Object-C语言Block的实现方式
开场白
Block基本概念
中间态转换方法
Block编译后结果分析
Block运行时状态与编译状态对比
开场白
Object-C语言是对C语言的扩展，所以将OC源码进行编译的时候，会将OC源码会被转换成C\C++，所以想了解OC源码的实现细节，还是需要手动编译成中间状态进行观察。
命令1:
clang -rewrite-objc main.m
如果Xcode版本较高，可能会出现报错：
./block_VC.h:9:9: fatal error: 'UIKit/UIKit.h' file not found
此时可尝试另一个命令：
clang -x objective-c -rewrite-objc -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator.sdk main.m
命令好长，每次输入这么长的指令很麻烦，此时可以考虑使用命令别名：alias
Block的基本概念
_NSConcreteGlobalBlock 全局的静态 block，不会访问任何外部变量。
_NSConcreteStackBlock 保存在栈中的 block，当函数返回时会被销毁。
_NSConcreteMallocBlock 保存在堆中的 block，当引用计数为 0 时会被销毁。
堆中的 block何时转换：
1.调用copy方法时
2.从一个函数返回时
3.将block传入dispatch等系统IPA参数block或者传递给带有usingBlock的Cocoa框架函数时。
4.将block赋给带有__strong修饰符的id类型或者Block类型时。
Block类型变换方式
1.没有捕获任何局部变量的block为_NSConcreteGlobalBlock，它以static函数的形式存储在代码区
2.捕获了局部变量的block为_NSConcreteStackBlock
3.当_NSConcreteStackBlock出现上面四种情况时，会变成_NSConcreteMallocBlock。（注意此时是调用了Block_Copy函数后）
Block编译后结果分析
基本类型变量
OC源码：
NSInteger age = 10; void(^completeBlock)(NSString *) = ^(NSString *name) { NSString *info = [NSString stringWithFormat:@"name:%@ - age:%d",name,age]; NSLog(@"%@",info); }; completeBlock(@"jack");
编译后的C\C++源码：
// __block_impl为block的基础struct //1.*isa：当前block对象所属的类型（注意：在OC语言中带有isa指针的都是对象） //2.Flags：标示位，当类型为NSConcreteMallocBlock时，表示需要ARC内存管理，其他默认不需要内存管理 //3.Reserved：预留标示位，暂时不管它 //4.block的实际static内部函数地址指针 struct __block_impl { void *isa; int Flags; int Reserved; void *FuncPtr; }; // __main_block_impl_0为block的最终struct //1.impl：为上面的__block_impl结构体实例 //2.Desc：为block的描述结构体__main_block_desc_0，里面包含了block结构体的信息描述，在下面讲解 //3.age：从外面补货的变量，block采用的方式是直接在block实例中定义一个变量，将捕获的值赋给它 //4.__main_block_impl_0：为block结构体的构造函数，参数中包含从外面捕获的变量值 struct __main_block_impl_0 { struct __block_impl impl; struct __main_block_desc_0* Desc; NSInteger age; __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, NSInteger _age, int flags=0) : age(_age) { impl.isa = &_NSConcreteStackBlock; impl.Flags = flags; impl.FuncPtr = fp; Desc = desc; } }; //__main_block_func_0为block具体实现的函数载体，具体的block的任务实现在此函数内 //参数：1是block的实例指针，2是block的外部传参name static void __main_block_func_0(struct __main_block_impl_0 *__cself, NSString *name) { //从block实例中获取age变量的值 NSInteger age = __cself->age; // bound by copy //生成字符串NSString *info = [NSString stringWithFormat:@"name:%@ - age:%d",name,age]; NSString *info = ((NSString * _Nonnull (*)(id, SEL, NSString * _Nonnull, ...))(void *)objc_msgSend)((id)objc_getClass("NSString"), sel_registerName("stringWithFormat:"), (NSString *)&__NSConstantStringImpl__var_folders_4y_ks8945f50k51_0j95ytw7ss80000gn_T_main_83d55e_mi_0, (NSString *)name, (NSInteger)age); //打印NSLog(@"%@",info); NSLog((NSString *)&__NSConstantStringImpl__var_folders_4y_ks8945f50k51_0j95ytw7ss80000gn_T_main_83d55e_mi_1,info); } //reserved：保留字0 //Block_size:block实例在内存中占据内存大小 //__main_block_desc_0_DATA：默认desc结构体实例 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, char * argv[]) { /* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool; NSInteger age = 10; void(*completeBlock)(NSString *) = ((void (*)(NSString *))&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, age)); ((void (*)(__block_impl *, NSString *))((__block_impl *)completeBlock)->FuncPtr)((__block_impl *)completeBlock, (NSString *)&__NSConstantStringImpl__var_folders_4y_ks8945f50k51_0j95ytw7ss80000gn_T_main_83d55e_mi_2); return UIApplicationMain(argc, argv, __null, NSStringFromClass(((Class (*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("AppDelegate"), sel_registerName("class")))); } }
block简单结构布局
struct __block_impl struct __main_block_impl_0 static void __main_block_func_0 static struct __main_block_desc_0
