objc_msgSend定义：id objc_msgSend(id self, SEL op, …);
self：A pointer that points to the instance of the class that is to receive the message.{消息的接收者}
op：The selector of the method that handles the message.{是selector}
…：A variable argument list containing the arguments to the method.{是参数列表}
{其实我没有找到这段的实现，就用大神的代码了}
id objc_msgSend(id self, SEL op, ...) {
if (!self) return nil;
IMP imp = class_getMethodImplementation(self->isa, SEL op);
imp(self, op, ...); //调用这个函数，伪代码...
}
// 源码
IMP class_getMethodImplementation(Class cls, SEL sel)
{
IMP imp;
if (!cls || !sel) return nil;
imp = lookUpImpOrNil(cls, sel, nil,
YES/*initialize*/, YES/*cache*/, YES/*resolver*/);
// Translate forwarding function to C-callable external version
if (!imp) {
// 如果找不到imp就返回消息转发
return _objc_msgForward;
}
return imp;
}
IMP lookUpImpOrNil(Class cls, SEL sel, id inst,
bool initialize, bool cache, bool resolver)
{
// 查找imp或决定转发
IMP imp = lookUpImpOrForward(cls, sel, inst, initialize, cache, resolver);
if (imp == _objc_msgForward_impcache) return nil;
else return imp;
}
IMP lookUpImpOrForward(Class cls, SEL sel, id inst,
bool initialize, bool cache, bool resolver)
{
Class curClass;
IMP methodPC = nil;
Method meth;
bool triedResolver = NO;
methodListLock.assertUnlocked();
// Optimistic cache lookup
if (cache) {
// 查找imp，如果有缓存，就返回缓存
methodPC = _cache_getImp(cls, sel);
if (methodPC) return methodPC;
}
// Check for freed class
// 查看释放的类
if (cls == _class_getFreedObjectClass())
// 如果类已经释放，返回_freedHandler
return (IMP) _freedHandler;
// Check for +initialize
// 检查有没有实现+initialize方法
if (initialize && !cls->isInitialized()) {
_class_initialize (_class_getNonMetaClass(cls, inst));
// If sel == initialize, _class_initialize will send +initialize and
// then the messenger will send +initialize again after this
// procedure finishes. Of course, if this is not being called
// from the messenger then it won't happen. 2778172
}
// 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:
// 加锁
methodListLock.lock();
// Try this class's cache.
// 查找类的缓存
methodPC = _cache_getImp(cls, sel);
if (methodPC) goto done;
// Try this class's method lists.
// 查找类的方法列表
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.
curClass = cls;
while ((curClass = curClass->superclass)) {
// Superclass cache.
// 查找父类的缓存
meth = _cache_getMethod(curClass, sel, _objc_msgForward_impcache);
if (meth) {
if (meth != (Method)1) {
// Found the method in a superclass. Cache it in this class.
// 在父类的缓存中查找
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.
// 查找父类的方法列表
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.
if (resolver && !triedResolver) {
// 解锁
methodListLock.unlock();
// 如果强制传入resolver为YES，且找不到imp，尝试一次_class_resolveMethod，不尝试消息转发了
_class_resolveMethod(cls, sel, inst);
triedResolver = YES;
goto retry;
}
// No implementation found, and method resolver didn't help.
// Use forwarding.
// 方法找不到：尝试一次resolver方法无效；开始转发
_cache_addForwardEntry(cls, sel);
// 返回_objc_msgForward_impcache表示转发
methodPC = _objc_msgForward_impcache;
done:
// 解锁
methodListLock.unlock();
return methodPC;
}
