OC类的原理探究一

前言

在前面的文章中我们知道在对象的isa指针中存储了类的信息，也知道了Class = isa & ISA_MASK，今天我们来探索一下类以及元类的继承链与类的数据结构。
ISA_MASK(掩码)

# if __arm64__
#   if __has_feature(ptrauth_calls) || TARGET_OS_SIMULATOR
#     define ISA_MASK        0x007ffffffffffff8ULL
#   else
#     define ISA_MASK        0x0000000ffffffff8ULL
elif __x86_64__
#   define ISA_MASK        0x00007ffffffffff8ULL

1.指针isa指向流程与类的继承链

1.1 isa指向流程

任何一个对象的isa指向当前类，当前类的isa指向元类，元类的isa指向NSObject(根元类)，根元类的isa指向根元类。如下图所示

验证一下，建立一个ZFPerson类利用llvm打印对象的isa指针指向流程

1.2 类继承链

• 任何类继承它的父类，父类继承根类(大部分都是NSObject)，根类继承一个nil,
  验证一下，建立两个类ZFStudent与ZFPerson。

@interface ZFPerson:NSObject
@end
@implementation ZFPerson
@end

@interface ZFStudent:ZFPerson
@end
@implementation ZFStudent

@end

从打印结果可以得到上面结论

• 任何类的元类继承它父类的元类，父类的元类继承根元类，根元类继承根类

配啥说perMeteSupClass是NSOject元类，perMeteSupSupClass是NSOject类呢？

从打印结果可以得到上面结论是正确的

2.类的结构

从底层源码可知道Class的本质是一个叫objc_class的结构体指针
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
    .....
}

对于着下面这一张图

3.类的bits探索

3.1获取bits数据

从类结构代码可以看到objc_class继承objc_object结构体,private继承objc_object的成员变量isa

struct objc_class : objc_object {
    // Class ISA;       //继承至objc_object的isa指针 8个字节
    Class superclass;   //结构体指针8个字节
    cache_t cache;      //16个字节       // formerly cache pointer and vtable
    class_data_bits_t bits; //类的地址偏移32位，就可以得到bits 
}

为啥说cache是16字节呢？

struct cache_t {
private:
    //typedef unsigned long           uintptr_t; 占用8字节
    explicit_atomic _bucketsAndMaybeMask;
    union {
        struct {
            explicit_atomic    _maybeMask; //typedef uint32_t mask_t; 4个字节
#if __LP64__
            uint16_t                   _flags; //2个字节
#endif
            uint16_t                   _occupied; //2个字节
        };
        explicit_atomic _originalPreoptCache;//8个字节
    };
 } 

所以cache是16字节，类的isa指针偏移32字节就可以得到bits数据

3.2 class_rw_t结构探索

从上面的bits数据中我们可以得到一个class_rw_t数据，class_rw_t在2020的wwdc大会的runtime视频有讲到，截图如下。里面包含了方法、属性、协议列表等

并且查看class_rw_t结构体源码，主要有以下几个属性

 struct class_rw_t {
   // Be warned that Symbolication knows the layout of this structure.
   uint32_t flags;
   uint16_t witness;
#if SUPPORT_INDEXED_ISA
   uint16_t index;
#endif

   explicit_atomic ro_or_rw_ext;

   Class firstSubclass;
   Class nextSiblingClass;
}

firstSubClass与nextSiblingClass：所有的类都会链接成一个树状结构，这是通过使用first subclass和Next Sibling Class指针实现的，这允许运行时遍历当前使用的所有类。

Demangled Name:swift类会使用demangled name字段，并且swift类不需要这个字段，除非有东西询问它们的objective-c名称时才需要

ro_or_rw_ext:存储class_ro_t或者class_rw_ext_t信息，下面这张图片是从苹果2020wwdc视频中扒取的

通过查看class_rw_t结构体源码，可以发现它提供了获取方法、属性、协议等方法

const method_array_t methods() const {
        auto v = get_ro_or_rwe();        
        if (v.is()) {
            return v.get(&ro_or_rw_ext)->methods;
        } else {
            return method_array_t{v.get(&ro_or_rw_ext)->baseMethods()};
        }
}
const property_array_t properties() const {
        auto v = get_ro_or_rwe();
        if (v.is()) {
            return v.get(&ro_or_rw_ext)->properties;
        } else {
            return property_array_t{v.get(&ro_or_rw_ext)->baseProperties};
        }
 }

const protocol_array_t protocols() const {
        auto v = get_ro_or_rwe();
        if (v.is()) {
            return v.get(&ro_or_rw_ext)->protocols;
        } else {
            return protocol_array_t{v.get(&ro_or_rw_ext)->baseProtocols};
        }
 }

