iOS-Block本質(zhì)

iOS-Block本質(zhì)

參考篇:iOS-Block淺談

前言:本文簡(jiǎn)述Block本質(zhì),如有錯(cuò)誤請(qǐng)留言指正。

第一部分:Block本質(zhì)

Q:什么是Block,Block的本質(zhì)是什么?

  • block本質(zhì)上也是一個(gè)OC對(duì)象,它內(nèi)部也有個(gè)isa指針
  • 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(duì)象
  • block是封裝函數(shù)及其上下文的OC對(duì)象
image

查看block源碼:

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;
  // 構(gòu)造函數(shù)(類(lèi)似于OC的init方法),返回結(jié)構(gòu)體對(duì)象
  __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;
  }
};

// 封裝了block執(zhí)行邏輯的函數(shù)
static void __main_block_func_0(struct __main_block_impl_0 *__cself) {

            NSLog((NSString *)&__NSConstantStringImpl__var_folders_2r__m13fp2x2n9dvlr8d68yry500000gn_T_main_c60393_mi_0);
        }

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;
        // 定義block變量
        void (*block)(void) = &__main_block_impl_0(
                                                   __main_block_func_0,
                                                   &__main_block_desc_0_DATA
                                                   );

        // 執(zhí)行block內(nèi)部的代碼
        block->FuncPtr(block);
    }
    return 0;
}

image
  • FuncPtr:指向調(diào)用函數(shù)的地址
  • __main_block_desc_0 :block描述信息
  • Block_size:block的大小

第二部分:Block捕獲變量

Q:下述代碼輸出值為多少?

int age=10;
void (^Block)(void) = ^{
    NSLog(@"age:%d",age);
};
age = 20;
Block();

輸出值為 age:10
原因:創(chuàng)建block的時(shí)候,已經(jīng)把a(bǔ)ge的值存儲(chǔ)在里面了。

Q:下列代碼輸出值分別為多少?

auto int age = 10;
static int num = 25;
void (^Block)(void) = ^{
    NSLog(@"age:%d,num:%d",age,num);
};
age = 20;
num = 11;
Block();

輸出結(jié)果為:age:10,num:11
愿意:auto變量block訪(fǎng)問(wèn)方式是值傳遞,static變量block訪(fǎng)問(wèn)方式是指針傳遞
源碼證明:

int age = __cself->age; // bound by copy
int *num = __cself->num; // bound by copy

NSLog((NSString *)&__NSConstantStringImpl__var_folders_2r__m13fp2x2n9dvlr8d68yry500000gn_T_main_d2875b_mi_0, age, (*num));

int age = 10;
static int num = 25;

block = ((void (*)())&__test_block_impl_0((void *)__test_block_func_0, &__test_block_desc_0_DATA, age, &num));

age = 20;
num = 11;

上述代碼可查看 static修飾的變量,是根據(jù)指針訪(fǎng)問(wèn)的

Q:為什么block對(duì)auto和static變量捕獲有差異?

auto自動(dòng)變量可能會(huì)銷(xiāo)毀的,內(nèi)存可能會(huì)消失,不采用指針訪(fǎng)問(wèn);static變量一直保存在內(nèi)存中,指針訪(fǎng)問(wèn)即可

Q:block對(duì)全局變量的捕獲方式是?

block不需要對(duì)全局變量捕獲,都是直接采用取值的

Q:為什么局部變量需要捕獲?

考慮作用域的問(wèn)題,需要跨函數(shù)訪(fǎng)問(wèn),就需要捕獲

Q:block的變量捕獲(capture)

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

image

Q:block里訪(fǎng)問(wèn)self是否會(huì)捕獲?

會(huì),self是當(dāng)調(diào)用block函數(shù)的參數(shù),參數(shù)是局部變量,self指向調(diào)用者

Q:block里訪(fǎng)問(wèn)成員變量是否會(huì)捕獲?

