為了能夠保證block正常訪(fǎng)問(wèn)外部的變量愉阎，block有個(gè)變量捕獲機(jī)制择克，如下圖
捕獲的變量
auto:自動(dòng)變量宾毒，平時(shí)我們定義int age = 10,前面有個(gè)auto幻捏，auto int age = 10盆犁，系統(tǒng)幫我們默認(rèn)的加上了一個(gè)auto。-----值傳遞-----
static:靜態(tài)變量 -----指針傳遞-----
全局變量 -----直接訪(fǎng)問(wèn)-----
根據(jù)上面結(jié)論篡九，我們一個(gè)個(gè)展開(kāi)討論和分析谐岁。
一、自動(dòng)變量auto修飾的變量，auto int age = 10伊佃；
int main(int argc, const char * argv[]) {
@autoreleasepool {
// insert code here...
auto int age = 10;
void (^block)(void) = ^() {
NSLog(@"age = %d,",age);
};
age = 20;
block();
}
return 0;
}
此main函數(shù)中的block窜司，是我們常見(jiàn)的OC代碼，那我們從聲明 int age = 10;到將age的值改變age = 20,最后block輸出的age會(huì)等于20嗎
帶著這個(gè)疑問(wèn)航揉，我們來(lái)分析一下底層代碼的實(shí)現(xiàn)塞祈，OC-->C++的轉(zhuǎn)換過(guò)程：cd到程序main.m的目錄下，執(zhí)行命令xcrun -sdk iphoneos clang -arch arm64 -rewrite-objc main.m帅涂，可以得到一個(gè)main.cpp文件织咧，拖到工程目錄中。
struct __main_block_impl_0 {
struct __block_impl impl;
struct __main_block_desc_0* Desc;
int age;
__main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, int _age, int flags=0) : age(_age) {
impl.isa = &_NSConcreteStackBlock;
impl.Flags = flags;
impl.FuncPtr = fp;
Desc = desc;
}
};
static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
int age = __cself->age; // bound by copy
NSLog((NSString *)&__NSConstantStringImpl__var_folders_5l_0xn052bn6dgb9z7pfk8bbg740000gn_T_main_6a0dd1_mi_0,age);
}
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;
void (*block)(void) = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, age));
age = 20;
((void (*)(__block_impl *))((__block_impl *)block)->FuncPtr)((__block_impl *)block);
}
return 0;
}
這堆代碼漠秋，大家都很熟悉了吧笙蒙，從上一章節(jié)講block-底層數(shù)據(jù)結(jié)構(gòu)就分析過(guò)的，看看定義了int age = 10 的區(qū)別
auto變量的源碼.png
從上圖中的源碼中庆锦，我們可以留意到外部聲明的
int age = 10在block里使用做的事情為了能夠保證block正常訪(fǎng)問(wèn)外部的變量捅位，block有個(gè)變量捕獲機(jī)制，那什么類(lèi)型會(huì)捕獲到block內(nèi)部搂抒，什么類(lèi)型不會(huì)捕獲呢艇搀，我們來(lái)探討一下
- 1.在main函數(shù)中，在block定義時(shí)求晶，block將外部的age當(dāng)做參數(shù)傳給了
__main_block_impl_0 - 2.
__main_block_impl_0已經(jīng)將外面聲明age捕捉賦值到block內(nèi)部焰雕，既block內(nèi)部也定義了一個(gè)int age；類(lèi)型 - 3.block把main函數(shù)定義的
int age = 10的值當(dāng)做參數(shù)賦值給了block里面的age - 4.此時(shí)改變age的值芳杏，只是改變main函數(shù)里的age的值矩屁，無(wú)法改變block內(nèi)部的age的值，兩個(gè)是屬于不同函數(shù)體內(nèi)的age爵赵，互不相關(guān)吝秕。
- 5.
block()調(diào)用時(shí)，輸出的age = __cselef->age,就是等于block內(nèi)部（__main_block_impl_0）的age值
auto變量總結(jié)空幻，由上面的分析烁峭，我們可驗(yàn)證自動(dòng)變量auto修飾的變量，是有捕捉到block內(nèi)部秕铛，并且是屬于值傳遞约郁。
二、靜態(tài)變量static修飾的變量但两，static int age = 10鬓梅；
int main(int argc, const char * argv[]) {
@autoreleasepool {
// insert code here...
static int age = 10;
void (^block)(void) = ^() {
NSLog(@"age = %d,",age);
};
age = 20;
block();
}
return 0;
}
// 控制臺(tái)的輸出
2018-06-13 15:30:13.674073+0800 block-變量的捕獲[92711:10064124] age = 20,
Program ended with exit code: 0
static修飾的變量age，打印的時(shí)候age = 20镜遣，為什么用static就能修改了呢己肮？我們來(lái)看一下下面的轉(zhuǎn)換成C++后的代碼士袄，分析一下底層的實(shí)現(xiàn)悲关。
struct __main_block_impl_0 {
struct __block_impl impl;
struct __main_block_desc_0* Desc;
int *age;
__main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, int *_age, int flags=0) : age(_age) {
impl.isa = &_NSConcreteStackBlock;
impl.Flags = flags;
impl.FuncPtr = fp;
Desc = desc;
}
};
static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
int *age = __cself->age; // bound by copy
NSLog((NSString *)&__NSConstantStringImpl__var_folders_5l_0xn052bn6dgb9z7pfk8bbg740000gn_T_main_73b8c0_mi_0,(*age));
}
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;
static int age = 10;
void (*block)(void) = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, &age));
age = 20;
((void (*)(__block_impl *))((__block_impl *)block)->FuncPtr)((__block_impl *)block);
}
return 0;
}
static變量的源碼.png
static變量總結(jié)谎僻，由上圖的分析，我們可以得知寓辱，static 修飾的age艘绍，是會(huì)捕獲進(jìn)block內(nèi)部，并且捕獲的是age的地址秫筏，所以外面age變成20的時(shí)候诱鞠，block內(nèi)部輸出的age也是等于20，因?yàn)橥獠亢蚥lock內(nèi)部的age指向的是同一地址这敬。
三航夺、全局變量
int age = 10;
int main(int argc, const char * argv[]) {
@autoreleasepool {
// insert code here...
void (^block)(void) = ^() {
NSLog(@"age = %d,",age);
};
age = 20;
block();
}
return 0;
}
// 控制臺(tái)的打印
2018-06-13 16:08:54.737079+0800 block-變量的捕獲[96358:10119188] age = 20,
Program ended with exit code: 0
當(dāng)age時(shí)全局變量的時(shí)候，打印的結(jié)果為20崔涂，按常理來(lái)分析阳掐，全部變量的作用域是默認(rèn)的情況下是所有的函數(shù)，生命周期是程序結(jié)束時(shí)冷蚂，所以block里打印等于20缭保，應(yīng)該大家都能理解，那我們來(lái)看一下block內(nèi)部有沒(méi)有捕獲到age呢蝙茶，分析一下源碼艺骂，如下
int age = 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_5l_0xn052bn6dgb9z7pfk8bbg740000gn_T_main_164d03_mi_0,age);
}
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;
void (*block)(void) = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA));
age = 20;
((void (*)(__block_impl *))((__block_impl *)block)->FuncPtr)((__block_impl *)block);
}
return 0;
}
