一.Block类型
block是Block_layout类型
Block_layout结构如下：
struct Block_layout {
void *isa;
volatile int32_t flags; // contains ref count
int32_t reserved;
BlockInvokeFunction invoke;
struct Block_descriptor_1 *descriptor; //
// imported variables
};
Block_descriptor结构如下：
#define BLOCK_DESCRIPTOR_1 1
struct Block_descriptor_1 {
uintptr_t reserved;
uintptr_t size;
};
\
// 可选
#define BLOCK_DESCRIPTOR_2 1
struct Block_descriptor_2 {
// requires BLOCK_HAS_COPY_DISPOSE
BlockCopyFunction copy;
BlockDisposeFunction dispose;
};
\
#define BLOCK_DESCRIPTOR_3 1
struct Block_descriptor_3 {
// requires BLOCK_HAS_SIGNATURE
const char *signature;
const char *layout; // contents depend on BLOCK_HAS_EXTENDED_LAYOUT
};
Block_layout中的flag枚举：
Values for Block_layout->flags to describe block objects
enum {
BLOCK_DEALLOCATING = (0x0001), // runtime
BLOCK_REFCOUNT_MASK = (0xfffe), // runtime
BLOCK_NEEDS_FREE = (1 << 24), // runtime
BLOCK_HAS_COPY_DISPOSE = (1 << 25), // compiler
BLOCK_HAS_CTOR = (1 << 26), // compiler: helpers have C++ code
BLOCK_IS_GC = (1 << 27), // runtime
BLOCK_IS_GLOBAL = (1 << 28), // compiler
BLOCK_USE_STRET = (1 << 29), // compiler: undefined if !BLOCK_HAS_SIGNATURE
BLOCK_HAS_SIGNATURE = (1 << 30), // compiler
BLOCK_HAS_EXTENDED_LAYOUT=(1 << 31) // compiler
};
block在编译时就已经对进行了拷贝，无论你是否调用。
Block的三层拷贝：
一.block为啥能捕获变量：
1.在创建block时，block自身被当作型参数传入了block内部，只是在OC层面是隐形参数。
Block的拷贝顺序：
1.block第一层拷贝是调用_Block_copy对自身进行拷贝。
// 栈 -> 堆 研究拷贝
void *_Block_copy(const void *arg) {
struct Block_layout *aBlock;
\
if (!arg) return NULL;
// The following would be better done as a switch statement
aBlock = (struct Block_layout *)arg;
if (aBlock->flags & BLOCK_NEEDS_FREE) {
// latches on high
latching_incr_int(&aBlock->flags);
return aBlock;
}
else if (aBlock->flags & BLOCK_IS_GLOBAL) {
return aBlock; // 不需要
}
else { // 栈
// Its a stack block. Make a copy.
struct Block_layout *result =
(struct Block_layout *)malloc(aBlock->descriptor->size);
if (!result) return NULL;
memmove(result, aBlock, aBlock->descriptor->size); // bitcopy first
#if __has_feature(ptrauth_calls)
// Resign the invoke pointer as it uses address authentication.
result->invoke = aBlock->invoke;
#endif
// reset refcount
result->flags &= ~(BLOCK_REFCOUNT_MASK|BLOCK_DEALLOCATING); // XXX not needed
result->flags |= BLOCK_NEEDS_FREE | 2; // logical refcount 1
_Block_call_copy_helper(result, aBlock);
// Set isa last so memory analysis tools see a fully-initialized object.
result->isa = _NSConcreteMallocBlock;
return result;
}
}
2.block如果捕获了外界外界变量时，会对变量进行一次拷贝，保存变量和block在同一片内存。(捕获一般变量，如：int，float)
// __block 变量
void _Block_object_assign(void *destArg, const void *object, const int flags) {
const void **dest = (const void **)destArg;
switch (os_assumes(flags & BLOCK_ALL_COPY_DISPOSE_FLAGS)) {
case BLOCK_FIELD_IS_OBJECT:
/*******
id object = ...;
[^{ object; } copy];
********/
// objc 指针地址 weakSelf (self)
// arc
_Block_retain_object(object);
// 持有
*dest = object;
break;
case BLOCK_FIELD_IS_BLOCK:
/*******
void (^object)(void) = ...;
[^{ object; } copy];
********/
// block 被一个 block 捕获
*dest = _Block_copy(object);
break;
case BLOCK_FIELD_IS_BYREF | BLOCK_FIELD_IS_WEAK:
case BLOCK_FIELD_IS_BYREF:
/*******
// copy the onstack __block container to the heap
// Note this __weak is old GC-weak/MRC-unretained.
// ARC-style __weak is handled by the copy helper directly.
__block ... x;
__weak __block ... x;
[^{ x; } copy];
********/
*dest = _Block_byref_copy(object);
break;
case BLOCK_BYREF_CALLER | BLOCK_FIELD_IS_OBJECT:
case BLOCK_BYREF_CALLER | BLOCK_FIELD_IS_BLOCK:
/*******
// copy the actual field held in the __block container
// Note this is MRC unretained __block only.
// ARC retained __block is handled by the copy helper directly.
__block id object;
__block void (^object)(void);
[^{ object; } copy];
********/
*dest = object;
break;
case BLOCK_BYREF_CALLER | BLOCK_FIELD_IS_OBJECT | BLOCK_FIELD_IS_WEAK:
case BLOCK_BYREF_CALLER | BLOCK_FIELD_IS_BLOCK | BLOCK_FIELD_IS_WEAK:
/*******
// copy the actual field held in the __block container
// Note this __weak is old GC-weak/MRC-unretained.
// ARC-style __weak is handled by the copy helper directly.
__weak __block id object;
__weak __block void (^object)(void);
[^{ object; } copy];
********/
*dest = object;
break;
default:
break;
}
}
3.block捕获__block修饰的外界变量时，有如下情况：
struct Block_byref_2 {
// requires BLOCK_BYREF_HAS_COPY_DISPOSE
BlockByrefKeepFunction byref_keep; // 结构体 __block 对象
BlockByrefDestroyFunction byref_destroy;
};
__block修饰的变量就会多出 Block_byref_2
如何查看block底层原理：
1.Object-C的编译前端是LLVM，所以只需要使用LLVM的子集clang进行如下操作：
clang -rewrite-objc main.m -o main.cpp 把目标文件编译成c++文件 UIKit报错问题
clang -rewrite-objc -fobjc-arc -fobjc-runtime=ios-13.0.0 -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/ iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator14.0.sdk main.m
2.Xcode对clang进行了封装，所以使用xcrun编译更方便，一般不用考虑SDK的版本，所以相对好用些。如下：
`xcode`安装的时候顺带安装了`xcrun`命令，`xcrun`命令在`clang`的基础上进行了 一些封装，要更好用一些
xcrun -sdk iphonesimulator clang -arch arm64 -rewrite-objc ViewController.m -o ViewController-arm64.cpp (模拟器)
xcrun -sdk iphoneos clang -arch arm64 -rewrite-objc ViewController.m -o ViewController-arm64.cpp (手机)
操作如下，cd到文件目录，进行上面的命令就可以，如下图：
剩下就自己查看.cpp文件了。 如果是swift的话，这个命令是无效的，因为swift的编译前端是swift，后续在更新swift是如何编译的。
