方法查找不到报错原因

慢速查找过程中，如果最终查找不到，会返回一个forward_imp

const IMP forward_imp = (IMP)_objc_msgForward_impcache;
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全局搜索_objc_msgForward_impcache

       STATIC_ENTRY __objc_msgForward_impcache

	// No stret specialization.
	b	__objc_msgForward

	END_ENTRY __objc_msgForward_impcache

	
	ENTRY __objc_msgForward

	adrp	x17, __objc_forward_handler@PAGE
	ldr	p17, [x17, __objc_forward_handler@PAGEOFF]
	TailCallFunctionPointer x17
	
	END_ENTRY __objc_msgForward
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可以发现最终走到了__objc_forward_handler，全局搜索__objc_forward_handler，搜索不到，去掉__再次搜索，

__attribute__((noreturn, cold)) 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);
}
void *_objc_forward_handler = (void*)objc_defaultForwardHandler;
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可以发现错误打印函数objc_defaultForwardHandler。

动态方法决议

慢速查找流程中，如果查找不到，会进入下面流程behavior为3，LOOKUP_RESOLVER为2，behavior & LOOKUP_RESOLVER 和behavior ^= LOOKUP_RESOLVER结合，实现单例模式，只能进入一次。

    if (slowpath(behavior & LOOKUP_RESOLVER)) {
        behavior ^= LOOKUP_RESOLVER;
        return resolveMethod_locked(inst, sel, cls, behavior);
    }
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查看resolveMethod_locked函数

static NEVER_INLINE IMP resolveMethod_locked(id inst, SEL sel, Class cls, int behavior)
{
    runtimeLock.assertLocked();
    ASSERT(cls->isRealized());

    runtimeLock.unlock();

    if (! cls->isMetaClass()) {
        // try [cls resolveInstanceMethod:sel]
        resolveInstanceMethod(inst, sel, cls);
    } 
    else {
        // try [nonMetaClass resolveClassMethod:sel]
        // and [cls resolveInstanceMethod:sel]
        resolveClassMethod(inst, sel, cls);
        if (!lookUpImpOrNilTryCache(inst, sel, cls)) {
            resolveInstanceMethod(inst, sel, cls);
        }
    }
    return lookUpImpOrForwardTryCache(inst, sel, cls, behavior);
}
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`resolveInstanceMethod`

如果! cls->isMetaClass()不是元类，即对象方法，进入resolveInstanceMethod函数

void resolveInstanceMethod(id inst, SEL sel, Class cls)
{
    runtimeLock.assertUnlocked();
    ASSERT(cls->isRealized());
    SEL resolve_sel = @selector(resolveInstanceMethod:);

    if (!lookUpImpOrNilTryCache(cls, resolve_sel, cls->ISA(/*authenticated*/true))) {
        // Resolver not implemented.
        return;
    }

    BOOL (*msg)(Class, SEL, SEL) = (typeof(msg))objc_msgSend;
    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
    IMP imp = lookUpImpOrNilTryCache(inst, sel, cls);

    if (resolved  &&  PrintResolving) {
        if (imp) {
            _objc_inform("RESOLVE: method %c[%s %s] "
                         "dynamically resolved to %p", 
                         cls->isMetaClass() ? '+' : '-', 
                         cls->nameForLogging(), sel_getName(sel), imp);
        }
        else {
            // Method resolver didn't add anything?
            _objc_inform("RESOLVE: +[%s resolveInstanceMethod:%s] returned YES"
                         ", but no new implementation of %c[%s %s] was found",
                         cls->nameForLogging(), sel_getName(sel), 
                         cls->isMetaClass() ? '+' : '-', 
                         cls->nameForLogging(), sel_getName(sel));
        }
    }
}
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可以看到系统会主动给resolveInstanceMethod函数发送消息

 BOOL (*msg)(Class, SEL, SEL) = (typeof(msg))objc_msgSend;
 bool resolved = msg(cls, resolve_sel, sel);
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然后再次进入缓存查找imp，如果我们在resolveInstanceMethod函数给sel返回一个imp，是否程序就可以补救

IMP imp = lookUpImpOrNilTryCache(inst, sel, cls);
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`resolveInstanceMethod`实现