下面我们通过提供的方法尝试获取一下它的方法、属性、协议如下

3.2.1 lldb获取类方法列表

(lldb) p/x ZFPerson.class  //获取类地址
(Class) $0 = 0x0000000100002850 ZFPerson
(lldb) p/x 0x0000000100002850 + 0x20 //地址偏移32位获取bits数据
(long) $1 = 0x0000000100002870
(lldb) p (class_data_bits_t*)0x0000000100002870 //强制转换bits为class_data_bits_t指针
(class_data_bits_t *) $2 = 0x0000000100002870
(lldb) p $2->data() //获取class_rw_t 数据，data方法是结构体class_data_bits_t提供的，具体可以看源码
(class_rw_t *) $3 = 0x000000010060e620
(lldb) p *$3
(class_rw_t) $4 = {
  flags = 2148007936
  witness = 0
  ro_or_rw_ext = {
    std::__1::atomic = 4294976080
  }
  firstSubclass = nil
  nextSiblingClass = NSUUID
}
(lldb) p $4.methods() //获取方法列表
(const method_array_t) $5 = {
  list_array_tt = {
     = {
      list = {
        ptr = 0x0000000100002298
      }
      arrayAndFlag = 4294976152
    }
  }
}
(lldb) p $5.list.ptr
(method_list_t *const) $6 = 0x0000000100002298
(lldb) p *$6
(method_list_t) $7 = {
  entsize_list_tt = (entsizeAndFlags = 27, count = 7)
}
(lldb) p $7.get(0)
(method_t) $8 = {}
(lldb) p $7.get(2)
(method_t) $9 = {}
(lldb) p $7.get(1)
(method_t) $10 = {}
(lldb) p $7.get(0).big
(method_t::big) $11 = {
  name = "killPig" //sel
  types = 0x0000000100001ed7 "v16@0:8" //类型编码，
  imp = 0x0000000100001b10 (ZFObjcBuild`-[ZFPerson killPig])
}
  Fix-it applied, fixed expression was: 
    $7.get(0).big()
(lldb) p $7.get(1).big
(method_t::big) $12 = {
  name = "eat"
  types = 0x0000000100001ed7 "v16@0:8"
  imp = 0x0000000100001b00 (ZFObjcBuild`-[ZFPerson eat])
}
  Fix-it applied, fixed expression was: 
    $7.get(1).big()
(lldb) p $7.get(2).big
(method_t::big) $13 = {
  name = "init"
  types = 0x0000000100001ecf "@16@0:8"
  imp = 0x0000000100001aa0 (ZFObjcBuild`-[ZFPerson init])
}
  Fix-it applied, fixed expression was: 
    $7.get(2).big()
(lldb) p $7.get(3).big
(method_t::big) $14 = {
  name = "name"
  types = 0x0000000100001ecf "@16@0:8"
  imp = 0x0000000100001b20 (ZFObjcBuild`-[ZFPerson name])
}
  Fix-it applied, fixed expression was: 
    $7.get(3).big()
(lldb) p $7.get(4).big
(method_t::big) $15 = {
  name = "setName:"
  types = 0x0000000100001edf "v24@0:8@16"
  imp = 0x0000000100001b50 (ZFObjcBuild`-[ZFPerson setName:])
}
  Fix-it applied, fixed expression was: 
    $7.get(4).big()
(lldb) p $7.get(5).big
(method_t::big) $16 = {
  name = "age"
  types = 0x0000000100001f91 "i16@0:8"
  imp = 0x0000000100001b80 (ZFObjcBuild`-[ZFPerson age])
}
  Fix-it applied, fixed expression was: 
    $7.get(5).big()
(lldb) p $7.get(6).big
(method_t::big) $17 = {
  name = "setAge:"
  types = 0x0000000100001f99 "v20@0:8i16"
  imp = 0x0000000100001ba0 (ZFObjcBuild`-[ZFPerson setAge:])
}
  Fix-it applied, fixed expression was: 
    $7.get(6).big()
(lldb) 