详细解释见上面的中间编译状态。
block实例中定义了一个age变量，并将外部的age传了过去
__block基本类型变量
OC源码：
__block NSInteger age = 10; void(^completeBlock)(NSString *) = ^(NSString *name) { NSString *info = [NSString stringWithFormat:@"name:%@ - age:%d",name,age]; NSLog(@"%@",info); }; completeBlock(@"jack");
编译后的C\C++源码：
struct __block_impl { void *isa; int Flags; int Reserved; void *FuncPtr; }; //age的block变量实例__Block_byref_age //__isa：age的block变量实例所属的类型 //__forwarding：age的block变量实例的指针 //__flags：变量是否被内存管理标示 //__size:所占内存大小 //age:age变量值 struct __Block_byref_age_0 { void *__isa; __Block_byref_age_0 *__forwarding; int __flags; int __size; NSInteger age; }; //原来的age变量值，变成了__Block_byref_age_0变量指针 struct __main_block_impl_0 { struct __block_impl impl; struct __main_block_desc_0* Desc; __Block_byref_age_0 *age; // by ref __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, __Block_byref_age_0 *_age, int flags=0) : age(_age->__forwarding) { impl.isa = &_NSConcreteStackBlock; impl.Flags = flags; impl.FuncPtr = fp; Desc = desc; } }; static void __main_block_func_0(struct __main_block_impl_0 *__cself, NSString *name) { //通过block实例获取变量的指针 __Block_byref_age_0 *age = __cself->age; // bound by ref NSLog((NSString *)&__NSConstantStringImpl__var_folders_4y_ks8945f50k51_0j95ytw7ss80000gn_T_main_7acc63_mi_0,name,(age->__forwarding->age)); } //当block从栈复制到堆上时，会对__Block_byref_age_0变量进行拷贝，也会从栈拷贝到堆上 static void __main_block_copy_0(struct __main_block_impl_0*dst, struct __main_block_impl_0*src) {_Block_object_assign((void*)&dst->age, (void*)src->age, 8/*BLOCK_FIELD_IS_BYREF*/);} //__Block_byref_age_0变量的析构函数，当block实例的引用计数为0时，是否__Block_byref_age_0变量的内存空间 static void __main_block_dispose_0(struct __main_block_impl_0*src) {_Block_object_dispose((void*)src->age, 8/*BLOCK_FIELD_IS_BYREF*/);} //desc结构体中新增了copy函数和dispose函数指针 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, char * argv[]) { /* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool; //block变量实例 __attribute__((__blocks__(byref))) __Block_byref_age_0 age = {(void*)0,(__Block_byref_age_0 *)&age, 0, sizeof(__Block_byref_age_0), 10}; //block实例 void(*completeBlock)(NSString *) = ((void (*)(NSString *))&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, (__Block_byref_age_0 *)&age, 570425344)); ((void (*)(__block_impl *, NSString *))((__block_impl *)completeBlock)->FuncPtr)((__block_impl *)completeBlock, (NSString *)&__NSConstantStringImpl__var_folders_4y_ks8945f50k51_0j95ytw7ss80000gn_T_main_7acc63_mi_1); return UIApplicationMain(argc, argv, __null, NSStringFromClass(((Class (*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("AppDelegate"), sel_registerName("class")))); } }
简单结构布局
struct __block_impl struct __main_block_impl_0 static void __main_block_func_0 static struct __main_block_desc_0 //新增 struct __Block_byref_age_0 static void __main_block_copy_0( static void __main_block_dispose_0(
__block NSInteger age = 10;后，在简单布局结构中新增了3项
struct __Block_byref_age_0 static void __main_block_copy_0( static void __main_block_dispose_0(
__Block_byref_age_0将基本变量转换成__block变量结构体实例指针
__main_block_copy_0和__main_block_dispose_0是对__block变量结构体实例的内存管理方法。
__block变量结构体实例从栈拷贝到堆时，调用方法：__main_block_copy_0
__block变量结构体实例引用计数为0时，调用方法：__main_block_dispose_0
指针类型变量
OC源码：
NSMutableArray *friends = [NSMutableArray array];; void(^completeBlock)(NSString *) = ^(NSString *name) { NSLog(@"%@--%@",name,friends); }; completeBlock(@"jack");
编译后的C\C++源码：
