文章地址:http://blog.csdn.net/uxyheaven
http://blog.csdn.net/uxyheaven/article/details/38120335
进入正题:
本篇文章将探究一下objc里的关于方法的函数是如何实现的
首先看下方法的定义，方法是一个objc_method结构体
objc_method
objc_method是类的一个方法的描述
定义如下
typedef struct objc_method *Method;
struct objc_method {
SEL method_name; // 方法名称
charchar *method_typesE; // 参数和返回类型的描述字串
IMP method_imp; // 方法的具体的实现的指针
}
方法class_getInstanceMethod（Class aClass，SEL aSelector）
返回ACLASS的名为aSelector方法的
定义如下
Method class_getInstanceMethod(Class cls, SEL sel)
{
if (!cls || !sel) return NULL;
return look_up_method(cls, sel, YES/*cache*/, YES/*resolver*/);
}
static Method look_up_method(Class cls, SEL sel, BOOL withCache, BOOL withResolver)
{
Method meth = NULL;
// 1. 找缓存,有过有就返回
if (withCache) {
meth = _cache_getMethod(cls, sel, &_objc_msgForward_internal);
if (meth == (Method)1) {
// Cache contains forward:: . Stop searching.
return NULL;
}
}
// 2. 找自身
if (!meth) meth = _class_getMethod(cls, sel);
// 3. 找转发
if (!meth && withResolver) meth = _class_resolveMethod(cls, sel);
return meth;
}
IMP class_getMethodImplementation（Class cls，SEL名称）
返回CLS的名称的方法调用地址
定义如下
IMP class_getMethodImplementation(Class cls, SEL sel)
{
IMP imp;
if (!cls || !sel) return NULL;
imp = lookUpMethod(cls, sel, YES/*initialize*/, YES/*cache*/);
// Translate forwarding function to C-callable external version
if (imp == (IMP)&_objc_msgForward_internal) {
return (IMP)&_objc_msgForward;
}
return imp;
}
PRIVATE_EXTERN IMP lookUpMethod(Class cls, SEL sel,
BOOL initialize, BOOL cache)
{
Class curClass;
IMP methodPC = NULL;
Method meth;
BOOL triedResolver = NO;
// Optimistic cache lookup
// 1. 先找下缓存
if (cache) {
methodPC = _cache_getImp(cls, sel);
if (methodPC) return methodPC;
}
// realize, +initialize, and any special early exit
// 2. 初始化下这个类,为接下来做准备
methodPC = prepareForMethodLookup(cls, sel, initialize);
if (methodPC) return methodPC;
// The lock is held to make method-lookup + cache-fill atomic
// with respect to method addition. Otherwise, a category could
// be added but ignored indefinitely because the cache was re-filled
// with the old value after the cache flush on behalf of the category.
retry:
lockForMethodLookup();
// Ignore GC selectors
if (ignoreSelector(sel)) {
methodPC = _cache_addIgnoredEntry(cls, sel);
goto done;
}
// Try this class's cache.
// 3. 先试着找缓存
methodPC = _cache_getImp(cls, sel);
if (methodPC) goto done;
// Try this class's method lists.
// 4. 找自己的method列表
meth = _class_getMethodNoSuper_nolock(cls, sel);
if (meth) {
log_and_fill_cache(cls, cls, meth, sel);
methodPC = method_getImplementation(meth);
goto done;
}
// Try superclass caches and method lists.
// 5. 找父类的缓存和method列表
curClass = cls;
while ((curClass = _class_getSuperclass(curClass))) {
// Superclass cache.
meth = _cache_getMethod(curClass, sel, &_objc_msgForward_internal);
if (meth) {
if (meth != (Method)1) {
// Found the method in a superclass. Cache it in this class.
log_and_fill_cache(cls, curClass, meth, sel);
methodPC = method_getImplementation(meth);
goto done;
}
else {
// Found a forward:: entry in a superclass.
// Stop searching, but don't cache yet; call method
// resolver for this class first.
break;
}
}
// Superclass method list.
meth = _class_getMethodNoSuper_nolock(curClass, sel);
if (meth) {
log_and_fill_cache(cls, curClass, meth, sel);
methodPC = method_getImplementation(meth);
goto done;
}
}
// No implementation found. Try method resolver once.
// 6. 如果还是找不到就转发
if (!triedResolver) {
unlockForMethodLookup();
_class_resolveMethod(cls, sel);
// Don't cache the result; we don't hold the lock so it may have
// changed already. Re-do the search from scratch instead.
triedResolver = YES;
goto retry;
}
// No implementation found, and method resolver didn't help.
// Use forwarding.
