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iOS 的 NSObject 类没有开源, 可是呢 runtime开源了,里面有个类 Object 看接口和NSObject差点儿相同,以下我就对着 Object 的代码来分析下 NSObject
runtime代码在下载,Object在<Object.h>, 这里的目录写着Obsolete, 呃.
是一个指向Class的指针,详细请看这篇文章
实例方法返回的是isa指针, 类方法返回的是本身
代码实现例如以下:
- class{ return (id)isa; }+ class { return self;} 返回父类
代码实现例如以下:
+ superclass { return class_getSuperclass((Class)self); }- superclass { return class_getSuperclass(isa); } 调用的是runtime中的class_getSuperclass方法, 跟踪到最后实例方法返回的是isa->superclass,类方法返回的是self->superclass
static class_t *getSuperclass(class_t *cls){ if (!cls) return NULL; return cls->superclass;} 就是直接比較
- (BOOL)isEqual:anObject{ return anObject == self; } - (BOOL)isMemberOf:aClass{ return isa == (Class)aClass;} 看代码能够得知是通过比較实例对象的isa是否和 传过来的[类 Class] 一样来推断的.而实例对象的isa确实就是指着实例对象的类的.
- (BOOL)isKindOf:aClass{ register Class cls; for (cls = isa; cls; cls = class_getSuperclass(cls)) if (cls == (Class)aClass) return YES; return NO;}// class_getSuperclass 展开后例如以下static class_t *getSuperclass(class_t *cls){ if (!cls) return NULL; return cls->superclass;} 代码思路也非常好理解,假设自己的isa等于aClass(aClass的父类,此处循环)就返回YES,否则返回NO
- init{ return self;} 没什么好说的
+ alloc{ return (*_zoneAlloc)((Class)self, 0, malloc_default_zone()); } 这里有一个函数指针和一个结构体,我们跟进去看
id (*_zoneAlloc)(Class, size_t, void *) = _class_createInstanceFromZone;PRIVATE_EXTERN id _class_createInstanceFromZone(Class cls, size_t extraBytes, void *zone){ id obj; size_t size; // Can't create something for nothing if (!cls) return nil; // Allocate and initialize size = _class_getInstanceSize(cls) + extraBytes; // CF requires all objects be at least 16 bytes. if (size < 16) size = 16;#if SUPPORT_GC if (UseGC) { obj = (id)auto_zone_allocate_object(gc_zone, size, AUTO_OBJECT_SCANNED, 0, 1); } else #endif if (zone) { obj = (id)malloc_zone_calloc (zone, 1, size); } else { obj = (id)calloc(1, size); } if (!obj) return nil; obj->isa = cls; if (_class_hasCxxStructors(cls)) { obj = _objc_constructOrFree(cls, obj); } return obj;} 上面那段代码的作用是
static BOOL object_cxxConstructFromClass(id obj, Class cls){ id (*ctor)(id); Class supercls; // Stop if neither this class nor any superclass has ctors. if (!_class_hasCxxStructors(cls)) return YES; // no ctor - ok supercls = _class_getSuperclass(cls); // Call superclasses' ctors first, if any. if (supercls) { BOOL ok = object_cxxConstructFromClass(obj, supercls); if (!ok) return NO; // some superclass's ctor failed - give up } // Find this class's ctor, if any. ctor = (id(*)(id))lookupMethodInClassAndLoadCache(cls, SEL_cxx_construct); if (ctor == (id(*)(id))&_objc_msgForward_internal) return YES; // no ctor - ok // Call this class's ctor. if (PrintCxxCtors) { _objc_inform("CXX: calling C++ constructors for class %s", _class_getName(cls)); } if ((*ctor)(obj)) return YES; // ctor called and succeeded - ok // This class's ctor was called and failed. // Call superclasses's dtors to clean up. if (supercls) object_cxxDestructFromClass(obj, supercls); return NO;} 大意是,先看自己有没有父类,有就递归调用自己,然后给自己加入方法,然后加入类别
+ new{ id newObject = (*_alloc)((Class)self, 0); Class metaClass = self->isa; if (class_getVersion(metaClass) > 1) return [newObject init]; else return newObject;} 跟进去看一下, 发现是和 alloc差点儿相同
id (*_alloc)(Class, size_t) = _class_createInstance;static id _class_createInstance(Class cls, size_t extraBytes){ return _class_createInstanceFromZone (cls, extraBytes, NULL);} - free { return (*_dealloc)(self); }+ free{ return nil; } 跟进去看一下
static id _object_dispose(id anObject) { if (anObject==nil) return nil; objc_destructInstance(anObject);#if SUPPORT_GC if (UseGC) { auto_zone_retain(gc_zone, anObject); // gc free expects rc==1 } else #endif { // only clobber isa for non-gc anObject->isa = _objc_getFreedObjectClass (); } free(anObject); return nil;}void *objc_destructInstance(id obj) { if (obj) { Class isa = _object_getClass(obj); if (_class_hasCxxStructors(isa)) { object_cxxDestruct(obj); } if (_class_instancesHaveAssociatedObjects(isa)) { _object_remove_assocations(obj); } if (!UseGC) objc_clear_deallocating(obj); } return obj;} 是查找有没有实现某个方法
- (BOOL)respondsTo:(SEL)aSelector { return class_respondsToMethod(isa, aSelector);}BOOL class_respondsToMethod(Class cls, SEL sel){ OBJC_WARN_DEPRECATED; return class_respondsToSelector(cls, sel);}BOOL class_respondsToSelector(Class cls, SEL sel){ IMP imp; if (!sel || !cls) return NO; // Avoids +initialize because it historically did so. // We're not returning a callable IMP anyway. imp = lookUpMethod(cls, sel, NO/*initialize*/, YES/*cache*/); return (imp != (IMP)_objc_msgForward_internal) ? YES : NO;} perform是发送消息到指定的接收器并返回值, 以下是代码:
- perform:(SEL)aSelector { if (aSelector) return objc_msgSend(self, aSelector); else return [self error:_errBadSel, sel_getName(_cmd), aSelector];} 原来就是objc_msgSend这玩意.objc_msgSend实现有非常多个版本号, 大体逻辑应该差点儿相同, 首先在找缓存,找到就跳转过去,找不到就在Class的方法列表里找方法, 假设还是没找到就转发.
