动态方法决议
objc_msgSend发送消息会先从缓存里面imp缓存中查找，如果没有查找到会进入lookUpImpOrForward查找，先从当前类的methods()查找，没有就去父类缓存，不断循环查找。如果都没有则会进入动态方法决议流程。resolveMethod_locked
- resolveMethod_locked
static NEVER_INLINE IMP
resolveMethod_locked(id inst, SEL sel, Class cls, int behavior)
{
runtimeLock.assertLocked();
ASSERT(cls->isRealized());
runtimeLock.unlock();
//实例方法
if (! cls->isMetaClass()) {
// try [cls resolveInstanceMethod:sel]
resolveInstanceMethod(inst, sel, cls);
}
else {
//类方法
// try [nonMetaClass resolveClassMethod:sel]
// and [cls resolveInstanceMethod:sel]
resolveClassMethod(inst, sel, cls);
if (!lookUpImpOrNilTryCache(inst, sel, cls)) {
resolveInstanceMethod(inst, sel, cls);
}
}
// chances are that calling the resolver have populated the cache
// so attempt using it 给一次机会
return lookUpImpOrForwardTryCache(inst, sel, cls, behavior);
}
- 判断当前是类还是元类，再执行
resolveInstanceMethod，判断类是否实现了-(void)resolveInstanceMethod方法 - 最后给一次机会
lookUpImpOrForwardTryCache去处理
- objc->resolveInstanceMethod
static void resolveInstanceMethod(id inst, SEL sel, Class cls)
{
runtimeLock.assertUnlocked();
ASSERT(cls->isRealized());
SEL resolve_sel = @selector(resolveInstanceMethod:);
//判断当前类的缓存方法中没有resolveInstanceMethod
if (!lookUpImpOrNilTryCache(cls, resolve_sel, cls->ISA(/*authenticated*/true))) {
// Resolver not implemented.
return;
}
BOOL (*msg)(Class, SEL, SEL) = (typeof(msg))objc_msgSend;
//向类的方法resolveInstanceMethod发送消息，并处理
bool resolved = msg(cls, resolve_sel, sel);
// Cache the result (good or bad) so the resolver doesn't fire next time.
// +resolveInstanceMethod adds to self a.k.a. cls
//从缓存中查找方法
IMP imp = lookUpImpOrNilTryCache(inst, sel, cls);
.....
}
- 判断当前类的缓存方法中没有
resolveInstanceMethod - 没有就向
resolveInstanceMethod发送消息，去处理 - 执行lookUpImpOrNilTryCache
- _lookUpImpTryCache
lookUpImpOrNilTryCache->_lookUpImpTryCache
ALWAYS_INLINE
static IMP _lookUpImpTryCache(id inst, SEL sel, Class cls, int behavior)
{
runtimeLock.assertUnlocked();
if (slowpath(!cls->isInitialized())) {
// see comment in lookUpImpOrForward
return lookUpImpOrForward(inst, sel, cls, behavior);
}
//缓存中有没有imp
IMP imp = cache_getImp(cls, sel);
if (imp != NULL) goto done;
#if CONFIG_USE_PREOPT_CACHES
if (fastpath(cls->cache.isConstantOptimizedCache(/* strict */true))) {
imp = cache_getImp(cls->cache.preoptFallbackClass(), sel);
}
#endif
//缓存没有imp，执行lookUpImpOrForward
if (slowpath(imp == NULL)) {
return lookUpImpOrForward(inst, sel, cls, behavior);
}
done:
if ((behavior & LOOKUP_NIL) && imp == (IMP)_objc_msgForward_impcache) {
return nil;
}
return imp;
}
- 缓存中有没有imp
- 没有执行
lookUpImpOrForward，有就返回imp - 此时
lookUpImpOrForward会将我们在-(void)resolveInstanceMethod处理的消息，先去methodlist使用二分法查找，找出后添加进缓存
此时resolveMethod_locked的objc->resolveInstanceMethod执行完毕，已经将我们需要调用的imp添加进methodlist中，并插入缓存 - 在
resolveMethod_locked执行完objc->resolveInstanceMethod，则又会执行_lookUpImpTryCache给了第二次机会，从而调用cache_getImp(cls, sel)->CacheLookup找出imp，如果没有再lookUpImpOrForward
消息转发
当resolveInstanceMethod/resolveClassMethod处理不了，会进入coreFoundation中的_CF_forwarding_prep_0处理。随后会调用下面几个方法：
- forwardingTargetForSelector
- methodSignatureForSelector
- forwardInvocation
备用接收者forwardingTargetForSelector
当前类处理不了，交给其他类处理，如果找到了imp就返回，如果没有就返回nil
-(id)forwardingTargetForSelector:(SEL)aSelector{
return [LGStudent alloc];
}
forwardingTargetForSelector中还是没有找到，进入最后的次挽救机会，重写methodSignatureForSelector，如下所示
-(NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector{
return [NSMethodSignature signatureWithObjCTypes:"v@:"];
}
- (void)forwardInvocation:(NSInvocation *)anInvocation{
anInvocation.target = [LGStudent alloc];
[anInvocation invoke];
}
image.png
在methodSignatureForSelector和forwardInvocation又调用了一次resolveInstanceMethod，即又进行了一次动态方法决议
