深入理解Block之Block的類(lèi)型

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深入理解Block之Block的類(lèi)型

當(dāng)我在 2012 年剛剛開(kāi)始從事 iOS 開(kāi)發(fā)工作時(shí),對(duì) Block 的使用開(kāi)始逐漸在 iOS 開(kāi)發(fā)者中推廣開(kāi)來(lái)(Block 的第一個(gè)穩(wěn)定 ABI 版本是在 Mac OS X 10.6 被引入的。)。作為 iOS 開(kāi)發(fā)中非常吸引我的一個(gè)特性,對(duì)其的深入分析自然必不可少。

重要聲明:雖然我已經(jīng)仔細(xì)的檢查了自己的相關(guān)代碼和相關(guān)的措辭,但是請(qǐng)不要盲目相信本文的正確性。我已經(jīng)見(jiàn)過(guò)非常多的經(jīng)驗(yàn)開(kāi)發(fā)者對(duì)于 Block 有錯(cuò)誤的理解(我也不會(huì)例外)。請(qǐng)一定保持一顆懷疑的心。

<a name="1"></a>類(lèi)型簡(jiǎn)介

對(duì) block 稍微有所了解的人都知道,block 會(huì)在編譯過(guò)程中,會(huì)被當(dāng)做結(jié)構(gòu)體進(jìn)行處理。 其結(jié)構(gòu)Block-ABI-Apple大概是這樣的:

struct Block_literal_1 {
    void *isa; // initialized to &_NSConcreteStackBlock or &_NSConcreteGlobalBlock
    int flags;
    int reserved;
    void (*invoke)(void *, ...);
    struct Block_descriptor_1 {
    unsigned long int reserved;         // NULL
        unsigned long int size;         // sizeof(struct Block_literal_1)
        // optional helper functions
        void (*copy_helper)(void *dst, void *src);     // IFF (1<<25)
        void (*dispose_helper)(void *src);             // IFF (1<<25)
        // required ABI.2010.3.16
        const char *signature;                         // IFF (1<<30)
    } *descriptor;
    // imported variables
};

isa 指針會(huì)指向 block 所屬的類(lèi)型,用于幫助運(yùn)行時(shí)系統(tǒng)進(jìn)行處理。

Block 常見(jiàn)的類(lèi)型有三種,分別是 _NSConcreteStackBlock _NSConcreteMallocBlock _NSConcreteGlobalBlock

另外還包括只在GC環(huán)境下使用的 _NSConcreteFinalizingBlock _NSConcreteAutoBlock _NSConcreteWeakBlockVariable

下面摘自 libclosure-65 - Block_private.h-213

// the raw data space for runtime classes for blocks
// class+meta used for stack, malloc, and collectable based blocks
BLOCK_EXPORT void * _NSConcreteMallocBlock[32]
    __OSX_AVAILABLE_STARTING(__MAC_10_6, __IPHONE_3_2);
BLOCK_EXPORT void * _NSConcreteAutoBlock[32]
    __OSX_AVAILABLE_STARTING(__MAC_10_6, __IPHONE_3_2);
BLOCK_EXPORT void * _NSConcreteFinalizingBlock[32]
    __OSX_AVAILABLE_STARTING(__MAC_10_6, __IPHONE_3_2);
BLOCK_EXPORT void * _NSConcreteWeakBlockVariable[32]
    __OSX_AVAILABLE_STARTING(__MAC_10_6, __IPHONE_3_2);
// declared in Block.h
// BLOCK_EXPORT void * _NSConcreteGlobalBlock[32];
// BLOCK_EXPORT void * _NSConcreteStackBlock[32];

<a name="1.1"></a>_NSConcreteGlobalBlock & _NSConcreteStackBlock

_NSConcreteGlobalBlock & _NSConcreteStackBlock 是 block 初始化時(shí)設(shè)置的類(lèi)型(上文中 Block-ABI-Apple 已經(jīng)提及,并且 CGBlocks_8cpp_source.html#l00141 也提到過(guò))。

