iOS下RunTime分析一（OC文件编译为C++）
以类MyObject为例来进行分析。
头文件：
@interface MyObject : NSObject
@end
源文件:
@interface MyObject ()
{
int a;
char *b;
NSObject *obj;
}
@property (nonatomic, strong) NSString *name;
@end
@implementation MyObject
+ (void)calClassMethod
{
NSLog(@"class method");
}
- (void)showDescription:(NSString *)str
{
NSLog(@"%@%@%d", str, self.name, a);
}
@end
通过 clang -rewrite-objc MyObject.m编译成功后，会生成一个MyObject.cpp文件。
现在就来看下这个cpp文件里面的内容是什么。
1. MyObject的类定义生成一个类似的结构体：
struct MyObject_IMPL {
struct NSObject_IMPL NSObject_IVARS;
int a;
char *b;
NSObject *obj;
NSString *_name;
};
struct NSObject_IMPL {
Class isa;
};
以及与runtime相关的一系列结构体：
struct _prop_t {
const char *name;
const char *attributes;
};
struct _protocol_t;
struct _objc_method {
struct objc_selector * _cmd;
const char *method_type;
void *_imp;
};
struct _protocol_t {
void * isa; // NULL
const char *protocol_name;
const struct _protocol_list_t * protocol_list; // super protocols
const struct method_list_t *instance_methods;
const struct method_list_t *class_methods;
const struct method_list_t *optionalInstanceMethods;
const struct method_list_t *optionalClassMethods;
const struct _prop_list_t * properties;
const unsigned int size; // sizeof(struct _protocol_t)
const unsigned int flags; // = 0
const char ** extendedMethodTypes;
};
struct _ivar_t {
unsigned long int *offset; // pointer to ivar offset location
const char *name;
const char *type;
unsigned int alignment;
unsigned int size;
};
struct _class_ro_t {
unsigned int flags;
unsigned int instanceStart;
unsigned int instanceSize;
unsigned int reserved;
const unsigned char *ivarLayout;
const char *name;
const struct _method_list_t *baseMethods;
const struct _objc_protocol_list *baseProtocols;
const struct _ivar_list_t *ivars;
const unsigned char *weakIvarLayout;
const struct _prop_list_t *properties;
};
struct _class_t {
struct _class_t *isa;
struct _class_t *superclass;
void *cache;
void *vtable;
struct _class_ro_t *ro;
};
struct _category_t {
const char *name;
struct _class_t *cls;
const struct _method_list_t *instance_methods;
const struct _method_list_t *class_methods;
const struct _protocol_list_t *protocols;
const struct _prop_list_t *properties;
};
通过看结构体的名字可以看出来是什么意思，不做详细解释。
2. 会生成4个static类型的C函数，分别是：
static void _C_MyObject_calClassMethod(Class self, SEL _cmd);
static void _I_MyObject_showDescription_(MyObject * self, SEL _cmd, NSString *str);
//是根据@property自动生成的getter和setter
static NSString * _I_MyObject_name(MyObject * self, SEL _cmd);
static void _I_MyObject_setName_(MyObject * self, SEL _cmd, NSString *name);
函数中C代表的是class method，I代表的是instance method。
3. 会创建4个很重要的结构体对象，在OC中其实就是类对象。
struct _class_ro_t _OBJC_METACLASS_RO_$_MyObject;//简化为:METACLASS_RO_MyObject
