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Runtime源码 方法调用的过程

前言

Objective-C语言的一大特性就是动态的,根据官方文档的描述:在runtime之前,消息和方法并不是绑定在一起的,编译器会把方法调用转换为objc_msgSend(receiver, selector),如果方法中带有参数则转换为objc_msgSend(receiver, selector, arg1, arg2, ...)接下来我们通过源码一窥究竟,在次之前我们先了解几个基本概念

  • SEL 在objc.h文件中我们可以看到如下代码:
/// An opaque type that represents a method selector.
typedef struct objc_selector *SEL;
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SEL其实就是一个不透明的类型它代表一个方法选择子,在编译期,会根据方法名字生成一个ID。

  • IMP 在objc.h文件中我们可以看到IMP:
/// A pointer to the function of a method implementation. 
#if !OBJC_OLD_DISPATCH_PROTOTYPES
typedef void (*IMP)(void /* id, SEL, ... */ ); 
#else
typedef id _Nullable (*IMP)(id _Nonnull, SEL _Nonnull, ...); 
#endif
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他是一个函数指针,指向方法实现的首地址。

  • Method
/// An opaque type that represents a method in a class definition.
typedef struct objc_method *Method;  

struct objc_method {
    SEL _Nonnull method_name                                 OBJC2_UNAVAILABLE;
    char * _Nullable method_types                            OBJC2_UNAVAILABLE;
    IMP _Nonnull method_imp                                  OBJC2_UNAVAILABLE;
}                                                            OBJC2_UNAVAILABLE;
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它保存了SEL到IMP和方法类型,所以我们可以通过SEL调用对应的IMP

方法调用的流程

objc_msgSend的消息分发分为以下几个步骤: 我们找到objc _msgSend源码,都是汇编,不过注释比较详尽

/********************************************************************
 *
 * id objc_msgSend(id self, SEL	_cmd,...);
 * IMP objc_msgLookup(id self, SEL _cmd, ...);
 *
 * objc_msgLookup ABI:
 * IMP returned in r11
 * Forwarding returned in Z flag
 * r10 reserved for our use but not used
 *
 ********************************************************************/
	
	.data
	.align 3
	.globl _objc_debug_taggedpointer_classes
_objc_debug_taggedpointer_classes:
	.fill 16, 8, 0
	.globl _objc_debug_taggedpointer_ext_classes
_objc_debug_taggedpointer_ext_classes:
	.fill 256, 8, 0

	ENTRY _objc_msgSend
	UNWIND _objc_msgSend, NoFrame
	MESSENGER_START

	NilTest	NORMAL

	GetIsaFast NORMAL		// r10 = self->isa
	CacheLookup NORMAL, CALL	// calls IMP on success

	NilTestReturnZero NORMAL

	GetIsaSupport NORMAL

// cache miss: go search the method lists
LCacheMiss:
	// isa still in r10
	MESSENGER_END_SLOW
	jmp	__objc_msgSend_uncached

	END_ENTRY _objc_msgSend

	
	ENTRY _objc_msgLookup

	NilTest	NORMAL

	GetIsaFast NORMAL		// r10 = self->isa
	CacheLookup NORMAL, LOOKUP	// returns IMP on success

	NilTestReturnIMP NORMAL

	GetIsaSupport NORMAL

// cache miss: go search the method lists
LCacheMiss:
	// isa still in r10
	jmp	__objc_msgLookup_uncached

	END_ENTRY _objc_msgLookup

	
	ENTRY _objc_msgSend_fixup
	int3
	END_ENTRY _objc_msgSend_fixup

	
	STATIC_ENTRY _objc_msgSend_fixedup
	// Load _cmd from the message_ref
	movq	8(%a2), %a2
	jmp	_objc_msgSend
	END_ENTRY _objc_msgSend_fixedup

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就此我们大概可以了解到其调用流程:

  • 判断receiver是否为nil,也就是objc_msgSend的第一个参数self,也就是要调用的那个方法所属对象

  • 从缓存里寻找,找到了则分发,否则

  • 利用objc-class.mm中_ class _lookupMethodAndLoadCache3方法去寻找selector

    • 如果支持GC,忽略掉非GC环境的方法(retain等)
    • 从本class的method list寻找selector,如果找到,填充到缓存中,并返回selector,否则
    • 寻找父类的method list,并依次往上寻找,直到找到selector,填充到缓存中,并返回selector,否则
    • 调用_class_resolveMethod,如果可以动态resolve为一个selector,不缓存,方法返回,否则
    • 转发这个selector,否则
    • 报错,抛出异常

这里的**_ class _lookupMethodAndLoadCache3其实就是对lookUpImpOrForward**方法的调用:

