objc_msgSend(receiver, selector)

objc_msgSend(receiver, selector, arg1, arg2, ...)

id objc_msgSend ( id self, SEL op, ... );

typedef struct objc_selector *SEL;

typedef struct objc_object *id;

struct objc_object {
private:
    isa_t isa;

public:

    // ISA() assumes this is NOT a tagged pointer object
    Class ISA();

    // getIsa() allows this to be a tagged pointer object
    Class getIsa();
    ... 此處省略其他方法宣告
}

typedef struct objc_class *Class;

struct objc_class : objc_object {
    // Class ISA;
    Class superclass;
    cache_t cache;             // formerly cache pointer and vtable
    class_data_bits_t bits;    // class_rw_t * plus custom rr/alloc flags
    class_rw_t *data() { 
        return bits.data();
    }
    ... 省略其他方法
}

struct cache_t {
    struct bucket_t *_buckets;
    mask_t _mask;
    mask_t _occupied;
    ... 省略其他方法
}

struct bucket_t {
private:
    cache_key_t _key;
    IMP _imp;

public:
    inline cache_key_t key() const { return _key; }
    inline IMP imp() const { return (IMP)_imp; }
    inline void setKey(cache_key_t newKey) { _key = newKey; }
    inline void setImp(IMP newImp) { _imp = newImp; }

    void set(cache_key_t newKey, IMP newImp);
};

struct class_data_bits_t {

	// Values are the FAST_ flags above.
	uintptr_t bits;
	class_rw_t* data() {
	   return (class_rw_t *)(bits & FAST_DATA_MASK);
	}
... 省略其他方法
}

// class is a Swift class
#define FAST_IS_SWIFT           (1UL<<0)
// class's instances requires raw isa
#define FAST_REQUIRES_RAW_ISA   (1UL<<1)
// class or superclass has .cxx_destruct implementation
//   This bit is aligned with isa_t->hasCxxDtor to save an instruction.
#define FAST_HAS_CXX_DTOR       (1UL<<2)
// data pointer
#define FAST_DATA_MASK          0x00007ffffffffff8UL
// class or superclass has .cxx_construct implementation
#define FAST_HAS_CXX_CTOR       (1UL<<47)
// class or superclass has default alloc/allocWithZone: implementation
// Note this is is stored in the metaclass.
#define FAST_HAS_DEFAULT_AWZ    (1UL<<48)
// class or superclass has default retain/release/autorelease/retainCount/
//   _tryRetain/_isDeallocating/retainWeakReference/allowsWeakReference
#define FAST_HAS_DEFAULT_RR     (1UL<<49)
// summary bit for fast alloc path: !hasCxxCtor and 
//   !instancesRequireRawIsa and instanceSize fits into shiftedSize
#define FAST_ALLOC              (1UL<<50)
// instance size in units of 16 bytes
//   or 0 if the instance size is too big in this field
//   This field must be LAST
#define FAST_SHIFTED_SIZE_SHIFT 51

bool isSwift() {
   return getBit(FAST_IS_SWIFT);
}

void setIsSwift() {
   setBits(FAST_IS_SWIFT);
}

class_rw_t* data() {
   return (class_rw_t *)(bits & FAST_DATA_MASK);
}

struct class_ro_t {
    uint32_t flags;
    uint32_t instanceStart;
    uint32_t instanceSize;
#ifdef __LP64__
    uint32_t reserved;
#endif

    const uint8_t * ivarLayout;

    const char * name;
    method_list_t * baseMethodList;
    protocol_list_t * baseProtocols;
    const ivar_list_t * ivars;

    const uint8_t * weakIvarLayout;
    property_list_t *baseProperties;

    method_list_t *baseMethods() const {
        return baseMethodList;
    }
};

#define RO_META               (1<<0) // class is a metaclass
#define RO_ROOT               (1<<1) // class is a root class
#define RO_HAS_CXX_STRUCTORS  (1<<2) // class has .cxx_construct/destruct implementations
// #define RO_HAS_LOAD_METHOD    (1<<3) // class has +load implementation
#define RO_HIDDEN             (1<<4) // class has visibility=hidden set
#define RO_EXCEPTION          (1<<5) // class has attribute(objc_exception): OBJC_EHTYPE_$_ThisClass is non-weak
// #define RO_REUSE_ME           (1<<6) // this bit is available for reassignment
#define RO_IS_ARC             (1<<7) // class compiled with ARC
#define RO_HAS_CXX_DTOR_ONLY  (1<<8) // class has .cxx_destruct but no .cxx_construct (with RO_HAS_CXX_STRUCTORS)
#define RO_HAS_WEAK_WITHOUT_ARC (1<<9) // class is not ARC but has ARC-style weak ivar layout 

