block的本质

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。

//
//  Created by SK on 2020/7/7.
//  Copyright © 2020 SK. All rights reserved.
//

#import <Foundation/Foundation.h>
#import "Person.h"

NSString *name = @"jack";

int main(int argc, const char * argv[]) {
    @autoreleasepool {

        int age = 20;
        static int sex = 0;
        Person *person1 = [[Person alloc] init];
        Person *person2 = [[Person alloc] init];
        Person *person3 = [[Person alloc] init];
        
        __strong Person *strongPerson = person1;
        __weak Person *weakPerson = person2;
        __block Person *blockPerson = person3;
        
        void (^block)(void) = ^{
            NSLog(@"%d-%d-%@", age, sex, name);
            [strongPerson run];
            [weakPerson run];
            [blockPerson run];
        };
        block();
        
    }
    return 0;
}

我们通过执行
xcrun -sdk iphoneos clang -arch arm64 -rewrite-objc -fobjc-arc -fobjc-runtime=ios-9.0 main.m
编译一下上述代码，来看一下block的底层结构

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

struct __Block_byref_blockPerson_0 {
  void *__isa;
__Block_byref_blockPerson_0 *__forwarding;
 int __flags;
 int __size;
 void (*__Block_byref_id_object_copy)(void*, void*);
 void (*__Block_byref_id_object_dispose)(void*);
 Person *__strong blockPerson;
};

struct __main_block_impl_0 {
  struct __block_impl impl;
  struct __main_block_desc_0* Desc;
  int age;
  int *sex;
  Person *__strong strongPerson;
  Person *__weak weakPerson;
  __Block_byref_blockPerson_0 *blockPerson; // by ref
}


static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
  __Block_byref_blockPerson_0 *blockPerson = __cself->blockPerson; // bound by ref
  int age = __cself->age; // bound by copy
  int *sex = __cself->sex; // bound by copy
  Person *__strong strongPerson = __cself->strongPerson; // bound by copy
  Person *__weak weakPerson = __cself->weakPerson; // bound by copy
            NSLog((NSString *)&__NSConstantStringImpl__var_folders_h6_tk2fn5rd5r92tqhsv84hkj540000gn_T_main_e976c4_mi_1, age, (*sex), name);
            ((void (*)(id, SEL))(void *)objc_msgSend)((id)strongPerson, sel_registerName("run"));
            ((void (*)(id, SEL))(void *)objc_msgSend)((id)weakPerson, sel_registerName("run"));
            ((void (*)(id, SEL))(void *)objc_msgSend)((id)(blockPerson->__forwarding->blockPerson), sel_registerName("run"));
}

static void __main_block_copy_0(struct __main_block_impl_0*dst, struct __main_block_impl_0*src) {_Block_object_assign((void*)&dst->strongPerson, (void*)src->strongPerson, 3/*BLOCK_FIELD_IS_OBJECT*/);_Block_object_assign((void*)&dst->weakPerson, (void*)src->weakPerson, 3/*BLOCK_FIELD_IS_OBJECT*/);_Block_object_assign((void*)&dst->blockPerson, (void*)src->blockPerson, 8/*BLOCK_FIELD_IS_BYREF*/);}

static void __main_block_dispose_0(struct __main_block_impl_0*src) {_Block_object_dispose((void*)src->strongPerson, 3/*BLOCK_FIELD_IS_OBJECT*/);_Block_object_dispose((void*)src->weakPerson, 3/*BLOCK_FIELD_IS_OBJECT*/);_Block_object_dispose((void*)src->blockPerson, 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; 

        int age = 20;
        static int sex = 0;
        Person *person1 = ((Person *(*)(id, SEL))(void *)objc_msgSend)((id)((Person *(*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("Person"), sel_registerName("alloc")), sel_registerName("init"));
        Person *person2 = ((Person *(*)(id, SEL))(void *)objc_msgSend)((id)((Person *(*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("Person"), sel_registerName("alloc")), sel_registerName("init"));
        Person *person3 = ((Person *(*)(id, SEL))(void *)objc_msgSend)((id)((Person *(*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("Person"), sel_registerName("alloc")), sel_registerName("init"));

        __attribute__((objc_ownership(strong))) Person *strongPerson = person1;
        __attribute__((objc_ownership(weak))) Person *weakPerson = person2;
        __attribute__((__blocks__(byref))) __Block_byref_blockPerson_0 blockPerson = {(void*)0,(__Block_byref_blockPerson_0 *)&blockPerson, 33554432, sizeof(__Block_byref_blockPerson_0), __Block_byref_id_object_copy_131, __Block_byref_id_object_dispose_131, person3};

        void (*block)(void) = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, age, &sex, strongPerson, weakPerson, (__Block_byref_blockPerson_0 *)&blockPerson, 570425344));
        ((void (*)(__block_impl *))((__block_impl *)block)->FuncPtr)((__block_impl *)block);

    }
    return 0;
}

从上述代码可以看出，block的底层结构如下：

Block底层结构.png

block的类型

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 ）
没有访问auto变量
NSStackBlock （ _NSConcreteStackBlock ）
访问了auto变量
NSMallocBlock （ _NSConcreteMallocBlock ）
NSStackBlock调用了copy

block的变量捕获

为了保证block内部能够正常访问外部的变量，block有个变量捕获机制

image.png

_NSConcreteGlobalBlock类型的block调动copy，无变化
_NSConcreteStackBlock类型的block调用copy，从栈复制到堆
_NSConcreteMallocBlock类型的block调用copy, 引用计数增加

在ARC环境下，编译器会根据情况自动将栈上的block复制到堆上，例如：

block作为函数返回值时
将block赋值给__strong指针时
block作为Cocoa API中方法名含有usingBlock的方法参数时
block作为GCD API的方法参数时

Auto类型变量

当block内部访问了对象类型的auto变量时

如果block是在栈上，将不会对auto变量产生强引用
如果block被拷贝到堆上，会调用block内部的copy函数
copy函数内部会调用_Block_object_assign函数
_Block_object_assign函数会根据auto变量的修饰符（__strong、__weak、__unsafe_unretained）做出相应的操作，形成强引用（retain）或者弱引用
如果block从堆上移除，会调用block内部的dispose函数
dispose函数内部会调用_Block_object_dispose函数
_Block_object_dispose函数会自动释放引用的auto变量（release）

__block修饰符

__block可以用于解决block内部无法修改auto变量值的问题
__block不能修饰全局变量、静态变量（static）
编译器会将__block变量包装成一个对象

struct __Block_byref_blockPerson_0 {
  void *__isa;
__Block_byref_blockPerson_0 *__forwarding;
 int __flags;
 int __size;
 void (*__Block_byref_id_object_copy)(void*, void*);
 void (*__Block_byref_id_object_dispose)(void*);
 Person *__strong blockPerson;
};

__block的内存管理

当block在栈上时，并不会对__block变量产生强引用
当block被copy到堆时
- 会调用block内部的copy函数
- copy函数内部会调用_Block_object_assign函数
- _Block_object_assign函数会对__block变量形成强引用（retain）
当block从堆中移除时
- 会调用block内部的dispose函数
- dispose函数内部会调用_Block_object_dispose函数
- _Block_object_dispose函数会自动释放引用的__block变量（release）

循环引用

__weak typeof(self) weakSelf = self;

探寻block