深入理解Block

什么是block

• 带有自动变量（局部变量）的匿名函数。它是C语言的扩充功能
• block本质上是一个OC对象，它内部也有isa指针
• block是封装了函数调用以及函数调用环境的OC对象

通过Clang编译器了解block结构

void(^block)(void) = ^{
          
            NSLog(@"This is a block");
        };
        
        block();

将OC代码转换为C/C++代码

xcrun -sdk iphoneos clang -arch arm64 -rewrite-objc main.m -o main.cpp

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

struct __main_block_impl_0 {
  struct __block_impl impl; // 上面的结构体
  struct __main_block_desc_0* Desc;
  // 构造函数啊
  __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, int flags=0) {
    impl.isa = &_NSConcreteStackBlock; // block类型
    impl.Flags = flags;
    impl.FuncPtr = fp; // block内部执行逻辑
    Desc = desc;    // 一些描述信息
  }
};
// NSLog(@"This is a block"); 这段代码
static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
     NSLog((NSString *)&__NSConstantStringImpl__var_folders_sk_84zmdndx0m3b5lg8frrxkxf00000gn_T_main_deb9d2_mi_0);
}
// 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)};

// 把block信息传递到__main_block_impl_0结构体中
void(*block)(void) = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA));

// 通过__main_block_impl_0.impl->FuncPtr�(block内部执行逻辑)
((void (*)(__block_impl *))((__block_impl *)block)->FuncPtr)((__block_impl *)block);

调用顺序

• 1.保存block到__main_block_impl_0结构体中
• 2.__main_block_impl_0构造函数进行赋值
• 3.通过__main_block_impl_0中的impl调用FuncPtr

block变量如何捕获

第一种

int age = 10;
void(^block)(void) = ^{ 
    NSLog(@"This is a jack. age is %d",age);
};
age = 20;
block();
//打印结果:
// This is a jack. age is 10

大家可以导出C++文件查看，__main_block_impl_0结构体内部会自动添加 int age 变量

xcrun -sdk iphoneos clang -arch arm64 -rewrite-objc main.m -o main.cpp

block内部捕获变量age的值是10，然后保存在main_block_impl_0结构体中，虽然设置了age = 20，这样做只是修改了外部age的值，并没有修改main_block_impl_0结构体内部中已经保存的age = 10，所以这种局部变量捕获是值传递

第二种

// auto: 自动变量，离开作用域就会销毁
auto int age = 10; 等价 int age = 10;
// 静态局部变量
static int height = 10;

static int height = 10;
void(^block)(void) = ^{
  
NSLog(@"This is a jack. height is %d",height);
};
height = 20;
block();
// 打印结果：
// This is a jack. height is 20

block内部捕获静态变量height地址指向是10，然后保存在main_block_impl_0结构体中，设置了height = 20，其实是height地址指向是20，这样做改变了height地址，并修改main_block_impl_0结构体内部中已经保存的height地址，所以这种局部变量捕获是指针传递

第三种

int age = 10;
static int height = 10;
int main(int argc, const char * argv[]) {
    @autoreleasepool {
    void(^block)(void) = ^{
      
        NSLog(@"This is a jack. age is %d. height is %d",age,height);
    };
        age = 20;
        height = 20;
        block();
    }
    return 0;
}
// 打印结果:
This is a jack. age is 20. height is 20

block内部没有捕获，而是直接访问

图片.png

block类型

void(^block)(void) = ^{
          
        NSLog(@"This is a jack.");
   };
        NSLog(@"%@  %@  %@  %@",[block class],[[block class] superclass],[[[block class] superclass] superclass],[[[[block class] superclass] superclass] superclass]);
// 打印结果:
// __NSGlobalBlock__  __NSGlobalBlock  NSBlock  NSObject

从打印也能证明block是一个OC对象

block有3种类型，可以通过调用class方法或者isa指针查看具体类型，最终都是继承自NSBlock类型

• NSGlobalBlock （ _NSConcreteGlobalBlock

• NSStackBlock （ _NSConcreteStackBlock ）

• NSMallocBlock （ _NSConcreteMallocBlock ）

  
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block环境变换

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block的copy

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__block修饰符

block可以解决block内部无法修改auto变量值的问题

__block int age = 10;
        
        void(^block)(void) = ^{
            age = 20;
            NSLog(@"This is a jack. age is %d",age);
        };
        block();

编译器会把__block变量包装成一个对象

struct __Block_byref_age_0 {
  void *__isa;
__Block_byref_age_0 *__forwarding;
 int __flags;
 int __size;
 int age;
};

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;
  }
};

编译后的block代码

__attribute__((__blocks__(byref))) __Block_byref_age_0 age = {(void*)0,(__Block_byref_age_0 *)&age, 0, sizeof(__Block_byref_age_0), 10};

void(*block)(void) = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, (__Block_byref_age_0 *)&age, 570425344));
((void (*)(__block_impl *))((__block_impl *)block)->FuncPtr)((__block_impl *)block);

图片.png

__block内存管理

__block修饰变量类型

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__block修饰对象类型

struct __Block_byref_person_0 {
  void *__isa;
__Block_byref_person_0 *__forwarding;
 int __flags;
 int __size;
 void (*__Block_byref_id_object_copy)(void*, void*);
 void (*__Block_byref_id_object_dispose)(void*);
 LPPerson *person;
};

struct __main_block_impl_0 {
  struct __block_impl impl;
  struct __main_block_desc_0* Desc;
  __Block_byref_person_0 *person; // by ref
  __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, __Block_byref_person_0 *_person, int flags=0) : person(_person->__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_person_0 *person = __cself->person; // bound by ref


            NSLog((NSString *)&__NSConstantStringImpl__var_folders_sk_84zmdndx0m3b5lg8frrxkxf00000gn_T_main_a9918c_mi_0,(person->__forwarding->person));
        }
static void __main_block_copy_0(struct __main_block_impl_0*dst, struct __main_block_impl_0*src) {_Block_object_assign((void*)&dst->person, (void*)src->person, 8/*BLOCK_FIELD_IS_BYREF*/);}

static void __main_block_dispose_0(struct __main_block_impl_0*src) {_Block_object_dispose((void*)src->person, 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};

编译后的block代码

__attribute__((__blocks__(byref))) __Block_byref_person_0 person = {(void*)0,(__Block_byref_person_0 *)&person, 33554432, sizeof(__Block_byref_person_0), __Block_byref_id_object_copy_131, __Block_byref_id_object_dispose_131, ((LPPerson *(*)(id, SEL))(void *)objc_msgSend)((id)((LPPerson *(*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("LPPerson"), sel_registerName("alloc")), sel_registerName("init"))};

void(*block)(void) = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, (__Block_byref_person_0 *)&person, 570425344));
((void (*)(__block_impl *))((__block_impl *)block)->FuncPtr)((__block_impl *)block);

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block解决循环引用

• ARC

  1. 用weak、unsafe_unretained解决循环引用

  2. __block也可以解决循环引用(对象设置为nil，最后必须要调用block)

• MRC

  1. 用__unsafe_unretained解决循环引用

  2. __block也可以解决循环引用