在以下情況中,block 會(huì)初始化為 _NSConcreteGlobalBlock

 if (!block->hasCaptures()) {
   info.StructureType =
     llvm::StructType::get(CGM.getLLVMContext(), elementTypes, true);
   info.CanBeGlobal = true;
   return;
 }
統(tǒng)計(jì)需要布局(layout)的變量:
* `this` (為了訪問(wèn) `c++` 的成員變量和函數(shù),需要 `this` 指針)
* 依次按下列規(guī)則處理捕獲的變量:
    * 不需要計(jì)算布局的變量:
        * 生命周期為靜態(tài)的變量(被 `const` `static` 修飾的變量,不被函數(shù)包含的靜態(tài)常量,c++中生命周期為靜態(tài)的變量)
        * 函數(shù)參數(shù)
    * 需要計(jì)算布局的變量:被 `__block` 修飾的變量,以上未提到的類(lèi)型(比如block)

 
**Tips**:當(dāng)需要布局(layout)的變量的統(tǒng)計(jì)完畢后,會(huì)按照以下順序進(jìn)行一次穩(wěn)定排序。
 
*  __strong 修飾的變量
*  ByRef 類(lèi)型
*  __weak 修飾的變量
*  其它類(lèi)型

<a name="1.2"></a>_NSConcreteMallocBlock

在非垃圾收集環(huán)境下,當(dāng) _NSConcreteStackBlock 類(lèi)型的block 被真正復(fù)制時(shí),在 _Block_copy_internal 方法內(nèi)部,會(huì)轉(zhuǎn)換為 _NSConcreteMallocBlock libclosure-65/runtime.c

// Its a stack block.  Make a copy.
if (!isGC) {
    struct Block_layout *result = malloc(aBlock->descriptor->size);
    if (!result) return NULL;
    memmove(result, aBlock, aBlock->descriptor->size); // bitcopy first
    // reset refcount
    result->flags &= ~(BLOCK_REFCOUNT_MASK|BLOCK_DEALLOCATING);    // XXX not needed
    result->flags |= BLOCK_NEEDS_FREE | 2;  // logical refcount 1
    result->isa = _NSConcreteMallocBlock;
    _Block_call_copy_helper(result, aBlock);
    return result;
}

<a name="1.3"></a>_NSConcreteFinalizingBlock&_NSConcreteAutoBlock

在垃圾收集環(huán)境下,當(dāng) block 被復(fù)制時(shí),如果block 有 ctors & dtors 時(shí),則會(huì)轉(zhuǎn)換為 _NSConcreteFinalizingBlock 類(lèi)型,反之,則會(huì)轉(zhuǎn)換為 _NSConcreteAutoBlock 類(lèi)型

if (hasCTOR) {
    result->isa = _NSConcreteFinalizingBlock;
}
else {
    result->isa = _NSConcreteAutoBlock;
}

_NSConcreteWeakBlockVariable

GC環(huán)境下,當(dāng)對(duì)象被 __weak __block 修飾,且從棧復(fù)制到堆時(shí),block 會(huì)被標(biāo)記為 _NSConcreteWeakBlockVariable 類(lèi)型。

bool isWeak = ((flags & (BLOCK_FIELD_IS_BYREF|BLOCK_FIELD_IS_WEAK)) == (BLOCK_FIELD_IS_BYREF|BLOCK_FIELD_IS_WEAK));
// if its weak ask for an object (only matters under GC)
struct Block_byref *copy = (struct Block_byref *)_Block_allocator(src->size, false, isWeak);
copy->flags = src->flags | _Byref_flag_initial_value; // non-GC one for caller, one for stack
copy->forwarding = copy; // patch heap copy to point to itself (skip write-barrier)
src->forwarding = copy;  // patch stack to point to heap copy
copy->size = src->size;
if (isWeak) {
  copy->isa = &_NSConcreteWeakBlockVariable;  // mark isa field so it gets weak scanning
}

