Block的好处,我总结了下主要有2点:1.用于回调特别方便,2.可以延长对象的作用区域。但是,Block的内存管理这个模块一直不是很清楚,这个周末好好的看了下Block的原理,有些许心得。
为了性能,默认Block都是分配在stack上面的,所以它的作用区域就是当前函数。
#include
int main()
{
int i = 1024;
void (^blk)(void) = ^ {
printf("%d\n", i);
};
blk();
return 0;
}
在blk这个block里面是不能修改i的。Why?我们可以通过clang看看编译器处理后的这块代码
struct __main_block_impl_0 {
struct __block_impl impl;
struct __main_block_desc_0* Desc;
int i;
__main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, int _i, int flags=0) : i(_i) {
impl.isa = &_NSConcreteStackBlock;
impl.Flags = flags;
impl.FuncPtr = fp;
Desc = desc;
}
};
static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
int i = __cself->i; // bound by copy
printf("%d\n", i);
}
static struct __main_block_desc_0 {
unsigned long reserved;
unsigned long Block_size;
} __main_block_desc_0_DATA = { 0, sizeof(struct __main_block_impl_0)};
int main()
{
int i = 1024;
void (*blk)(void) = (void (*)(void))&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, i);
((void (*)(struct __block_impl *))((struct __block_impl *)blk)->FuncPtr)((struct __block_impl *)blk);
return 0;
}
struct __block_impl是Block的一个内部结构体,原型是
struct __block_impl {
void *isa;
int Flags;
int Reserved;
void *FuncPtr;
};
每个block都有个默认的构造函数
__main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, int _i, int flags=0) : i(_i) 所以只能读取i,而不能修改i,当你试图修改它时,编译器就在预处理阶段直接报错。
只要在i前加__Block变量就可以在Block里面修改i值了,此时由值类型变为引用类型
struct __main_block_impl_0 {
struct __block_impl impl;
struct __main_block_desc_0* Desc;
int *i;
__main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, int *_i, int flags=0) : i(_i) {
impl.isa = &_NSConcreteStackBlock;
impl.Flags = flags;
impl.FuncPtr = fp;
Desc = desc;
}
};
static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
int *i = __cself->i; // bound by copy
printf("%d\n", (*i));
}
static struct __main_block_desc_0 {
unsigned long reserved;
unsigned long Block_size;
} __main_block_desc_0_DATA = { 0, sizeof(struct __main_block_impl_0)};
int main()
{
static int i = 1024;
void (*blk)(void) = (void (*)(void))&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, &i);
((void (*)(struct __block_impl *))((struct __block_impl *)blk)->FuncPtr)((struct __block_impl *)blk);
return 0;
}
上面的代码块是将int i的类型修改为__Block int i = 1024;后编译器生成代码块,可以看到__main_block_impl_0中的 i类型已经改变为int *,所以我们可以修改它的值。
所以只要没对Block进行copy操作,它一直存在stack里面。不管是否有__block修饰符
要想延长Block的作于域,我们可以对它进行copy操作,apple提供的接口是Block_Copy()方法
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通过观察apple提供的block源码,我们可以看到copy方法将block从statck拷贝到heap里面,所以它的作用区域延长了
待完成:1.block与oc的混合
2.__block修饰符与oc的混合
总结
1.block默认都是分配在stack,当copy后,它移到heap里
2.block中的变量默认是不能修改的,只有添加__Block修饰符后才能修改
3.block中有oc对象时,会_Block_retain_object(object),直到block销毁后才会_Block_release_object(object);
4.对block进行copy时
If you access an instance variable by reference, a strong reference is made to self;
If you access an instance variable by value, a strong reference is made to the variable.
它会将self进行copy,此时改对象的dealloc方法不会执行(因为它的引用计数归0),解决此问题有2种方法:在block执行完成后面立即Block_Release(),或者将改变量声明为__Block类型(Why?)
继续补充block的oc的混合
View Code
编译器生成代码为
View Code
blk的原型为
void (*blk)(void) = (void (*)(void))&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, str, 570425344);
其中570425344 = BLOCK_HAS_COPY_DISPOSE | BLOCK_HAS_DESCRIPTOR;
通过构造函数,我们看到仍然是值传递,所以blk中是能不修改str的。
至于上面的__main_block_copy_0和__main_block_dispose_0 就是用于Block_Copy()和Block_Release();
当我将str的类型修改为__block NSMutableString时,生成如下代码
View Code
可以看到str的类型实际上是__Block_byref_str_0,其中33554432 = BLOCK_HAS_COPY_DISPOSE = (1 << 25) 而blk的构造函数中传递的是__Block_byref_str_0类型的指针,所以我们能在blk中修改str