方法缓存与查找

前面我们去查看了类的结构,其中有个cache_t cache;字段没有去分析,现在我们就去源码中探寻一下。
cache顾名思义,就是缓存的意思,它是对曾经调用过的方法进行缓存,为什么要缓存呢?首先我们先说下方法的调用,在我们调用[objc message]的时候会编译成objc_msgSend(objc,@selector(message)),这时候就对象就会根据自己的isa指针去寻找自己的类,在找到类后根据类得到class_rw_t结构体,然后遍历class_rw_t中的method_array_t methods数组,如果找不到则根据superclass指针去遍历父类的方法列表直到找到为止(类方法同理),如果每次调用方法都去查下的话是一个很浪费资源的做法,所以OC就设计了一个cache去缓存曾经调用过的方法列表。
cache_t是一个散列表,所以可以使用键值对的方式去读取,这样的效率就肯定比遍历快的多,我们简单看下方法是怎么缓存的

方法调用

首先我们在lookUpImpOrForward打个断点

可以看到调用 _objc_msgSend后会调用该方法

IMP lookUpImpOrForward(Class cls, SEL sel, id inst, 
                       bool initialize, bool cache, bool resolver)
{
    Class curClass;
    IMP imp = nil;
    Method meth;
    bool triedResolver = NO;

    runtimeLock.assertUnlocked();
    if (cache) {
    //1,如果本身有缓存就直接返回缓存中的
        imp = cache_getImp(cls, sel);
        if (imp) return imp;
    }

    if (!cls->isRealized()) {
   // 2,查看类是否加载,若未加载 直接加载
        rwlock_writer_t lock(runtimeLock);
        realizeClass(cls);
    }

    if (initialize  &&  !cls->isInitialized()) {
    //3,调用initialize方法
        _class_initialize (_class_getNonMetaClass(cls, inst));
    }
 retry:
    runtimeLock.read();

    // Try this class iss cache.
    //4,在类缓存中查找,如果存在 直接返回
    imp = cache_getImp(cls, sel);
    if (imp) goto done;

    // Try this class is method lists.
    //5,在自己的方法列表中查找
    meth = getMethodNoSuper_nolock(cls, sel);
    if (meth) {
    //6,如果查找到了,加载进缓存,结束查找
        log_and_fill_cache(cls, meth->imp, sel, inst, cls);
        imp = meth->imp;
        goto done;
    }

    // Try superclass caches and method lists.

    curClass = cls;
    //7,循环在类的父类中查找方法
    while ((curClass = curClass->superclass)) {
        // Superclass cache.
        //8,在父类缓存中查找
        imp = cache_getImp(curClass, sel);
        if (imp) {
            if (imp != (IMP)_objc_msgForward_impcache) {
            //9,在父类中找到方法并且不是`_objc_msgForward_impcache`(消息转发)就把方法添加的自身的缓存中,并返回
                // 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 do not cache yet; call method 
                // resolver for this class first.
                break;
            }
        }
        // Superclass method list.
        //10,缓存中查不到在父类的方法列表中查找
        meth = getMethodNoSuper_nolock(curClass, sel);
        if (meth) {
        //11查找到,就把方法添加的自身的缓存中,并返回
            log_and_fill_cache(cls, meth->imp, sel, inst, curClass);
            imp = meth->imp;
            goto done;
        }
    }


    if (resolver  &&  !triedResolver) {
    //12,方法解析
        runtimeLock.unlockRead();
        _class_resolveMethod(cls, sel, inst);
        triedResolver = YES;
        goto retry;
    }
    //13,查找不到直接返回`_objc_msgForward_impcache`(方法转发)
    imp = (IMP)_objc_msgForward_impcache;
    cache_fill(cls, sel, imp, inst);

 done:
    runtimeLock.unlockRead();

    return imp;
}

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1,如果本身有缓存就直接返回缓存中的

    if (cache) {
        imp = cache_getImp(cls, sel);
        if (imp) return imp;
    }
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2,查看类是否加载,若未加载直接加载

  if (!cls->isRealized()) {
        rwlock_writer_t lock(runtimeLock);
        realizeClass(cls);//类如果没有加入内存 则加入
    }
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3,调用initialize方法

if (initialize  &&  !cls->isInitialized()) {
 _class_initialize (_class_getNonMetaClass(cls, inst));
 }
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由此也能看出initialize与类加载是否到内存中无关,只要实现了该方法切第一次调用类就会执行该方法

