oc 元类 元数据

首先认清一些概念。

struct objc_object {    Class isa;};typedef struct objc_object *id;

struct objc_class {    Class isa;    Class super_class;    const char *name;    long version;    long info;    long instance_size;    struct objc_ivar_list *ivars;    struct objc_method_list **methodLists;    struct objc_cache *cache;    struct objc_protocol_list *protocols;};typedef struct objc_class *Class;

此结构体可以储存类的诸多信息，例如类型名、父类类型、实例变量列表、方法列表等，这些信息被称作类的元数据（metadata）。该结构体也有一个Class类型的成员isa，说明Class本身也是一个OC对象（被称为类对象或类型对象），而它的对象类型（isa所指向的类型）被称为元类（metaclass），元类中储存的是类对象的元数据，比如类方法就储存在这里。每个类可以有无数个对象，但仅有一个类对象，也仅有一个与之对应的元类。

object_getClass(obj)与[obj class]区别？

object_getClass(obj)返回的是obj中的isa指针；而[obj class]则分两种情况：一是当obj为实例对象时，[obj class]中class是实例方法：- (Class)class，返回的obj对象中的isa指针；二是当obj为类对象（包括元类和根类以及根元类）时，调用的是类方法：+ (Class)class，返回的结果为其本身。

    Class f = object_getClass([sunClasssuperclass]);

    Class g = object_getClass([[sunClasssuperclass] class]);

  f == g

Object-C的对象模型图

    metaSunClass * sunClass = [[metaSunClass alloc] init];       Class a = object_getClass(object_getClass([sunClass class]));        Class b = object_getClass(object_getClass([[sunClass superclass] class]));

//此时 a == b 证明上图最右侧虚线元类的class是root[既:NSObject]。

下面的测试代码，是用来获取实例对象的方法和相应的元类方法，证明上图流程。

@interface metaBaseClass : NSObject {    NSInteger iNum;}@property (assign, nonatomic, readonly ) NSInteger uNum;- (NSInteger) getINumber;- (void) setINumber:(NSInteger) num;+ (void)printILog;+ (void)printIILog;@end

@implementation metaBaseClass- (NSInteger) getINumber {    return iNum;}- (void) setINumber:(NSInteger) num {    iNum = num;}+ (void)printILog {    NSLog(@"show log");}+ (void)printIILog {    NSLog(@"show II log");}@end

@interface metaSunClass : metaBaseClass {    NSInteger yNum;}@property (assign, nonatomic, readonly ) NSInteger uYNum;- (NSInteger) getYNumber;- (void) setYNumber:(NSInteger) num;+ (void)printYLog;@end

@implementation metaSunClass- (NSInteger) getYNumber {    return yNum;}- (void) setYNumber:(NSInteger) num {    yNum = num;}+ (void)printILog {    NSLog(@"show Y log");}+ (void)printYLog {    NSLog(@"show YY log");}@end

    Method * mm = class_copyMethodList([sunClass class], &count);    NSLog(@"%u",count);    for(int j  =0; j < count; j++) {        Method mt = mm[j];        SEL sel =method_getName(mt);        NSString *strName = [NSString stringWithCString:sel_getName(sel)encoding:NSUTF8StringEncoding];        NSLog(@"%@",strName);    }

打印结果如下：

2016-05-16 20:17:03.698 test[17841:1055828] 3

2016-05-16 20:17:03.698 test[17841:1055828] getYNumber

2016-05-16 20:17:03.699 test[17841:1055828] setYNumber:

2016-05-16 20:17:03.699 test[17841:1055828] uYNum

    Method * mm = class_copyMethodList([[sunClass superclass] class], &count);    NSLog(@"%u",count);    for(int j  =0; j < count; j++) {        Method mt = mm[j];        SEL sel =method_getName(mt);        NSString *strName = [NSString stringWithCString:sel_getName(sel)encoding:NSUTF8StringEncoding];        NSLog(@"%@",strName);    }

2016-05-16 20:17:49.717 test[17863:1056796] 3

2016-05-16 20:17:49.717 test[17863:1056796] getINumber

2016-05-16 20:17:49.717 test[17863:1056796] setINumber:

2016-05-16 20:17:49.717 test[17863:1056796] uNum

    Method * mm = class_copyMethodList(object_getClass([sunClassclass]), &count);    NSLog(@"%u",count);    for(int j  =0; j < count; j++) {        Method mt = mm[j];        SEL sel =method_getName(mt);        NSString *strName = [NSString stringWithCString:sel_getName(sel)encoding:NSUTF8StringEncoding];        NSLog(@"%@",strName);    }

2016-05-16 20:19:21.936 test[17888:1058041] 2

2016-05-16 20:19:21.936 test[17888:1058041] printILog

2016-05-16 20:19:21.936 test[17888:1058041] printYLog

    Method * mm = class_copyMethodList(object_getClass([[sunClasssuperclass]class]), &count);   //equal to  Method * mm = class_copyMethodList(object_getClass([[sunClassclass]superclass]), &count);    NSLog(@"%u",count);    for(int j  =0; j < count; j++) {        Method mt = mm[j];        SEL sel =method_getName(mt);        NSString *strName = [NSString stringWithCString:sel_getName(sel)encoding:NSUTF8StringEncoding];        NSLog(@"%@",strName);    }

2016-05-16 20:22:32.720 test[17930:1060301] 2

2016-05-16 20:22:32.720 test[17930:1060301] printILog

2016-05-16 20:22:32.720 test[17930:1060301] printIILog