block的内部实现，结构体是什么样的？

block的结构体如下：

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：由此可知，block也是一个对象类型，具体类型包括_NSConcreteGlobalBlock、_NSConcreteStackBlock、_NSConcreteMallocBlock。

flags：block 的负载信息（引用计数和类型信息），按位存储，也可以获取block版本兼容的相关信息。以下是flags按bit位取与的所有可能值：

enum {
    // Set to true on blocks that have captures (and thus are not true
    // global blocks) but are known not to escape for various other
    // reasons. For backward compatibility with old runtimes, whenever
    // BLOCK_IS_NOESCAPE is set, BLOCK_IS_GLOBAL is set too. Copying a
    // non-escaping block returns the original block and releasing such a
    // block is a no-op, which is exactly how global blocks are handled.
    BLOCK_IS_NOESCAPE      =  (1 << 23),

    BLOCK_HAS_COPY_DISPOSE =  (1 << 25),
    BLOCK_HAS_CTOR =          (1 << 26), // helpers have C++ code
    BLOCK_IS_GLOBAL =         (1 << 28),
    BLOCK_HAS_STRET =         (1 << 29), // IFF BLOCK_HAS_SIGNATURE
    BLOCK_HAS_SIGNATURE =     (1 << 30),
};

switch (flags & (3<<29)) {
  case (0<<29):      10.6.ABI, no signature field available
  case (1<<29):      10.6.ABI, no signature field available
  case (2<<29): ABI.2010.3.16, regular calling convention, presence of signature field
  case (3<<29): ABI.2010.3.16, stret calling convention, presence of signature field,
}

由此可知：当flags & (3<<29) is BLOCK_HAS_COPY_DISPOSE的时候，才会有copy_helper和dispose_helper函数指针。

invoke：是block具体实现函数指针地址，可以通过此地址直接调用block。

Block_descriptor_1：block的描述文内容，它包括如下：

size：block所占的内存大小

copy_helper：copy函数指针（不同版本不一定存在）

dispose_helper：dispose函数指针（不同版本不一定存在）

signature：block的实现函数的签名（不同版本不一定存在），可以通过此指针获取block的参数内容描述、返回值内容描述等

获取block的方法签名，可以参考这篇文章