會(huì),成員變量的訪(fǎng)問(wèn)其實(shí)是self->xx,先捕獲self,再通過(guò)self訪(fǎng)問(wèn)里面的成員變量

第三部分 Block類(lèi)型

Q:block有哪幾種類(lèi)型?

block的類(lèi)型,取決于isa指針,可以通過(guò)調(diào)用class方法或者isa指針查看具體類(lèi)型,最終都是繼承自NSBlock類(lèi)型

  • __NSGlobalBlock __ ( _NSConcreteGlobalBlock )
  • __NSStackBlock __ ( _NSConcreteStackBlock )
  • __NSMallocBlock __ ( _NSConcreteMallocBlock )

代碼示例

void (^block1)(void) = ^{
    NSLog(@"block1");
};
NSLog(@"%@",[block1 class]);
NSLog(@"%@",[[block1 class] superclass]);
NSLog(@"%@",[[[block1 class] superclass] superclass]);
NSLog(@"%@",[[[[block1 class] superclass] superclass] superclass]);
NSLog(@"%@",[[[[[block1 class] superclass] superclass] superclass] superclass]);

輸出結(jié)果:
NSGlobalBlock
__NSGlobalBlock
NSBlock
NSObject
null

上述代碼輸出了block1的類(lèi)型,也證實(shí)了block是對(duì)象,最終繼承NSObject

代碼展示block的三種類(lèi)型:

int age = 1;
void (^block1)(void) = ^{
    NSLog(@"block1");
};

void (^block2)(void) = ^{
    NSLog(@"block2:%d",age);
};

NSLog(@"%@/%@/%@",[block1 class],[block2 class],[^{
    NSLog(@"block3:%d",age);
} class]);

輸出結(jié)果:
__NSGlobalBlock __/__NSMallocBlock __/__NSStackBlock __

Q:各類(lèi)型的block在內(nèi)存中如何分配的?

  • __NSGlobalBlock __ 在數(shù)據(jù)區(qū)
  • __NSMallocBlock __ 在堆區(qū)
  • __NSStackBlock __ 在棧區(qū)
  • 堆:動(dòng)態(tài)分配內(nèi)存,需要程序員自己申請(qǐng),程序員自己管理
  • 棧:自動(dòng)分配內(nèi)存,自動(dòng)銷(xiāo)毀,先入后出,棧上的內(nèi)容存在自動(dòng)銷(xiāo)毀的情況

[圖片上傳失敗...(image-17266-1605599404838)]

Q:如何判斷block是哪種類(lèi)型?

  • 沒(méi)有訪(fǎng)問(wèn)auto變量的block是__NSGlobalBlock __ ,放在數(shù)據(jù)段
  • 訪(fǎng)問(wèn)了auto變量的block是__NSStackBlock __
  • [__NSStackBlock __ copy]操作就變成了__NSMallocBlock __

Q:對(duì)每種類(lèi)型block調(diào)用copy操作后是什么結(jié)果?

  • __NSGlobalBlock __ 調(diào)用copy操作后,什么也不做
  • __NSStackBlock __ 調(diào)用copy操作后,復(fù)制效果是:從棧復(fù)制到堆;副本存儲(chǔ)位置是
  • __NSMallocBlock __ 調(diào)用copy操作后,復(fù)制效果是:引用計(jì)數(shù)增加;副本存儲(chǔ)位置是

Q:在ARC環(huán)境下,編譯器會(huì)根據(jù)情況自動(dòng)將棧上的block復(fù)制到堆上的幾種情況?

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

MRC下block屬性的建議寫(xiě)法
@property (copy, nonatomic) void (^block)(void);

ARC下block屬性的建議寫(xiě)法
@property (copy, nonatomic) void (^block)(void);

第四部分:對(duì)象類(lèi)型的auto變量

Q:ARC下述代碼中Person對(duì)象是否會(huì)釋放?