_cache_addForwardEntry(cls, sel);
methodPC = &_objc_msgForward_internal;
done:
unlockForMethodLookup();
// paranoia: look for ignored selectors with non-ignored implementations
assert(!(ignoreSelector(sel) && methodPC != (IMP)&_objc_ignored_method));
return methodPC;
}
不同的类可以有相同的方法名，方法链表中根据方法名去查找具体的方法实现的。
IMP是一个函数指针，这个被指向的函数包含一个接收消息的对象id（自指针），调用方法的选标SEL（方法名）和不定个数的方法参数，并返回一个id。
BOOL class_addMethod（Class cls，SEL名称，IMP imp，const char *类型）
给CLS添加一个新的方法，若干CLS这个存在则方法报道查看失败
下面来看代码
class_addMethod(Class cls, SEL name, IMP imp, const charchar *types)
{
if (!cls) return NO;
rwlock_write(&runtimeLock);
IMP old = addMethod(newcls(cls), name, imp, types ?: "", NO);
rwlock_unlock_write(&runtimeLock);
return old ? NO : YES;
}
static IMP addMethod(class_t *cls, SEL name, IMP imp, const charchar *types, BOOL replace)
{
IMP result = NULL;
rwlock_assert_writing(&runtimeLock);
assert(types);
assert(isRealized(cls));
method_t *m;
// 1. 在自己的类的方法列表里找这个方法
if ((m = getMethodNoSuper_nolock(cls, name))) {
// already exists
if (!replace) {
// 不取代, 返回 m->imp
result = _method_getImplementation(m);
} else {
// 取代, 设置 cls 的 m 方法实现为 imp
result = _method_setImplementation(cls, m, imp);
}
} else {
// fixme optimize
// 2. 建立一个method_list_t节点
method_list_t *newlist;
newlist = (method_list_t *)_calloc_internal(sizeof(*newlist), 1);
newlist->entsize_NEVER_USE = (uint32_t)sizeof(method_t) | fixed_up_method_list;
newlist->count = 1;
newlist->first.name = name;
newlist->first.types = strdup(types);
if (!ignoreSelector(name)) {
newlist->first.imp = imp;
} else {
newlist->first.imp = (IMP)&_objc_ignored_method;
}
// 3. 把newlist加到cls的方法列表里
BOOL vtablesAffected = NO;
attachMethodLists(cls, &newlist, 1, NO, &vtablesAffected);
// 4. 刷新cls缓存
flushCaches(cls);
if (vtablesAffected) flushVtables(cls);
result = NULL;
}
return result;
}
泪目, 这里就是直接设置replace == YES.
void method_exchangeImplementations(Method m1_gen, Method m2_gen)
交换2个方法的实现指针
void method_exchangeImplementations(Method m1_gen, Method m2_gen)
{
method_t *m1 = newmethod(m1_gen);
method_t *m2 = newmethod(m2_gen);
if (!m1 || !m2) return;
rwlock_write(&runtimeLock);
if (ignoreSelector(m1->name) || ignoreSelector(m2->name)) {
// Ignored methods stay ignored. Now they're both ignored.
m1->imp = (IMP)&_objc_ignored_method;
m2->imp = (IMP)&_objc_ignored_method;
rwlock_unlock_write(&runtimeLock);
return;
}
// 交换2个方法的实现指针
IMP m1_imp = m1->imp;
m1->imp = m2->imp;
m2->imp = m1_imp;
if (vtable_containsSelector(m1->name) ||
vtable_containsSelector(m2->name))
{
// Don't know the class - will be slow if vtables are affected
// fixme build list of classes whose Methods are known externally?
flushVtables(NULL);
}
// fixme catch NSObject changing to custom RR
// cls->setCustomRR();
// fixme update monomorphism if necessary
rwlock_unlock_write(&runtimeLock);
}
其实这里有个坑, Method是怎么来的呢, 通过class_getInstanceMethod,如果子类没有的话,会返回父类的方法, 如果这个时候在用method_exchangeImplementations替换,会把父类替的方法替换掉,这显然不是我们想要的.所以呢,我们的method swizzle通常是这么写
static void XY_swizzleInstanceMethod(Class c, SEL original, SEL replacement)
{
Method a = class_getInstanceMethod(c, original);
Method b = class_getInstanceMethod(c, replacement);
if (class_addMethod(c, original, method_getImplementation(b), method_getTypeEncoding(b)))
{
class_replaceMethod(c, replacement, method_getImplementation(a), method_getTypeEncoding(a));
}
else
{
method_exchangeImplementations(a, b);
}
}
IMP method_getImplementation(Method method)
返回method的实现指针
代码如下, 没什么好说的,其实就是返回method->imp
IMP method_getImplementation(Method m)
{
return _method_getImplementation(newmethod(m));
}
static IMP _method_getImplementation(method_t *m)
{
if (!m) return NULL;
return m->imp;
}
IMP method_setImplementation(Method method, IMP imp)
设置方法的新的实现指针, 返回旧的实现指针
IMP method_setImplementation(Method m, IMP imp)
{
// Don't know the class - will be slow if vtables are affected
// fixme build list of classes whose Methods are known externally?
IMP result;
rwlock_write(&runtimeLock);
result = _method_setImplementation(Nil, newmethod(m), imp);
rwlock_unlock_write(&runtimeLock);
return result;
}
static IMP _method_setImplementation(class_t *cls, method_t *m, IMP imp)
{
rwlock_assert_writing(&runtimeLock);
if (!m) return NULL;
if (!imp) return NULL;
if (ignoreSelector(m->name)) {
// Ignored methods stay ignored
return m->imp;
}
// 替换方法的实现指针
IMP old = _method_getImplementation(m);
m->imp = imp;
// No cache flushing needed - cache contains Methods not IMPs.
if (vtable_containsSelector(newmethod(m)->name)) {
// Will be slow if cls is NULL (i.e. unknown)
// fixme build list of classes whose Methods are known externally?
flushVtables(cls);
}
// fixme catch NSObject changing to custom RR
// cls->setCustomRR();
// fixme update monomorphism if necessary
return old;
}
method_getTypeEncoding(Method m)
返回方法m的参数和返回值的描述的字串
这个就是直接返回m->types
