Block的本質
Block本質是個OC對象,它內部也有個isa指針,是封裝了函數(shù)調用以及函數(shù)調用環(huán)境的OC對象.
int age = 10;
void (^block)(int,int) = ^(int a,int b){
NSLog(@"this is a block ----%d",age);
};
block(10,10);
終端運行以下命令
xcrun -sdk iphoneos clang -arch arm64 -rewrite-objc main.m
struct __block_impl {
void *isa;
int Flags;
int Reserved;
void *FuncPtr;
};
struct __main_block_impl_0 {
struct __block_impl impl;
struct __main_block_desc_0* Desc;
int age;
//構造函數(shù) 類似于oc的init 返回結構體對象
__main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, int _age, int flags=0) : age(_age) {
impl.isa = &_NSConcreteStackBlock;//block類型
impl.Flags = flags;
impl.FuncPtr = fp;//__main_block_func_0
Desc = desc;
}
};
static void __main_block_func_0(struct __main_block_impl_0 *__cself, int a, int b) {
int age = __cself->age; // bound by copy
NSLog((NSString *)&__NSConstantStringImpl__var_folders_bc_vdlk454n49l_cspn85mp8dc40000gn_T_main_865327_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;
//定義block變量
void (*block)(int,int) = ((void (*)(int, int))&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, age));
//執(zhí)行block代碼 block->FuncPtr(block)
((void (*)(__block_impl *, int, int))((__block_impl *)block)->FuncPtr)((__block_impl *)block, 10, 10);
}
return 0;
}
block其實就是一個__main_block_impl_0的結構體,其內部有個同名的構造函數(shù),其傳入的參數(shù)一個是該block實現(xiàn)的函數(shù)地址,一個是該block的信息.block的調用其實質就是通過結構體的指針拿到函數(shù)的地址并將自身作為參數(shù)傳入調用.
變量捕獲
auto int age = 10; //離開作用域就銷毀 值傳遞
static int age2 = 30;
void (^block)(int,int) = ^(int a,int b){
NSLog(@"this is a block ----%d ----%d",a,b);
NSLog(@"age is %d---- age2 is %d",age,age2);
};
age = 20;
age2 = 40;
block(10,10);
輸出 age = 10 age2 = 40;
struct __main_block_impl_0 {
struct __block_impl impl;
struct __main_block_desc_0* Desc;
int age;
int *age2;
__main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, int _age, int *_age2, int flags=0) : age(_age), age2(_age2) {
impl.isa = &_NSConcreteStackBlock;
impl.Flags = flags;
impl.FuncPtr = fp;
Desc = desc;
}
};
static void __main_block_func_0(struct __main_block_impl_0 *__cself, int a, int b) {
int age = __cself->age; // bound by copy
NSLog((NSString *)&__NSConstantStringImpl__var_folders_bc_vdlk454n49l_cspn85mp8dc40000gn_T_main_2b925f_mi_0,a,b);
NSLog((NSString *)&__NSConstantStringImpl__var_folders_bc_vdlk454n49l_cspn85mp8dc40000gn_T_main_2b925f_mi_1,age);
}
auto int age = 10;
static int age2 = 30;
void (*block)(int,int) = ((void (*)(int, int))&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, age, &age2));
age = 20;
age2 = 40;
}
因為自動變量離開作用域就銷毀的原因,block需要記錄的是這個值,而不是地址.因為這塊地址這時候可能另作他用.而靜態(tài)局部變量不同一直存在內存.
接下來看看全局變量
int age_ = 10;
static int height_ = 10;
int main(int argc, const char * argv[]) {
@autoreleasepool {
void (^block)(void) = ^{
NSLog(@"age is %d---- height_ is %d",age_,height_);
};
age_ = 20;
height_ = 20;
block();
}
return 0;
}
40.807363+0800 Block[34211:1225423] age is 20---- height_ is 20
查看Cpp文件
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_bc_vdlk454n49l_cspn85mp8dc40000gn_T_main_f716de_mi_0,age_,height_);
}
可以發(fā)現(xiàn)block并沒有捕獲進去,直接就是傳入的全局變量.
image.png
Block類型
先來看看block的class
void (^block)(void) = ^{
NSLog(@"age is %d---- height_ is %d",age_,height_);
};
age_ = 20;
height_ = 20;
block();
NSLog(@"%@",[block class]);
NSLog(@"%@",[[block class] superclass]);
NSLog(@"%@",[[[block class] superclass]superclass]);
NSLog(@"%@",[[[[block class]
superclass]superclass]superclass]);
Block[34708:1251319] __NSGlobalBlock__
Block[34708:1251319] __NSGlobalBlock
Block[34708:1251319] NSBlock
Block[34708:1251319] NSObject
可以發(fā)現(xiàn)其是繼承與NSBlock,而NSBlock又是繼承于 NSObject的.
void (^block)(void) = ^{
NSLog(@"hello");
};
int age = 10;
void (^block2)(void) = ^{
NSLog(@"hello--age %d",age);
};
NSLog(@"\n%@ \n %@ \n %@",[block class],[block2 class],[^{
NSLog(@"hell3-age%d",age);
} class]);
輸出:
__NSGlobalBlock__
__NSMallocBlock__
__ NSStackBlock__
image.png
- NSGlobalBlock 沒有訪問auto變量
- NSMallocBlock (ARC 訪問auto變量 棧上自動copy到堆)
- NSStackBlock (非ARC 訪問auto 變量 棧上會自動銷毀)
xcrun -sdk iphoneos clang -arch arm64 -rewrite-objc -fobjc-arc -fobjc-runtime=ios-8.0.0 main.m
__block
再次編譯,__block int age 被轉成如下結構:
__attribute__((__blocks__(byref))) __Block_byref_age_0 age = {
(void*)0,
(__Block_byref_age_0 *)&age,
0,
sizeof(__Block_byref_age_0),
10};
再來看__Block_byref_age_0
struct __Block_byref_age_0 {
void *__isa;
__Block_byref_age_0 *__forwarding;
int __flags;
int __size;
int age;
};
這里可以看出__block 修飾的變量被包裝成一個對象,__forwarding指向的就是自身所在的地址.
在看block的結構:
TestBlock block = ((void (*)())&__main_block_impl_0((void*)__main_block_func_0,
&__main_block_desc_0_DATA,
(__Block_byref_age_0 *)&age,
570425344));
block內部有個指針指向__Block_byref_age_0結構體,這個結構體內部又有個成員存儲age這個變量的值.
static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
__Block_byref_age_0 *age = __cself->age; // bound by ref
(age->__forwarding->age) = 20;
}
可以看出block內部是通過拿到__Block_byref_age_0在拿到__forwarding,因為這個是指向自己的,再拿到age.
