说明：阅读本文章，请参考之前的block文章加以理解；
一、栈区block分析
//代码
//ARC void test1() { { Person *per = [[Person alloc] init]; per.age = 10; ^{ NSLog(@"age:%d", per.age); }; } NSLog(@"-------1"); }
//打印
2019-01-14 17:24:12.118653+0800 MJ_TEST[6638:285938] Person dealloc 2019-01-14 17:24:12.118934+0800 MJ_TEST[6638:285938] -------1 Program ended with exit code: 0
分析：
<1>block代码内部引用的Person实例对象先于输出语句销毁，因为per仅限于大括号内，但此时block销毁了没有？往下看；
<2>上述block代码块并没有被指针持有，接下来看看指针持有的情况；
//代码
typedef void(^MyBlock)(void); //ARC void test2() { MyBlock block; { Person *per = [[Person alloc] init]; per.age = 10; block = ^{ NSLog(@"age:%d", per.age); }; } NSLog(@"-------1"); }
//打印
2019-01-14 17:34:58.473267+0800 MJ_TEST[6824:293129] -------1 2019-01-14 17:34:58.473705+0800 MJ_TEST[6824:293129] Person dealloc Program ended with exit code: 0
分析：Person实例对象后于输出语句销毁，为什么有指针持有，顺序就变了？
<1>等号左边：是一个auto类型的局部的block指针变量，存放在栈区；等号右边：是一个block代码块（对象），也是一个局部对象，存放在栈区；
<2>在ARC模式下，如果有指针持有（默认是强指针，修饰符为__strong）一个局部的block对象，系统会自动copy该block对象从栈区到堆区；
补充：其他三种情况——block作为函数返回值、含usingBlock方法（如数据的枚举方法）、GCD的应用（自己可以验证，此处不再赘述）；
那么，我们再看看MRC的情况
//打印————test1()和test2()
2019-01-14 17:56:46.641171+0800 MJ_TEST[7171:306788] -------1 Program ended with exit code: 0
分析：为什么per对象没有销毁？——因为需要手动释放；
//代码
[per release];
//打印————test1()和test2()
2019-01-14 17:59:39.091313+0800 MJ_TEST[7243:309139] Person dealloc 2019-01-14 17:59:39.091974+0800 MJ_TEST[7243:309139] -------1 2019-01-14 17:59:39.092013+0800 MJ_TEST[7243:309139] Person dealloc 2019-01-14 17:59:39.092086+0800 MJ_TEST[7243:309139] -------1 Program ended with exit code: 0
分析：
<1>此时的block对象的作用域在第一个大括号范围内，超出则被释放；
<2>Person实例对象被捕获到block对象结构体体中，同时其作用域也仅限于第一个大括号内，因此超出同样被释放；
二、堆区block分析
1）类型分析——ARC
//strong类型
执行上述test2()方法，我们知道系统会自动将block对象从栈区copy到堆区；同时，Person实例对象会被捕捉到block对象的结构体中，如下
struct __test2_block_impl_0 { struct __block_impl impl; struct __test2_block_desc_0* Desc; Person *per; __test2_block_impl_0(void *fp, struct __test2_block_desc_0 *desc, Person *_per, int flags=0) : per(_per) { impl.isa = &_NSConcreteStackBlock; impl.Flags = flags; impl.FuncPtr = fp; Desc = desc; } };
分析：可以看到per是一个指针变量，而该指针变量默认修饰符为__strong；修改代码
__strong Person *per = [[Person alloc] init];
clang命令：xcrun -sdk iphoneos clang -arch arm64 -rewrite-objc -fobjc-arc -fobjc-runtime=ios-9.0.0 main.m
说明：
<1>该命令行，只针对ARC模式下，MRC模式下，如果有release语句会报错；
<2>该命令行，是解决ARC模式下，实例对象为__weak类型，转成C++代码；
struct __test2_block_impl_0 { struct __block_impl impl; struct __test2_block_desc_0* Desc; Person *__strong per; __test2_block_impl_0(void *fp, struct __test2_block_desc_0 *desc, Person *__strong _per, int flags=0) : per(_per) { impl.isa = &_NSConcreteStackBlock; impl.Flags = flags; impl.FuncPtr = fp; Desc = desc; } };
分析：因此，一般的指针变量，默认修饰符为__strong；
//weak类型
//代码
//ARC void test2() { MyBlock block; { Person *per = [[Person alloc] init]; per.age = 10; __weak Person *weakPer = per; block = ^{ NSLog(@"age:%d", weakPer.age); }; // [per release]; } NSLog(@"-------1"); }
//C++代码
struct __test2_block_impl_0 { struct __block_impl impl; struct __test2_block_desc_0* Desc; Person *__weak weakPer; __test2_block_impl_0(void *fp, struct __test2_block_desc_0 *desc, Person *__weak _weakPer, int flags=0) : weakPer(_weakPer) { impl.isa = &_NSConcreteStackBlock; impl.Flags = flags; impl.FuncPtr = fp; Desc = desc; } };
