有些東西我覺得還是寫出來比較好鞍爱，你知道是你知道的，但是寫出來總結(jié)出來又是不一樣的一種收獲。
先看這斷代碼吧蠕趁！
-(void)test{
testPerson*person = [[testPerson alloc]init];
__weak testPerson * weakPerson = person;
person.mitBlock = ^{
__strong testPerson * strongPerson = weakPerson;
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
[strongPerson test];
});
};
person.mitBlock();
}
首先person是一個局部變量娜搂，所以在test方法執(zhí)行完person對象就會被銷毀迁霎。（如果沒有寫_strong來修飾的話。）
用了strong來修飾百宇，則會在gcd里面形成一個強引用指針考廉。當mitblock執(zhí)行完，gcd外面的strongPerson釋放掉携御，這時候gcd里面的由于copy出來一份在堆上昌粤，所以還會存在不會釋放掉，當gcd執(zhí)行結(jié)束啄刹，strongperson釋放掉涮坐。沒有循環(huán)引用問題。
Block的分類
下面這三個我相信看字面就知道什么意思
_NSConcreteStackBlock 棧鸵膏，_NSConcreteMallocBlock 堆膊升，_NSConcreteGlobalBlock 全局
那么什么時候是stack 什么時候是malloc 什么時候是global？
首先先來認識下c語言中的這幾種變量
自動變量
函數(shù)參數(shù)
靜態(tài)變量
靜態(tài)全局變量
全局變量
int global_i = 1;//全局變量
static int static_global_j = 2;//靜態(tài)全局變量
int main(int argc, const char * argv[]) {
static int static_k = 3;//靜態(tài)變量
int val = 4; //自動變量
void (^myBlock)(void) = ^{
global_i ++;
static_global_j ++;
static_k ++;
};
第一種stack
只用到外部局部變量谭企、成員屬性變量廓译，且沒有強指針引用的block都是StackBlock。
StackBlock的生命周期由系統(tǒng)控制的债查，一旦返回之后非区，就被系統(tǒng)銷毀了
在mrc下
__block int i = 0;
NSLog(@"%p",&i);
void (^myBlock)(void) = ^{
i ++;
NSLog(@"Block 里面的%p",&i);
};
NSLog(@"%@",myBlock);
myBlock();
第二種 malloc
有強指針引用或copy修飾的成員屬性引用的block會被復(fù)制一份到堆中成為MallocBlock，沒有強指針引用即銷毀盹廷，生命周期由程序員控制,一般在arc下征绸，不管你加不加_block ,都會是堆。
arc下
__block int i = 0;
NSLog(@"%p",&i);
void (^myBlock)(void) = ^{
i ++;
NSLog(@"Block 里面的%p",&i);
};
NSLog(@"%@",myBlock);
myBlock();
第三種 global
沒有用到外界變量或只用到全局變量、靜態(tài)變量的block為_NSConcreteGlobalBlock管怠，生命周期從創(chuàng)建到應(yīng)用程序結(jié)束淆衷。
int global_i = 1;
static int static_global_j = 2;
int main(int argc, const char * argv[]) {
static int static_k = 3;
void (^myBlock)(void) = ^{
NSLog(@"Block中 變量 = %d %d %d",static_global_j ,static_k, global_i);
};
NSLog(@"%@",myBlock)
myBlock();
return 0;
}
分析block函數(shù)
int global_i = 1；
static int static_global_j = 2;
int main(int argc, const char * argv[]) {
static int static_k = 3;
int val = 4;
void (^myBlock)(void) = ^{
global_i ++;
static_global_j ++;
static_k ++;
NSLog(@"Block中 global_i = %d,static_global_j = %d,static_k = %d,val = %d",global_i,static_global_j,static_k,val);
};
下面看一下上面這段代碼源碼渤弛。
int global_i = 1;
static int static_global_j = 2;
struct __main_block_impl_0 {
struct __block_impl impl;
struct __main_block_desc_0* Desc;
int *static_k;
int val;
__main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, int *_static_k, int _val, int flags=0) : static_k(_static_k), val(_val) {
impl.isa = &_NSConcreteStackBlock;
impl.Flags = flags;
impl.FuncPtr = fp;
Desc = desc;
}
};
static void __main_block_func_0(struct __main_block_impl_0 *__cself) { //這里是block
int *static_k = __cself->static_k; // bound by copy 這里靜態(tài)變量傳進來的是一個指針
int val = __cself->val; // bound by copy 這里自動變量傳進來的是值祝拯。 所以靜態(tài)變量可以改變值，而自動變量是不能變的她肯。
global_i ++;
static_global_j ++;
(*static_k) ++;
NSLog((NSString *)&__NSConstantStringImpl__var_folders_45_k1d9q7c52vz50wz1683_hk9r0000gn_T_main_6fe658_mi_0,global_i,static_global_j,(*static_k),val);
}
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)};
上面代碼中我已經(jīng)把原因?qū)懬宄思淹贰Ｏ旅婵聪录觃block的實現(xiàn)。