示例代碼:

typedef void(^XBTBlock)(void);
XBTBlock block;
{
    Person *p = [[Person alloc] init];
    p.age = 10;

    block = ^{
        NSLog(@"======= %d",p.age);
    };
}

Person.m
- (void)dealloc{
    NSLog(@"Person - dealloc");
}

輸出結(jié)果:不會(huì)打印Person - dealloc
轉(zhuǎn)化C++代碼后:

struct __main_block_impl_0 {
  struct __block_impl impl;
  struct __main_block_desc_0* Desc;
  Person *person;
  __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, MJPerson *__strong _person, int flags=0) : person(_person) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};

上述
block為堆block,block里面有一個(gè)Person指針,Person指針指向Person對(duì)象。只要block還在,Person就還在。block強(qiáng)引用了Person對(duì)象。

Q:上述代碼改換成MRC,Person對(duì)象會(huì)釋放么?

會(huì)的!
堆空間的block會(huì)對(duì)Person對(duì)象retain操作,擁有一次Person對(duì)象。

Q:下列代碼中Person是否會(huì)被釋放?

@autoreleasepool {
        XBTBlock block;

        {
            Person *p = [[Person alloc] init];
            p.age = 10;
            __weak Person *weakPersn = p;
            block = ^{
                NSLog(@"======= %d",weakPersn.age);
            };
        }
        NSLog(@"--------------");
}

答案:會(huì)釋放

小結(jié)

無(wú)論MRC還是ARC,??臻g上的block,不會(huì)持有對(duì)象;堆空間的block,會(huì)持有對(duì)象。

Q:當(dāng)block內(nèi)部訪(fǎng)問(wèn)了對(duì)象類(lèi)型的auto變量時(shí),是否會(huì)強(qiáng)引用?

答案:分情況討論,分為棧block和堆block

棧block
a) 如果block是在棧上,將不會(huì)對(duì)auto變量產(chǎn)生強(qiáng)引用
b) 棧上的block隨時(shí)會(huì)被銷(xiāo)毀,也沒(méi)必要去強(qiáng)引用其他對(duì)象

堆block
1.如果block被拷貝到堆上:
a) 會(huì)調(diào)用block內(nèi)部的copy函數(shù)
b) copy函數(shù)內(nèi)部會(huì)調(diào)用_Block_object_assign函數(shù)
c) _Block_object_assign函數(shù)會(huì)根據(jù)auto變量的修飾符(__strong、__weak、__unsafe_unretained)做出相應(yīng)的操作,形成強(qiáng)引用(retain)或者弱引用

2.如果block從堆上移除
a) 會(huì)調(diào)用block內(nèi)部的dispose函數(shù)
b) dispose函數(shù)內(nèi)部會(huì)調(diào)用_Block_object_dispose函數(shù)
c) _Block_object_dispose函數(shù)會(huì)自動(dòng)釋放引用的auto變量(release)

正確答案:

  • 如果block在空間,不管外部變量是強(qiáng)引用還是弱引用,block都會(huì)弱引用訪(fǎng)問(wèn)對(duì)象
  • 如果block在空間,如果外部強(qiáng)引用,block內(nèi)部也是強(qiáng)引用;如果外部弱引用,block內(nèi)部也是弱引用

Q:__weak 在使用clang轉(zhuǎn)換OC為C++代碼時(shí),可能會(huì)遇到以下問(wèn)題cannot create __weak reference in file using manual reference

解決方案:支持ARC、指定運(yùn)行時(shí)系統(tǒng)版本,比如
xcrun -sdk iphoneos clang -arch arm64 -rewrite-objc -fobjc-arc -fobjc-runtime=ios-8.0.0 main.m

Q1:gcd的block中引用 Person對(duì)象什么時(shí)候銷(xiāo)毀?

-(void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event{
    Person *person = [[Person alloc] init];
    person.age = 10;

    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2.0 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        NSLog(@"age:%d",person.age);
    });

    NSLog(@"touchesBegan");
}

輸出結(jié)果:
14:36:03.395120+0800 test[1032:330314] touchesBegan
14:36:05.395237+0800 test[1032:330314] age:10
14:36:05.395487+0800 test[1032:330314] Person-dealloc

原因:gcd的block默認(rèn)會(huì)做copy操作,即dispatch_after的block是堆block,block會(huì)對(duì)Person強(qiáng)引用,block銷(xiāo)毀時(shí)候Person才會(huì)被釋放。

Q2:上述代碼如果換成__weak,Person什么時(shí)候釋放?

-(void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event{

    Person *person = [[Person alloc] init];
    person.age = 10;

    __weak Person *weakPerson = person;
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2.0 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        NSLog(@"age:%p",weakPerson);
    });

    NSLog(@"touchesBegan");
}

輸出結(jié)果:
14:38:42.996990+0800 test[1104:347260] touchesBegan
14:38:42.997481+0800 test[1104:347260] Person-dealloc
14:38:44.997136+0800 test[1104:347260] age:0x0

原因:使用__weak修飾過(guò)后的對(duì)象,堆block會(huì)采用弱引用,無(wú)法延時(shí)Person的壽命,所以在touchesBegan函數(shù)結(jié)束后,Person就會(huì)被釋放,gcd就無(wú)法捕捉到Person。

Q3:如果gcd內(nèi)包含gcd,Person會(huì)什么時(shí)候釋放?

-(void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event{

    Person *person = [[Person alloc] init];
    person.age = 10;

    __weak Person *weakPerson = person;
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(4.0 * NSEC_PER_SEC)),
                   dispatch_get_main_queue(), ^{

        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(3.0 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
            NSLog(@"2-----age:%p",person);
        });
        NSLog(@"1-----age:%p",weakPerson);
    });

    NSLog(@"touchesBegan");
}

輸出結(jié)果:
14:48:01.293818+0800 test[1199:403589] touchesBegan
14:48:05.294127+0800 test[1199:403589] 1-----age:0x604000015eb0
14:48:08.582807+0800 test[1199:403589] 2-----age:0x604000015eb0
14:48:08.583129+0800 test[1199:403589] Person-dealloc

原因:gcd內(nèi)部只要有強(qiáng)引用Person,Person就會(huì)等待執(zhí)行完再銷(xiāo)毀!所以Person銷(xiāo)毀時(shí)間為7秒。

Q4:如果gcd內(nèi)部先強(qiáng)引用后弱引用,Person什么時(shí)候釋放?

-(void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event{

    Person *person = [[Person alloc] init];
    person.age = 10;

    __weak Person *weakPerson = person;
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(4.0 * NSEC_PER_SEC)),
                   dispatch_get_main_queue(), ^{

        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(3.0 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
            NSLog(@"2-----age:%p",weakPerson);
        });
        NSLog(@"1-----age:%p",person);
    });

    NSLog(@"touchesBegan");
}

輸出結(jié)果
14:52:29.036878+0800 test[1249:431302] touchesBegan
14:52:33.417862+0800 test[1249:431302] 1-----age:0x6000000178d0
14:52:33.418178+0800 test[1249:431302] Person-dealloc
14:52:36.418204+0800 test[1249:431302] 2-----age:0x0

原因:Person會(huì)等待強(qiáng)引用執(zhí)行完畢后釋放,只要強(qiáng)引用執(zhí)行完,就不會(huì)等待后執(zhí)行的弱引用,會(huì)直接釋放的,所以Person釋放時(shí)間為4秒。

第五部分:__block修飾符

Q:block在修改NSMutableArray,需不需要添加__block?

不需要。

Q:block能否修改變量值?

auto修飾變量,block無(wú)法修改,因?yàn)閎lock使用的時(shí)候是內(nèi)部創(chuàng)建了變量來(lái)保存外部的變量的值,block只有修改內(nèi)部自己變量的權(quán)限,無(wú)法修改外部變量的權(quán)限。
static修飾變量,block可以修改,因?yàn)閎lock把外部static修飾變量的指針存入,block直接修改指針指向變量值,即可修改外部變量值。
全局變量值,全局變量無(wú)論哪里都可以修改,當(dāng)然block內(nèi)部也可以修改。

Q:__block int age = 10,系統(tǒng)做了哪些?