//打印
2019-01-15 10:21:36.280746+0800 MJ_TEST[997:45906] Person dealloc 2019-01-15 10:21:36.281063+0800 MJ_TEST[997:45906] -------1 Program ended with exit code: 0
分析：
<1>此时的per由weakPer弱指针指向，并被捕捉到block对象结构体中；
<2>结合上述强指针引用Person实例对象的打印结果，weak修饰的指针变量先于输出语句销毁，可以肯定系统是没有对block对象copy到堆区的；
那针对不同类型指针的引用，系统是如果判断操作的呢？往下看
2）调用原理
//C++代码
typedef void(*MyBlock)(void); struct __test2_block_impl_0 { struct __block_impl impl; struct __test2_block_desc_0* Desc; Person *__weak weakPer; __test2_block_impl_0(void *fp, struct __test2_block_desc_0 *desc, Person *__weak _weakPer, int flags=0) : weakPer(_weakPer) { impl.isa = &_NSConcreteStackBlock; impl.Flags = flags; impl.FuncPtr = fp; Desc = desc; } }; static void __test2_block_func_0(struct __test2_block_impl_0 *__cself) { Person *__weak weakPer = __cself->weakPer; // bound by copy NSLog((NSString *)&__NSConstantStringImpl__var_folders_tb_zgsq5gq15rd3zvbdmw1c09y80000gn_T_main_3e374f_mi_2, ((int (*)(id, SEL))(void *)objc_msgSend)((id)weakPer, sel_registerName("age"))); } static void __test2_block_copy_0(struct __test2_block_impl_0*dst, struct __test2_block_impl_0*src) {_Block_object_assign((void*)&dst->weakPer, (void*)src->weakPer, 3/*BLOCK_FIELD_IS_OBJECT*/);} static void __test2_block_dispose_0(struct __test2_block_impl_0*src) {_Block_object_dispose((void*)src->weakPer, 3/*BLOCK_FIELD_IS_OBJECT*/);} static struct __test2_block_desc_0 { size_t reserved; size_t Block_size; void (*copy)(struct __test2_block_impl_0*, struct __test2_block_impl_0*); void (*dispose)(struct __test2_block_impl_0*); } __test2_block_desc_0_DATA = { 0, sizeof(struct __test2_block_impl_0), __test2_block_copy_0, __test2_block_dispose_0}; void test2() { MyBlock block; { Person *per = ((Person *(*)(id, SEL))(void *)objc_msgSend)((id)((Person *(*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("Person"), sel_registerName("alloc")), sel_registerName("init")); ((void (*)(id, SEL, int))(void *)objc_msgSend)((id)per, sel_registerName("setAge:"), 10); __attribute__((objc_ownership(weak))) Person *weakPer = per; block = ((void (*)())&__test2_block_impl_0((void *)__test2_block_func_0, &__test2_block_desc_0_DATA, weakPer, 570425344)); } NSLog((NSString *)&__NSConstantStringImpl__var_folders_tb_zgsq5gq15rd3zvbdmw1c09y80000gn_T_main_3e374f_mi_3); } int main(int argc, const char * argv[]) { /* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool; test2(); } return 0; }
分析：
<1>根据前面的文章分析，我们发现block描述结构体（__test2_block_desc_0）中多了两个函数指针的成员变量
void (*copy)(struct __test2_block_impl_0*, struct __test2_block_impl_0*); void (*dispose)(struct __test2_block_impl_0*);
其中，copy函数指针指向__test2_block_copy_0函数，dispose指向__test2_block_dispose_0；
<2>__test2_block_copy_0函数主要是通过_Block_object_assign函数来确定对per对象是否强引用，其根据就是per的引用类型——如果是__strong类型，则block对象对per对对象进行强引用（per的生命周期可控）；如果是__weak类型，则进行弱引用（per的生命周期不可控）；
<3>当block对像从堆区销毁时，会调用__test2_block_dispose_0函数会自动释放引用的per对象（相当于release）——注：严格意义上，此处的释放指的断开是block对象对per的引用即retainCount减1，至于per对象所占的内存是否被释放（回收）则在所不问（也许还有其他的指针变量引用），只有retainCount变为0零，其内存才会被回收；
补充：当block访问的外部的auto类型的局部数据为对象时，则会产生上述两个函数指针；如果是非实例对象（如基础数据类型），则不会有上述两个函数指针——原因：实例对象一般是在堆区开辟的内存，需要对其进行内存管理————注：如果是__block修饰前述变量（包括实例对象），也会产生上述两个指针函数，具体后面文章会写到！
//代码
void test3() { int age = 10; ^{ NSLog(@"%d", age); }; }
