runtime04 - initlialize方法

initialize调用时机

第一次调用跟类或者子类相关的方法的时候, 会利用消息发送机制调用类的+initialize方法

initialize调用顺序

在利用消息发送机制调用子类的+initialize方法前, 会递归的调用所有继承链上的父类的+initialize方法, 但是所有的类都会根据metaclass的flags&RW_INITIALIZED, 来控制+initialize方法只会主动调用一次

1. 先父类,后子类
2. 类和分类中同时实现+initialize方法时,优先调用分类中的+initialize方法

initialize注意事项

1. +initialize方法调用是走的消息发送机制
2. 类主动调用它的+initialize方法只会调用一次, 但是由于消息发送的机制, 在子类没实现+initialize的情况下,消息寻址会找到他的父类的+initialize方法地址
3. 由于2的原因, 如果希望+initialize中的代码只执行一次,需要用dispatch_once包一下

源码

/***********************************************************************
* class_initialize.  Send the '+initialize' message on demand to any
* uninitialized class. Force initialization of superclasses first.
**********************************************************************/
void initializeNonMetaClass(Class cls)
{
    ASSERT(!cls->isMetaClass());

    Class supercls;
    bool reallyInitialize = NO;

    // Make sure super is done initializing BEFORE beginning to initialize cls.
    // See note about deadlock above.
    supercls = cls->getSuperclass();
    if (supercls  &&  !supercls->isInitialized()) {
        initializeNonMetaClass(supercls);
    }
    
    // Try to atomically set CLS_INITIALIZING.
    SmallVector<_objc_willInitializeClassCallback, 1> localWillInitializeFuncs;
    {
        monitor_locker_t lock(classInitLock);
        if (!cls->isInitialized() && !cls->isInitializing()) {
            cls->setInitializing();
            reallyInitialize = YES;

            // Grab a copy of the will-initialize funcs with the lock held.
            localWillInitializeFuncs.initFrom(willInitializeFuncs);
        }
    }
    
    if (reallyInitialize) {
        // We successfully set the CLS_INITIALIZING bit. Initialize the class.
        
        // Record that we're initializing this class so we can message it.
        _setThisThreadIsInitializingClass(cls);

        if (MultithreadedForkChild) {
            // LOL JK we don't really call +initialize methods after fork().
            performForkChildInitialize(cls, supercls);
            return;
        }
        
        for (auto callback : localWillInitializeFuncs)
            callback.f(callback.context, cls);

        // Send the +initialize message.
        // Note that +initialize is sent to the superclass (again) if 
        // this class doesn't implement +initialize. 2157218
        if (PrintInitializing) {
            _objc_inform("INITIALIZE: thread %p: calling +[%s initialize]",
                         objc_thread_self(), cls->nameForLogging());
        }

        // Exceptions: A +initialize call that throws an exception 
        // is deemed to be a complete and successful +initialize.
        //
        // Only __OBJC2__ adds these handlers. !__OBJC2__ has a
        // bootstrapping problem of this versus CF's call to
        // objc_exception_set_functions().
#if __OBJC2__
        @try
#endif
        {
            callInitialize(cls);

            if (PrintInitializing) {
                _objc_inform("INITIALIZE: thread %p: finished +[%s initialize]",
                             objc_thread_self(), cls->nameForLogging());
            }
        }
#if __OBJC2__
        @catch (...) {
            if (PrintInitializing) {
                _objc_inform("INITIALIZE: thread %p: +[%s initialize] "
                             "threw an exception",
                             objc_thread_self(), cls->nameForLogging());
            }
            @throw;
        }
        @finally
#endif
        {
            // Done initializing.
            lockAndFinishInitializing(cls, supercls);
        }
        return;
    }
    
    else if (cls->isInitializing()) {
        // We couldn't set INITIALIZING because INITIALIZING was already set.
        // If this thread set it earlier, continue normally.
        // If some other thread set it, block until initialize is done.
        // It's ok if INITIALIZING changes to INITIALIZED while we're here, 
        //   because we safely check for INITIALIZED inside the lock 
        //   before blocking.
        if (_thisThreadIsInitializingClass(cls)) {
            return;
        } else if (!MultithreadedForkChild) {
            waitForInitializeToComplete(cls);
            return;
        } else {
            // We're on the child side of fork(), facing a class that
            // was initializing by some other thread when fork() was called.
            _setThisThreadIsInitializingClass(cls);
            performForkChildInitialize(cls, supercls);
        }
    }
    
    else if (cls->isInitialized()) {
        // Set CLS_INITIALIZING failed because someone else already 
        //   initialized the class. Continue normally.
        // NOTE this check must come AFTER the ISINITIALIZING case.
        // Otherwise: Another thread is initializing this class. ISINITIALIZED 
        //   is false. Skip this clause. Then the other thread finishes 
        //   initialization and sets INITIALIZING=no and INITIALIZED=yes. 
        //   Skip the ISINITIALIZING clause. Die horribly.
        return;
    }
    
    else {
        // We shouldn't be here. 
        _objc_fatal("thread-safe class init in objc runtime is buggy!");
    }
}


void callInitialize(Class cls)
{
    ((void(*)(Class, SEL))objc_msgSend)(cls, @selector(initialize));
    asm("");
}

源码路径

objc_sendMsg

IMP lookUpImpOrForward(id obj, SEL, Class cls, int behavior);

static Class realizeAndInitializeIfNeeded_locked(id inst, Class cls, bool initialize)

static Class initializeAndLeaveLocked(Class cls, id obj, mutex_t& lock)

static Class initializeAndMaybeRelock(Class cls, id inst, mutex_t& lock, bool leaveLocked)

void initializeNonMetaClass(Class cls)

void callInitialize(Class cls)