上一篇文章中咱们得知了OC的block其实就是C的函数指针,这一篇文章咱们来研究下block与局部变量的各类化学反应。函数
咱们主要截获三种变量,分别是全局变量、静态全局变量、布局变量、静态局部变量,代码以下:布局
/** 全局变量 */
int global_count = 10;
/** 静态全局变量 */
static int static_global_count = 10;
int main(int argc, const char * argv[]) {
@autoreleasepool {
// 声明block
void (^block)(void);
// 局部变量
int count = 10;
// 静态局部变量
static int static_count = 10;
// block的实现
block = ^{
global_count = 11;
static_global_count = 11;
NSLog(@"count = %@", @(count));
static_count = 11;
};
// block的调用
block();
// 从新赋值
global_count = 12;
static_global_count = 12;
count = 12;
static_count = 12;
}
return 0;
}
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上面代码,咱们向block传了一个参数count,如今咱们clang一下,获得下面的C/C++代码:ui
// __block_imp结构体
struct __block_impl {
void *isa;
int Flags;
int Reserved;
void *FuncPtr;
};
int global_count = 10;
static int static_global_count = 10;
// __main_block_impl_0结构体
struct __main_block_impl_0 {
struct __block_impl impl;
struct __main_block_desc_0* Desc;
int count;
int *static_count;
__main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, int _count, int *_static_count, int flags=0) : count(_count), static_count(_static_count) {
impl.isa = &_NSConcreteStackBlock;
impl.Flags = flags;
impl.FuncPtr = fp;
Desc = desc;
}
};
// __main_block_func_0函数
static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
int count = __cself->count; // bound by copy
int *static_count = __cself->static_count; // bound by copy
global_count = 11;
static_global_count = 11;
NSLog((NSString *)&__NSConstantStringImpl__var_folders_0r_hkkmpct143n4wd3xxk0l1j8c0000gn_T_main_23de66_mi_0, ((NSNumber *(*)(Class, SEL, int))(void *)objc_msgSend)(objc_getClass("NSNumber"), sel_registerName("numberWithInt:"), (int)(count)));
(*static_count) = 11;
}
// __main_block_desc_0结构体
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)};
// main函数
int main(int argc, const char * argv[]) {
/* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool;
void (*block)(void);
int count = 10;
static int static_count = 10;
block = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, count, &static_count));
((void (*)(__block_impl *))((__block_impl *)block)->FuncPtr)((__block_impl *)block);
global_count = 12;
static_global_count = 12;
count = 12;
static_count = 12;
}
return 0;
}
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咱们重点看一下__main_block_impl_0
这个结构体:spa
// __main_block_impl_0结构体
struct __main_block_impl_0 {
struct __block_impl impl;
struct __main_block_desc_0* Desc;
int count;
int *static_count;
/** 这里须要说明的是:冒号语法后面的内容至关于为参数成员count初始化 int count = _count; */
__main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, int _count, int *_static_count, int flags=0) : count(_count), static_count(_static_count) {
impl.isa = &_NSConcreteStackBlock;
impl.Flags = flags;
impl.FuncPtr = fp;
Desc = desc;
}
};
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咱们能够看到__main_block_impl_0
结构体多了两个成员变量count
和static_count
。 在声明block时,count
的值就已经被保存到了__main_block_impl_0
结构体内部,注意这里是值传递,也就是说在调用前对count
从新赋值,并不会影响到__main_block_impl_0
结构体内部的count
成员变量的值。 一样的,在声明block是,静态变量static_count
的指针被保存到了__main_block_impl_0
结构体内部,由于是引用传递,因此block内部对static_count
的修改会直接影响到static_count
的值。 全局变量global_count
和静态全局变量static_global_count
由于做用域是全局的,因此block内部的修改会直接影响到它们的值。3d
咱们都知道block是没法修改外部的局部变量的值的,可是__block
却可让咱们修改外部的局部变量的值,接下来咱们探索一下缘由。 首先,上OC代码:指针
// 声明block
void (^block)(void);
__block int count = 10;
// block的实现
block = ^{
count = 11;
};
// block的调用
block();
// 给count从新赋值
count = 12;
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咱们clang一下,获得下面代码:code
// __block_impl结构体
struct __block_impl {
void *isa;
int Flags;
int Reserved;
void *FuncPtr;
};
// __Block_byref_count_0结构体
struct __Block_byref_count_0 {
void *__isa;
__Block_byref_count_0 *__forwarding;
int __flags;
int __size;
int count;
};
// __main_block_impl_0结构体
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;
}
};
// __main_block_func_0函数
static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
__Block_byref_count_0 *count = __cself->count; // bound by ref
(count->__forwarding->count) = 11;
}
// __main_block_copy_0函数
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*/);}
// __main_block_dispose_0函数
static void __main_block_dispose_0(struct __main_block_impl_0*src) {_Block_object_dispose((void*)src->count, 8/*BLOCK_FIELD_IS_BYREF*/);}
// __main_block_desc_0结构体
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;
void (*block)(void);
__attribute__((__blocks__(byref))) __Block_byref_count_0 count = {(void*)0,(__Block_byref_count_0 *)&count, 0, sizeof(__Block_byref_count_0), 10};
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);
(count.__forwarding->count) = 12;
}
return 0;
}
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咱们发现,__main_block_desc_0
结构体多了成员变量copy
和dispose
,另外还多了几个函数和结构体,分别是__Block_byref_count_0
结构体、__main_block_copy_0
函数和__main_block_dispose_0
函数。对象
__Block_byref_count_0
结构体咱们首先看一下__Block_byref_count_0
这个结构体,代码以下:作用域
// __Block_byref_count_0结构体
struct __Block_byref_count_0 {
void *__isa;
__Block_byref_count_0 *__forwarding;
int __flags;
int __size;
int count
};
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这个结构体是__block变量的包装,至关于OC的对象,咱们看一下main()
函数中该结构体的初始化函数:get
__attribute__((__blocks__(byref))) __Block_byref_count_0 count = {(void*)0,(__Block_byref_count_0 *)&count, 0, sizeof(__Block_byref_count_0), 10};
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咱们能够看到结构体中最后的成员变量至关于原局部变量,从成员变量名和初始化函数也能够看出。 咱们来具体看一下__block变量的赋值:
// block的实现
block = ^{
count = 11;
};
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这段代码的转换以下:
// __main_block_func_0函数
static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
__Block_byref_count_0 *count = __cself->count; // bound by ref
(count->__forwarding->count) = 11;
}
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__main_block_impl_0
结构体实例持有指向__block变量的__Block_byref_count_0
结构体实例的指针。 __Block_byref_count_0
结构体实例的成员变量__forwarding
持有指向该实例自身的指针。经过成员变量__forwarding
访问成员变量count
。成员变量count
是该实例自身持有的变量,它至关于局部变量。
咱们总结一下,block捕获变量分如下几种状况: