block本質探尋五之atuto類型局部實例對象

說明：閱讀本文章，請參考之前的block文章加以理解；一、棧區block分析 //代碼 //列印分析： <1>block代碼內部引用的Person實例對象先於輸出語句銷毀，因為per僅限於大括弧內，但此時block銷毀了沒有？往下看； <2>上述block代碼塊並沒有被指針持有，接下來看看指針持 ...

說明：閱讀本文章，請參考之前的block文章加以理解；

一、棧區block分析

//代碼

//ARC
void test1()
{
    {
        Person *per = [[Person alloc] init];
        per.age = 10;
        ^{
            NSLog(@"age:%d", per.age);
        };
    }
    
    NSLog(@"-------1");
}

//列印

2019-01-14 17:24:12.118653+0800 MJ_TEST[6638:285938] Person dealloc
2019-01-14 17:24:12.118934+0800 MJ_TEST[6638:285938] -------1
Program ended with exit code: 0

分析：

<1>block代碼內部引用的Person實例對象先於輸出語句銷毀，因為per僅限於大括弧內，但此時block銷毀了沒有？往下看；

<2>上述block代碼塊並沒有被指針持有，接下來看看指針持有的情況；

//代碼

typedef void(^MyBlock)(void);

//ARC
void test2()
{
    MyBlock block;
    
    {
        Person *per = [[Person alloc] init];
        per.age = 10;
        block = ^{
            NSLog(@"age:%d", per.age);
        };
    }
    
    NSLog(@"-------1");
}

//列印

2019-01-14 17:34:58.473267+0800 MJ_TEST[6824:293129] -------1
2019-01-14 17:34:58.473705+0800 MJ_TEST[6824:293129] Person dealloc
Program ended with exit code: 0

分析：Person實例對象後於輸出語句銷毀，為什麼有指針持有，順序就變了？

<1>等號左邊：是一個auto類型的局部的block指針變數，存放在棧區；等號右邊：是一個block代碼塊（對象），也是一個局部對象，存放在棧區；

<2>在ARC模式下，如果有指針持有（預設是強指針，修飾符為__strong）一個局部的block對象，系統會自動copy該block對象從棧區到堆區；

補充：其他三種情況——block作為函數返回值、含usingBlock方法（如數據的枚舉方法）、GCD的應用（自己可以驗證，此處不再贅述）；

那麼，我們再看看MRC的情況

//列印————test1()和test2()

2019-01-14 17:56:46.641171+0800 MJ_TEST[7171:306788] -------1
Program ended with exit code: 0

分析：為什麼per對象沒有銷毀？——因為需要手動釋放；

//代碼

[per release];

//列印————test1()和test2()

2019-01-14 17:59:39.091313+0800 MJ_TEST[7243:309139] Person dealloc
2019-01-14 17:59:39.091974+0800 MJ_TEST[7243:309139] -------1
2019-01-14 17:59:39.092013+0800 MJ_TEST[7243:309139] Person dealloc
2019-01-14 17:59:39.092086+0800 MJ_TEST[7243:309139] -------1
Program ended with exit code: 0

分析：

<1>此時的block對象的作用域在第一個大括弧範圍內，超出則被釋放；

<2>Person實例對象被捕獲到block對象結構體體中，同時其作用域也僅限於第一個大括弧內，因此超出同樣被釋放；

二、堆區block分析

1）類型分析——ARC

//strong類型

執行上述test2()方法，我們知道系統會自動將block對象從棧區copy到堆區；同時，Person實例對象會被捕捉到block對象的結構體中，如下

struct __test2_block_impl_0 {
  struct __block_impl impl;
  struct __test2_block_desc_0* Desc;
  Person *per;
  __test2_block_impl_0(void *fp, struct __test2_block_desc_0 *desc, Person *_per, int flags=0) : per(_per) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};

分析：可以看到per是一個指針變數，而該指針變數預設修飾符為__strong；修改代碼

 __strong Person *per = [[Person alloc] init];

clang命令：xcrun -sdk iphoneos clang -arch arm64 -rewrite-objc -fobjc-arc -fobjc-runtime=ios-9.0.0 main.m

說明：

<1>該命令行，只針對ARC模式下，MRC模式下，如果有release語句會報錯；

<2>該命令行，是解決ARC模式下，實例對象為__weak類型，轉成C++代碼；

struct __test2_block_impl_0 {
  struct __block_impl impl;
  struct __test2_block_desc_0* Desc;
  Person *__strong per;
  __test2_block_impl_0(void *fp, struct __test2_block_desc_0 *desc, Person *__strong _per, int flags=0) : per(_per) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};

分析：因此，一般的指針變數，預設修飾符為__strong；

//weak類型

//代碼

//ARC
void test2()
{
    MyBlock block;
    
    {
        Person *per = [[Person alloc] init];
        per.age = 10;
        __weak Person *weakPer = per;
        block = ^{
            NSLog(@"age:%d", weakPer.age);
        };
//        [per release];
    }
    
    NSLog(@"-------1");
}

//C++代碼

struct __test2_block_impl_0 {
  struct __block_impl impl;
  struct __test2_block_desc_0* Desc;
  Person *__weak weakPer;
  __test2_block_impl_0(void *fp, struct __test2_block_desc_0 *desc, Person *__weak _weakPer, int flags=0) : weakPer(_weakPer) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};

//列印

2019-01-15 10:21:36.280746+0800 MJ_TEST[997:45906] Person dealloc
2019-01-15 10:21:36.281063+0800 MJ_TEST[997:45906] -------1
Program ended with exit code: 0

分析：

<1>此時的per由weakPer弱指針指向，並被捕捉到block對象結構體中；

<2>結合上述強指針引用Person實例對象的列印結果，weak修飾的指針變數先於輸出語句銷毀，可以肯定系統是沒有對block對象copy到堆區的；

那針對不同類型指針的引用，系統是如果判斷操作的呢？往下看

2）調用原理

//C++代碼

typedef void(*MyBlock)(void);



struct __test2_block_impl_0 {
  struct __block_impl impl;
  struct __test2_block_desc_0* Desc;
  Person *__weak weakPer;
  __test2_block_impl_0(void *fp, struct __test2_block_desc_0 *desc, Person *__weak _weakPer, int flags=0) : weakPer(_weakPer) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};

static void __test2_block_func_0(struct __test2_block_impl_0 *__cself) {
  Person *__weak weakPer = __cself->weakPer; // bound by copy

            NSLog((NSString *)&__NSConstantStringImpl__var_folders_tb_zgsq5gq15rd3zvbdmw1c09y80000gn_T_main_3e374f_mi_2, ((int (*)(id, SEL))(void *)objc_msgSend)((id)weakPer, sel_registerName("age")));
        }

static void __test2_block_copy_0(struct __test2_block_impl_0*dst, struct __test2_block_impl_0*src) {_Block_object_assign((void*)&dst->weakPer, (void*)src->weakPer, 3/*BLOCK_FIELD_IS_OBJECT*/);}

static void __test2_block_dispose_0(struct __test2_block_impl_0*src) {_Block_object_dispose((void*)src->weakPer, 3/*BLOCK_FIELD_IS_OBJECT*/);}

static struct __test2_block_desc_0 {
  size_t reserved;
  size_t Block_size;
  void (*copy)(struct __test2_block_impl_0*, struct __test2_block_impl_0*);
  void (*dispose)(struct __test2_block_impl_0*);
} __test2_block_desc_0_DATA = { 0, sizeof(struct __test2_block_impl_0), __test2_block_copy_0, __test2_block_dispose_0};

void test2()
{
    MyBlock block;

    {
        Person *per = ((Person *(*)(id, SEL))(void *)objc_msgSend)((id)((Person *(*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("Person"), sel_registerName("alloc")), sel_registerName("init"));
        ((void (*)(id, SEL, int))(void *)objc_msgSend)((id)per, sel_registerName("setAge:"), 10);
        __attribute__((objc_ownership(weak))) Person *weakPer = per;
        block = ((void (*)())&__test2_block_impl_0((void *)__test2_block_func_0, &__test2_block_desc_0_DATA, weakPer, 570425344));

    }

    NSLog((NSString *)&__NSConstantStringImpl__var_folders_tb_zgsq5gq15rd3zvbdmw1c09y80000gn_T_main_3e374f_mi_3);
}

int main(int argc, const char * argv[]) {
    /* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool; 

        test2();
    }
    return 0;
}

分析：

<1>根據前面的文章分析，我們發現block描述結構體（__test2_block_desc_0）中多了兩個函數指針的成員變數

void (*copy)(struct __test2_block_impl_0*, struct __test2_block_impl_0*);
  void (*dispose)(struct __test2_block_impl_0*);