<a name="2"></a>ARC環(huán)境的特殊處理

下面的代碼均通過(guò)添加 objc_retainBlock _Block_copy_Block_copy_internal 符號(hào)斷點(diǎn)進(jìn)行測(cè)試

  • 在 ARC 下,block 類(lèi)型通過(guò)=進(jìn)行傳遞時(shí),會(huì)導(dǎo)致調(diào)用objc_retainBlock->_Block_copy->_Block_copy_internal方法鏈。并導(dǎo)致 __NSStackBlock__ 類(lèi)型的 block 轉(zhuǎn)換為 __NSMallocBlock__ 類(lèi)型。


objc4-680/runtime/NSObject.mm-193
提及到了這一點(diǎn)。

```
//
// The -fobjc-arc flag causes the compiler to issue calls to objc_{retain/release/autorelease/retain_block}
//

id objc_retainBlock(id x) {
    return (id)_Block_copy(x);
}

...

void *_Block_copy(const void *arg) {
   return _Block_copy_internal(arg, true);
}
```

測(cè)試代碼:

void test() {
    __block int i = 0;
    dispatch_block_t block  = ^(){NSLog(@"%@", @(i)); };
    dispatch_block_t block1 = block;
    NSLog(@"初始化為變量后再打印:%@", block1);

    NSLog(@"直接打印:%@", ^(){NSLog(@"%@", @(i)); });
}

日志:


"objc_retainBlock 函數(shù)被調(diào)用"

"_Block_copy 函數(shù)被調(diào)用"

"_Block_copy_internal 函數(shù)被調(diào)用"

"objc_retainBlock 函數(shù)被調(diào)用"

"_Block_copy 函數(shù)被調(diào)用"

"_Block_copy_internal 函數(shù)被調(diào)用"

初始化為變量后再打印:<__NSMallocBlock__: 0x7fb05b605800>
直接打印:<__NSStackBlock__: 0x7fff55ccc568>

  • 在 ARC 下,不同的屬性修飾符以及不同賦值、取值方式均會(huì)對(duì)方法調(diào)用產(chǎn)生影響。下表為測(cè)試結(jié)果。
\ strong retain copy
直接賦值 _Block_copy->_Block_copy_internal _Block_copy->_Block_copy_internal 無(wú)
間接賦值 _Block_copy->_Block_copy_internal _Block_copy->_Block_copy_internal _Block_copy->_Block_copy_internal
通過(guò)屬性取值 _Block_copy->_Block_copy_internal-> _Block_copy->_Block_copy_internal _Block_copy->_Block_copy_internal-> _Block_copy->_Block_copy_internal _Block_copy->_Block_copy_internal-> _Block_copy->_Block_copy_internal _Block_copy->_Block_copy_internal-> _Block_copy->_Block_copy_internal
通過(guò)變量取值 無(wú) 無(wú) 無(wú)

直接賦值:

NSString *str = @"sun";
dispatch_block_t block = ^(){
    NSLog(@"%@", str);
};
self.block = block;

間接賦值:

self.block = ^(){
    NSLog(@"%@", str);
};

通過(guò)屬性取值

self.block

通過(guò)變量取值

self->_block

測(cè)試代碼:

  
- (void)test {

    NSString *str = @"sun";
    {
        NSLog(@"直接賦值開(kāi)始");
        {
            self.copyBlock = ^(){
                NSLog(@"%@", str);
            };

            NSLog(@"copy 屬性修飾的 block:%@", self->_copyBlock);
        }
        {
            self.strongBlock = ^(){
                NSLog(@"%@", str);
            };

            NSLog(@"strong 屬性修飾的 block:%@", self->_strongBlock);
        }
        {
            self.retainBlock = ^(){
                NSLog(@"%@", str);
            };