int main(int argc, const char * argv[]) {
__block int i = 0;
void (^myBlock)(void) = ^{
i ++;
NSLog(@"%d",i);
};
myBlock();
return 0;
}
源碼如下
struct __Block_byref_i_0 {
void *__isa;
__Block_byref_i_0 *__forwarding;
int __flags;
int __size;
int I;
};
struct __main_block_impl_0 {
struct __block_impl impl;
struct __main_block_desc_0* Desc;
__Block_byref_i_0 *i; // by ref
__main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, __Block_byref_i_0 *_i, int flags=0) : i(_i->__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_i_0 *i = __cself->i; // bound by ref 主要代碼是這兩行
(i->__forwarding->i) ++;
NSLog((NSString *)&__NSConstantStringImpl__var_folders_45_k1d9q7c52vz50wz1683_hk9r0000gn_T_main_3b0837_mi_0,(i->__forwarding->i));
}
static void __main_block_copy_0(struct __main_block_impl_0*dst, struct __main_block_impl_0*src) {_Block_object_assign((void*)&dst->i, (void*)src->i, 8/*BLOCK_FIELD_IS_BYREF*/);}
static void __main_block_dispose_0(struct __main_block_impl_0*src) {_Block_object_dispose((void*)src->i, 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[]) {
__attribute__((__blocks__(byref))) __Block_byref_i_0 i = {(void*)0,(__Block_byref_i_0 *)&i, 0, sizeof(__Block_byref_i_0), 0};
void (*myBlock)(void) = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, (__Block_byref_i_0 *)&i, 570425344));
((void (*)(__block_impl *))((__block_impl *)myBlock)->FuncPtr)((__block_impl *)myBlock);
return 0;
}
相信你注意到了__Block_byref_i_0這個結(jié)構(gòu)體晴氨，__Block_byref_i_0,這個結(jié)構(gòu)體有5個成員變量康嘉。第一個是isa指針，第二個是指向自身類型的__forwarding指針籽前，第三個是一個標記flag亭珍，第四個是它的大小，第五個是變量值聚假，名字和變量名同名
說下我的理解块蚌，加了_block之后，在arc下會把block copy到堆上膘格，則原來指向棧上的forwarding轉(zhuǎn)而指向堆上面的forwarding，而forwarding又會指向自身财松，(i->__forwarding->i)瘪贱。
下面盜下霜神博客里的圖。這樣理解的更深刻一些辆毡。
1194012-5f5f486bab68191f.jpg
MRC環(huán)境下菜秦，只有copy，__block才會被復(fù)制到堆上舶掖，否則球昨，__block一直都在棧上，block也只是__NSStackBlock眨攘，這個時候__forwarding指針就只指向自己了主慰。
下面看下帶對象的block
__block id block_obj = [[NSObject alloc]init];
id obj = [[NSObject alloc]init];
NSLog(@"block_obj = [%@ , %p] , obj = [%@ , %p]",block_obj , &block_obj , obj , &obj);
void (^myBlock)(void) = ^{
NSLog(@"***Block中****block_obj = [%@ , %p] , obj = [%@ , %p]",block_obj , &block_obj , obj , &obj);
};
myBlock();
struct __Block_byref_block_obj_0 {
void *__isa;
__Block_byref_block_obj_0 *__forwarding;
int __flags;
int __size;
void (*__Block_byref_id_object_copy)(void*, void*);
void (*__Block_byref_id_object_dispose)(void*);
id block_obj;
};
struct __main_block_impl_0 {
struct __block_impl impl;
struct __main_block_desc_0* Desc;
id obj;
__Block_byref_block_obj_0 *block_obj; // by ref
__main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, id _obj, __Block_byref_block_obj_0 *_block_obj, int flags=0) : obj(_obj), block_obj(_block_obj->__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_block_obj_0 *block_obj = __cself->block_obj; // bound by ref
id obj = __cself->obj; // bound by copy
NSLog((NSString *)&__NSConstantStringImpl__var_folders_45_k1d9q7c52vz50wz1683_hk9r0000gn_T_main_e64910_mi_1,(block_obj->__forwarding->block_obj) , &(block_obj->__forwarding->block_obj) , obj , &obj);
}
