接着上篇文章(系统底层源码分析(18)——objc_msgSend)继续说：
IMP lookUpImpOrForward(Class cls, SEL sel, id inst,
bool initialize, bool cache, bool resolver)
{
...
if (!cls->isRealized()) {
realizeClass(cls);//准备-父类
}
...
retry:
runtimeLock.assertLocked();
// Try this class's cache.
imp = cache_getImp(cls, sel);//从缓存获取imp
if (imp) goto done;//找到返回
//缓存中没有找到就去查找方法列表
// Try this class's method lists.
{//局部作用域，可避免名字冲突
Method meth = getMethodNoSuper_nolock(cls, sel);//查找
if (meth) {
log_and_fill_cache(cls, meth->imp, sel, inst, cls);//缓存
imp = meth->imp;
goto done;
}
}
//子类找不到就递归找父类
// Try superclass caches and method lists.
{
unsigned attempts = unreasonableClassCount();
for (Class curClass = cls->superclass;
curClass != nil;
curClass = curClass->superclass)
{
// Halt if there is a cycle in the superclass chain.
if (--attempts == 0) { ... }
// Superclass cache.
imp = cache_getImp(curClass, sel);//从父类缓存获取imp
if (imp) {
if (imp != (IMP)_objc_msgForward_impcache) {
log_and_fill_cache(cls, imp, sel, inst, curClass);
goto done;
}
else {
break;
}
}
//缓存中没有找到就去查找父类方法列表
// Superclass method list.
Method meth = getMethodNoSuper_nolock(curClass, sel);//查找
if (meth) {
log_and_fill_cache(cls, meth->imp, sel, inst, curClass);//缓存
imp = meth->imp;
goto done;
}
}
}
// No implementation found. Try method resolver once.
if (resolver && !triedResolver) {
runtimeLock.unlock();
_class_resolveMethod(cls, sel, inst);//对找不到的方法进行处理
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.
triedResolver = YES;
goto retry;//再次查找
}
// No implementation found, and method resolver didn't help.
// Use forwarding.
imp = (IMP)_objc_msgForward_impcache;//报错
cache_fill(cls, sel, imp, inst);
done:
runtimeLock.unlock();
return imp;
}
-
报错
- 之前说到调用方法，底层会调用
objc_msgSend，进入快速查找流程，找不到就进入慢速查找流程，然后来到lookUpImpOrForward，如果最后还找不到，就会调用_objc_msgForward_impcache报错：
STATIC_ENTRY __objc_msgForward_impcache
// Method cache version
// THIS IS NOT A CALLABLE C FUNCTION
// Out-of-band Z is 0 (EQ) for normal, 1 (NE) for stret
beq __objc_msgForward
b __objc_msgForward_stret
END_ENTRY __objc_msgForward_impcache
ENTRY __objc_msgForward
// Non-stret version
MI_GET_EXTERN(r12, __objc_forward_handler)
ldr r12, [r12]
bx r12
END_ENTRY __objc_msgForward
上述是汇编代码，调用流程是：
__objc_msgForward_impcache -> __objc_msgForward -> __objc_forward_handler
-
__objc_forward_handler会回到源码中，查找时需要去掉一个下划线：
__attribute__((noreturn)) void
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);//打印的错误信息
}
void *_objc_forward_handler = (void*)objc_defaultForwardHandler;
打印信息是不是很熟悉，就是我们调用没有实现的方法时报的错：
报错信息
-
动态方法决议
- 如果调用没有实现的方法就会报错，但是系统给了挽救的机会，就在报错前进行处理的
_class_resolveMethod函数：
void _class_resolveMethod(Class cls, SEL sel, id inst)
{
if (! cls->isMetaClass()) {
// try [cls resolveInstanceMethod:sel]
_class_resolveInstanceMethod(cls, sel, inst);//实例方法决议
}
else {
// try [nonMetaClass resolveClassMethod:sel]
// and [cls resolveInstanceMethod:sel]
_class_resolveClassMethod(cls, sel, inst);//类方法决议
if (!lookUpImpOrNil(cls, sel, inst,
NO/*initialize*/, YES/*cache*/, NO/*resolver*/))
{//根据isa走位图，查找实例方法和类方法最终都走向NSObject类，所以再进行一次实例方法决议
_class_resolveInstanceMethod(cls, sel, inst);//实例方法决议
}
}
}
- 我们先来看对实例方法的处理，首先会查找
resolveInstanceMethod：
static void _class_resolveInstanceMethod(Class cls, SEL sel, id inst)
{
if (! lookUpImpOrNil(cls->ISA(), SEL_resolveInstanceMethod, cls,
NO/*initialize*/, YES/*cache*/, NO/*resolver*/)) //查找resolveInstanceMethod并缓存
{
// Resolver not implemented.
return;
}
BOOL (*msg)(Class, SEL, SEL) = (typeof(msg))objc_msgSend;//消息发送，快速查找
bool resolved = msg(cls, SEL_resolveInstanceMethod, sel);
// Cache the result (good or bad) so the resolver doesn't fire next time.
// +resolveInstanceMethod adds to self a.k.a. cls
IMP imp = lookUpImpOrNil(cls, sel, inst,
NO/*initialize*/, YES/*cache*/, NO/*resolver*/);//再找一次
...