答案:編譯器會(huì)將__block變量包裝成一個(gè)對(duì)象
查看c++源碼:

struct __Block_byref_age_0 {
  void *__isa;
__Block_byref_age_0 *__forwarding;//age的地址
 int __flags;
 int __size;
 int age;//age 的值
};

Q:__block 修飾符作用?

  • __block可以用于解決block內(nèi)部無(wú)法修改auto變量值的問(wèn)題
  • __block不能修飾全局變量、靜態(tài)變量(static)
  • 編譯器會(huì)將__block變量包裝成一個(gè)對(duì)象
  • __block修改變量:age->__forwarding->age
  • __Block_byref_age_0結(jié)構(gòu)體內(nèi)部地址和外部變量age是同一地址
image

Q:block可以向NSMutableArray添加元素么?

NSMutableArray *arr = [NSMutableArray array];

Block block = ^{
    [arr addObject:@"123"];
    [arr addObject:@"2345"];
};

答案:可以,因?yàn)槭莂ddObject是使用NSMutableArray變量,而不是通過(guò)指針改變NSMutableArray,如果是arr = nil,這就是改變了NSMutableArray變量,會(huì)報(bào)錯(cuò)。

5.1 __block的內(nèi)存管理

當(dāng)block在棧上時(shí),并不會(huì)對(duì)__block變量產(chǎn)生強(qiáng)引用

Q:block的屬性修飾詞為什么是copy?

block一旦沒(méi)有進(jìn)行copy操作,就不會(huì)在堆上
block在堆上,程序員就可以對(duì)block做內(nèi)存管理等操作,可以控制block的生命周期

Q:當(dāng)block被copy到堆時(shí),對(duì)__block修飾的變量做了什么?

  • 會(huì)調(diào)用block內(nèi)部的copy函數(shù)
  • copy函數(shù)內(nèi)部會(huì)調(diào)用_Block_object_assign函數(shù)
  • _Block_object_assign函數(shù)會(huì)對(duì)__block變量形成強(qiáng)引用(retain)
  • 對(duì)于__block 修飾的變量 assign函數(shù)對(duì)其強(qiáng)引用;對(duì)于外部對(duì)象 assign函數(shù)根據(jù)外部如何引用而引用
image
image

Q:當(dāng)block從堆中移除時(shí),對(duì)__block修飾的變量做了什么?

  • 會(huì)調(diào)用block內(nèi)部的dispose函數(shù)
  • dispose函數(shù)內(nèi)部會(huì)調(diào)用_Block_object_dispose函數(shù)
  • _Block_object_dispose函數(shù)會(huì)自動(dòng)釋放引用的__block變量(release)
image
image

Q:block對(duì)象類(lèi)型的auto變量、__block變量的區(qū)別?

從幾方面回答:

1.當(dāng)block在棧上時(shí),對(duì)它們都不會(huì)產(chǎn)生強(qiáng)引用
2.當(dāng)block拷貝到堆上時(shí)
  • 都會(huì)通過(guò)copy函數(shù)來(lái)處理它們
  • 對(duì)于__block 修飾的變量 assign函數(shù)對(duì)其強(qiáng)引用;對(duì)于外部對(duì)象 assign函數(shù)根據(jù)外部如何引用而引用
    __block變量(假設(shè)變量名叫做a)
_Block_object_assign((void*)&dst->a, (void*)src->a, 8/*BLOCK_FIELD_IS_BYREF*/);

對(duì)象類(lèi)型的auto變量(假設(shè)變量名叫做p)

_Block_object_assign((void*)&dst->p, (void*)src->p, 3/*BLOCK_FIELD_IS_OBJECT*/);

3.當(dāng)block從堆上移除時(shí)

  • 都會(huì)通過(guò)dispose函數(shù)來(lái)釋放它們
    __block變量(假設(shè)變量名叫做a)
_Block_object_dispose((void*)src->a, 8/*BLOCK_FIELD_IS_BYREF*/);

對(duì)象類(lèi)型的auto變量(假設(shè)變量名叫做p)

_Block_object_dispose((void*)src->p, 3/*BLOCK_FIELD_IS_OBJECT*/);

4.__block的__forwarding指針

  • 棧上__block的__forwarding指向本身
  • 棧上__block復(fù)制到堆上后,棧上block的__forwarding指向堆上的block,堆上block的__forwarding指向本身
image

Q:被__block修飾的對(duì)象類(lèi)型在block上如何操作的?

