OC底层

OC 类&类结构分析

2020-09-18  本文已影响0人  H丶ym

OC底层原理学习

学习OC时，常听的就是万物皆对象，对象都有isa指针，那为什么有isa指针，isa指针到底是谁创建的？怎么创建的？我们来探索一下

探索的方法

使用Clang或者xcrun将.m文件转换成.cpp文件，通过.cpp文件观察底层的方法和实现
Clang是由苹果主导编写，基于LLVM 的C/C++/OC的编译器
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，要更好一些
打开终端，cd到当前文件下，利用以下几种命令行可以编译成.cpp文件
创建一个继承于NSObject的Person类，添加一个字符串name属性

//使用xcode工具 xcrun
//1、指定模拟器文件编译
xcrun -sdk iphonesimulator clang -arch x86_64 -rewrite-objc Person.m -o Person.cpp

//2、指定真机文件编译
xcrun -sdk iphoneos clang -arch arm64 -rewrite-objc Person.m -o Person-arm64.cpp

//使用Clang 将 main.m 编译成 main.cpp
clang -rewrite-objc main.m -o main.cpp

objc_class& objc_object

打开编译好的.cpp文件，内容多的不知道从何入手了，不急，一步步来

  1. 搜索我们的类名Person，看看能发现啥

我找了OC的源码,但是被注释掉了，在它的上面发现了Person_IMPL这个结构体，它包含了我们的属性name,由此可证明，我的Person类被编译成了结构体，这个结构体中，不仅包含了name，还有一个struct NSObject_IMPL NSObject_IVARS;找到NSObject_IMPL的定义

  1. 打开我们之前编译好的objc4-781的源码，搜索Class isa,cmd Class找到它的定义
  1. cmd objc_class,来到 objc_class的源码

objc_class 没有isa啊，进它的父类objc_object看看

objc_class结构分析

struct objc_class : objc_object {
    // Class ISA; //8字节
    Class superclass; //Class 类型 8字节
    cache_t cache;             // formerly cache pointer and vtable
    class_data_bits_t bits;    // class_rw_t * plus custom rr/alloc flags
    //....方法部分省略
}

cache 内存大小

cache是cache_t类型，看下cache_t的源码

无论走if还是 else if 都是16字节

通过指针偏移读取类内存中bits 中的信息

bits = 类的首地址偏移32字节
上一小节中我们提到bits存储的属性、方法等信息，现在我们添加一个成员变量、一个类方法、一个对象方法

接下来打印一下bits

step 1. 获取类地址

(lldb) p/x person.class
(Class) $0 = 0x00000001000022d8 Person

step 2. 首地址偏移32字节，32字节转16进制 20
0x00000001000022d8 -> 0x00000001000022f8,打印时，强转成bits的类型,强转以后我们才能调用class_data_bits_t的方法

(lldb) p (class_data_bits_t *)0x00000001000022f8
(class_data_bits_t *) $1 = 0x00000001000022f8

step 3. 调用class_data_bits_t提供的data()方法,获取class_rw_t* data,看下data的信息

(lldb) p $1->data()
(class_rw_t *) $2 = 0x0000000102004680
(lldb) p *$2
(class_rw_t) $3 = {
  flags = 2148007936
  witness = 1
  ro_or_rw_ext = {
    std::__1::atomic<unsigned long> = 4294975800
  }
  firstSubclass = nil
  nextSiblingClass = NSUUID
}

暂时看不懂，class_rw_t提供了一个方法methods(),看起来应该是获取方法列表

step 4.调用methods,看下输出

(lldb) p $3.methods()
(const method_array_t) $4 = {
  list_array_tt<method_t, method_list_t> = {
     = {
      list = 0x0000000100002180
      arrayAndFlag = 4294975872
    }
  }
}

返回的是method_array_t 继承于 list_array_tt,使用 list_array_tt提供的list属性，打印

(lldb) p $4.list
(method_list_t *const) $5 = 0x0000000100002180
(lldb) p *$5
(method_list_t) $6 = {
  entsize_list_tt<method_t, method_list_t, 3> = {
    entsizeAndFlags = 26
    count = 4
    first = {
      name = "objcMethod"
      types = 0x0000000100000f80 "v16@0:8"
      imp = 0x0000000100000dd0 (Example`-[Person objcMethod])
    }
  }
}

count = 4，证明我们有4个方法，分别打印一下其它的方法

(lldb) p $6.get(0)
(method_t) $7 = {
  name = "objcMethod"
  types = 0x0000000100000f80 "v16@0:8"
  imp = 0x0000000100000dd0 (Example`-[Person objcMethod])
}
(lldb) p $6.get(1)
(method_t) $8 = {
  name = ".cxx_destruct"
  types = 0x0000000100000f80 "v16@0:8"
  imp = 0x0000000100000e40 (Example`-[Person .cxx_destruct])
}
(lldb) p $6.get(2)
(method_t) $9 = {
  name = "name"
  types = 0x0000000100000f94 "@16@0:8"
  imp = 0x0000000100000de0 (Example`-[Person name])
}
(lldb) p $6.get(3)
(method_t) $10 = {
  name = "setName:"
  types = 0x0000000100000f9c "v24@0:8@16"
  imp = 0x0000000100000e10 (Example`-[Person setName:])
}
(lldb) p $6.get(4)
Assertion failed: (i < count), function get, file /Users/star/Desktop/objc4-781/runtime/objc-runtime-new.h, line 438.
error: Execution was interrupted, reason: signal SIGABRT.
The process has been returned to the state before expression evaluation.