}
IMP lookUpImpOrNil(Class cls, SEL sel, id inst,
bool initialize, bool cache, bool resolver)
{
IMP imp = lookUpImpOrForward(cls, sel, inst, initialize, cache, resolver);//查找
if (imp == _objc_msgForward_impcache) return nil;
else return imp;
}
- 找到之后因为会缓存，所以下次查找时，可以通过
objc_msgSend更快查找并调用，这里要注意找的是系统方法resolveInstanceMethod：
+ (BOOL)resolveInstanceMethod:(SEL)sel {
return NO;
}
- 这时可以通过在
Person类中重写resolveInstanceMethod来进行处理（动态添加方法）：
//Person类中
+ (BOOL)resolveInstanceMethod:(SEL)sel{
if (sel == @selector(say)) {
IMP sayHelloIMP = class_getMethodImplementation(self, @selector(sayHello));
Method sayHelloMethod = class_getInstanceMethod(self, @selector(sayHello));
const char *sayHelloType = method_getTypeEncoding(sayHelloMethod);
return class_addMethod(self, sel, sayHelloIMP, sayHelloType);
}
return [super resolveInstanceMethod:sel];
}
返回后，回到第2步，这时再次
lookUpImpOrNil，由于动态添加了方法，所以这次可以找到方法，然后返回后就回到第1步进行retry，重新查找并找到，所以就不会报错。另外类方法处理也是差不多：
/***********************************************************************
* _class_resolveClassMethod
* Call +resolveClassMethod, looking for a method to be added to class cls.
* cls should be a metaclass.
* Does not check if the method already exists.
**********************************************************************/
static void _class_resolveClassMethod(Class cls, SEL sel, id inst)
{
assert(cls->isMetaClass());
if (! lookUpImpOrNil(cls, SEL_resolveClassMethod, inst,
NO/*initialize*/, YES/*cache*/, NO/*resolver*/)) //查找resolveClassMethod并缓存
{
// Resolver not implemented.
return;
}
BOOL (*msg)(Class, SEL, SEL) = (typeof(msg))objc_msgSend;//消息发送，快速查找
bool resolved = msg(_class_getNonMetaClass(cls, inst),
SEL_resolveClassMethod, sel);
// Cache the result (good or bad) so the resolver doesn't fire next time.
// +resolveClassMethod adds to self->ISA() a.k.a. cls
IMP imp = lookUpImpOrNil(cls, sel, inst,
NO/*initialize*/, YES/*cache*/, NO/*resolver*/);
...
}
//Person类中
+ (BOOL)resolveClassMethod:(SEL)sel{
if (sel == @selector(cls_say)) {
IMP sayHelloIMP = class_getMethodImplementation(objc_getMetaClass("Person"), @selector(cls_sayHello));
Method sayHelloMethod = class_getInstanceMethod(objc_getMetaClass("Person"), @selector(cls_sayHello));
const char *sayHelloType = method_getTypeEncoding(sayHelloMethod);
return class_addMethod(objc_getMetaClass("Person"), sel, sayHelloIMP, sayHelloType);
}
return [super resolveClassMethod:sel];
}
-
消息转发
如果动态方法决议也没有处理，就会进入消息转发流程（简单来说就是交给别人处理）：
- 首先会进入快速转发：
//Person类中
- (id)forwardingTargetForSelector:(SEL)aSelector{
if (aSelector == @selector(say)) {
return [Teacher alloc];//返回对象
}
return [super forwardingTargetForSelector:aSelector];
}
- 如果快速转发也没有处理，就进入慢速转发流程：
2-1. methodSignatureForSelector返回SEL方法的签名，返回的签名是根据方法的参数来封装的。这个函数让重载方有机会抛出一个函数的签名，用于生成NSInvocation，再由后面的 forwardInvocation去执行：
//Person类中
- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector{
if (aSelector == @selector(say)) {
return [NSMethodSignature signatureWithObjCTypes:"v@:"];//方法签名
}
return [super methodSignatureForSelector:aSelector];
}
2-2. 在forwardInvocation里可以将NSInvocation多次转发到多个对象中，这也是这种方式灵活的地方。（而forwarding TargetForSelectorSelec只能以Selector的形式转向一个对象)：
//Person类中
- (void)forwardInvocation:(NSInvocation *)anInvocation{
SEL aSelector = [anInvocation selector];
Teacher *teacher = [Teacher alloc];
if ([teacher respondsToSelector:aSelector])
[anInvocation invokeWithTarget:teacher];
else
[super forwardInvocation:anInvocation];
}
- 实现
forwardInvocation后，就算不处理应用也不会再崩溃。
没有实现的方法处理流程图
-
补充
- 为了更充分说明这些流程的有效性，我们可以通过
instrumentObjcMessageSends获取系统日志：
extern void instrumentObjcMessageSends(BOOL flag);//系统方法
int main(int argc, const char * argv[]) {
@autoreleasepool {
Student *student = [Student alloc] ;
instrumentObjcMessageSends(true);//路径在/tmp/
[student say];
instrumentObjcMessageSends(false);
}
return 0;
}
- 然后直接前往
/tmp（文件夹路径），然后打开msgSends-前缀的文件：
//动态方法决议
+ Student NSObject resolveInstanceMethod:
+ Student NSObject resolveInstanceMethod:
//消息转发-快速
- Student NSObject forwardingTargetForSelector:
- Student NSObject forwardingTargetForSelector:
//消息转发-慢速
- Student Student methodSignatureForSelector:
- Student Student methodSignatureForSelector:
...
//还会调用一次resolveInstanceMethod，因为methodSignatureForSelector返回的签名需要查找方法来匹配，所以进行lookUpImpOrForward查询导致又调用了resolveInstanceMethod
+ Student NSObject resolveInstanceMethod:
+ Student NSObject resolveInstanceMethod:
//
- Student Student forwardInvocation:
+ NSInvocation NSObject initialize
+ NSInvocation NSInvocation _invocationWithMethodSignature:frame:
- NSMethodSignature NSMethodSignature frameLength
...
