NSObject 的实现分析
转载请注名出处 http://blog.csdn.net/uxyheaven
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http://opensource.apple.com/tarballs/objc4/objc4-493.9.tar.gz下载,Object在<Object.h>, 这里的目录写着Obsolete, 呃.
属性
isa
是一个指向Class的指针,详细请看这篇文章Objective-C objc_class 介绍
方法
class
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;
}
superclass
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;
}
isEqual
就是直接比較
- (BOOL)isEqual:anObject
{
return anObject == self;
}
isMemberOf:
- (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.
isKindOf:
- (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
- init
{
return self;
}
没什么好说的
alloc
+ 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
- 得到这个类占用多少空间,最小占16 bytes
- 然后就给这个实例分配多少空间, 假设失败的话就返回nil
- 把这个实例的isa设置成这个类对象
- 假设cls的info设置了get属性就用cls这个类在obj这个空间去构造一个实例,跟进去是
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
+ 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
- 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;
}
- 运行一个叫object_cxxDestruct的东西干了点什么事(沿着继承链逐层向上搜寻SEL_cxx_destruct这个selector, 找到函数实现(void (*)(id)(函数指针)并运行)
- 运行_object_remove_assocations去除和这个对象关联的对象
- 运行objc_clear_deallocating。清空引用计数表并清除弱引用表。将全部weak引用指nil
respondsTo:
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:
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 lists
LMsgSendCacheMiss:
ldmfd sp!, {a4,v1}
b _objc_msgSend_uncached
LMsgSendExit:
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
conformsTo:
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
copy
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