            NSLog(@"retain 屬性修飾的 block:%@", self->_retainBlock);
        }
        NSLog(@"直接賦值結(jié)束");
    }
    {
        dispatch_block_t copyBlock = ^(){
            NSLog(@"%@", str);
        };
        dispatch_block_t strongBlock = ^(){
            NSLog(@"%@", str);
        };
        dispatch_block_t retainBlock = ^(){
            NSLog(@"%@", str);
        };
        NSLog(@"間接賦值開(kāi)始");
        {
            self.copyBlock = copyBlock;

            NSLog(@"copy 屬性修飾的 block:%@", self->_copyBlock);
        }
        {
            self.strongBlock = strongBlock;

            NSLog(@"strong 屬性修飾的 block:%@", self->_strongBlock);
        }
        {
            self.retainBlock = retainBlock;

            NSLog(@"retain 屬性修飾的 block:%@", self->_retainBlock);
        }
        NSLog(@"間接賦值結(jié)束");
    }
    {
        NSLog(@"通過(guò)屬性獲取開(kāi)始");
        {
            NSLog(@"copy 屬性修飾的 block:%@", self.copyBlock);

            NSLog(@"strong 屬性修飾的 block:%@", self.strongBlock);

            NSLog(@"retain 屬性修飾的 block:%@", self.retainBlock);
        }

        NSLog(@"獲取結(jié)束");
    }

    {
        NSLog(@"通過(guò)變量獲取開(kāi)始");
        {
            NSLog(@"copy 屬性修飾的 block:%@", self->_copyBlock);

            NSLog(@"strong 屬性修飾的 block:%@", self->_strongBlock);

            NSLog(@"retain 屬性修飾的 block:%@", self->_retainBlock);
        }

        NSLog(@"獲取結(jié)束");
    }
}

日志:


間接賦值開(kāi)始


"_Block_copy 函數(shù)被調(diào)用"

"_Block_copy_internal 函數(shù)被調(diào)用"

copy 屬性修飾的 block:<__NSMallocBlock__: 0x7fab00fa4c30>


"_Block_copy 函數(shù)被調(diào)用"

"_Block_copy_internal 函數(shù)被調(diào)用"

strong 屬性修飾的 block:<__NSMallocBlock__: 0x7fab00d1a970>


"_Block_copy 函數(shù)被調(diào)用"

"_Block_copy_internal 函數(shù)被調(diào)用"

retain 屬性修飾的 block:<__NSMallocBlock__: 0x7fab00f08ad0>


間接賦值結(jié)束


通過(guò)屬性獲取開(kāi)始


"_Block_copy 函數(shù)被調(diào)用"

"_Block_copy_internal 函數(shù)被調(diào)用"

"_Block_copy 函數(shù)被調(diào)用"

"_Block_copy_internal 函數(shù)被調(diào)用"

copy 屬性修飾的 block:<__NSMallocBlock__: 0x7fab00fa4c30>


"_Block_copy 函數(shù)被調(diào)用"

"_Block_copy_internal 函數(shù)被調(diào)用"

"_Block_copy 函數(shù)被調(diào)用"

"_Block_copy_internal 函數(shù)被調(diào)用"

strong 屬性修飾的 block:<__NSMallocBlock__: 0x7fab00d1a970>


"_Block_copy 函數(shù)被調(diào)用"

"_Block_copy_internal 函數(shù)被調(diào)用"

"_Block_copy 函數(shù)被調(diào)用"

"_Block_copy_internal 函數(shù)被調(diào)用"

retain 屬性修飾的 block:<__NSMallocBlock__: 0x7fab00f08ad0>


獲取結(jié)束

通過(guò)變量獲取開(kāi)始

copy 屬性修飾的 block:<__NSMallocBlock__: 0x7fab00fa4c30>
strong 屬性修飾的 block:<__NSMallocBlock__: 0x7fab00d1a970>
retain 屬性修飾的 block:<__NSMallocBlock__: 0x7fab00f08ad0>

獲取結(jié)束
(lldb) 
  
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