/***********************************************************************
* _class_lookupMethodAndLoadCache.
* Method lookup for dispatchers ONLY. OTHER CODE SHOULD USE lookUpImp().
* This lookup avoids optimistic cache scan because the dispatcher 
* already tried that.
**********************************************************************/
IMP _class_lookupMethodAndLoadCache3(id obj, SEL sel, Class cls)
{        
    return lookUpImpOrForward(cls, sel, obj, 
                              YES/*initialize*/, NO/*cache*/, YES/*resolver*/);
}
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对第五个参数cache传值为NO,因为在此之前已经做了一个查找这里CacheLookup NORMAL, CALL,这里是对缓存查找的一个优化。

接下来看一下lookUpImpOrForward的一些关键实现细节

  • 缓存查找优化
 // Optimistic cache lookup
 if (cache)  
     methodPC = _cache_getImp(cls, sel);
     if (methodPC) return methodPC;    
 }
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这里有个判断,是否需要缓存查找,如果cache为NO则进入下一步

  • 检查被释放类
// Check for freed class
if (cls == _class_getFreedObjectClass())
	return (IMP) _freedHandler;
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_class _getFreedObjectClass的实现:

/***********************************************************************
* _class_getFreedObjectClass.  Return a pointer to the dummy freed
* object class.  Freed objects get their isa pointers replaced with
* a pointer to the freedObjectClass, so that we can catch usages of
* the freed object.
**********************************************************************/
static Class _class_getFreedObjectClass(void)
{
    return (Class)freedObjectClass;
}
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注释写到,这里返回的被释放对象的指针,不是太理解,备注这以后再看看

  • 懒加载+initialize
// Check for +initialize
    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
    }
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在方法调用过程中,如果类没有被初始化的时候,会调用_class_initialize对类进行初始化,关于+initialize可以看之前的Runtime源码 +load 和 +initialize

  • 加锁保证原子性
	// 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:
    methodListLock.lock();

    // Try this class's cache.

    methodPC = _cache_getImp(cls, sel);
    if (methodPC) goto done;
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这里又做了一次缓存查找,因为上一步执行了+initialize

加锁这一部分只有一行简单的代码,其主要目的保证方法查找以及缓存填充(cache-fill)的原子性,保证在运行以下代码时不会有新方法添加导致缓存被冲洗(flush)。

  • 本类的方法列表查找
// Try this class's method lists.

meth = _class_getMethodNoSuper_nolock(cls, sel);
if (meth) {
log_and_fill_cache(cls, cls, meth, sel);
 methodPC = method_getImplementation(meth);
 goto done;
}
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这里调用了log_ and_ fill_cache这个后面来看,接下里就是

  • 父类方法列表查找
// Try superclass caches and method lists.

    curClass = cls;
    while ((curClass = curClass->superclass)) {
        // Superclass cache.
        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.
                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;
        }
    }
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关于消息在列表方法查找的过程,根据官方文档如下:

messaging1

这里沿着集成体系对父类的方法列表进行查找,找到了就调用log_ and_ fill_cache

log_ and_ fill_cach的实现:
记录:

/***********************************************************************
* log_and_fill_cache
* Log this method call. If the logger permits it, fill the method cache.
* cls is the method whose cache should be filled. 
* implementer is the class that owns the implementation in question.
**********************************************************************/
static void
log_and_fill_cache(Class cls, Class implementer, Method meth, SEL sel)
{
#if SUPPORT_MESSAGE_LOGGING
    if (objcMsgLogEnabled) {
        bool cacheIt = logMessageSend(implementer->isMetaClass(), 
                                      cls->nameForLogging(),
                                      implementer->nameForLogging(), 
                                      sel);
        if (!cacheIt) return;
    }
#endif
    _cache_fill (cls, meth, sel);
}
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内部调用了**_cache _fill**,填充缓存:

/***********************************************************************
* _cache_fill.  Add the specified method to the specified class' cache.
* Returns NO if the cache entry wasn't added: cache was busy, 
*  class is still being initialized, new entry is a duplicate.
*
* Called only from _class_lookupMethodAndLoadCache and
* class_respondsToMethod and _cache_addForwardEntry.
*
* Cache locks: cacheUpdateLock must not be held.
**********************************************************************/
bool _cache_fill(Class cls, Method smt, SEL sel)
{
    uintptr_t newOccupied;
    uintptr_t index;
    cache_entry **buckets;
    cache_entry *entry;
    Cache cache;

    cacheUpdateLock.assertUnlocked();

    // Never cache before +initialize is done
    if (!cls->isInitialized()) {
        return NO;
    }

    // Keep tally of cache additions
    totalCacheFills += 1;

    mutex_locker_t lock(cacheUpdateLock);

    entry = (cache_entry *)smt;

    cache = cls->cache;