4,在类缓存中查找,如果存在直接返回

    imp = cache_getImp(cls, sel);
    if (imp) goto done;
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5,在自己的方法列表中查找
6,如果查找到了,加载进缓存,结束查找

    meth = getMethodNoSuper_nolock(cls, sel);
    if (meth) {
        log_and_fill_cache(cls, meth->imp, sel, inst, cls);
        imp = meth->imp;
        goto done;
    }
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7,循环在类的父类中查找方法
8,在父类缓存中查找
9,在父类中找到方法并且不是_objc_msgForward_impcache(消息转发)就把方法添加的自身的缓存中,并返回

 while ((curClass = curClass->superclass)) {
        // 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 do not cache yet; call method 
                // resolver for this class first.
                break;
            }
        }
    }
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10,缓存中查不到在父类的方法列表中查找
11,查找到,就把方法添加的自身的缓存中,并返回

while ((curClass = curClass->superclass)) {
  meth = getMethodNoSuper_nolock(curClass, sel);
        if (meth) {
            log_and_fill_cache(cls, meth->imp, sel, inst, curClass);
            imp = meth->imp;
            goto done;
        }
    }
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12,方法解析

    if (resolver  &&  !triedResolver) {
		//12,方法解析
        runtimeLock.unlockRead();
        _class_resolveMethod(cls, sel, inst);
        // 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;
    }
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在这个函数中实现了复杂的方法解析逻辑。如果 cls 是元类则会发送 +resolveClassMethod，然后根据 lookUpImpOrNil(cls, sel, inst, NO/*initialize*/, YES/*cache*/, NO/*resolver*/) 函数的结果来判断是否发送 +resolveInstanceMethod；如果不是元类，则只需要发送 +resolveInstanceMethod 消息。这里调用 +resolveInstanceMethod 或 +resolveClassMethod 时再次用到了 objc_msgSend，而且第三个参数正是传入 lookUpImpOrForward 的那个 sel。在发送方法解析消息之后还会调用 lookUpImpOrNil(cls, sel, inst, NO/*initialize*/, YES/*cache*/, NO/*resolver*/) 来判断是否已经添加上 sel 对应的 IMP 了，打印出结果。

13,查找不到直接返回_objc_msgForward_impcache(方法转发)

    imp = (IMP)_objc_msgForward_impcache;
    cache_fill(cls, sel, imp, inst);
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方法缓存

在上面代码中我们可以看到有调用cache_fill方法

void cache_fill(Class cls, SEL sel, IMP imp, id receiver)
{
#if !DEBUG_TASK_THREADS
    mutex_locker_t lock(cacheUpdateLock);
    cache_fill_nolock(cls, sel, imp, receiver);
#else
    _collecting_in_critical();
    return;
#endif
}
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又调用了cache_fill_nolock

static void cache_fill_nolock(Class cls, SEL sel, IMP imp, id receiver)
{
    cacheUpdateLock.assertLocked();

    // Never cache before +initialize is done
	//类如果没有初始化直接返回
    if (!cls->isInitialized()) return;

    // Make sure the entry was not added to the cache by some other thread 
    // before we grabbed the cacheUpdateLock.
	//读取缓存 如果存在,直接返回
    if (cache_getImp(cls, sel)) return;

	//获取缓存表,以`sel`地址为key
    cache_t *cache = getCache(cls);
    cache_key_t key = getKey(sel);

    // Use the cache as-is if it is less than 3/4 full
	//获取缓存中已被占用多少
    mask_t newOccupied = cache->occupied() + 1;
	//缓存一共多大
    mask_t capacity = cache->capacity();
	
    if (cache->isConstantEmptyCache()) {
        // Cache is read-only. Replace it.
		//如果是个空表则初始化
        cache->reallocate(capacity, capacity ?: INIT_CACHE_SIZE);
    }
    else if (newOccupied <= capacity / 4 * 3) {
		//如果缓存小于3/4
        // Cache is less than 3/4 full. Use it as-is.
    }
    else {
        // Cache is too full. Expand it.
		//缓存超过3/4 则要重新分配缓存空间
        cache->expand();
    }

    // Scan for the first unused slot and insert there.
    // There is guaranteed to be an empty slot because the 
    // minimum size is 4 and we resized at 3/4 full.
	//获取到可用的bucket_t
    bucket_t *bucket = cache->find(key, receiver);
	//判断当前的位置是否为空,如果是的 则将占用量+1
    if (bucket->key() == 0) cache->incrementOccupied();
	//存入缓存
    bucket->set(key, imp);
}