struct _class_ro_t _OBJC_CLASS_RO_$_MyObject;//简化为:RO_MyObject
struct _class_t OBJC_METACLASS_$_MyObject;//简化为:METACLASS_MyObject
struct _class_t OBJC_CLASS_$_MyObject;//简化为:MyObject
下面就来看看这4个结构体对象是如何定义及如何组织的，为方便阅读，在这里会省去一些不必要的代码。
(used, section ("__DATA,__objc_data"))
struct _class_t MyObject = {
0, //_class_t *isa = &METACLASS_MyObject,
0, //_class_t *superclass = &NSObject,
0, //void *cache = (void *)&_objc_empty_cache,
0, //void *vtable = (void *)&_objc_empty_vtable, unused
&RO_MyObject, //_class_ro_t *ro,
};
(used, section ("__DATA,__objc_data"))
struct _class_t METACLASS_MyObject = {
0, // &METACLASS_NSObject,
0, // &METACLASS_NSObject,
0, // (void *)&_objc_empty_cache,
0, // (void *)&_objc_empty_vtable,unused
&METACLASS_RO_MyObject,
};
(used, section ("__DATA,__objc_const"))
static struct _class_ro_t RO_MyObject = {
0, //flags
__OFFSETOFIVAR__(struct MyObject, a), //instanceStart
sizeof(struct MyObject_IMPL), //instanceSize
0, //reserved
0, //ivarLayout
"MyObject", //name
&_OBJC_$_INSTANCE_METHODS_MyObject, //baseMethods
0, //baseProtocols
&_OBJC_$_INSTANCE_VARIABLES_MyObject, //ivars
0, //weakIvarLayout
0, //properties
};
(used, section ("__DATA,__objc_const"))
static struct _class_ro_t METACLASS_RO_MyObject = {
1, //flags
sizeof(struct _class_t), //instanceStart
sizeof(struct _class_t), //instanceSize
0, //reserved
0, //ivarLayout
"MyObject", //name
&_OBJC_$_CLASS_METHODS_MyObject, //baseMethods
0, //baseProtocols
0, //ivars
0, //weakIvarLayout
0, //properties
};
4. MyObject类中声明的“成员变量”、“Property”、“方法”的生成及组织方式
在上面的RO_MyObject和METACLASS_RO_MyObject中出现的_OBJC_$_INSTANCE_METHODS_MyObject、_OBJC_$_INSTANCE_VARIABLES_MyObject，_OBJC_$_CLASS_METHODS_MyObject又是什么呢？
源码一目了然：
(used, section ("__DATA,__objc_const"))
static struct /*_ivar_list_t*/ {
unsigned int entsize;
unsigned int count;
struct _ivar_t ivar_list[4];
} _OBJC_$_INSTANCE_VARIABLES_MyObject = {
sizeof(_ivar_t),
4,
{ {(unsigned long int *)&OBJC_IVAR_$_MyObject$a, "a", "i", 2, 4},
{(unsigned long int *)&OBJC_IVAR_$_MyObject$b, "b", "*", 3, 8},
{(unsigned long int *)&OBJC_IVAR_$_MyObject$obj, "obj", "@"NSObject"", 3, 8},
{(unsigned long int *)&OBJC_IVAR_$_MyObject$_name, "_name", "@"NSString"", 3, 8} }
};
(used, section ("__DATA,__objc_const"))
static struct /*_method_list_t*/ {
unsigned int entsize;
unsigned int method_count;
struct _objc_method method_list[3];
} _OBJC_$_INSTANCE_METHODS_MyObject = {
sizeof(_objc_method),
3,
{ {(struct objc_selector *)"showDescription:", "v24@0:8@16", (void *)_I_MyObject_showDescription_},
{(struct objc_selector *)"name", "@16@0:8", (void *)_I_MyObject_name},
{(struct objc_selector *)"setName:", "v24@0:8@16", (void *)_I_MyObject_setName_} }
