在iOS-慢速方法查找和iOS-快速方法查找中我們分別提到了objc_msgSend的快速查找和慢速查找袍暴，如果經(jīng)歷這兩步仍未找到該方法的imp會怎么樣呢佛呻？
Apple給了我們兩次補(bǔ)救的機(jī)會，
動態(tài)方法決議：慢速查找流程未找到后，會執(zhí)行一次動態(tài)方法決議
消息轉(zhuǎn)發(fā)：如果動態(tài)方法決議仍然沒有找到實現(xiàn)，則進(jìn)行消息轉(zhuǎn)發(fā)
如果這兩次機(jī)會都沒有做任何操作，就會報我們?nèi)粘ｉ_發(fā)中常見的方法未實現(xiàn)的崩潰報錯——unrecognized selector sent to。
我們來看看是怎么從慢速查找進(jìn)入到第一個機(jī)會——動態(tài)方法決議的吧跨扮。

動態(tài)方法決議入口

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

    runtimeLock.unlock();
    //對象 -- 類
    if (! cls->isMetaClass()) { //類不是元類，調(diào)用對象的解析方法
        // try [cls resolveInstanceMethod:sel]
        resolveInstanceMethod(inst, sel, cls);
    } 
    else {//如果是元類验毡，調(diào)用類的解析方法衡创， 類 -- 元類
        // try [nonMetaClass resolveClassMethod:sel]
        // and [cls resolveInstanceMethod:sel]
        resolveClassMethod(inst, sel, cls);
        //為什么要有這行代碼？ -- 類方法在元類中是對象方法晶通，所以還是需要查詢元類中對象方法的動態(tài)方法決議
        if (!lookUpImpOrNil(inst, sel, cls)) { //如果沒有找到或者為空璃氢，在元類的對象方法解析方法中查找
            resolveInstanceMethod(inst, sel, cls);
        }
    }

    // chances are that calling the resolver have populated the cache
    // so attempt using it
    //如果方法解析中將其實現(xiàn)指向其他方法，則繼續(xù)走方法查找流程
    return lookUpImpOrForward(inst, sel, cls, behavior | LOOKUP_CACHE);
}

先看看對象方法的動態(tài)決議源碼

static 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))) {//查找resolveInstanceMethod是否實現(xiàn)狮辽，其實源碼中NSObject實現(xiàn)了該方法一也。
        // Resolver not implemented.
        return;
    }
   //消息轉(zhuǎn)發(fā)巢寡，執(zhí)行resolveInstanceMethod方法
    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);
    //感覺是在打日志
    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));
        }
    }
}

我們來代碼驗證一下，給LGPerson.h添加如下代碼

@interface LGPerson : NSObject

+ (void)say1;//只聲明椰苟，未實現(xiàn)
+ (void)say4;

- (void)say2;
- (void)say3;//只聲明抑月，未實現(xiàn)
@end

給LGPerson.m添加如下代碼

@implementation LGPerson

//- (void)say1
//{
//  NSLog(@"%s",__func__);
//}
- (void)say2
{
  NSLog(@"%s",__func__);
}
//- (void)say3
//{
//  NSLog(@"%s",__func__);
//}
+ (void)say4
{
  NSLog(@"%s",__func__);
}


+ (BOOL)resolveInstanceMethod:(SEL)sel{
    if (sel == @selector(say3)) {
        NSLog(@"%@ 來了", NSStringFromSelector(sel));
    }
    return [super resolveInstanceMethod:sel];
}
@end

在 main中調(diào)用say3,執(zhí)行結(jié)果如下：

image.png

image.png

如果想抓住這個防止崩潰的機(jī)會，可以修改resolveInstanceMethod代碼如下

+ (BOOL)resolveInstanceMethod:(SEL)sel{
    if (sel == @selector(say3)) {
        NSLog(@"%@ 來了", NSStringFromSelector(sel));
        //獲取say2方法的imp
        IMP imp = class_getMethodImplementation(self, @selector(say2));
        //獲取say2的實例方法
        Method sayMethod  = class_getInstanceMethod(self, @selector(say2));
        //獲取say2的簽名
        const char *type = method_getTypeEncoding(sayMethod);
        //將sel的實現(xiàn)指向say2
        return class_addMethod(self, sel, imp, type);
    }
//
    return [super resolveInstanceMethod:sel];
}