OC底层探索（十二）： 消息动态决议

入口

• 当lookUpImpOrForward通过sel查找imp找不到时

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

enum {
    LOOKUP_INITIALIZE = 1,
    LOOKUP_RESOLVER = 2,
    LOOKUP_NIL = 4,
    LOOKUP_NOCACHE = 8,
};
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• 根据汇编我们看到 behavior = LOOKUP_INITIALIZE | LOOKUP_RESOLVER, 所以 behavior = 1 | 2 = 3
• behavior & LOOKUP_RESOLVER = 3 & 2 = 2
• behavior ^= LOOKUP_RESOLVER; = 2 ^ 2 = 0
• behavior = 0, 当其在与上其他时，一直都是0，其实就相当于一个标记，所以这里方法只会走进来一次

resolveMethod_locked分析

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

    runtimeLock.unlock();

    // cls 不是元类
    if (! cls->isMetaClass()) {
        // try [cls resolveInstanceMethod:sel]
        // 对象方法
        resolveInstanceMethod(inst, sel, cls);
    } 
    else {
       // 类方法
        // try [nonMetaClass resolveClassMethod:sel]
        // and [cls resolveInstanceMethod:sel]
        resolveClassMethod(inst, sel, cls);
        if (!lookUpImpOrNilTryCache(inst, sel, cls)) {
            resolveInstanceMethod(inst, sel, cls);
        }
    }

    // chances are that calling the resolver have populated the cache
    // so attempt using it
    // 再查找一次
    return lookUpImpOrForwardTryCache(inst, sel, cls, behavior);
}
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对象方法动态解析

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

    // 查看缓存里有没有 resolve_sel， 就是有没有实现 resolveInstanceMethod 方法
    // 系统默认有实现 所以不会走 return
    if (!lookUpImpOrNilTryCache(cls, resolve_sel, cls->ISA(/*authenticated*/true))) {
        // Resolver not implemented.
        return;
    }

   // 消息发送调用 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
    // 在查看一下调用的方法有没有
    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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• 如果查找方法没有找到，根据上面代码的逻辑可以看出，我们只要对象方法实现了 resolveInstanceMethod

• 没有实现look 方法，但是我们通过resolveInstanceMethod,将 look的sel绑定到了hear的imp上
• 最终调用的是hear方法

类方法动态解析

• 我们知道类方法，存在元类里，类方法就是元类的实例方法
• 所以可以将 eat 方法 动态解析到 say 方法

NSObject+Catagory

• 对象方法 -> 类 -> 父类 > NSObject
• 类方法 -> 元类 -> 父类 -> 根元类 -> NSObject
• 最终都会走到NSOject, 所以我们只需要在NSObject 的类别里实现 resolveInstanceMethod 就可以拦截到所有未实现的 sel

• 一样不会crash, 会调用对应的方法
• 这样就可以hook所有未实现发方法，对其进行处理分析，也可以防止这种崩溃，通过这个方法上传崩溃的信息给后台