其中，copy函數指針指向__test2_block_copy_0函數，dispose指向__test2_block_dispose_0；

<2>__test2_block_copy_0函數主要是通過_Block_object_assign函數來確定對per對象是否強引用，其根據就是per的引用類型——如果是__strong類型，則block對象對per對對象進行強引用（per的生命周期可控）；如果是__weak類型，則進行弱引用（per的生命周期不可控）；

<3>當block對像從堆區銷毀時，會調用__test2_block_dispose_0函數會自動釋放引用的per對象（相當於release）——註：嚴格意義上，此處的釋放指的斷開是block對象對per的引用即retainCount減1，至於per對象所占的記憶體是否被釋放（回收）則在所不問（也許還有其他的指針變數引用），只有retainCount變為0零，其記憶體才會被回收；

補充：當block訪問的外部的auto類型的局部數據為對象時，則會產生上述兩個函數指針；如果是非實例對象（如基礎數據類型），則不會有上述兩個函數指針——原因：實例對象一般是在堆區開闢的記憶體，需要對其進行記憶體管理————註：如果是__block修飾前述變數（包括實例對象），也會產生上述兩個指針函數，具體後面文章會寫到！

//代碼

void test3()
{
    int age = 10;
    ^{
        NSLog(@"%d", age);
    };
}

//clang

static struct __test3_block_desc_0 {
  size_t reserved;
  size_t Block_size;
} __test3_block_desc_0_DATA = { 0, sizeof(struct __test3_block_impl_0)};

三、結論

【1】棧區block：不論是ARC還是MRC模式，指向該block對象的指針變數，不會對引用的auto類型的局部的實例對象進行強引用；

【2】堆區block：不論是ARC還是MRC模式，指向該block對象的指針變數，根據引用的auto類型的局部的實例對象的引用類型，通過調用block結構體中的copy函數指針來調用_Block_object_assign函數，來決定對實例對象是否進行強引用——__strong類型強引用，__weak類型弱引用；

【3】堆區block釋放：系統會通過調用block結構體中的dispose函數指針來調用__test2_block_dispose_0函數，自動釋放引用的外部auto類型的局部實例對象；

說明：

<1>block對象本身，即代碼塊（位於等號右邊），非block指針變數（位於等號左邊）；

<2>所謂的強引用，類似於retain操作即保留實例對象（所占記憶體不隨作用域限制而被自動回收），保證手動管理記憶體釋放，達到可控的目的；

四、拓展——GCD引用分析

1）

//代碼

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
{
    Person *per = [[Person alloc] init];
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)NSEC_PER_SEC*3), dispatch_get_main_queue(), ^{
        NSLog(@"%@", per);
    });
    
    NSLog(@"touchesBegan");
}

//列印

2019-01-15 11:56:19.867668+0800 GCD_Refrence[1558:90109] touchesBegan
2019-01-15 11:56:22.867823+0800 GCD_Refrence[1558:90109] <Person: 0x600003bb8cf0>
2019-01-15 11:56:22.867996+0800 GCD_Refrence[1558:90109] Person dealloc

2）

//代碼

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
{
    Person *per = [[Person alloc] init];
    __weak Person *weakPer = per;
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)NSEC_PER_SEC*3), dispatch_get_main_queue(), ^{
        NSLog(@"%@", weakPer);
    });
    
    NSLog(@"touchesBegan");
}

//列印

2019-01-15 11:58:58.381396+0800 GCD_Refrence[1619:93062] touchesBegan
2019-01-15 11:58:58.381583+0800 GCD_Refrence[1619:93062] Person dealloc
2019-01-15 11:59:01.381697+0800 GCD_Refrence[1619:93062] (null)

3）

//代碼

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
{
    Person *per = [[Person alloc] init];
    __weak Person *weakPer = per;
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)NSEC_PER_SEC*1), dispatch_get_main_queue(), ^{
        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)NSEC_PER_SEC*2), dispatch_get_main_queue(), ^{
            NSLog(@"%@", per);
        });
    });
    
