1. 不同类型变量捕获
主要有全局变量、全局静态变量、局部变量、局部静态变量,代码如下:
// 全局变量
int global_count = 10;
// 静态全局变量
static int static_globbal_count = 10;
int main(int argc, const char * argv[]) {
@autoreleasepool {
// 局部变量
int count = 10;
// 静态局部变量
static int static_count = 10;
void (^block)(void);
// 访问全局变量,并修改
block = ^{
NSLog(@"global_count:%d 地址:%p", global_count, &global_count);
global_count = 11;
NSLog(@"global_count:%d 地址:%p", global_count, &global_count);
};
block();
NSLog(@"global_count:%d 地址:%p", global_count, &global_count);
global_count = 12;
NSLog(@"global_count:%d 地址:%p", global_count, &global_count);
// 访问全局静态变量,并修改
block = ^{
NSLog(@"static_globbal_count:%d 地址:%p", static_globbal_count, &static_globbal_count);
static_globbal_count = 11;
NSLog(@"static_globbal_count:%d 地址:%p", static_globbal_count, &static_globbal_count);
};
block();
NSLog(@"static_globbal_count:%d 地址:%p", static_globbal_count, &static_globbal_count);
static_globbal_count = 12;
NSLog(@"static_globbal_count:%d 地址:%p", static_globbal_count, &static_globbal_count);
// 访问局部变量,并修改
NSLog(@"count:%d 地址:%p", count, &count);
block = ^{
NSLog(@"count:%d 地址:%p", count, &count);
// count = 11;
// NSLog(@"count:%d 地址:%p", count, &count);
};
block();
NSLog(@"count:%d 地址:%p", count, &count);
count = 12;
NSLog(@"count:%d 地址:%p", count, &count);
// 访问静态局部变量,并修改
block = ^{
NSLog(@"static_count:%d 地址:%p", static_count, &static_count);
static_count = 11;
NSLog(@"static_count:%d 地址:%p", static_count, &static_count);
};
block();
NSLog(@"static_count:%d 地址:%p", static_count, &static_count);
static_count = 12;
NSLog(@"static_count:%d 地址:%p", static_count, &static_count);
}
return 0;
}
执行以上代码,获取打印数据:
global_count:10 地址:0x100002030
global_count:11 地址:0x100002030
global_count:11 地址:0x100002030
global_count:12 地址:0x100002030
static_globbal_count:10 地址:0x100002038
static_globbal_count:11 地址:0x100002038
static_globbal_count:11 地址:0x100002038
static_globbal_count:12 地址:0x100002038
count:10 地址:0x7ffeefbff57c
count:10 地址:0x100703e50
count:10 地址:0x7ffeefbff57c
count:12 地址:0x7ffeefbff57c
static_count:10 地址:0x100002034
static_count:11 地址:0x100002034
static_count:11 地址:0x100002034
static_count:12 地址:0x100002034
从打印输出,我们可以得出一个结论:
- 除了局部变量,其它的类型我们都可以在block中直接访问并修改,且存储空间不变,这说明指针引用
- 而局部变量我们可以在block中获取,但是并不能修改(编译器会直接报错),并且在block访问局部变量后,其内存地址发生了改变,这代表着在访问外部变量的时候并不是指针引用,而是值引用
2. 底层分析
通过clang -rewrite-objc main.m生成main.cpp文件,从中获取到:
int global_count = 10;
static int static_globbal_count = 10;
struct __main_block_impl_0 {
struct __block_impl impl;
struct __main_block_desc_0* Desc;
__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;
}
};
static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
NSLog((NSString *)&__NSConstantStringImpl__var_folders_7x_02rxw4ms63z1hyly7yn3nj040000gn_T_main_bd7e26_mi_0, global_count, &global_count);
global_count = 11;
NSLog((NSString *)&__NSConstantStringImpl__var_folders_7x_02rxw4ms63z1hyly7yn3nj040000gn_T_main_bd7e26_mi_1, global_count, &global_count);
}
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)};
struct __main_block_impl_1 {
struct __block_impl impl;
struct __main_block_desc_1* Desc;
__main_block_impl_1(void *fp, struct __main_block_desc_1 *desc, int flags=0) {
impl.isa = &_NSConcreteStackBlock;
impl.Flags = flags;
impl.FuncPtr = fp;
Desc = desc;
}
};
static void __main_block_func_1(struct __main_block_impl_1 *__cself) {
NSLog((NSString *)&__NSConstantStringImpl__var_folders_7x_02rxw4ms63z1hyly7yn3nj040000gn_T_main_bd7e26_mi_4, static_globbal_count, &static_globbal_count);
static_globbal_count = 11;
NSLog((NSString *)&__NSConstantStringImpl__var_folders_7x_02rxw4ms63z1hyly7yn3nj040000gn_T_main_bd7e26_mi_5, static_globbal_count, &static_globbal_count);
}
static struct __main_block_desc_1 {
size_t reserved;
size_t Block_size;
