Method could not find the cause of the error

During a slow lookup, a forward_IMP is returned if the final lookup is not found

const IMP forward_imp = (IMP)_objc_msgForward_impcache;
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Search globally for _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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__objc_forward_handler = __objc_forward_handler = __objc_forward_handler

__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)) ? '+' : The '-', 
                object_getClassName(self), sel_getName(sel), self);
}
void *_objc_forward_handler = (void*)objc_defaultForwardHandler;
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You can find the error printing function objc_defaultForwardHandler.

Dynamic method resolution

Behavior is 3, LOOKUP_RESOLVER is 2, behavior & LOOKUP_RESOLVER is combined with behavior ^= LOOKUP_RESOLVER to realize singleton mode. You can only enter once.

    if (slowpath(behavior & LOOKUP_RESOLVER)) {
        behavior ^= LOOKUP_RESOLVER;
        return resolveMethod_locked(inst, sel, cls, behavior);
    }
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Check the resolveMethod_locked function

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

If! CLS ->isMetaClass() is not a metaclass, that is, the object method, into the resolveInstanceMethod function

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)); }}}Copy the code

As you can see, the system actively sends a message to the resolveInstanceMethod function

 BOOL (*msg)(Class, SEL, SEL) = (typeof(msg))objc_msgSend;
 bool resolved = msg(cls, resolve_sel, sel);
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If the resolveInstanceMethod function returns an IMP to sel, then the program can fix it

IMP imp = lookUpImpOrNilTryCache(inst, sel, cls);
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resolveInstanceMethodimplementation

Add a class LKPerson, add the method sayHello, but do not implement it. Implementation method

- (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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You can see that the program doesn’t crash, and print teacherSayHello.

resolveClassMethod

If! CLS ->isMetaClass() ->isMetaClass() ->isMetaClass(

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() ? '+' : The '-', 
                         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() ? '+' : The '-', cls->nameForLogging(), sel_getName(sel)); }}}Copy the code

ResolveClassMethod is similar to the resolveInstanceMethod function

    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); According to the metaclass, get the current class, send the resolveClassMethod message to the class, and then look for imp again.

resolveClassMethodimplementation

Add instance methods, but do not implement them.

+ (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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Print classTeacherSayHello.