block的本質(zhì)

block本質(zhì)上也是一個(gè)OC對象闷堡，它內(nèi)部也有個(gè)isa指針喉前。
block是封裝了函數(shù)調(diào)用以及函數(shù)調(diào)用環(huán)境的OC對象佳吞。

//
//  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;
}

我們通過執(zhí)行
xcrun -sdk iphoneos clang -arch arm64 -rewrite-objc -fobjc-arc -fobjc-runtime=ios-9.0 main.m
編譯一下上述代碼苍姜，來看一下block的底層結(jié)構(gòu)

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;
}

從上述代碼可以看出译蒂，block的底層結(jié)構(gòu)如下：

Block底層結(jié)構(gòu).png

block的類型

block有3種類型，可以通過調(diào)用class方法或者isa指針查看具體類型谊却，最終都是繼承自NSBlock類型柔昼。

NSGlobalBlock （ _NSConcreteGlobalBlock ）
沒有訪問auto變量
NSStackBlock （ _NSConcreteStackBlock ）
訪問了auto變量
NSMallocBlock （ _NSConcreteMallocBlock ）
NSStackBlock調(diào)用了copy

block的變量捕獲

為了保證block內(nèi)部能夠正常訪問外部的變量，block有個(gè)變量捕獲機(jī)制

image.png

_NSConcreteGlobalBlock類型的block調(diào)動copy炎辨，無變化
_NSConcreteStackBlock類型的block調(diào)用copy捕透，從棧復(fù)制到堆
_NSConcreteMallocBlock類型的block調(diào)用copy, 引用計(jì)數(shù)增加

在ARC環(huán)境下，編譯器會根據(jù)情況自動將棧上的block復(fù)制到堆上碴萧，例如：

block作為函數(shù)返回值時(shí)
將block賦值給__strong指針時(shí)
block作為Cocoa API中方法名含有usingBlock的方法參數(shù)時(shí)
block作為GCD API的方法參數(shù)時(shí)

Auto類型變量

當(dāng)block內(nèi)部訪問了對象類型的auto變量時(shí)

如果block是在棧上乙嘀，將不會對auto變量產(chǎn)生強(qiáng)引用
如果block被拷貝到堆上，會調(diào)用block內(nèi)部的copy函數(shù)
copy函數(shù)內(nèi)部會調(diào)用_Block_object_assign函數(shù)
_Block_object_assign函數(shù)會根據(jù)auto變量的修飾符（__strong破喻、__weak虎谢、__unsafe_unretained）做出相應(yīng)的操作，形成強(qiáng)引用（retain）或者弱引用
如果block從堆上移除曹质，會調(diào)用block內(nèi)部的dispose函數(shù)
dispose函數(shù)內(nèi)部會調(diào)用_Block_object_dispose函數(shù)
_Block_object_dispose函數(shù)會自動釋放引用的auto變量（release）

__block修飾符

__block可以用于解決block內(nèi)部無法修改auto變量值的問題
__block不能修飾全局變量婴噩、靜態(tài)變量（static）
編譯器會將__block變量包裝成一個(gè)對象

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的內(nèi)存管理

當(dāng)block在棧上時(shí)，并不會對__block變量產(chǎn)生強(qiáng)引用
當(dāng)block被copy到堆時(shí)
- 會調(diào)用block內(nèi)部的copy函數(shù)
- copy函數(shù)內(nèi)部會調(diào)用_Block_object_assign函數(shù)
- _Block_object_assign函數(shù)會對__block變量形成強(qiáng)引用（retain）
當(dāng)block從堆中移除時(shí)
- 會調(diào)用block內(nèi)部的dispose函數(shù)
- dispose函數(shù)內(nèi)部會調(diào)用_Block_object_dispose函數(shù)
- _Block_object_dispose函數(shù)會自動釋放引用的__block變量（release）

循環(huán)引用

__weak typeof(self) weakSelf = self;

探尋block