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这个方法主要就是去查找表中的空余位置,然后将对应的方法放入该位置
我们看到缓存如果超出了,则需要重新分配空间

void cache_t::expand()
{
    cacheUpdateLock.assertLocked();
    
    uint32_t oldCapacity = capacity();
    uint32_t newCapacity = oldCapacity ? oldCapacity*2 : INIT_CACHE_SIZE;

    if ((uint32_t)(mask_t)newCapacity != newCapacity) {
        // mask overflow - ca not grow further
        // fixme this wastes one bit of mask
        newCapacity = oldCapacity;
    }

    reallocate(oldCapacity, newCapacity);
}
void cache_t::reallocate(mask_t oldCapacity, mask_t newCapacity)
{
    bool freeOld = canBeFreed();

    bucket_t *oldBuckets = buckets();
    bucket_t *newBuckets = allocateBuckets(newCapacity);

    // Cache is old contents are not propagated. 
    // This is thought to save cache memory at the cost of extra cache fills.
    // fixme re-measure this

    assert(newCapacity > 0);
    assert((uintptr_t)(mask_t)(newCapacity-1) == newCapacity-1);

    setBucketsAndMask(newBuckets, newCapacity - 1);
    
    if (freeOld) {
        cache_collect_free(oldBuckets, oldCapacity);
        cache_collect(false);
    }
}
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在上述代码中可以看到,重新分配缓存空间是原先空间的2倍,然后后将原先的空间释放了,但是没有将原先的数据存储下来,这是为什么呢?主要因为这个表是个散列表,它获取到的位置是拿key与mask去进行&运算得出的,我们可以在find方法中查看到

bucket_t * cache_t::find(cache_key_t k, id receiver)
{
    assert(k != 0);

    bucket_t *b = buckets();
    mask_t m = mask();
    //根据`key`与`mask`获取到存储在表中的位置
    mask_t begin = cache_hash(k, m);
    mask_t i = begin;
    do {
        if (b[i].key() == 0  ||  b[i].key() == k) {
            return &b[i];
        }
    } while ((i = cache_next(i, m)) != begin);

    // hack
    Class cls = (Class)((uintptr_t)this - offsetof(objc_class, cache));
    cache_t::bad_cache(receiver, (SEL)k, cls);
}
//该方法就是进行简单的`&`运算获取,因为是`&`运算,所以得到的结果肯定比`mask`小
static inline mask_t cache_hash(cache_key_t key, mask_t mask) 
{
    return (mask_t)(key & mask);
}
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mask其实就是当前最大存储量-1

void cache_t::reallocate(mask_t oldCapacity, mask_t newCapacity)
{
    bool freeOld = canBeFreed();

    bucket_t *oldBuckets = buckets();
    bucket_t *newBuckets = allocateBuckets(newCapacity);

    assert(newCapacity > 0);
    assert((uintptr_t)(mask_t)(newCapacity-1) == newCapacity-1);

   //设置新表,设置mask值
    setBucketsAndMask(newBuckets, newCapacity - 1);
    
    if (freeOld) {
        cache_collect_free(oldBuckets, oldCapacity);
        cache_collect(false);
    }
}
void cache_t::setBucketsAndMask(struct bucket_t *newBuckets, mask_t newMask)
{
    mega_barrier();

    _buckets = newBuckets;
    
    // ensure other threads see new buckets before new mask
    mega_barrier();
    
    _mask = newMask;
    _occupied = 0;
}
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因为是个mask就只有那么大,如果容量变大了,那么mask的值也会变话,那样再拿key与新的mask进行&运算得出的结果肯定就不是原先的值了
因为key值不确定,进行&运算后很可能得到相同的值,这也是我们常说的hash冲突,苹果是以cache_next的方式去解决冲突(arm64)

static inline mask_t cache_next(mask_t i, mask_t mask) {
    return i ? i-1 : mask;
}
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从当前位子一直向下找,找到了空位置,就填充进去,如果找不到,就从后面向前遍历查找mask,一直找到(i = cache_next(i, m)) != begin

总结
1,当方法调用的时候首先回去缓存中查找,找不到会遍历方法列表,在找不到会查找父类方法缓存,然后查找父类方法列表,一直找到rootClass,找到后会加入本身类的方法缓存中,如果一直找不到就进行消息转发;
2,方法缓存是使用cache_t缓存,它是一个散列表,当缓存空间大于3/4时就需要扩容,并将先前存储的所有方法清空;
3,当第二次调用该方法时会在objc_msgSend中查找一遍然后调用,所以可以打断点尝试下第二次调用的时候就不会在调用lookUpImpOrForward了

方法调用

方法缓存

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