概述

代碼塊Block是蘋(píng)果在iOS4開(kāi)始引入的對(duì)C語(yǔ)言的擴(kuò)展,用來(lái)實(shí)現(xiàn)匿名函數(shù)的特性,Block是一種特殊的數(shù)據(jù)類型,其可以正常定義變量却特、作為參數(shù)堪藐、作為返回值,特殊地,Block還可以保存一段代碼,在需要的時(shí)候調(diào)用,目前Block已經(jīng)廣泛應(yīng)用于iOS開(kāi)發(fā)中,常用于GCD曾棕、動(dòng)畫(huà)获列、排序及各類回調(diào)

block 會(huì)在編譯過(guò)程中粱胜，會(huì)被當(dāng)做結(jié)構(gòu)體進(jìn)行處理烛占。 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 所屬的類型，用于幫助運(yùn)行時(shí)系統(tǒng)進(jìn)行處理胁出。
Block 常見(jiàn)的類型有三種离唬，分別是` _NSConcreteStackBlock ` ` _NSConcreteMallocBlock` `_NSConcreteGlobalBlock` 。
另外還包括只在GC環(huán)境下使用的 `_NSConcreteFinalizingBlock` `_NSConcreteAutoBlock`  ` _NSConcreteWeakBlockVariable`划鸽。

下面摘自 [libclosure-65 – Block_private.h-213](http://opensource.apple.com/source/libclosure/libclosure-65/runtime.c)
```objectivec
// 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];```

##Block的幾種類型
1._NSConcreteGlobalBlock &  _NSConcreteStackBlock
2._NSConcreteMallocBlock
3._NSConcreteFinalizingBlock &  _NSConcreteAutoBlock
4._NSConcreteWeakBlockVariable

### _NSConcreteGlobalBlock &  _NSConcreteStackBlock 
`_NSConcreteGlobalBlock`& ` _NSConcreteStackBlock `是 block 初始化時(shí)設(shè)置的類型。
在以下情況中，block 會(huì)初始化為` _NSConcreteGlobalBlock`:

1.未捕獲外部變量裸诽。
在 [static void computeBlockInfo(CodeGenModule &CGM, CodeGenFunction *CGF,CGBlockInfo &info)](http://clang.llvm.org/doxygen/CGBlocks_8cpp_source.html#l00326) 函數(shù)內(nèi)的 [334行](http://clang.llvm.org/doxygen/CGBlocks_8cpp_source.html#l00334) 至 [339行](http://clang.llvm.org/doxygen/CGBlocks_8cpp_source.html#l00339)嫂用，通過(guò)判斷 block(以及嵌套的block) 是否捕捉了本地存儲(chǔ)(原文為：local storage)，未捕獲時(shí)丈冬，block 會(huì)初始化為`_NSConcreteGlobalBlock`
```objectivec
 if (!block->hasCaptures()) {
   info.StructureType =
     llvm::StructType::get(CGM.getLLVMContext(), elementTypes, true);
   info.CanBeGlobal = true;
   return;
 }

2.當(dāng)需要布局（layout）的變量的數(shù)量為0時(shí)嘱函。
在static void computeBlockInfo(CodeGenModule &CGM, CodeGenFunction *CGF,CGBlockInfo &info)函數(shù)內(nèi)，通過(guò)計(jì)算 block 的布局(layout)埂蕊。當(dāng)需要布局的變量為0時(shí)往弓，block 會(huì)初始化為_NSConcreteGlobalBlock

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 修飾的變量蓄氧，不被函數(shù)包含的靜態(tài)常量函似，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，以上未提到的類型（比如block）
    Tips：當(dāng)需要布局（layout）的變量的統(tǒng)計(jì)完畢后喉童，會(huì)按照以下順序進(jìn)行一次穩(wěn)定排序撇寞。
• __strong 修飾的變量
• ByRef 類型
• __weak 修飾的變量
• 其它類型

_NSConcreteMallocBlock

在非垃圾收集環(huán)境下，當(dāng)_NSConcreteStackBlock類型的block 被真正復(fù)制時(shí)堂氯，在_Block_copy_internal 方法內(nèi)部蔑担，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;
}```

###_NSConcreteFinalizingBlock & _NSConcreteAutoBlock
在垃圾收集環(huán)境下，當(dāng) block 被復(fù)制時(shí)咽白，如果block 有 ctors & dtors 時(shí)啤握，則會(huì)轉(zhuǎn)換為 `_NSConcreteFinalizingBlock`類型，反之晶框，則會(huì)轉(zhuǎn)換為`_NSConcreteAutoBlock` 類型

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

_NSConcreteWeakBlockVariable

GC環(huán)境下排抬，當(dāng)對(duì)象被__weak __block修飾，且從棧復(fù)制到堆時(shí)三妈，block 會(huì)被標(biāo)記為 _NSConcreteWeakBlockVariable
類型畜埋。

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
}```