    // Make sure the entry wasn't added to the cache by some other thread 
    // before we grabbed the cacheUpdateLock.
    // Don't use _cache_getMethod() because _cache_getMethod() doesn't 
    // return forward:: entries.
    if (_cache_getImp(cls, sel)) {
        return NO; // entry is already cached, didn't add new one
    }

    // Use the cache as-is if it is less than 3/4 full
    newOccupied = cache->occupied + 1;
    if ((newOccupied * 4) <= (cache->mask + 1) * 3) {
        // Cache is less than 3/4 full.
        cache->occupied = (unsigned int)newOccupied;
    } else {
        // Cache is too full. Expand it.
        cache = _cache_expand (cls);

        // Account for the addition
        cache->occupied += 1;
    }

    // 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.
    buckets = (cache_entry **)cache->buckets;
    for (index = CACHE_HASH(sel, cache->mask); 
         buckets[index] != NULL; 
         index = (index+1) & cache->mask)
    {
        // empty
    }
    buckets[index] = entry;

    return YES; // successfully added new cache entry
}
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这里还没找到实现则进入下一步,动态方法解析和消息转发,关于消息转发的细节我们下篇再看。

方法缓存

在上面截出的源码中我们多次看到了cache,下面我们就来看看这个,在runtime.hobjc-runtime-newcache的定义如下

struct objc_cache {
    unsigned int mask /* total = mask + 1 */                 OBJC2_UNAVAILABLE;
    unsigned int occupied                                    OBJC2_UNAVAILABLE;
    Method _Nullable buckets[1]                              OBJC2_UNAVAILABLE;
};
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struct cache_t {
  struct bucket_t *_buckets;
  mask_t _mask;
  mask_t _occupied;
  ...
}
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这就是cache在runtime层面的表示,里面的字段和代表的含义类似

  • buckets
    数组表示的hash表,每个元素代表一个方法缓存
  • mask
    当前能达到的最大index(从0开始),,所以缓存的size(total)是mask+1
  • occupied
    被占用的槽位,因为缓存是以散列表的形式存在的,所以会有空槽,而occupied表示当前被占用的数目

而在_ buckets中包含了一个个的cache_entrybucket_t(objc2.0的变更):

typedef struct {
    SEL name;     // same layout as struct old_method
    void *unused;
    IMP imp;  // same layout as struct old_method
} cache_entry;
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cache_entry定义也包含了三个字段,分别是:

  • name,被缓存的方法名字
  • unused,保留字段,还没被使用。
  • imp,方法实现
struct bucket_t {
private:
    cache_key_t _key;
    IMP _imp;
    ...
}
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而bucket_t则没有了老的unused,包含了两个字段:

  • key,方法的标志(和之前的name对应)
  • imp, 方法的实现

后记

从runtime的源码我们知道了方法调用的流程和方法缓存,有些附带的问题答案也就呼之欲出了:

  • 方法缓存在元类的上,由第一节(Runtime源码 类、对象、isa)我们就知道在objc_class的isa指向了他的元类,所以每个类都只有一份方法缓存,而不是每一个类的object都保存一份。
  • 在方法调用的父类方法列表查找过程中,如果命中了也会调用_cache_fill (cls, meth, sel);,所以即便是从父类取到的方法,也会存在类本身的方法缓存里。而当用一个父类对象去调用那个方法的时候,也会在父类的metaclass里缓存一份。
  • 缓存容量限制,在上面的代码中我们注意到这个判断:
// Use the cache as-is if it is less than 3/4 full
mask_t newOccupied = cache->occupied() + 1;
mask_t capacity = cache->capacity();
if (cache->isConstantEmptyCache()) {
    // Cache is read-only. Replace it.
    cache->reallocate(capacity, capacity ?: INIT_CACHE_SIZE);
}
else if (newOccupied <= capacity / 4 * 3) {
     // Cache is less than 3/4 full. Use it as-is.
}
else {
     // Cache is too full. Expand it.
     cache->expand();
}
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当cache为空时创建;当新的被占用槽数小于等于其容量的3/4时,直接使用;否则调用cache->expand();扩充容量:

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 - can't grow further
        // fixme this wastes one bit of mask
        newCapacity = oldCapacity;
    }

    reallocate(oldCapacity, newCapacity);
}
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  • 为什么类的方法列表不直接做成散列表呢,做成list,还要单独缓存,多费事?
    散列表是没有顺序的,Objective-C的方法列表是一个list,是有顺序的 这个问题么,我觉得有以下三个原因:
    • Objective-C在查找方法的时候会顺着list依次寻找,并且category的方法在原始方法list的前面,需要先被找到,如果直接用hash存方法,方法的顺序就没法保证。
    • list的方法还保存了除了selector和imp之外其他很多属性。
    • 散列表是有空槽的,会浪费空间。

相关资料:
美团酒旅博文:深入理解Objective-C:方法缓存
官方文档:Messaging

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