};
(used, section ("__DATA,__objc_const"))
static struct /*_method_list_t*/ {
unsigned int entsize;
unsigned int method_count;
struct _objc_method method_list[1];
} _OBJC_$_CLASS_METHODS_MyObject = {
sizeof(_objc_method),
1,
{ {(struct objc_selector *)"calClassMethod", "v16@0:8", (void *)_C_MyObject_calClassMethod} }
};
在OC中执行的方法或者调用的变量及property都是从这里面去找的。
5. 生成的其他源码
#define __OFFSETOFIVAR__(TYPE, MEMBER) ((long long) &((TYPE *)0)->MEMBER)
(used, section ("__DATA,__objc_ivar"))
unsigned long int OBJC_IVAR_$_MyObject$a = __OFFSETOFIVAR__(struct MyObject, a);
unsigned long int OBJC_IVAR_$_MyObject$b = __OFFSETOFIVAR__(struct MyObject, b);
unsigned long int OBJC_IVAR_$_MyObject$obj = __OFFSETOFIVAR__(struct MyObject, obj);
unsigned long int OBJC_IVAR_$_MyObject$_name = __OFFSETOFIVAR__(struct MyObject, _name);
这个__OFFSETOFIVAR__的宏定义的使用寻找变量地址感觉很新奇，以后开发中可以借鉴。
static void OBJC_CLASS_SETUP_$_MyObject(void ) {
METACLASS_MyObject.isa = &METACLASS_NSObject;
METACLASS_MyObject.superclass = &METACLASS_NSObject;
METACLASS_MyObject.cache = &_objc_empty_cache;
MyObject.isa = &METACLASS_MyObject;
MyObject.superclass = &NSObject;
MyObject.cache = &_objc_empty_cache;
}
static void *OBJC_CLASS_SETUP[] = {
(void *)&OBJC_CLASS_SETUP_$_MyObject,
};
static struct _class_t *L_OBJC_LABEL_CLASS_$ [1] = {
&MyObject,
};
static struct IMAGE_INFO { unsigned version; unsigned flag; } _OBJC_IMAGE_INFO = { 0, 2 };
备注
在OC的源文件里有个属性的声明：@property (nonatomic, strong) NSString *name;
在编译之后的C++代码里面有个：struct _prop_t结构体，这个就是表示属性的一个结构体，可是在C++代码里面并没有体现出来name这个属性的结构体对象。如果把@property (nonatomic, strong) NSString *name;放到头文件就会在C++源码中看到这个name属性的结构体对象。编译之后的源码如下：
//_prop_list_t
(used, section ("__DATA,__objc_const"))
static struct {
unsigned int entsize; // sizeof(struct _prop_t)
unsigned int count_of_properties;
struct _prop_t prop_list[1];
} _OBJC_$_PROP_LIST_MyObject = {
sizeof(_prop_t),
1,
name
};
static struct _class_ro_t RO_MyObject = {
0,
__OFFSETOFIVAR__(struct MyObject, a),
sizeof(struct MyObject_IMPL),
(unsigned int)0,
0,
"MyObject",
&_OBJC_$_INSTANCE_METHODS_MyObject,
0,
&_OBJC_$_INSTANCE_VARIABLES_MyObject,
0,
&_OBJC_$_PROP_LIST_MyObject,
};
说明
在生成的C++源码中会出现(used, section (“__DATA,__objc_const”))、(used, section (“__DATA,__objc_ivar”))、(used, section (“__DATA,__objc_const”))类似于这种样式的代码，这个表示用这些标示的变量会被放到程序的数据段，程序启动时runtime会从数据段加载这些信息。
End
以上就是对OC代码通过clang编译为C++代码的一些源码的解析及理解。不过在期间也产生了一些疑问：
- 以上4个结构体对象创建出来时都只是一些默认赋值，真正的赋值是在OBJC_CLASS_SETUP_$_MyObject函数里面做的，那么该函数是在什么情况下执行呢？
- 上面第4点中的成员变量、property、方法都是在编译的情况下生成的是静态数组，那怎么在runtime下动态的添加成员变量、property、方法呢？
- 源代码中生成的都是类对象，如果根据类对象创建一个实例对象时，实例对象结构是怎么样的？
- 在app启动时，这些类对象及在编译阶段生成的对象是如何加入到runtime中的？
- 在有协议、类别和KVO时源码又是什么样的？
- 当调用实例对象和类对象的方法时是怎么样一个查询过程？
后续会一一解决。