接上一篇文章：iOS下RunTime分析一（OC文件编译为C++）

这里主要是解释iOS下RunTime分析一（OC文件编译为C++）中的第5点疑问。

这里看类别部分的OC源码

头文件：

@interface MyObject (Extensions)
- (void)methodA;
- (void)methodB:(NSString *)str;
@property (nonatomic, strong) NSObject *methodObj;
@end

@interface MyObject (Common)
- (void)methodCommon;
+ (void)classCommonMethod;
@end

源文件：

@implementation MyObject (Extensions)

- (void)methodA
{
    return;
}

- (void)methodB:(NSString *)str
{
    return;
}

- (NSObject *)methodObj
{
    return nil;
}

- (void)setMethodObj:(NSObject *)methodObj
{
    return;
}

@end

@implementation MyObject (Common)

- (void)methodCommon
{
    return;
}

+ (void)classCommonMethod
{
    return;
}

@end

现在来看下编译成C++之后的源代码，与RunTime分析一相似的部分，不在重新写了。

会生成一个_category_t的结构体：

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;
};

以及相关的静态结构体对象：

(used, section ("__DATA,__objc_const"))
static struct /*_method_list_t*/ {
	unsigned int entsize;  // sizeof(struct _objc_method)
	unsigned int method_count;
	struct _objc_method method_list[4];
} _OBJC_$_CATEGORY_INSTANCE_METHODS_MyObject_$_Extensions = {
	sizeof(_objc_method),
	4,
	{ {(struct objc_selector *)"methodA", "v16@0:8", (void *)_I_MyObject_Extensions_methodA},
	{(struct objc_selector *)"methodB:", "v24@0:8@16", (void *)_I_MyObject_Extensions_methodB_},
	{(struct objc_selector *)"methodObj", "@16@0:8", (void *)_I_MyObject_Extensions_methodObj},
	{(struct objc_selector *)"setMethodObj:", "v24@0:8@16", (void *)_I_MyObject_Extensions_setMethodObj_} }
};

(used, section ("__DATA,__objc_const"))
static struct /*_prop_list_t*/ {
	unsigned int entsize;  // sizeof(struct _prop_t)
	unsigned int count_of_properties;
	struct _prop_t prop_list[1];
} _OBJC_$_PROP_LIST_MyObject_$_Extensions = {
	sizeof(_prop_t),
	1,
	{ {"methodObj","T@\"NSObject\",&,N"}}
};

(used, section ("__DATA,__objc_const"))
static struct _category_t _OBJC_$_CATEGORY_MyObject_$_Extensions = 
{
	"MyObject",
	0, // &MyObject,
	&_OBJC_$_CATEGORY_INSTANCE_METHODS_MyObject_$_Extensions,
	0,
	0,
	(const struct _prop_list_t *)&_OBJC_$_PROP_LIST_MyObject_$_Extensions,
};

static void OBJC_CATEGORY_SETUP_$_MyObject_$_Extensions(void ) {
	_OBJC_$_CATEGORY_MyObject_$_Extensions.cls = &MyObject;
}

以上生成的是@interface MyObject (Extensions)这个类别中的相关结构体对象。

下面看下@interface MyObject (Common)中的结构体对象：

OBJC_CATEGORY_SETUP_$_MyObject_
static struct /*_method_list_t*/ {
	unsigned int entsize;  // sizeof(struct _objc_method)
	unsigned int method_count;
	struct _objc_method method_list[1];
} _OBJC_$_CATEGORY_INSTANCE_METHODS_MyObject_$_Common  = {
	sizeof(_objc_method),
	1,
	{ {"methodCommon", "v16@0:8", (void *)_I_MyObject_Common_methodCommon} }
};

OBJC_CATEGORY_SETUP_$_MyObject_
static struct /*_method_list_t*/ {
	unsigned int entsize;  // sizeof(struct _objc_method)
	unsigned int method_count;
	struct _objc_method method_list[1];
} _OBJC_$_CATEGORY_CLASS_METHODS_MyObject_$_Common = {
	sizeof(_objc_method),
	1,
	{ {"classCommonMethod", "v16@0:8", (void*)_C_MyObject_Common_classCommonMethod}}
};

OBJC_CATEGORY_SETUP_$_MyObject_
static struct _category_t _OBJC_$_CATEGORY_MyObject_$_Common = 
{
	"MyObject",
	0, // &MyObject,
	&_OBJC_$_CATEGORY_INSTANCE_METHODS_MyObject_$_Common,
	)&_OBJC_$_CATEGORY_CLASS_METHODS_MyObject_$_Common,
	0,
	0,
};
static void OBJC_CATEGORY_SETUP_$_MyObject_$_Common(void ) {
	_OBJC_$_CATEGORY_MyObject_$_Common.cls = &MyObject;
}

以上两个结构体对象都是通过OBJC_CATEGORY_SETUP_$_MyObject_$_Extensions和OBJC_CATEGORY_SETUP_$_MyObject_$_Common与MyObject这个类关联起来的。

static void *OBJC_CATEGORY_SETUP[] = {
	(void *)&OBJC_CATEGORY_SETUP_$_MyObject_$_Extensions,
	(void *)&OBJC_CATEGORY_SETUP_$_MyObject_$_Common,
};

(used, section ("__DATA, __objc_catlist,regular,no_dead_strip"))
static struct _category_t *L_OBJC_LABEL_CATEGORY_$ [2] = {
	&_OBJC_$_CATEGORY_MyObject_$_Extensions,
	&_OBJC_$_CATEGORY_MyObject_$_Common,
};
static struct IMAGE_INFO { unsigned version; unsigned flag; } _OBJC_IMAGE_INFO = { 0, 2 };

总结

每个category都是一个结构体的对象，只是_category_t中的isa指向的是MyObject的对象。只是现在有一个疑问，当调用category中的方法时，MyObject是如何找到该方法的？