Blocks 笔记

Block 简述

Blocks是C语言的扩充功能，是一种带有自动变量的匿名函数。具体来说就是没有函数名称和能带自动变量。

Blocks的存储域有：NSConcreteStackBlock(在栈区)、NSConcreteGlobalBlock（数据区）、NSConcreteMallocBlock（堆区) ARC情况下栈区声明的Block会自动复制到堆区，这是为什么超过作用域还能存在的理由

看看block实现原理

int main(int argc, const char * argv[]) {
@autoreleasepool {
 
    void(^blk)(int count) = ^void(int count){
        
        NSLog(@"%d",count);
    };
    
    blk(10);
}
return 0;
}

用clang -rewrite-objc + 原文件名 可以看到源代码

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;
__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;
}
};

static void __main_block_func_0(struct __main_block_impl_0 *__cself, int count) {


        NSLog((NSString *)&__NSConstantStringImpl__var_folders__l_qvvd4l511c3bxr5z7px1xgx80000gn_T_XQBlockTest_ad2fef_mi_0,count);
    }

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; 

    void(*blk)(int count) = ((void (*)(int))&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA));

    ((void (*)(__block_impl *, int))((__block_impl *)blk)->FuncPtr)((__block_impl *)blk, 10);
}
return 0;

}

主要有三个结构体
1. struct __block_impl 结构体其形式如下

 struct __block_impl {
  void *isa;
  int Flags;
  int Reserved;
  void *FuncPtr;
};

isa用于存储其类对象的信息，给其赋值一个Block的实例，机指向Block的对象的具体实例 跟objective-c 处理运行时很像。FunPtr是指其函数指针，其他信息不太重要

2. 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)};
   这个主要包含内存地址和结构体版本信息

3. struct __main_block_impl_0 {
   struct __block_impl impl;
   struct __main_block_desc_0* Desc;
   __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;
   }
   };
   这个结构体又有上述两个结构体为其成员变量，而且初始化这个block时给他进行了赋值

Blocks 模式

语法

最详细的block语法模式是: ^ 返回值类型 参数列表 表达式

   void(^myBlock)(int count) = ^void(int count){
        
        printf("result is :%d \n",count*2);
    };
    
    myBlock(10);

但是block的各种省略模式眼花缭乱，无论怎么省略这个符号是一定不能省略的 ^

Blocks 类型变量

Objective-c中一般用typedef声明Blocks变量

typedef int(^myBlock3)(int count);
myBlock3 blk = ^int (int testCount){
    
        return testCount * 2;
    };
    
    printf("testCount is %d \n",blk(10));

Blocks 截获自动变量值

Blocks截获自动变量其实是保存了这个变量的瞬间值，并且blocks中增加了这个成员变量为这个变量，并且为其赋值为当时保存的瞬值，这里我们不能再对它进行赋值操作了，不然会报错的，这里我们可以理解为时截取到类对象的结构体实例指针，显然赋值是会报错的。

      int a = 10;
      TestBlk testBlk = ^{
    
        NSLog(@"a is  %d ",a);
    };
            
    testBlk();

其实截获自动变量值就是在在上述的struct __main_block_impl_0的机构体重自增新的的成员变量

struct __main_block_impl_0 {

struct __block_impl impl;
struct __main_block_desc_0* Desc;
int a;
__main_block_impl_0(void *fp, struct __main_block_desc_0   *desc, int _a int flags=0:a(_a)) {
impl.isa = &_NSConcreteStackBlock;
impl.Flags = flags;
impl.FuncPtr = fp;
Desc = desc;
}
};      

__block 说明符

用block说明符声明的变量可以进行赋值操作

      __block int a = 10;
    
    void(^blk5)(void) = ^{
    
        a = 11;
    };
    
    printf("a is %d \n",a);

    blk5();

打印结果为:

a is 11 

__block 修饰符将变量设置在哪个储存域中 看看源码

int main(int argc, const char * argv[]) {
@autoreleasepool {
 
    __block int objc_a = 10;
    
    void(^blk)(void) = ^{
        
        objc_a = 11;
    };
    
    NSLog(@"objc_a is %d",objc_a);
    
    blk();
    
}
return 0;
}

看看clang -rewrite 转换成C++的源码

struct __block_impl {
void *isa;
int Flags;
int Reserved;
void *FuncPtr;
};
  struct __Block_byref_objc_a_0 {
  void *__isa;
  __Block_byref_objc_a_0 *__forwarding;
 int __flags;
 int __size;
 int objc_a;
};

  struct __main_block_impl_0 {
  struct __block_impl impl;
  struct __main_block_desc_0* Desc;
  __Block_byref_objc_a_0 *objc_a; // by ref
  __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, __Block_byref_objc_a_0 *_objc_a, int flags=0) : objc_a(_objc_a->__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_objc_a_0 *objc_a = __cself->objc_a; // bound by ref


        (objc_a->__forwarding->objc_a) = 11;
    }
static void __main_block_copy_0(struct __main_block_impl_0*dst, struct __main_block_impl_0*src) {_Block_object_assign((void*)&dst->objc_a, (void*)src->objc_a, 8/*BLOCK_FIELD_IS_BYREF*/);}

static void __main_block_dispose_0(struct __main_block_impl_0*src) {_Block_object_dispose((void*)src->objc_a, 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[]) {
/* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool; 

    __attribute__((__blocks__(byref))) __Block_byref_objc_a_0 objc_a = {(void*)0,(__Block_byref_objc_a_0 *)&objc_a, 0, sizeof(__Block_byref_objc_a_0), 10};

    void(*blk)(void) = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, (__Block_byref_objc_a_0 *)&objc_a, 570425344));

    NSLog((NSString *)&__NSConstantStringImpl__var_folders__l_qvvd4l511c3bxr5z7px1xgx80000gn_T_XQBlockTest_906fa9_mi_0,(objc_a.__forwarding->objc_a));

    ((void (*)(__block_impl *))((__block_impl *)blk)->FuncPtr)((__block_impl *)blk);

}
return 0;
}

这里面比前面block实现的源代码多了一个结构体，而且这个结构体自增到struct __main_block_impl_0 这个结构体中当成员变量，这个新增的结构体神奇的地方就是 它有一个成员变量 __forwarding，而这个__forwarding又是指向该实例自身的指针，然后通过__forwarding访问其成员变量的值，这也是Block超出变量作用域可存在的理由。

 struct __Block_byref_objc_a_0 {
  void *__isa;
  __Block_byref_objc_a_0 *__forwarding;
 int __flags;
 int __size;
 int objc_a;
};

截获的自动变量

我们截获的自动变量，我们能使用它，但不能给他赋值。要给他赋值的话加上 __block修饰符。这里我们能使用它的原因是因为我们截取了NSMutableArray类对象用的结构体实例指针。而且在使用C语言字符串的数组用 * 代替[],不然会报错。

MutableArray *array = [NSMutableArray array];
    
    void(^blk)(void) = ^{
        
        TestClass *class = [[TestClass alloc] init];
        
        [array addObject:class];
        
    };
    
    blk();
    
    NSLog(@"array count is  %d",array.count);

未完待续………………