在iOS - objc_msgSend分析一文中我们提到了__class_lookupMethodAndLoadCache3方法可以通过,全局搜索找到,那么还有其他的方式能看到_class_lookupMethodAndLoadCache3方法在哪里面呢?下面就介绍另一种方法:
1、通过查看汇编,找到_class_lookupMethodAndLoadCache3
1.1打断点,查看汇编
1.2运行程序,由iOS - objc_msgSend分析一文我们知道,方法的快速查找会进入objc_msgSend方法中,因此我们在此处打断点,按住control点击Step into 进入该方法中;
1.3在_objc_msgSend_uncached出打断点,按住control点击Step into 进入方法中
至此我们完美的看到:_class_lookupMethodAndLoadCache3(id, SEL, Class) at objc-runtime-new.mm:4845证明了_class_lookupMethodAndLoadCache3方法实现在objc-runtime-new.mm文件中.
IMP _class_lookupMethodAndLoadCache3(id obj, SEL sel, Class cls)
{
/**
查找当前的imp或者转发
cls:如果是实例方法那就是类，如果是类方法那就是元类
sel：方法名
obj:方法调用者
NO/*cache*/ //因为是CheckMiss状态下点用的该方法,所以此处为NO
*/
return lookUpImpOrForward(cls, sel, obj,
YES/*initialize*/, NO/*cache*/, YES/*resolver*/);
}
2、方法查找流程
1.1 lookUpImpOrForward源码分析
/**此处省略部分代码*/
//为查找方法做准备条件，判断类有没有加载好，如果没有加载好，那就先加载一下类信息，准备好父类、元类
if (!cls->isRealized()) {
realizeClass(cls);
}
//确定类已经加载完毕
if (initialize && !cls->isInitialized()) {
runtimeLock.unlock();
_class_initialize (_class_getNonMetaClass(cls, inst));
runtimeLock.lock();
// 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.assertLocked();//加锁,防止多个方法调用时产生冲突
// Try this class's cache.
imp = cache_getImp(cls, sel);//从该类的方法缓存中找imp,明显缓存为NO,此处势必拿不到方法的,因此会继续走接下来的流程
if (imp) goto done;//拿到方法就返回
// Try this class's method lists.
{
Method meth = getMethodNoSuper_nolock(cls, sel);//在该类中查找该方法
if (meth) {
//如果找到了则进行方法的缓存, log_and_fill_cache->cache_fill->cache_fill_nolock
log_and_fill_cache(cls, meth->imp, sel, inst, cls);//进行缓存
imp = meth->imp;//拿到imp
goto done;
}
}
// Try superclass caches and method lists.
{
unsigned attempts = unreasonableClassCount();
//遍历父类,父类的父类等等一系列父类,直到为nil
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);//从父类的方法缓存中查找该方法,如果拿到该方法则直接goto done
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.
//如果在父类的方法缓存中没有找到该方法,则从发类的方法列表中查到
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.
//如果方法仍然没找到，就开始做方法消息动态解析了
if (resolver && !triedResolver) {
runtimeLock.unlock();
/*
实例方法解析：_class_resolveInstanceMethod
类方法解析：_class_resolveClassMethod
*/
_class_resolveMethod(cls, sel, inst);
runtimeLock.lock();
// Don't cache the result; we don't hold the lock so it may have
// changed already. Re-do the search from scratch instead.
//将triedResolver标记为YES，下次就不会再进入动态方法解析
triedResolver = YES;
goto retry;
}
// No implementation found, and method resolver didn't help.
// Use forwarding.
//动态解析阶段完成,进入消息转发阶段
imp = (IMP)_objc_msgForward_impcache;
cache_fill(cls, sel, imp, inst);
done:
runtimeLock.unlock();
return imp;
}
getMethodNoSuper_nolock获取方法列表
static method_t *
getMethodNoSuper_nolock(Class cls, SEL sel)
{
runtimeLock.assertLocked();
assert(cls->isRealized());
// fixme nil cls?
// fixme nil sel?
for (auto mlists = cls->data()->methods.beginLists(),
end = cls->data()->methods.endLists();
mlists != end;
++mlists)
{
method_t *m = search_method_list(*mlists, sel);//通过二分查找获取m,此处不做详解,有兴趣的可以自己看下源码
if (m) return m;
}
return nil;
}
_class_resolveMethod
void _class_resolveMethod(Class cls, SEL sel, id inst)
{
//首先会判断cls是不是元类，如果cls不是元类的话，说明调用的是实例方法，那就就会调用_class_resolveInstanceMethod函数，
if (! cls->isMetaClass()) {
// try [cls resolveInstanceMethod:sel]
_class_resolveInstanceMethod(cls, sel, inst);
}
else {
//如果是元类的话，说明调用的是类方法，那么就会调用_class_resolveClassMethod函数，
// try [nonMetaClass resolveClassMethod:sel]
// and [cls resolveInstanceMethod:sel]
_class_resolveClassMethod(cls, sel, inst);
if (!lookUpImpOrNil(cls, sel, inst,
NO/*initialize*/, YES/*cache*/, NO/*resolver*/))
{
_class_resolveInstanceMethod(cls, sel, inst);
}
}
}
_objc_msgForward_impcache
STATIC_ENTRY __objc_msgForward_impcache
// No stret specialization.
b __objc_msgForward //调用__objc_msgForward
END_ENTRY __objc_msgForward_impcache
ENTRY __objc_msgForward
adrp x17, __objc_forward_handler@PAGE
ldr p17, [x17, __objc_forward_handler@PAGEOFF]//调用该方法
TailCallFunctionPointer x17
END_ENTRY __objc_msgForward
通过全局搜索_objc_forward_handler我们可以找到
objc_defaultForwardHandler(id self, SEL sel)
{
_objc_fatal("%c[%s %s]: unrecognized selector sent to instance %p "
"(no message forward handler is installed)",
class_isMetaClass(object_getClass(self)) ? '+' : '-',
object_getClassName(self), sel_getName(sel), self);
}
void *_objc_forward_handler = (void*)objc_defaultForwardHandler;
这便是我们经常看到的,方法查找不到时所报的错误.
流程总结:
1.对需要的变量进行初始化操作和加锁操作
2.在该类的方法缓存中进行查找,如果找到就就直接goto done;
3.缓存中查找不到时,则从该类的方法列表中进行查找,如果找到,则在缓存中备份一份,方便下次查找,然后goto done;
4.如果该类的方法列表用依旧找不到,则从该类的父类,父类的父类,一直找到NSObject类,依旧是先从父类的缓存中查找,缓存中有则直接goto done,否则就遍历父类的方法列表,如果找到就直接goto done,找不到则break,进行方法决议;
5.实例方法会转发+(BOOL) resolveInstanceMethod:(SEL)sel；类方法会转发+(BOOL) resolveClassMethod:(SEL)sel，并且类方法转发完后会再次走查找流程，如果还没找到的话会走一下实例方法转发流程；转发逻辑完成后，会再次走一下方法查找逻辑
6.如果第一次转发依然后还没找到IMP，那么就会返回_objc_msgForward_impcache方法指针,调用_objc_forward_handler->objc_defaultForwardHandler,报出方法错误
附:方法查找流程图
方法查找流程