} __main_block_desc_1_DATA = { 0, sizeof(struct __main_block_impl_1)};
struct __main_block_impl_2 {
struct __block_impl impl;
struct __main_block_desc_2* Desc;
int count;
__main_block_impl_2(void *fp, struct __main_block_desc_2 *desc, int _count, int flags=0) : count(_count) {
impl.isa = &_NSConcreteStackBlock;
impl.Flags = flags;
impl.FuncPtr = fp;
Desc = desc;
}
};
static void __main_block_func_2(struct __main_block_impl_2 *__cself) {
int count = __cself->count; // bound by copy
NSLog((NSString *)&__NSConstantStringImpl__var_folders_7x_02rxw4ms63z1hyly7yn3nj040000gn_T_main_bd7e26_mi_8, count, &count);
NSLog((NSString *)&__NSConstantStringImpl__var_folders_7x_02rxw4ms63z1hyly7yn3nj040000gn_T_main_bd7e26_mi_9, count, &count);
}
static struct __main_block_desc_2 {
size_t reserved;
size_t Block_size;
} __main_block_desc_2_DATA = { 0, sizeof(struct __main_block_impl_2)};
struct __main_block_impl_3 {
struct __block_impl impl;
struct __main_block_desc_3* Desc;
int *static_count;
__main_block_impl_3(void *fp, struct __main_block_desc_3 *desc, int *_static_count, int flags=0) : static_count(_static_count) {
impl.isa = &_NSConcreteStackBlock;
impl.Flags = flags;
impl.FuncPtr = fp;
Desc = desc;
}
};
static void __main_block_func_3(struct __main_block_impl_3 *__cself) {
int *static_count = __cself->static_count; // bound by copy
NSLog((NSString *)&__NSConstantStringImpl__var_folders_7x_02rxw4ms63z1hyly7yn3nj040000gn_T_main_bd7e26_mi_12, (*static_count), &(*static_count));
(*static_count) = 11;
NSLog((NSString *)&__NSConstantStringImpl__var_folders_7x_02rxw4ms63z1hyly7yn3nj040000gn_T_main_bd7e26_mi_13, (*static_count), &(*static_count));
}
static struct __main_block_desc_3 {
size_t reserved;
size_t Block_size;
} __main_block_desc_3_DATA = { 0, sizeof(struct __main_block_impl_3)};
int main(int argc, const char * argv[]) {
/* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool;
int count = 10;
static int static_count = 10;
void (*block)(void);
block = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA));
((void (*)(__block_impl *))((__block_impl *)block)->FuncPtr)((__block_impl *)block);
NSLog((NSString *)&__NSConstantStringImpl__var_folders_7x_02rxw4ms63z1hyly7yn3nj040000gn_T_main_bd7e26_mi_2, global_count, &global_count);
global_count = 12;
NSLog((NSString *)&__NSConstantStringImpl__var_folders_7x_02rxw4ms63z1hyly7yn3nj040000gn_T_main_bd7e26_mi_3, global_count, &global_count);
block = ((void (*)())&__main_block_impl_1((void *)__main_block_func_1, &__main_block_desc_1_DATA));
((void (*)(__block_impl *))((__block_impl *)block)->FuncPtr)((__block_impl *)block);
NSLog((NSString *)&__NSConstantStringImpl__var_folders_7x_02rxw4ms63z1hyly7yn3nj040000gn_T_main_bd7e26_mi_6, static_globbal_count, &static_globbal_count);
static_globbal_count = 12;
NSLog((NSString *)&__NSConstantStringImpl__var_folders_7x_02rxw4ms63z1hyly7yn3nj040000gn_T_main_bd7e26_mi_7, static_globbal_count, &static_globbal_count);
block = ((void (*)())&__main_block_impl_2((void *)__main_block_func_2, &__main_block_desc_2_DATA, count));
((void (*)(__block_impl *))((__block_impl *)block)->FuncPtr)((__block_impl *)block);
NSLog((NSString *)&__NSConstantStringImpl__var_folders_7x_02rxw4ms63z1hyly7yn3nj040000gn_T_main_bd7e26_mi_10, count, &count);
count = 12;
NSLog((NSString *)&__NSConstantStringImpl__var_folders_7x_02rxw4ms63z1hyly7yn3nj040000gn_T_main_bd7e26_mi_11, count, &count);
block = ((void (*)())&__main_block_impl_3((void *)__main_block_func_3, &__main_block_desc_3_DATA, &static_count));
((void (*)(__block_impl *))((__block_impl *)block)->FuncPtr)((__block_impl *)block);
NSLog((NSString *)&__NSConstantStringImpl__var_folders_7x_02rxw4ms63z1hyly7yn3nj040000gn_T_main_bd7e26_mi_14, static_count, &static_count);
static_count = 12;