method_t中的结构以init方法举例

name = "init" //方法的名称(sel),选择器是字符串，它具有唯一性，所以它们可以使用指针相等来进行比较

types = 0x0000000100001ecf "@16@0:8" //类型编码：表示参数和返回类型的字符串，它不是用来发送消息的，但它是运行时introspection和消息forwarding所必需的

//指向方法的实现的指针
imp = 0x0000000100001aa0 (ZFObjcBuild`-[ZFPerson init])

从上面可以看到我们输出的方法列表中是没有类方法，因为类方法是存储在类的元类中
在ZFPerson声明一个类方法sleep,验证的具体步骤我就不写了，跟上面获取lldb差不多，只是用类的元类地址去做偏移

为什么类方法会放在元类中呢？这是为了避免与同名的对象方法产生冲突，毕竟方法在底层存储方式就是sel与imp

3.2.2 lldb获取类的属性列表

3.2.3 lldb获取类的协议列表

声明了一个ZFPersonDelegate协议，让ZFPerson遵循它

@protocol ZFPersonDelegate
- (void)killPig;
@end

(lldb) p/x ZFPerson.class
(Class) $13 = 0x0000000100002850 ZFPerson
(lldb) p 0x0000000100002850 + 0x20
(long) $14 = 4294977648
(lldb) p (class_data_bits_t*)4294977648
(class_data_bits_t *) $15 = 0x0000000100002870
(lldb) p $15->data()
(class_rw_t *) $16 = 0x0000000100627ba0
(lldb) p *$16
(class_rw_t) $17 = {
  flags = 2148007936
  witness = 0
  ro_or_rw_ext = {
    std::__1::atomic = 4294976080
  }
  firstSubclass = nil
  nextSiblingClass = NSUUID
}
(lldb) p $17.protocols()
(const protocol_array_t) $18 = {
  list_array_tt = {
     = {
      list = {
        ptr = 0x0000000100002348
      }
      arrayAndFlag = 4294976328
    }
  }
}
(lldb) p $18.list
(RawPtr) $19 = {
  ptr = 0x0000000100002348
}
(lldb) p $19.ptr
(protocol_list_t *const) $20 = 0x0000000100002348
(lldb) p *$20
(protocol_list_t) $21 = (count = 1, list = protocol_ref_t [] @ 0x00007fd025b50828)
(lldb) p $21.list[0]
(protocol_ref_t) $22 = 4294977696
(lldb) p/x 4294977696
(long) $23 = 0x00000001000028a0
(lldb) p (protocol_t *)0x00000001000028a0
(protocol_t *) $24 = 0x00000001000028a0
(lldb) p *$24
(protocol_t) $25 = {
  objc_object = {
    isa = {
      bits = 4298453192
      cls = Protocol
       = {
        nonpointer = 0
        has_assoc = 0
        has_cxx_dtor = 0
        shiftcls = 537306649
        magic = 0
        weakly_referenced = 0
        unused = 0
        has_sidetable_rc = 0
        extra_rc = 0
      }
    }
  }
  mangledName = 0x0000000100001ea9 "ZFPersonDelegate"
  protocols = 0x0000000100002430
  instanceMethods = 0x0000000100002448
  classMethods = 0x0000000000000000
  optionalInstanceMethods = 0x0000000000000000
  optionalClassMethods = 0x0000000000000000
  instanceProperties = 0x0000000000000000
  size = 96
  flags = 0
  _extendedMethodTypes = 0x0000000100002468
  _demangledName = 0x0000000000000000
  _classProperties = 0x0000000000000000
}
(lldb) 

通过上面lldb步骤，输出了class_rw_t结构中的protocol信息

4.补充

4.1 类型编码
获取方法的类型编码 method_getTypeEncoding

苹果关于类型编码文档