分幾方面回答:

1.當(dāng)__block變量在棧上時(shí),不會(huì)對(duì)指向的對(duì)象產(chǎn)生強(qiáng)引用
2.當(dāng)__block變量被copy到堆時(shí)
  • 會(huì)調(diào)用__block變量?jī)?nèi)部的copy函數(shù)
  • copy函數(shù)內(nèi)部會(huì)調(diào)用_Block_object_assign函數(shù)
  • _Block_object_assign函數(shù)會(huì)根據(jù)所指向?qū)ο蟮男揎椃╛_strong、__weak、__unsafe_unretained)做出相應(yīng)的操作,形成強(qiáng)引用(retain)或者弱引用(注意:這里僅限于ARC時(shí)會(huì)retain,MRC時(shí)不會(huì)retain)
  • MRC環(huán)境下不會(huì)根據(jù)對(duì)象的修飾符引用,都是弱引用
3.如果__block變量從堆上移除
  • 會(huì)調(diào)用__block變量?jī)?nèi)部的dispose函數(shù)
  • dispose函數(shù)內(nèi)部會(huì)調(diào)用_Block_object_dispose函數(shù)
    _Block_object_dispose函數(shù)會(huì)自動(dòng)釋放指向的對(duì)象(release)

第六部分 block循環(huán)引用

Q:ARC下如何解決block循環(huán)引用的問(wèn)題?

三種方式:__weak、__unsafe_unretained、__block

1.第一種方式:__weak
Person *person = [[Person alloc] init];
//        __weak Person *weakPerson = person;
__weak typeof(person) weakPerson = person;

person.block = ^{
    NSLog(@"age is %d", weakPerson.age);
};

2.第二種方式:__unsafe_unretained
__unsafe_unretained Person *person = [[Person alloc] init];
person.block = ^{
    NSLog(@"age is %d", weakPerson.age);
};

3.第三種方式:__block
__block Person *person = [[Person alloc] init];
person.block = ^{
    NSLog(@"age is %d", person.age);
    person = nil;
};
person.block();

4.三種方法比較
  • __weak:不會(huì)產(chǎn)生強(qiáng)引用,指向的對(duì)象銷(xiāo)毀時(shí),會(huì)自動(dòng)讓指針置為nil
  • __unsafe_unretained:不會(huì)產(chǎn)生強(qiáng)引用,不安全,指向的對(duì)象銷(xiāo)毀時(shí),指針存儲(chǔ)的地址值不變
  • __block:必須把引用對(duì)象置位nil,并且要調(diào)用該block
image
image

Q:MRC下如何解決block循環(huán)引用的問(wèn)題?

兩種方式:__unsafe_unretained、__block

1.第一種方式:__unsafe_unretained
__unsafe_unretained Person *person = [[Person alloc] init];
person.block = ^{
    NSLog(@"age is %d", weakPerson.age);
};

2.第二種方式:__block
__block Person *person = [[Person alloc] init];
person.block = ^{
    NSLog(@"age is %d", person.age);
};

最后編輯于
?著作權(quán)歸作者所有,轉(zhuǎn)載或內(nèi)容合作請(qǐng)聯(lián)系作者
【社區(qū)內(nèi)容提示】社區(qū)部分內(nèi)容疑似由AI輔助生成,瀏覽時(shí)請(qǐng)結(jié)合常識(shí)與多方信息審慎甄別。
平臺(tái)聲明:文章內(nèi)容(如有圖片或視頻亦包括在內(nèi))由作者上傳并發(fā)布,文章內(nèi)容僅代表作者本人觀(guān)點(diǎn),簡(jiǎn)書(shū)系信息發(fā)布平臺(tái),僅提供信息存儲(chǔ)服務(wù)。

友情鏈接更多精彩內(nèi)容