return 0;
}
block的copy
对象类型的auto变量
#import <Foundation/Foundation.h>
#import "CHPerson.h"
typedef void(^CHBlock)(void);
int main(int argc, const char * argv[]) {
@autoreleasepool {
CHPerson *person = [[CHPerson alloc] init];
// 在ARC下 block被q强引用指针引用被拷贝的堆中
CHBlock block = ^{
NSLog(@"----%@", person);
};
block();
NSLog(@"---------------------");
}
return 0;
}
clang编译器生成对应的c++代码
#pragma clang assume_nonnull end
typedef void(*CHBlock)(void);
struct __main_block_impl_0 {
struct __block_impl impl;
struct __main_block_desc_0* Desc;
CHPerson *person; // block内部捕获CHPerson
__main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, CHPerson *_person, int flags=0) : person(_person) {
impl.isa = &_NSConcreteStackBlock;
impl.Flags = flags;
impl.FuncPtr = fp;
Desc = desc;
}
};
// block代码块函数
static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
CHPerson *person = __cself->person; // bound by copy
NSLog((NSString *)&__NSConstantStringImpl__var_folders_zz_1js09xx15fz7j7qyrylx4xm80000gn_T_main_b0818f_mi_0, person);
}
// __NSStackBlock__类型block赋值到堆中执行的copy操作
static void __main_block_copy_0(struct __main_block_impl_0*dst, struct __main_block_impl_0*src) {
_Block_object_assign((void*)&dst->person, (void*)src->person, 3/*BLOCK_FIELD_IS_OBJECT*/);
}
// block销毁会对内部引用的对象类型进行release
static void __main_block_dispose_0(struct __main_block_impl_0*src) {
_Block_object_dispose((void*)src->person, 3/*BLOCK_FIELD_IS_OBJECT*/);
}
static struct __main_block_desc_0 {
size_t reserved;
size_t Block_size;
// __NSStackBlock__类型block赋值到堆中执行的copy操作 函数指针
void (*copy)(struct __main_block_impl_0*, struct __main_block_impl_0*);
// block销毁会对内部引用的对象类型进行release 函数指针
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, const char * argv[]) {
/* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool;
// 创建Person对象
CHPerson *person = ((CHPerson *(*)(id, SEL))(void *)objc_msgSend)((id)((CHPerson *(*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("CHPerson"), sel_registerName("alloc")), sel_registerName("init"));
CHBlock block = &__main_block_impl_0(__main_block_func_0, &__main_block_desc_0_DATA, person, 570425344));
// 执行block
block->FuncPtr(block);
NSLog((NSString *)&__NSConstantStringImpl__var_folders_zz_1js09xx15fz7j7qyrylx4xm80000gn_T_main_b0818f_mi_1);
}
return 0;
}
__bock修饰符
默认block内部无法修改auto变量的值
#import <Foundation/Foundation.h>
#import "CHPerson.h"
typedef void(^CHBlock)(void);
int height = 170;
int main(int argc, const char * argv[]) {
@autoreleasepool {
// __block修饰的auto变量
int age = 10;
static int weight = 120;
CHPerson *person = [[CHPerson alloc] init];
person.age = 12;
NSMutableArray *array = [NSMutableArray array];
CHBlock block = ^{
[array addObject:@"123"];
[array addObject:@"jake"];
person.age = 22;
// 下列写法报错
// age = 20;
// person = nil;
// weight = 125;
// array = nil;
};
block();
NSLog(@"-------------");
}
return 0;
}
auto变量在block内部捕获只是值传递，内部无法修改auto变量的值。而全局变量与static修饰的变量是可以修改的，那是因为他们作用域是整个程序并且static修饰变量在block捕获的是变量地址，因此可以block内部修改。
虽然可以通过定义static或者全局变量来实现在block内部修改变量，但是全局变量和static会修改变量的作用域，因此开发中一般使用__block。__block不能修饰全局变量、静态变量（static）
#import <Foundation/Foundation.h>
#import "CHPerson.h"
typedef void(^CHBlock)(void);
int main(int argc, const char * argv[]) {
@autoreleasepool {
// __block修饰的auto变量
__block int age = 10;
__block CHPerson *person = [[CHPerson alloc] init];
__block NSMutableArray *array = [NSMutableArray array];
CHBlock block = ^{
// 下列写法报错
age = 20;
person = nil;
array = nil;
};
block();
NSLog(@"-------------");
}
return 0;
}
clang编译器生成对应的c++代码
#pragma clang assume_nonnull end
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);