NSLog((NSString *)&__NSConstantStringImpl__var_folders_7x_02rxw4ms63z1hyly7yn3nj040000gn_T_main_bd7e26_mi_15, static_count, &static_count);
}
return 0;
}
通过分析:
- 在捕获全局变量的时候,__main_block_impl_0和__main_block_impl_1中没有新增成员变量,因为全局变量的作用域是全局的,所以block内部修改会直接改变他们的值
- 而捕获局部变量的时候:
- 1.如果是局部变量,会在__main_block_impl_2结构体重生成了一个int count的成员变量,这表示在声明block的时候就将count值保存到了结构体内部,这是一个值传递,同时说明了再调用前修改外部变量的值并不会影响到block内部的count值
- 2.如果是局部静态变量,在__main_block_impl_3中生成了一个int *static_count的指针,在声明block的时候就会将外部变量的指针保存到结构体中,因为是指针引用,所以在block内部队static_count的修改会直接影响到static_count的值
3. __block修饰符
int main(int argc, const char * argv[]) {
@autoreleasepool {
// 局部变量
__block int count = 10;
void (^block)(void);
// 访问局部变量,并修改
block = ^{
NSLog(@"count:%d 地址:%p", count, &count);
count = 11;
NSLog(@"count:%d 地址:%p", count, &count);
};
block();
NSLog(@"count:%d 地址:%p", count, &count);
count = 12;
NSLog(@"count:%d 地址:%p", count, &count);
}
}
通过clang生成main.cpp文件:
struct __Block_byref_count_0 {
void *__isa;
__Block_byref_count_0 *__forwarding;
int __flags;
int __size;
int count;
};
struct __main_block_impl_0 {
struct __block_impl impl;
struct __main_block_desc_0* Desc;
__Block_byref_count_0 *count; // by ref
__main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, __Block_byref_count_0 *_count, int flags=0) : count(_count->__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_count_0 *count = __cself->count; // bound by ref
NSLog((NSString *)&__NSConstantStringImpl__var_folders_7x_02rxw4ms63z1hyly7yn3nj040000gn_T_main_05e572_mi_0, (count->__forwarding->count), &(count->__forwarding->count));
(count->__forwarding->count) = 11;
NSLog((NSString *)&__NSConstantStringImpl__var_folders_7x_02rxw4ms63z1hyly7yn3nj040000gn_T_main_05e572_mi_1, (count->__forwarding->count), &(count->__forwarding->count));
}
static void __main_block_copy_0(struct __main_block_impl_0*dst, struct __main_block_impl_0*src) {_Block_object_assign((void*)&dst->count, (void*)src->count, 8/*BLOCK_FIELD_IS_BYREF*/);}
static void __main_block_dispose_0(struct __main_block_impl_0*src) {_Block_object_dispose((void*)src->count, 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;
__attribute__((__blocks__(byref))) __Block_byref_count_0 count = {(void*)0,(__Block_byref_count_0 *)&count, 0, sizeof(__Block_byref_count_0), 10};
void (*block)(void);
block = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, (__Block_byref_count_0 *)&count, 570425344));
((void (*)(__block_impl *))((__block_impl *)block)->FuncPtr)((__block_impl *)block);
NSLog((NSString *)&__NSConstantStringImpl__var_folders_7x_02rxw4ms63z1hyly7yn3nj040000gn_T_main_05e572_mi_2, (count.__forwarding->count), &(count.__forwarding->count));
(count.__forwarding->count) = 12;
NSLog((NSString *)&__NSConstantStringImpl__var_folders_7x_02rxw4ms63z1hyly7yn3nj040000gn_T_main_05e572_mi_3, (count.__forwarding->count), &(count.__forwarding->count));
}
return 0;
}
通过分析我们发现:
在__main_block_impl_0结构体中多了一个__main_block_impl_0结构体
在__main_block_desc_0结构体中多了2个成员变量copy和dispose
多了一个__main_block_copy_0函数
多了一个__main_block_dispose_0函数
多了一个结构体__Block_byref_count_0(捕获变量的封装)
__Block_byref_count_0是_block变量的封装,相当于一个OC对象,我们在main函数中去看看
__attribute__((__blocks__(byref))) __Block_byref_count_0 count = {(void*)0,(__Block_byref_count_0 *)&count, 0, sizeof(__Block_byref_count_0), 10};
// 在声明block的时候将就会生成一个__Block_byref_count_0结构体,在其中将外部变量的地址赋值个__forwarding指针,最后的10就是外部变量的值
// 而在block中修改值的时候:
(count.__forwarding->count) = 12;
// 实际的修改是通过指针来修改的,指针的访问指引是:
__main_block_impl_0 *impl -> __Block_byref_count_0 *count -> __Block_byref_count_0 *__forwarding
// 所以block内部值的修改,外部变量会直接发生改变
4. 总结
通过分析,得出结论:
- 变量为全局变量和静态变量时,block不会做额外的处理,捕获的是变量的指针,会直接拿来用
- 变量为局部变量时,只能捕获声明在block前的变量的值,在block中无法修改
- 被__block修饰的变量,在声明block的时候回被封装成一个结构体(OC对象),改结构体的最后一个成员变量为被__block修饰过的局部变量,block捕获改结构体的指针,在block内部可以修改变量的值
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