//clang
static struct __test3_block_desc_0 { size_t reserved; size_t Block_size; } __test3_block_desc_0_DATA = { 0, sizeof(struct __test3_block_impl_0)};
三、结论
【1】栈区block：不论是ARC还是MRC模式，指向该block对象的指针变量，不会对引用的auto类型的局部的实例对象进行强引用；
【2】堆区block：不论是ARC还是MRC模式，指向该block对象的指针变量，根据引用的auto类型的局部的实例对象的引用类型，通过调用block结构体中的copy函数指针来调用_Block_object_assign函数，来决定对实例对象是否进行强引用——__strong类型强引用，__weak类型弱引用；
【3】堆区block释放：系统会通过调用block结构体中的dispose函数指针来调用__test2_block_dispose_0函数，自动释放引用的外部auto类型的局部实例对象；
说明：
<1>block对象本身，即代码块（位于等号右边），非block指针变量（位于等号左边）；
<2>所谓的强引用，类似于retain操作即保留实例对象（所占内存不随作用域限制而被自动回收），保证手动管理内存释放，达到可控的目的；
四、拓展——GCD引用分析
1）
//代码
- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event { Person *per = [[Person alloc] init]; dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)NSEC_PER_SEC*3), dispatch_get_main_queue(), ^{ NSLog(@"%@", per); }); NSLog(@"touchesBegan"); }
//打印
2019-01-15 11:56:19.867668+0800 GCD_Refrence[1558:90109] touchesBegan 2019-01-15 11:56:22.867823+0800 GCD_Refrence[1558:90109] <Person: 0x600003bb8cf0> 2019-01-15 11:56:22.867996+0800 GCD_Refrence[1558:90109] Person dealloc
2）
//代码
- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event { Person *per = [[Person alloc] init]; __weak Person *weakPer = per; dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)NSEC_PER_SEC*3), dispatch_get_main_queue(), ^{ NSLog(@"%@", weakPer); }); NSLog(@"touchesBegan"); }
//打印
2019-01-15 11:58:58.381396+0800 GCD_Refrence[1619:93062] touchesBegan 2019-01-15 11:58:58.381583+0800 GCD_Refrence[1619:93062] Person dealloc 2019-01-15 11:59:01.381697+0800 GCD_Refrence[1619:93062] (null)
3）
//代码
- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event { Person *per = [[Person alloc] init]; __weak Person *weakPer = per; dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)NSEC_PER_SEC*1), dispatch_get_main_queue(), ^{ dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)NSEC_PER_SEC*2), dispatch_get_main_queue(), ^{ NSLog(@"%@", per); }); }); NSLog(@"touchesBegan"); }
//打印
2019-01-15 12:00:44.996108+0800 GCD_Refrence[1653:94592] touchesBegan 2019-01-15 12:00:48.088426+0800 GCD_Refrence[1653:94592] <Person: 0x6000010f4a20> 2019-01-15 12:00:48.088664+0800 GCD_Refrence[1653:94592] Person dealloc
4）
//代码
- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event { Person *per = [[Person alloc] init]; __weak Person *weakPer = per; dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)NSEC_PER_SEC*1), dispatch_get_main_queue(), ^{ dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)NSEC_PER_SEC*2), dispatch_get_main_queue(), ^{ NSLog(@"%@", weakPer); }); }); NSLog(@"touchesBegan"); }
//打印
2019-01-15 12:01:42.122836+0800 GCD_Refrence[1672:95546] touchesBegan 2019-01-15 12:01:42.123038+0800 GCD_Refrence[1672:95546] Person dealloc 2019-01-15 12:01:45.123256+0800 GCD_Refrence[1672:95546] (null)
5）
//代码
- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event { Person *per = [[Person alloc] init]; __weak Person *weakPer = per; dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)NSEC_PER_SEC*1), dispatch_get_main_queue(), ^{ NSLog(@"%@", weakPer); dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)NSEC_PER_SEC*2), dispatch_get_main_queue(), ^{ NSLog(@"%@", per); }); }); NSLog(@"touchesBegan"); }