下的是arm下的代码
ENTRY objc_msgSend# check whether receiver is nil teq a1, #0 itt eq moveq a2, #0 bxeq lr# save registers and load receiver's class for CacheLookup stmfd sp!, {a4,v1} ldr v1, [a1, #ISA]# receiver is non-nil: search the cache CacheLookup a2, v1, LMsgSendCacheMiss# cache hit (imp in ip) and CacheLookup returns with nonstret (eq) set, restore registers and call ldmfd sp!, {a4,v1} bx ip# cache miss: go search the method listsLMsgSendCacheMiss: ldmfd sp!, {a4,v1} b _objc_msgSend_uncachedLMsgSendExit: END_ENTRY objc_msgSend STATIC_ENTRY objc_msgSend_uncached# Push stack frame stmfd sp!, {a1-a4,r7,lr} add r7, sp, #16# Load class and selector ldr a1, [a1, #ISA] /* class = receiver->isa */ # MOVE a2, a2 /* selector already in a2 */# Do the lookup MI_CALL_EXTERNAL(__class_lookupMethodAndLoadCache) MOVE ip, a1# Prep for forwarding, Pop stack frame and call imp teq v1, v1 /* set nonstret (eq) */ ldmfd sp!, {a1-a4,r7,lr} bx ip 返回是否遵循了某个协议
- (BOOL) conformsTo: (Protocol *)aProtocolObj{ return [(id)isa conformsTo:aProtocolObj];}+ (BOOL) conformsTo: (Protocol *)aProtocolObj{ Class class; for (class = self; class; class = class_getSuperclass(class)) { if (class_conformsToProtocol(class, aProtocolObj)) return YES; } return NO;} 终于用的是class_conformsToProtocol, 返回一个布尔值,表示一个类是否符合给定的协议。
class_conformsToProtocol的实例如以下
BOOL class_conformsToProtocol(Class cls_gen, Protocol *proto_gen){ struct old_class *cls = oldcls(cls_gen); struct old_protocol *proto = oldprotocol(proto_gen); if (!cls_gen) return NO; if (!proto) return NO; if (cls->isa->version >= 3) { struct old_protocol_list *list; for (list = cls->protocols; list != NULL; list = list->next) { int i; for (i = 0; i < list->count; i++) { if (list->list[i] == proto) return YES; if (protocol_conformsToProtocol((Protocol *)list->list[i], proto_gen)) return YES; } if (cls->isa->version <= 4) break; } } return NO;} 能够看到是在cls->protocols里面找.protocols 是协议的数组
浅拷贝
- copy { return [self copyFromZone: [self zone]];}// 返回指定区域的指针- (void *)zone{ void *z = malloc_zone_from_ptr(self); return z ? z : malloc_default_zone(); } - copyFromZone:(void *)z { return (*_zoneCopy)(self, 0, z); } id (*_zoneCopy)(id, size_t, void *) = _object_copyFromZone; static id _object_copyFromZone(id oldObj, size_t extraBytes, void *zone) { id obj; size_t size; if (!oldObj) return nil; // 用旧对象的isa生成一个新的对象的空间 obj = (*_zoneAlloc)(oldObj->isa, extraBytes, zone); size = _class_getInstanceSize(oldObj->isa) + extraBytes; // fixme need C++ copy constructor // 把旧对象的内存复制到新对象 objc_memmove_collectable(obj, oldObj, size); }
本文转自mfrbuaa博客园博客，原文链接：http://www.cnblogs.com/mfrbuaa/p/5073937.html，如需转载请自行联系原作者