iOS进阶之动态方法解析（四）

其他文章请点击下方：

1. iOS进阶之Runtime初探(一)
2. iOS进阶之对象与方法的本质（二）
3. iOS进阶之方法查找（三）
4. iOS进阶之消息转发(五)

源码分析

通过方法查找还没有找到，就会进入下面的逻辑

 // 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;
    }

• _class_resolveMethod

1. _class_resolveInstanceMethod

    BOOL (*msg)(Class, SEL, SEL) = (__typeof__(msg))objc_msgSend;
    bool resolved = msg(cls, SEL_resolveInstanceMethod, sel);

2. _class_resolveClassMethod 最终还是会调用 _class_resolveInstanceMethod

/***********************************************************************
* _class_resolveMethod
* Call +resolveClassMethod or +resolveInstanceMethod.
* Returns nothing; any result would be potentially out-of-date already.
* Does not check if the method already exists.
**********************************************************************/
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);
        }
    }
}

如果当前方法没有实现且父类也没有实现，就会调用类或实例方法，那么我们就开启下面的动态方法解析

#pragma mark - 动态方法解析
+ (BOOL)resolveInstanceMethod:(SEL)sel{
    NSLog(@"开启动态方法解析");
   return [super resolveInstanceMethod:sel];
}

+ (BOOL)resolveClassMethod:(SEL)sel{
    NSLog(@"开启动态方法解");
    return  [super resolveClassMethod:sel];
}

程度运行后，控制台输出两次NSLog信息，这是为什么呢？往下看

/***********************************************************************
* _class_resolveInstanceMethod
* Call +resolveInstanceMethod, looking for a method to be added to class cls.
* cls may be a metaclass or a non-meta class.
* Does not check if the method already exists.
**********************************************************************/
static void _class_resolveInstanceMethod(Class cls, SEL sel, id inst)
{
    if (! lookUpImpOrNil(cls->ISA(), SEL_resolveInstanceMethod, cls, 
                         NO/*initialize*/, YES/*cache*/, NO/*resolver*/)) 
    {
        // Resolver not implemented.
        return;
    }

    BOOL (*msg)(Class, SEL, SEL) = (__typeof__(msg))objc_msgSend;
    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*/);

    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));
        }
    }
}

其中有一行代码：msg(cls, SEL_resolveInstanceMethod, sel)，系统会自动发送一次消息;

第二次调用是通过消息转发机制(后面文章会介绍)时，消息无法处理就会调用一次，接下来我们来看一张经典的消息转发的图：