打印第5个元素时，越界了，那我们的类方法去哪了？一会再分析，我们再去看看属性

step 5.class_rw_t提供了一个方法properties(),看起来应该是获取属性列表的

(lldb) p $3.properties()
(const property_array_t) $11 = {
  list_array_tt<property_t, property_list_t> = {
     = {
      list = 0x0000000100002230
      arrayAndFlag = 4294976048
    }
  }
}

返回的是property_array_t 继承于 list_array_tt,使用 list_array_tt提供的list属性，打印

(lldb) p $11.list
(property_list_t *const) $12 = 0x0000000100002230
(lldb) p *$12
(property_list_t) $13 = {
  entsize_list_tt<property_t, property_list_t, 0> = {
    entsizeAndFlags = 16
    count = 1
    first = (name = "name", attributes = "T@\"NSString\",C,N,V_name")
  }
}

count = 1,不会吧，只有一个属性？分别打印试下

(lldb) p $13.get(0)
(property_t) $14 = (name = "name", attributes = "T@\"NSString\",C,N,V_name")
(lldb) p $13.get(1)
Assertion failed: (i < count), function get, file /Users/star/Desktop/objc4-781/runtime/objc-runtime-new.h, line 438.
error: Execution was interrupted, reason: signal SIGABRT.
The process has been returned to the state before expression evaluation.

我们的成员变量也丢了
step 6.再看下class_rw_t的定义，还有一个ro的方法，返回了一个class_ro_t类型的结构体，class_ro_t有一个ivars属性，看着比较像，打印一下

(lldb) p $3.ro()
(const class_ro_t *) $15 = 0x0000000100002138
(lldb) p *$15
(const class_ro_t) $16 = {
  flags = 388
  instanceStart = 8
  instanceSize = 24
  reserved = 0
  ivarLayout = 0x0000000100000f3f "\x02"
  name = 0x0000000100000f38 "Person"
  baseMethodList = 0x0000000100002180
  baseProtocols = 0x0000000000000000
  ivars = 0x00000001000021e8
  weakIvarLayout = 0x0000000000000000
  baseProperties = 0x0000000100002230
  _swiftMetadataInitializer_NEVER_USE = {}
}
(lldb) p $16.ivars
(const ivar_list_t *const) $17 = 0x00000001000021e8
(lldb) p $17.get(0)
(ivar_t) $18 = {
  offset = 0x0000000100002250
  name = 0x0000000100000f4d "_cybl"
  type = 0x0000000100000f88 "@\"NSString\""
  alignment_raw = 3
  size = 8
}
  Fix-it applied, fixed expression was: 
    $17->get(0)

找到了 _cybl!

探索类方法存储

猜测一下，对象的isa指向了类，类里有对象方法，类的isa指向元类，类方法会不会在元类里呢？
step 1.获取元类地址

(lldb) x/4gx Person.class
0x1000022d8: 0x00000001000022b0 0x0000000100333140
0x1000022e8: 0x0000000102118040 0x0001802400000003
(lldb) p/x 0x00000001000022b0 & 0x0000000ffffffff8ULL
(unsigned long long) $1 = 0x00000001000022b0

step 2. 首地址偏移32字节，32字节转16进制 20

(lldb) p (class_data_bits_t *)0x00000001000022d0
(class_data_bits_t *) $2 = 0x00000001000022d0

step 3.调用data(),获取class_rw_t* data,看下data的信息

(lldb) p $2->data()
(class_rw_t *) $3 = 0x0000000101053d80
(lldb) p *$3
(class_rw_t) $4 = {
  flags = 2684878849
  witness = 1
  ro_or_rw_ext = {
    std::__1::atomic<unsigned long> = 4294975696
  }
  firstSubclass = nil
  nextSiblingClass = 0x00007fff8c6b0c60
}

step 4.调用methods,看下输出

(const method_array_t) $5 = {
  list_array_tt<method_t, method_list_t> = {
     = {
      list = 0x0000000100002118
      arrayAndFlag = 4294975768
    }
  }
}

返回的是method_array_t 继承于 list_array_tt,使用 list_array_tt提供的list属性，打印

(lldb) p $5.list
(method_list_t *const) $6 = 0x0000000100002118
(lldb) p *$6
(method_list_t) $7 = {
  entsize_list_tt<method_t, method_list_t, 3> = {
    entsizeAndFlags = 26
    count = 1
    first = {
      name = "classMethod"
      types = 0x0000000100000f80 "v16@0:8"
      imp = 0x0000000100000dc0 (Example`+[Person classMethod])
    }
  }
}

找到了 classMethod

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