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; NSMutableArray *friends; __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, NSMutableArray *_friends, int flags=0) : friends(_friends) { impl.isa = &_NSConcreteStackBlock; impl.Flags = flags; impl.FuncPtr = fp; Desc = desc; } }; static void __main_block_func_0(struct __main_block_impl_0 *__cself, NSString *name) { NSMutableArray *friends = __cself->friends; // bound by copy NSLog((NSString *)&__NSConstantStringImpl__var_folders_4y_ks8945f50k51_0j95ytw7ss80000gn_T_main_1b4a2f_mi_0,name,friends); } //block实例从栈复制到堆上时，将指针变量引用计数加一 static void __main_block_copy_0(struct __main_block_impl_0*dst, struct __main_block_impl_0*src) {_Block_object_assign((void*)&dst->friends, (void*)src->friends, 3/*BLOCK_FIELD_IS_OBJECT*/);} //block实例引用计数为0时，析构指针变量对象 static void __main_block_dispose_0(struct __main_block_impl_0*src) {_Block_object_dispose((void*)src->friends, 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, char * argv[]) { /* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool; NSMutableArray *friends = ((NSMutableArray * _Nonnull (*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("NSMutableArray"), sel_registerName("array"));; void(*completeBlock)(NSString *) = ((void (*)(NSString *))&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, friends, 570425344)); ((void (*)(__block_impl *, NSString *))((__block_impl *)completeBlock)->FuncPtr)((__block_impl *)completeBlock, (NSString *)&__NSConstantStringImpl__var_folders_4y_ks8945f50k51_0j95ytw7ss80000gn_T_main_1b4a2f_mi_1); return UIApplicationMain(argc, argv, __null, NSStringFromClass(((Class (*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("AppDelegate"), sel_registerName("class")))); } }
简单结构布局
struct __block_impl struct __main_block_impl_0 static void __main_block_func_0 static struct __main_block_desc_0 static void __main_block_copy_0( static void __main_block_dispose_0(
block实例中定义了一个NSMutableArray *friends;指针变量，并通过构造函数将指针传递给block实例
__main_block_copy_0：block实例从栈复制到堆上时，将指针变量引用计数加一
__main_block_dispose_0：block实例引用计数为0时，析构指针变量对象
__block指针类型变量
OC源码：
__block NSMutableArray *friends = [NSMutableArray array];; void(^completeBlock)(NSString *) = ^(NSString *name) { NSLog(@"%@--%@",name,friends); }; completeBlock(@"jack");
编译后的C\C++源码：
struct __block_impl { void *isa; int Flags; int Reserved; void *FuncPtr; }; //__Block_byref_friends_0结构体实例中包含了friends指针 //并且多了__Block_byref_id_object_copy函数：对friends进行copy //多了__Block_byref_id_object_dispose函数：对friends进行析构 struct __Block_byref_friends_0 { void *__isa; __Block_byref_friends_0 *__forwarding; int __flags; int __size; void (*__Block_byref_id_object_copy)(void*, void*); void (*__Block_byref_id_object_dispose)(void*); NSMutableArray *friends; }; struct __main_block_impl_0 { struct __block_impl impl; struct __main_block_desc_0* Desc; __Block_byref_friends_0 *friends; // by ref __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, __Block_byref_friends_0 *_friends, int flags=0) : friends(_friends->__forwarding) { impl.isa = &_NSConcreteStackBlock; impl.Flags = flags; impl.FuncPtr = fp; Desc = desc; } }; static void __main_block_func_0(struct __main_block_impl_0 *__cself, NSString *name) { __Block_byref_friends_0 *friends = __cself->friends; // bound by ref NSLog((NSString *)&__NSConstantStringImpl__var_folders_4y_ks8945f50k51_0j95ytw7ss80000gn_T_main_252d0c_mi_0,name,(friends->__forwarding->friends)); } //block实例从栈复制到堆上时，__block变量结构体实例将指针变量引用计数加一，同时__block变量结构体实例内的NSMutableArray *friends变量也递归加一 static void __main_block_copy_0(struct __main_block_impl_0*dst, struct __main_block_impl_0*src) {_Block_object_assign((void*)&dst->friends, (void*)src->friends, 8/*BLOCK_FIELD_IS_BYREF*/);} //block实例引用计数为0时，__block变量结构体实例被析构，同时__block变量结构体实例内的NSMutableArray *friends变量也递归析构 