添加一个类LKPerson，添加方法sayHello，但不实现。实现方法

- (void)teacherSayHello{
    NSLog(@"%@ - %s",self , __func__);
}

+ (BOOL)resolveInstanceMethod:(SEL)sel{
    NSLog(@"resolveInstanceMethod :%@-%@",self,NSStringFromSelector(sel));
    if (sel == @selector(sayHello)) {
        IMP sayHelloImp     = class_getMethodImplementation(self, @selector(teacherSayHello));
        Method method    = class_getInstanceMethod(self, @selector(teacherSayHello));
        const char *type = method_getTypeEncoding(method);
        return class_addMethod(self, sel, sayHelloImp, type);
    }
    return [super resolveInstanceMethod:sel];
}
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可以看到程序并没有崩溃，并且打印teacherSayHello。

`resolveClassMethod`

如果! cls->isMetaClass()是元类，即实例方法，进入resolveClassMethod函数

static void resolveClassMethod(id inst, SEL sel, Class cls)
{
    runtimeLock.assertUnlocked();
    ASSERT(cls->isRealized());
    ASSERT(cls->isMetaClass());

    if (!lookUpImpOrNilTryCache(inst, @selector(resolveClassMethod:), cls)){
        // Resolver not implemented.
        return;
    }

    Class nonmeta;
    {
        mutex_locker_t lock(runtimeLock);
        nonmeta = getMaybeUnrealizedNonMetaClass(cls, inst);
        // +initialize path should have realized nonmeta already
        if (!nonmeta->isRealized()) {
            _objc_fatal("nonmeta class %s (%p) unexpectedly not realized",
                        nonmeta->nameForLogging(), nonmeta);
        }
    }
    BOOL (*msg)(Class, SEL, SEL) = (typeof(msg))objc_msgSend;
    bool resolved = msg(nonmeta, @selector(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 = lookUpImpOrNilTryCache(inst, sel, cls);

    if (resolved  &&  PrintResolving) {
        if (imp) {
            _objc_inform("RESOLVE: method %c[%s %s] "
                         "dynamically resolved to %p", 
                         cls->isMetaClass() ? '+' : '-', 
                         cls->nameForLogging(), sel_getName(sel), imp);
        }
        else {
            // Method resolver didn't add anything?
            _objc_inform("RESOLVE: +[%s resolveClassMethod:%s] returned YES"
                         ", but no new implementation of %c[%s %s] was found",
                         cls->nameForLogging(), sel_getName(sel), 
                         cls->isMetaClass() ? '+' : '-', 
                         cls->nameForLogging(), sel_getName(sel));
        }
    }
}
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resolveClassMethod与resolveInstanceMethod函数类似

    Class nonmeta;
    {
        mutex_locker_t lock(runtimeLock);
        nonmeta = getMaybeUnrealizedNonMetaClass(cls, inst);
        // +initialize path should have realized nonmeta already
        if (!nonmeta->isRealized()) {
            _objc_fatal("nonmeta class %s (%p) unexpectedly not realized",
                        nonmeta->nameForLogging(), nonmeta);
        }
    }
    BOOL (*msg)(Class, SEL, SEL) = (typeof(msg))objc_msgSend;
    bool resolved = msg(nonmeta, @selector(resolveClassMethod:), sel);
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nonmeta = getMaybeUnrealizedNonMetaClass(cls, inst);根据元类，获取当前类
向类发送resolveClassMethod消息,然后再次查找imp。

`resolveClassMethod`实现

添加实例方法,但不实现。

+ (void)classSayHello;

+ (void)classTeacherSayHello{
    NSLog(@"%@ - %s",self , __func__);
}

+ (BOOL)resolveClassMethod:(SEL)sel{
    NSLog(@"resolveClassMethod :%@-%@",self,NSStringFromSelector(sel));
    if (sel == @selector(classSayHello)) {
        IMP sayHelloImp     = class_getMethodImplementation(objc_getMetaClass("LKPerson"), @selector(classTeacherSayHello));
        Method method    = class_getInstanceMethod(objc_getMetaClass("LKPerson"), @selector(classTeacherSayHello));
        const char *type = method_getTypeEncoding(method);
        return class_addMethod(objc_getMetaClass("LKPerson"), sel, sayHelloImp, type);
    }
    return [super resolveClassMethod:sel];
}
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