(一)Block的常见问题
首先我们先看下面几个问题：
-
block的原理是怎样的？本质是什么？
-
__block的作用是什么？有什么使用注意点？
-
block的属性修饰词为什么是copy？使用block有哪些使用注意？
-
block在修改NSMutableArray，需不需要添加__block？
如果上面的不能完整的回答，说明block的知识需要进行深刻的学习了
(二)block的本质及底层结构
- block本质上也是一个OC对象，它内部也有个isa指针
- block是封装了函数调用以及函数调用环境的OC对象
下面我们通过多个方法去印证。
(1)block的底层结构
我们看下面的代码：
int main(int argc, const char * argv[]) {
@autoreleasepool {
^{
NSLog(@"hello,world!");
}();
int age = 10;
void(^block)(int,int) = ^(int a,int b){
NSLog(@"this is a block == %d" , age);
NSLog(@"this is a block a == %d b == %d" , a, b);
};
block(20,30);
}
return 0;
}
通过clang编译器转化为C++代码后（截取主要代码）：
//struct __block_impl结构
struct __block_impl {
void *isa;//isa指针
int Flags;
int Reserved;
void *FuncPtr;
};
//第一个block的底层结构
struct __main_block_impl_0 {
struct __block_impl impl;
struct __main_block_desc_0* Desc;
//构造函数(C++)
__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;
}
};
//第一个block调用函数
static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
NSLog((NSString *)&__NSConstantStringImpl__var_folders_f9__q8q2xr131vf4h1cy7837sm00000gn_T_main_26fdd2_mi_0);
}
//第一个block描述信息
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)};
//第二个block的底层结构（包含变量age）
struct __main_block_impl_1 {
struct __block_impl impl;
struct __main_block_desc_1* Desc;
int age;
__main_block_impl_1(void *fp, struct __main_block_desc_1 *desc, int _age, int flags=0) : age(_age) {
impl.isa = &_NSConcreteStackBlock;
impl.Flags = flags;
impl.FuncPtr = fp;
Desc = desc;
}
};
//第二个block调用函数
static void __main_block_func_1(struct __main_block_impl_1 *__cself, int a, int b) {
int age = __cself->age; // bound by copy
NSLog((NSString *)&__NSConstantStringImpl__var_folders_f9__q8q2xr131vf4h1cy7837sm00000gn_T_main_26fdd2_mi_1 , age);
NSLog((NSString *)&__NSConstantStringImpl__var_folders_f9__q8q2xr131vf4h1cy7837sm00000gn_T_main_26fdd2_mi_2 , a, b);
}
//第二个block描述信息
static struct __main_block_desc_1 {
size_t reserved;
size_t Block_size;
} __main_block_desc_1_DATA = { 0, sizeof(struct __main_block_impl_1)};
int main(int argc, const char * argv[]) {
/* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool;
((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA))();
//简化后 &__main_block_impl_0(__main_block_func_0,&__main_block_desc_0_DATA)
int age = 10;
void(*block)(int,int) = ((void (*)(int, int))&__main_block_impl_1((void *)__main_block_func_1, &__main_block_desc_1_DATA, age));
//简化后 block = &__main_block_impl_1(__main_block_func_1, &__main_block_desc_1_DATA, age)
((void (*)(__block_impl *, int, int))((__block_impl *)block)->FuncPtr)((__block_impl *)block, 20, 30);
//简化后 (block -> FuncPtr)(block, 20, 30)
}
return 0;
}
我们通过转化后的代码可以证实。另外，我们通过，类型强转也可以查看block的底层结构
通过类型强转查看底层结构
image.png
总结
block的底层结构图解
(2)block的本质
int main(int argc, const char * argv[]) {
@autoreleasepool {
int age = 10;
void(^block)(int,int) = ^(int a,int b){
NSLog(@"this is a block == %d" , age);
NSLog(@"this is a block a == %d b == %d" , a, b);
};
block(20,30);
}
return 0;
}
通过clang编译器转化为C++代码后（截取主要代码）：
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;
int age;
//构造函数（类似于init方法）
__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;
}
};
//将block的内部执行逻辑封装到__main_block_func_0函数
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_f9__q8q2xr131vf4h1cy7837sm00000gn_T_main_142399_mi_1 , age);
NSLog((NSString *)&__NSConstantStringImpl__var_folders_f9__q8q2xr131vf4h1cy7837sm00000gn_T_main_142399_mi_2 , a, b);
}
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;
//定义block变量
void(*block)(int,int) = ((void (*)(int, int))&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, age));
//简化：block = &__main_block_impl_0(__main_block_func_0, &__main_block_desc_0_DATA, age) 参数1:函数地址 参数2：block的描述 参数3：age
//执行block内部代码
((void (*)(__block_impl *, int, int))((__block_impl *)block)->FuncPtr)((__block_impl *)block, 20, 30);
// //简化：(block->FuncPtr)(block, 20, 30) 函数调用
}
return 0;
}
(2)block的本质
int main(int argc, const char * argv[]) {
@autoreleasepool {
int age = 10;
void(^block)(int,int) = ^(int a,int b){
NSLog(@"this is a block a == %d" , age);
};
block(20,30);
NSLog(@"%@",[block class]);//__NSMallocBlock__
NSLog(@"%@",[[block class] superclass]);//__NSMallocBlock
NSLog(@"%@",[[[block class] superclass] superclass]);//NSBlock
NSLog(@"%@",[[[[block class] superclass] superclass] superclass]);//NSObject
NSLog(@"%@",[[[[[block class] superclass] superclass] superclass] superclass]);//(null)
}
return 0;
}
从输出结果来看，block是一个OC对象，基类是NSObject，下一章我们对block的类型进行进一步的学习。
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