static void __main_block_copy_0(struct __main_block_impl_0*dst, struct __main_block_impl_0*src) {_Block_object_assign((void*)&dst->block_obj, (void*)src->block_obj, 8/*BLOCK_FIELD_IS_BYREF*/);_Block_object_assign((void*)&dst->obj, (void*)src->obj, 3/*BLOCK_FIELD_IS_OBJECT*/);}
static void __main_block_dispose_0(struct __main_block_impl_0*src) {_Block_object_dispose((void*)src->block_obj, 8/*BLOCK_FIELD_IS_BYREF*/);_Block_object_dispose((void*)src->obj, 3/*BLOCK_FIELD_IS_OBJECT*/);}
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};
這個結(jié)構(gòu)體__Block_byref_block_obj_0，跟自動變量的結(jié)構(gòu)體差不多鲫售。原理也差不多共螺。
疑惑解決，為何appending不需要加_block也可以修改值情竹。
看下面的代碼和源碼
NSMutableString * str = [[NSMutableString alloc]initWithString:@"Hello,"];
void (^myBlock)(void) = ^{
[str appendString:@"World!"];
NSLog(@"Block中 str = %@",str);
};
NSLog(@"Block外 str = %@",str);
myBlock();
struct __main_block_impl_0 {
struct __block_impl impl;
struct __main_block_desc_0* Desc;
NSMutableString *str;
__main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, NSMutableString *_str, int flags=0) : str(_str) {
impl.isa = &_NSConcreteStackBlock;
impl.Flags = flags;
impl.FuncPtr = fp;
Desc = desc;
}
};
static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
NSMutableString *str = __cself->str; // bound by copy 主要看這里藐不，block里傳進來的是指針所以可以修改變量的值。
((void (*)(id, SEL, NSString *))(void *)objc_msgSend)((id)str, sel_registerName("appendString:"), (NSString *)&__NSConstantStringImpl__var_folders_45_k1d9q7c52vz50wz1683_hk9r0000gn_T_main_33ff12_mi_1);
NSLog((NSString *)&__NSConstantStringImpl__var_folders_45_k1d9q7c52vz50wz1683_hk9r0000gn_T_main_33ff12_mi_2,str);
}
static void __main_block_copy_0(struct __main_block_impl_0*dst, struct __main_block_impl_0*src) {_Block_object_assign((void*)&dst->str, (void*)src->str, 3/*BLOCK_FIELD_IS_OBJECT*/);}
static void __main_block_dispose_0(struct __main_block_impl_0*src) {_Block_object_dispose((void*)src->str, 3/*BLOCK_FIELD_IS_OBJECT*/);}
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[]) {
NSMutableString * str = ((NSMutableString *(*)(id, SEL, NSString *))(void *)objc_msgSend)((id)((NSMutableString *(*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("NSMutableString"), sel_registerName("alloc")), sel_registerName("initWithString:"), (NSString *)&__NSConstantStringImpl__var_folders_45_k1d9q7c52vz50wz1683_hk9r0000gn_T_main_33ff12_mi_0);
void (*myBlock)(void) = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, str, 570425344));
NSLog((NSString *)&__NSConstantStringImpl__var_folders_45_k1d9q7c52vz50wz1683_hk9r0000gn_T_main_33ff12_mi_3,str);
((void (*)(__block_impl *))((__block_impl *)myBlock)->FuncPtr)((__block_impl *)myBlock);
return 0;
}
最后總結(jié)下。
首先arc默認進行了copy操作雏蛮，而mrc必須手動進行copy涎嚼。
靜態(tài)全局變量，全局變量挑秉，函數(shù)參數(shù)法梯，也是可以在直接在Block中改變變量值的，但是他們并沒有變成Block結(jié)構(gòu)體__main_block_impl_0的成員變量衷模，因為他們的作用域大鹊汛，所以可以直接更改他們的值.靜態(tài)全局變量，全局變量阱冶，函數(shù)參數(shù)他們并不會被Block持有刁憋，也就是說不會增加retainCount值。
對非對象變量來說木蹬，自動變量的值至耻，被copy進了Block，不帶__block的自動變量只能在里面被訪問镊叁，并不能改變值尘颓。帶__block的自動變量 和 靜態(tài)變量 就是直接地址訪問。所以在Block里面可以直接改變變量的值晦譬。
對對象變量來說
在MRC環(huán)境下疤苹，__block根本不會對指針所指向的對象執(zhí)行copy操作，而只是把指針進行的復(fù)制敛腌。
而在ARC環(huán)境下卧土，對于聲明為__block的外部對象，在block內(nèi)部會進行retain像樊，以至于在block環(huán)境內(nèi)能安全的引用外部對象尤莺。