//打印
2019-01-15 12:02:50.591355+0800 GCD_Refrence[1693:96581] touchesBegan 2019-01-15 12:02:51.685830+0800 GCD_Refrence[1693:96581] <Person: 0x6000033a4470> 2019-01-15 12:02:53.686541+0800 GCD_Refrence[1693:96581] <Person: 0x6000033a4470> 2019-01-15 12:02:53.686810+0800 GCD_Refrence[1693:96581] Person dealloc
6）
//代码
- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event { Person *per = [[Person alloc] init]; __weak Person *weakPer = per; dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)NSEC_PER_SEC*1), dispatch_get_main_queue(), ^{ NSLog(@"%@", per); dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)NSEC_PER_SEC*2), dispatch_get_main_queue(), ^{ NSLog(@"%@", weakPer); }); }); NSLog(@"touchesBegan"); }
//打印
2019-01-15 12:03:47.349637+0800 GCD_Refrence[1715:97486] touchesBegan 2019-01-15 12:03:48.447971+0800 GCD_Refrence[1715:97486] <Person: 0x600000163900> 2019-01-15 12:03:48.448271+0800 GCD_Refrence[1715:97486] Person dealloc 2019-01-15 12:03:50.448553+0800 GCD_Refrence[1715:97486] (null)
7）
//代码
- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event { Person *per = [[Person alloc] init]; __weak Person *weakPer = per; dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)NSEC_PER_SEC*1), dispatch_get_main_queue(), ^{ NSLog(@"%@", per); dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)NSEC_PER_SEC*2), dispatch_get_main_queue(), ^{ NSLog(@"%@", per); }); }); NSLog(@"touchesBegan"); }
//打印
2019-01-15 12:04:37.584067+0800 GCD_Refrence[1735:98332] touchesBegan 2019-01-15 12:04:38.679922+0800 GCD_Refrence[1735:98332] <Person: 0x600003bd7570> 2019-01-15 12:04:40.876560+0800 GCD_Refrence[1735:98332] <Person: 0x600003bd7570> 2019-01-15 12:04:40.876803+0800 GCD_Refrence[1735:98332] Person dealloc
8）
//代码
- (void)test8
{
Person *per = [[Person alloc] init];
__weak Person *weakPer = per;
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)NSEC_PER_SEC*3), dispatch_get_main_queue(), ^{
NSLog(@"1---weakPer--%@", weakPer);
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)NSEC_PER_SEC*2), dispatch_get_main_queue(), ^{
NSLog(@"2---per--%@", per);
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)NSEC_PER_SEC*2), dispatch_get_main_queue(), ^{
NSLog(@"3---weakPer--%@", weakPer);
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)NSEC_PER_SEC*2), dispatch_get_main_queue(), ^{
NSLog(@"4---per--%@", per);
});
});
});
});
NSLog(@"touchesBegan");
}
//打印
2019-05-27 10:27:15.314844+0800 2-2 auto类型局部实例对象GCD[971:43547] touchesBegan 2019-05-27 10:27:18.315007+0800 2-2 auto类型局部实例对象GCD[971:43547] 1---weakPer--<Person: 0x60000392ab30> 2019-05-27 10:27:20.315417+0800 2-2 auto类型局部实例对象GCD[971:43547] 2---per--<Person: 0x60000392ab30> 2019-05-27 10:27:22.315839+0800 2-2 auto类型局部实例对象GCD[971:43547] 3---weakPer--<Person: 0x60000392ab30> 2019-05-27 10:27:24.316280+0800 2-2 auto类型局部实例对象GCD[971:43547] 4---per--<Person: 0x60000392ab30> 2019-05-27 10:27:24.316524+0800 2-2 auto类型局部实例对象GCD[971:43547] Person dealloc
分析：
<1>ARC环境下，使用GCD时，系统自动将block对象从栈区copy到堆区；
<2>根据以上打印结果，发现如果是单纯的对per进行强引用，则延时3秒后per对象才销毁；如果是弱引用，则立即销毁，再次使用时为空（此时已经被释放）；
<3>如果既对per强引用又有弱引用，在嵌套的GCD使用中，以最后一个强引用为准——即per对象在最后一个强引用执行完后就会释放（之后的弱引用则输出为空）；
结论：同一个auto类型的局部的实例对象，既有强引用，也有弱引用，以强引用为准；