源码

/***********************************************************************
* lookUpImpOrForward.
* The standard IMP lookup.
* initialize==NO tries to avoid +initialize (but sometimes fails)
* cache==NO skips optimistic unlocked lookup (but uses cache elsewhere)
* Most callers should use initialize==YES and cache==YES.
* inst is an instance of cls or a subclass thereof, or nil if none is known.
*   If cls is an un-initialized metaclass then a non-nil inst is faster.
* May return _objc_msgForward_impcache. IMPs destined for external use
*   must be converted to _objc_msgForward or _objc_msgForward_stret.
*   If you don't want forwarding at all, use lookUpImpOrNil() instead.
**********************************************************************/
IMP lookUpImpOrForward(Class cls, SEL sel, id inst, bool initialize, bool cache, bool resolver)
{IMP imp = nil;bool triedResolver = NO;runtimeLock.assertUnlocked();// Optimistic cache lookupif (cache) { /// 如果有缓存则从缓存里取imp = cache_getImp(cls, sel);if (imp) return imp;}// runtimeLock is held during isRealized and isInitialized checking// to prevent races against concurrent realization.// runtimeLock is held during method search 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.runtimeLock.read();if (!cls->isRealized()) {// Drop the read-lock and acquire the write-lock.// realizeClass() checks isRealized() again to prevent// a race while the lock is down.runtimeLock.unlockRead();runtimeLock.write();realizeClass(cls);runtimeLock.unlockWrite();runtimeLock.read();}if (initialize  &&  !cls->isInitialized()) {runtimeLock.unlockRead();_class_initialize (_class_getNonMetaClass(cls, inst));runtimeLock.read();// 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}retry:    runtimeLock.assertReading();// Try this class's cache./// 这里为啥又取了一次imp ////// 1. 保证并发///  2.remap(cls) -- 重映射imp = cache_getImp(cls, sel);if (imp) goto done;// Try this class's method lists.{Method meth = getMethodNoSuper_nolock(cls, sel);if (meth) {log_and_fill_cache(cls, meth->imp, sel, inst, cls);imp = meth->imp;goto done;}}// Try superclass caches and method lists.{unsigned attempts = unreasonableClassCount();for (Class curClass = cls->superclass;curClass != nil;curClass = curClass->superclass){// Halt if there is a cycle in the superclass chain.if (--attempts == 0) {_objc_fatal("Memory corruption in class list.");}// Superclass cache.imp = cache_getImp(curClass, sel);if (imp) {if (imp != (IMP)_objc_msgForward_impcache) {// Found the method in a superclass. Cache it in this class.log_and_fill_cache(cls, imp, sel, inst, curClass);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.Method meth = getMethodNoSuper_nolock(curClass, sel);if (meth) {log_and_fill_cache(cls, meth->imp, sel, inst, curClass);imp = meth->imp;goto done;}}}// No implementation found. Try method resolver once.if (resolver  &&  !triedResolver) {runtimeLock.unlockRead();_class_resolveMethod(cls, sel, inst);runtimeLock.read();// 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.imp = (IMP)_objc_msgForward_impcache;cache_fill(cls, sel, imp, inst);done:runtimeLock.unlockRead();return imp;
}
复制代码

lookUpImpOrForward

cache

首先声明了一个nil imp，然后判断cache 如果有则调用cache_ getImp，这个cache_ getimp是 是从缓存里去取并非递归，不会执行之前快速查找的代码（慢速查找绝对没有缓存，因为刚刚快速查找调用这个方法传进来的第二个参数是NO，第一个参数的意思是）

checkIsKnowClass

检查类是否是已知类，如果是未知的则抛异常。如果是已知类则判断是否已经实现，如果未实现则进行赋值，然后在判断是否已经初始化。如果类未初始化，对其进行初始化。如果这个消息是initialize，那么直接进行类的初始化

方法查找流程

此时他又执行cache_ getimp（之所以在此这么做1是因为防止并发进行资源抢夺，2是因为remap可能会已经持有就没必要浪费资源再去查找了），如果没有则会执行一个的过程

  // Try this class's method lists.{Method meth = getMethodNoSuper_nolock(cls, sel);if (meth) {log_and_fill_cache(cls, meth->imp, sel, inst, cls);imp = meth->imp;goto done;}}
复制代码

然后传入一个cls和sel,找到对应的imp如果存在则缓存后直接去done，如果没有找到的话则去父类查找

 // Try superclass caches and method lists.{unsigned attempts = unreasonableClassCount();for (Class curClass = cls->superclass;curClass != nil;curClass = curClass->superclass){// Halt if there is a cycle in the superclass chain.if (--attempts == 0) {_objc_fatal("Memory corruption in class list.");}// Superclass cache. ///查找父类方法缓存中取缓存imp = cache_getImp(curClass, sel);if (imp) {if (imp != (IMP)_objc_msgForward_impcache) {// Found the method in a superclass. Cache it in this class.log_and_fill_cache(cls, imp, sel, inst, curClass);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. ///娶不到的话查找父类方法列表Method meth = getMethodNoSuper_nolock(curClass, sel);if (meth) {log_and_fill_cache(cls, meth->imp, sel, inst, curClass);imp = meth->imp;goto done;}}}
复制代码

查找父类循环条件是 父类不为nil，则一直进行循环，如果父类方法有缓存则取缓存，然后缓存到子类的方法中，如果父类方法没有缓存，则去方法列表里查找然后进行缓存赋值。

  // No implementation found. Try method resolver once.if (resolver  &&  !triedResolver) {runtimeLock.unlockRead();_class_resolveMethod(cls, sel, inst);runtimeLock.read();// 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.imp = (IMP)_objc_msgForward_impcache;cache_fill(cls, sel, imp, inst);
复制代码

如果没有发现实现则会进入动态方法解析流程。

ps: 2019年04月04日10:33:21 补充：

cache_getImp不会从缓存查找IMP，这个方法不会递归调用因为cache_getImp是一个

他掉用的GETIMP方法当调用CacheHit Checkmiss的时候并没有发送objc_msgSend_uncached

慢速查找总结

当消息发送没有通过查找时则会进入慢速查找，如果未初始化则进行类的初始化，首先查找当前类的方法列表通过Selector找到对应IMP，如果有则返回并缓存，没有的话进行下一步遍历父类直到nil如果父类里存在则返回父类方法。并缓存到当前类，如果都没有则判断reslover参数是否支持消息转发机制，如果支持则进入消息转发流程，如果不支持则抛出异常。