今天无意这中遇到一个奇怪的崩溃，先上引起崩溃的代码：
- (void)dealloc
{
__weak __typeof(self)weak_self = self;
NSLog(@"%@", weak_self);
}
当执行到dealloc的时候，程序就crash 掉了。
崩溃信息如下：
objc[4572]: Cannot form weak reference to instance (0x160f6f890) of class MFChatRoomBoardController. It is possible that this object was over-released, or is in the process of deallocation.
(lldb)
error: empty command
(lldb) bt
* thread #1: tid = 0x35914d, 0x0000000182307aac libobjc.A.dylib`_objc_trap(), queue = 'com.apple.main-thread', stop reason = EXC_BREAKPOINT (code=1, subcode=0x182307aac)
* frame #0: 0x0000000182307aac libobjc.A.dylib`_objc_trap()
frame #1: 0x0000000182307b24 libobjc.A.dylib`_objc_fatal(char const*, ...) + 88
frame #2: 0x0000000182319890 libobjc.A.dylib`weak_register_no_lock + 316
frame #3: 0x0000000182320688 libobjc.A.dylib`objc_initWeak + 224
frame #4: 0x000000010022bf8c MakeFriends`-[MFChatRoomBoardController dealloc](self=0x0000000160f6f890, _cmd="dealloc") + 36 at MFChatRoomBoardController.m:31
其中，可以在控制台明确看到这样一段描述:
objc[4572]: Cannot form weak reference to instance (0x160f6f890) of class MFChatRoomBoardController. It is possible that this object was over-released, or is in the process of deallocation.
说明不允许在 dealloc 的时候取 weak self.
查看了一下 weak_register_no_lock 的函数代码，找到问题所在。
id
weak_register_no_lock(weak_table_t *weak_table, id referent_id, id *referrer_id)
{
objc_object *referent = (objc_object *)referent_id;
objc_object **referrer = (objc_object **)referrer_id;
if (!referent || referent->isTaggedPointer()) return referent_id;
// ensure that the referenced object is viable
bool deallocating;
if (!referent->ISA()->hasCustomRR()) {
deallocating = referent->rootIsDeallocating();
}
else {
BOOL (*allowsWeakReference)(objc_object *, SEL) =
(BOOL(*)(objc_object *, SEL))
object_getMethodImplementation((id)referent,
SEL_allowsWeakReference);
if ((IMP)allowsWeakReference == _objc_msgForward) {
return nil;
}
deallocating =
! (*allowsWeakReference)(referent, SEL_allowsWeakReference);
}
if (deallocating) {
_objc_fatal("Cannot form weak reference to instance (%p) of "
"class %s. It is possible that this object was "
"over-released, or is in the process of deallocation.",
(void*)referent, object_getClassName((id)referent));
}
// now remember it and where it is being stored
weak_entry_t *entry;
if ((entry = weak_entry_for_referent(weak_table, referent))) {
append_referrer(entry, referrer);
}
else {
weak_entry_t new_entry;
new_entry.referent = referent;
new_entry.out_of_line = 0;
new_entry.inline_referrers[0] = referrer;
for (size_t i = 1; i < WEAK_INLINE_COUNT; i++) {
new_entry.inline_referrers[i] = nil;
}
weak_grow_maybe(weak_table);
weak_entry_insert(weak_table, &new_entry);
}
// Do not set *referrer. objc_storeWeak() requires that the
// value not change.
return referent_id;
}
可以看出，runtime 是通过检查引用计数的个数来判断对象是否在 deallocting， 然后通过
if (deallocating) {
_objc_fatal("Cannot form weak reference to instance (%p) of "
"class %s. It is possible that this object was "
"over-released, or is in the process of deallocation.",
(void*)referent, object_getClassName((id)referent));
}
这段代码让程序crash。
再看一下 _objc_fatal 这个函数
void _objc_fatal(const char *fmt, ...)
{
va_list ap;
char *buf1;
char *buf2;
va_start(ap,fmt);
vasprintf(&buf1, fmt, ap);
va_end (ap);
asprintf(&buf2, "objc[%d]: %s\n", getpid(), buf1);
_objc_syslog(buf2);
_objc_crashlog(buf2);
_objc_trap();
}
可以看到这个函数实际会在控制台输出一段信息，然后调用 _bojc_trap() 引起 crash. 而最后一个函数调用刚好也对上我们之前的崩溃堆栈。