#define RO_FROM_BUNDLE        (1<<29) // class is in an unloadable bundle - must never be set by compiler
#define RO_FUTURE             (1<<30) // class is unrealized future class - must never be set by compiler
#define RO_REALIZED           (1<<31) // class is realized - must never be set by compiler

struct class_rw_t {
    // Be warned that Symbolication knows the layout of this structure.
    uint32_t flags;
    uint32_t version;

    const class_ro_t *ro;

    method_array_t methods;
    property_array_t properties;
    protocol_array_t protocols;

    Class firstSubclass;
    Class nextSiblingClass;

    char *demangledName;

#if SUPPORT_INDEXED_ISA
    uint32_t index;
#endif
	... 省略操作 flags 的相關方法
}

#define RW_REALIZED           (1<<31) // class_t->data is class_rw_t, not class_ro_t
#define RW_FUTURE             (1<<30) // class is unresolved future class
#define RW_INITIALIZED        (1<<29) // class is initialized
#define RW_INITIALIZING       (1<<28) // class is initializing
#define RW_COPIED_RO          (1<<27) // class_rw_t->ro is heap copy of class_ro_t
#define RW_CONSTRUCTING       (1<<26) // class allocated but not yet registered
#define RW_CONSTRUCTED        (1<<25) // class allocated and registered
// #define RW_24 (1<<24) // available for use; was RW_FINALIZE_ON_MAIN_THREAD
#define RW_LOADED             (1<<23) // class +load has been called
#if !SUPPORT_NONPOINTER_ISA
#define RW_INSTANCES_HAVE_ASSOCIATED_OBJECTS (1<<22) // class instances may have associative references
#endif
#define RW_HAS_INSTANCE_SPECIFIC_LAYOUT (1 << 21) // class has instance-specific GC layout
// #define RW_20       (1<<20) // available for use
#define RW_REALIZING          (1<<19) // class has started realizing but not yet completed it
#define RW_HAS_CXX_CTOR       (1<<18) // class or superclass has .cxx_construct implementation
#define RW_HAS_CXX_DTOR       (1<<17) // class or superclass has .cxx_destruct implementation
// class or superclass has default alloc/allocWithZone: implementation
// Note this is is stored in the metaclass.
#define RW_HAS_DEFAULT_AWZ    (1<<16)
#if SUPPORT_NONPOINTER_ISA
#define RW_REQUIRES_RAW_ISA   (1<<15) // class's instances requires raw isa
#endif

ro = (const class_ro_t *)cls->data();
if (ro->flags & RO_FUTURE) {
   // This was a future class. rw data is already allocated.
   rw = cls->data();
   ro = cls->data()->ro;
   cls->changeInfo(RW_REALIZED|RW_REALIZING, RW_FUTURE);
} else {
   // Normal class. Allocate writeable class data.
   rw = (class_rw_t *)calloc(sizeof(class_rw_t), 1);
   rw->ro = ro;
   rw->flags = RW_REALIZED|RW_REALIZING;
   cls->setData(rw);
}

#define RO_FUTURE             (1<<30)
#define RO_REALIZED           (1<<31)

#define RW_REALIZED           (1<<31)
#define RW_FUTURE             (1<<30)

typedef struct category_t *Category;

struct category_t {
    const char *name;
    classref_t cls;
    struct method_list_t *instanceMethods;
    struct method_list_t *classMethods;
    struct protocol_list_t *protocols;
    struct property_list_t *instanceProperties;
    // Fields below this point are not always present on disk.
    struct property_list_t *_classProperties;

    method_list_t *methodsForMeta(bool isMeta) {
        if (isMeta) return classMethods;
        else return instanceMethods;
    }

    property_list_t *propertiesForMeta(bool isMeta, struct header_info *hi);
};