Runtime-objc_msgSend

MJPerson *person = [MJPerson alloc] init];

[person personTest];

// 编译后对应的源码
MJPerson *person = objc_msgSend(objc_msgSend(objc_getClass("MJPerson"), sel_registerName("alloc")), sel_registerName("init"));

// 等价于 objc_msgSend(person, @selector(personTest));
objc_msgSend(person, sel_registerName("personTest"));

IMP _class_lookupMethodAndLoadCache3(id obj, SEL sel, Class cls)
{
    return lookUpImpOrForward(cls, sel, obj, 
                              YES/*initialize*/, NO/*cache*/, YES/*resolver*/);
}

static method_t *search_method_list(const method_list_t *mlist, SEL sel)
{
    int methodListIsFixedUp = mlist->isFixedUp();
    int methodListHasExpectedSize = mlist->entsize() == sizeof(method_t);
    
    if (__builtin_expect(methodListIsFixedUp && methodListHasExpectedSize, 1)) {
         // 有序的话就用二分查找
        return findMethodInSortedMethodList(sel, mlist);
    } else {
        // Linear search of unsorted method list
        // 无序的话就用线性遍历查找
        for (auto& meth : *mlist) {
            if (meth.name == sel) return &meth;
        }
    }
}

// No implementation found. Try method resolver once.

if (resolver  &&  !triedResolver) {
        runtimeLock.unlockRead();
        _class_resolveMethod(cls, sel, inst);
        runtimeLock.read();
        // 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;
    }

void _class_resolveMethod(Class cls, SEL sel, id inst)
{
    if (! cls->isMetaClass()) {
        // try [cls resolveInstanceMethod:sel]
        _class_resolveInstanceMethod(cls, sel, inst);
    } 
    else {
        // 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);
        }
    }
}

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

+ (BOOL)resolveInstanceMethod:(SEL)sel {
    if (sel == @selector(test)) {
        // 获取其他方法
        Method method = class_getInstanceMethod(self, @selector(other));

        // 动态添加test方法的实现
        // class_addMethod会把函数添加到Class里面的class_rw_t里面的methods
        class_addMethod(self, sel,
                        method_getImplementation(method),
                        method_getTypeEncoding(method));

        // 返回YES代表有动态添加方法
        return YES;
    }
    return [super resolveInstanceMethod:sel];
}

// No implementation found, and method resolver didn't help. 
// Use forwarding.

imp = (IMP)_objc_msgForward_impcache;

3   CoreFoundation 0x00007fff3077c8ef ___forwarding___ + 1485

- (id)forwardingTargetForSelector:(SEL)aSelector {
    if (aSelector == @selector(test)) {
        return [[MJCat alloc] init];
    }
    return [super forwardingTargetForSelector:aSelector];
}

// 方法签名：返回值类型、参数类型
- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector {
    if (aSelector == @selector(test)) {
        NSMethodSignature *methodSign = [NSMethodSignature signatureWithObjCTypes:"v@:"];
        return methodSign;
    }
    return [super methodSignatureForSelector:aSelector];
}

// NSInvocation封装了一个方法调用，包括：方法调用者、方法名、方法参数
// anInvocation.target 方法调用者
// anInvocation.selector 方法名
// [anInvocation getArgument:NULL atIndex:0]
- (void)forwardInvocation:(NSInvocation *)anInvocation {
    // 这个函数内部可以根据业务场景实现，没有具体要求
    // anInvocation.target = [[MJCat alloc] init];
    // [anInvocation invoke];
    // [anInvocation invokeWithTarget:[[MJCat alloc] init]];
}