static void __main_block_dispose_0(struct __main_block_impl_0*src) {_Block_object_dispose((void*)src->friends, 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, char * argv[]) { /* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool; __attribute__((__blocks__(byref))) __Block_byref_friends_0 friends = {(void*)0,(__Block_byref_friends_0 *)&friends, 33554432, sizeof(__Block_byref_friends_0), __Block_byref_id_object_copy_131, __Block_byref_id_object_dispose_131, ((NSMutableArray * _Nonnull (*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("NSMutableArray"), sel_registerName("array"))};; void(*completeBlock)(NSString *) = ((void (*)(NSString *))&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, (__Block_byref_friends_0 *)&friends, 570425344)); ((void (*)(__block_impl *, NSString *))((__block_impl *)completeBlock)->FuncPtr)((__block_impl *)completeBlock, (NSString *)&__NSConstantStringImpl__var_folders_4y_ks8945f50k51_0j95ytw7ss80000gn_T_main_252d0c_mi_1); return UIApplicationMain(argc, argv, __null, NSStringFromClass(((Class (*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("AppDelegate"), sel_registerName("class")))); } }
简单结构变量
struct __block_impl struct __main_block_impl_0 static void __main_block_func_0 static struct __main_block_desc_0 static void __main_block_copy_0( static void __main_block_dispose_0( //新增 struct __Block_byref_age_0
和__block普通变量一样，出现了__Block_byref_age_0结构体
__Block_byref_friends_0结构体实例中包含了friends指针
并且多了__Block_byref_id_object_copy函数：对friends进行copy
多了__Block_byref_id_object_dispose函数：对friends进行析构
同时在函数__main_block_copy_0和__main_block_dispose_0中，出现的也是对__Block_byref_age_0结构体的调用。它是在将__Block_byref_age_0结构体析构之前，先析构friends对象。
Block编译中间态与运行时状态对比
利用lldb打印出来的结果
局部自由变量
//局部变量 __block NSMutableArray *friends = [NSMutableArray array];; void(^completeBlock)(NSString *) = ^(NSString *name) { NSLog(@"%@--%@",name,friends); }; (lldb) expression -P 5 -- completeBlock (void (^)(NSString *)) $2 = 0x000000010a061130 { __isa = __NSMallocBlock__ __flags = -1023410170 __reserved = 0 __FuncPtr = 0x000000010a061130 (iOS_KnowledgeStructure`__main_block_invoke at main.m:19) {} }
利用clang转换后的C\C++源码
struct __main_block_impl_0 { struct __block_impl impl; struct __main_block_desc_0* Desc; __Block_byref_friends_0 *friends; // by ref __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, __Block_byref_friends_0 *_friends, int flags=0) : friends(_friends->__forwarding) { impl.isa = &_NSConcreteStackBlock; impl.Flags = flags; impl.FuncPtr = fp; Desc = desc; } };
证明clang编译器和llvm编译器编译后的结果有出入。发现isa指针不一致。
编译后的为：impl.isa = &_NSConcreteStackBlock;
运行时的为：__isa = __NSMallocBlock__
原因是block类型转换情况4:“将block赋给带有__strong修饰符的id类型或者Block类型时。”
注意：
_NSConcreteMallocBlock一般不会在源码中出现，它通常在block copy到堆时出现，变化的源码：
static void *_Block_copy_internal(const void *arg, const int flags) { struct Block_layout *aBlock; const bool wantsOne = (WANTS_ONE & flags) == WANTS_ONE; // 1 if (!arg) return NULL; // 2 aBlock = (struct Block_layout *)arg; // 3 if (aBlock->flags & BLOCK_NEEDS_FREE) { // latches on high latching_incr_int(&aBlock->flags); return aBlock; } // 4 else if (aBlock->flags & BLOCK_IS_GLOBAL) { return aBlock; } // 5 struct Block_layout *result = malloc(aBlock->descriptor->size); if (!result) return (void *)0; // 6 memmove(result, aBlock, aBlock->descriptor->size); // bitcopy first // 7 result->flags &= ~(BLOCK_REFCOUNT_MASK); // XXX not needed result->flags |= BLOCK_NEEDS_FREE | 1; // 8 result->isa = _NSConcreteMallocBlock; // 9 if (result->flags & BLOCK_HAS_COPY_DISPOSE) { (*aBlock->descriptor->copy)(result, aBlock); // do fixup } return result; }