}
typedef void(*CHBlock)(void);
// 编译器将__block int age 变量包装成一个对象
struct __Block_byref_age_0 {
void *__isa;
__Block_byref_age_0 *__forwarding;
int __flags;
int __size;
int age;
};
// 编译器将__block CHPerson *person 变量包装成一个对象
struct __Block_byref_person_1 {
void *__isa;
__Block_byref_person_1 *__forwarding; // 指向自己
int __flags;
int __size;
void (*__Block_byref_id_object_copy)(void*, void*);
void (*__Block_byref_id_object_dispose)(void*);
CHPerson *person;
};
// 编译器将__block NSMutableArray *array 变量包装成一个对象
struct __Block_byref_array_2 {
void *__isa;
__Block_byref_array_2 *__forwarding;
int __flags;
int __size;
void (*__Block_byref_id_object_copy)(void*, void*);
void (*__Block_byref_id_object_dispose)(void*);
NSMutableArray *array;
};
struct __main_block_impl_0 {
struct __block_impl impl;
struct __main_block_desc_0* Desc;
// 内部指向__block变量包装的对象
__Block_byref_age_0 *age; // by ref
__Block_byref_person_1 *person; // by ref
__Block_byref_array_2 *array; // by ref
__main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, __Block_byref_age_0 *_age, __Block_byref_person_1 *_person, __Block_byref_array_2 *_array, int flags=0) : age(_age->__forwarding), person(_person->__forwarding), array(_array->__forwarding) {
impl.isa = &_NSConcreteStackBlock;
impl.Flags = flags;
impl.FuncPtr = fp;
Desc = desc;
}
};
static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
__Block_byref_age_0 *age = __cself->age; // bound by ref
__Block_byref_person_1 *person = __cself->person; // bound by ref
__Block_byref_array_2 *array = __cself->array; // bound by ref
(age->__forwarding->age) = 20;
(person->__forwarding->person) = __null;
(array->__forwarding->array) = __null;
}
static void __main_block_copy_0(struct __main_block_impl_0*dst, struct __main_block_impl_0*src) {
_Block_object_assign(&dst->age, (void*)src->age, 8/*BLOCK_FIELD_IS_BYREF*/);
_Block_object_assign(&dst->person, (void*)src->person, 8/*BLOCK_FIELD_IS_BYREF*/);
_Block_object_assign((void*)&dst->array, (void*)src->array, 8/*BLOCK_FIELD_IS_BYREF*/);}
static void __main_block_dispose_0(struct __main_block_impl_0*src) {
_Block_object_dispose((void*)src->age, 8/*BLOCK_FIELD_IS_BYREF*/);
_Block_object_dispose((void*)src->person, 8/*BLOCK_FIELD_IS_BYREF*/);
_Block_object_dispose((void*)src->array, 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, const char * argv[]) {
/* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool;
// 编译器将__block int age 变量包装成一个对象
__Block_byref_age_0 age = {
0,
(__Block_byref_age_0 *)&age,
0,
sizeof(__Block_byref_age_0),
10 // age
};
// 编译器将__block CHPerson *person 变量包装成一个对象
__Block_byref_person_1 person = {
0,
(__Block_byref_person_1 *)&person,
33554432,
sizeof(__Block_byref_person_1),
__Block_byref_id_object_copy_131,
__Block_byref_id_object_dispose_131,
((CHPerson *(*)(id, SEL))(void *)objc_msgSend)((id)((CHPerson *(*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("CHPerson"), sel_registerName("alloc")), sel_registerName("init"))
};
// 编译器将__block NSMutableArray *array 变量包装成一个对象
__Block_byref_array_2 array = {
0,
(__Block_byref_array_2 *)&array,
33554432,
sizeof(__Block_byref_array_2),
__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"))
};
CHBlock block = &__main_block_impl_0(
__main_block_func_0,
&__main_block_desc_0_DATA,
(__Block_byref_age_0 *)&age,
(__Block_byref_person_1 *)&person,
(__Block_byref_array_2 *)&array,
570425344
));
block->FuncPtr(block);
NSLog((NSString *)&__NSConstantStringImpl__var_folders_zz_1js09xx15fz7j7qyrylx4xm80000gn_T_main_b553f0_mi_0);
}
return 0;
}
__block之所以可以修改外部的auto变量，是因为编译器将__block变量包装成一个对象。