Runtime底层原理探究(二) --- 消息发送机制(慢速查找)

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 lookup if (cache) { /// 若是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，而后判断cache 若是有则调用cache_ getImp，这个cache_ getimp是 是从缓存里去取并不是递归，不会执行以前快速查找的代码（慢速查找绝对没有缓存，由于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

检查类是不是已知类，若是是未知的则抛异常。若是是已知类则判断是否已经实现，若是未实现则进行赋值，而后在判断是否已经初始化。若是类未初始化，对其进行初始化。若是这个消息是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（之因此在此这么作1是由于防止并发进行资源抢夺，2是由于remap可能会已经持有就不必浪费资源再去查找了），若是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

而后传入一个cls和sel,找到对应的imp若是存在则缓存后直接去done，若是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，若是父类方法有缓存则取缓存，而后缓存到子类的方法中，若是父类方法没有缓存，则去方法列表里查找而后进行缓存赋值。

// 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

若是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由于cache_getImp是一个

他掉用的GETIMP方法当调用CacheHit Checkmiss的时候并无发送objc_msgSend_uncached

慢速查找总结

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