    NSLog(@"touchesBegan");
}

//列印

2019-01-15 12:00:44.996108+0800 GCD_Refrence[1653:94592] touchesBegan
2019-01-15 12:00:48.088426+0800 GCD_Refrence[1653:94592] <Person: 0x6000010f4a20>
2019-01-15 12:00:48.088664+0800 GCD_Refrence[1653:94592] Person dealloc

4）

//代碼

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
{
    Person *per = [[Person alloc] init];
    __weak Person *weakPer = per;
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)NSEC_PER_SEC*1), dispatch_get_main_queue(), ^{
        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)NSEC_PER_SEC*2), dispatch_get_main_queue(), ^{
            NSLog(@"%@", weakPer);
        });
    });
    
    NSLog(@"touchesBegan");
}

//列印

2019-01-15 12:01:42.122836+0800 GCD_Refrence[1672:95546] touchesBegan
2019-01-15 12:01:42.123038+0800 GCD_Refrence[1672:95546] Person dealloc
2019-01-15 12:01:45.123256+0800 GCD_Refrence[1672:95546] (null)

5）

//代碼

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
{
    Person *per = [[Person alloc] init];
    __weak Person *weakPer = per;
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)NSEC_PER_SEC*1), dispatch_get_main_queue(), ^{
        NSLog(@"%@", weakPer);
        
        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)NSEC_PER_SEC*2), dispatch_get_main_queue(), ^{
            NSLog(@"%@", per);
        });
    });
    
    NSLog(@"touchesBegan");
}

//列印

2019-01-15 12:02:50.591355+0800 GCD_Refrence[1693:96581] touchesBegan
2019-01-15 12:02:51.685830+0800 GCD_Refrence[1693:96581] <Person: 0x6000033a4470>
2019-01-15 12:02:53.686541+0800 GCD_Refrence[1693:96581] <Person: 0x6000033a4470>
2019-01-15 12:02:53.686810+0800 GCD_Refrence[1693:96581] Person dealloc

6）

//代碼

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
{
    Person *per = [[Person alloc] init];
    __weak Person *weakPer = per;
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)NSEC_PER_SEC*1), dispatch_get_main_queue(), ^{
        NSLog(@"%@", per);
        
        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)NSEC_PER_SEC*2), dispatch_get_main_queue(), ^{
            NSLog(@"%@", weakPer);
        });
    });
    
    NSLog(@"touchesBegan");
}

//列印

2019-01-15 12:03:47.349637+0800 GCD_Refrence[1715:97486] touchesBegan
2019-01-15 12:03:48.447971+0800 GCD_Refrence[1715:97486] <Person: 0x600000163900>
2019-01-15 12:03:48.448271+0800 GCD_Refrence[1715:97486] Person dealloc
2019-01-15 12:03:50.448553+0800 GCD_Refrence[1715:97486] (null)

7）

//代碼

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
{
    Person *per = [[Person alloc] init];
    __weak Person *weakPer = per;
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)NSEC_PER_SEC*1), dispatch_get_main_queue(), ^{
        NSLog(@"%@", per);
        
        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)NSEC_PER_SEC*2), dispatch_get_main_queue(), ^{
            NSLog(@"%@", per);
        });
    });
    
    NSLog(@"touchesBegan");
}

//列印

2019-01-15 12:04:37.584067+0800 GCD_Refrence[1735:98332] touchesBegan
2019-01-15 12:04:38.679922+0800 GCD_Refrence[1735:98332] <Person: 0x600003bd7570>
2019-01-15 12:04:40.876560+0800 GCD_Refrence[1735:98332] <Person: 0x600003bd7570>
2019-01-15 12:04:40.876803+0800 GCD_Refrence[1735:98332] Person dealloc

分析：

<1>ARC環境下，使用GCD時，系統自動將block對象從棧區copy到堆區；

<2>根據以上列印結果，發現如果是單純的對per進行強引用，則延時3秒後per對象才銷毀；如果是弱引用，則立即銷毀，再次使用時為空（此時已經被釋放）；

<3>如果既對per強引用又有弱引用，在嵌套的GCD使用中，始終以強引用為準，弱引用不影響強引用；

結論：同一個auto類型的局部的實例對象，既有強引用，也有弱引用，以強引用為準；

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