method及相关方法分析

转载请注名出处 http://blog.csdn.net/uxyheaven

本篇文章将探究一下objc里的关于方法的函数是如何实现的

首先看下方法的定义, Method 是一个objc_method结构体

objc_method

objc_method 是类的一个方法的描述

定义如下

Method class_getInstanceMethod(Class cls, SEL sel)
{
    if (!cls  ||  !sel) return NULL;

    return look_up_method(cls, sel, YES/*cache*/, YES/*resolver*/);
}

static Method look_up_method(Class cls, SEL sel, 
                             BOOL withCache, BOOL withResolver)
{
    Method meth = NULL;
    // 1. 找缓存,有过有就返回
    if (withCache) {
        meth = _cache_getMethod(cls, sel, &_objc_msgForward_internal);
        if (meth == (Method)1) {
            // Cache contains forward:: . Stop searching.
            return NULL;
        }
    }
    // 2. 找自身
    if (!meth) meth = _class_getMethod(cls, sel);

    // 3. 找转发
    if (!meth  &&  withResolver) meth = _class_resolveMethod(cls, sel);

    return meth;
}

IMP class_getMethodImplementation(Class cls, SEL name)

返回cls的name方法的调用地址

定义如下

BOOL class_addMethod(Class cls, SEL name, IMP imp, const char *types)
{
    if (!cls) return NO;

    rwlock_write(&runtimeLock);
    IMP old = addMethod(newcls(cls), name, imp, types ?: "", NO);
    rwlock_unlock_write(&runtimeLock);
    return old ? NO : YES;
}

static IMP addMethod(class_t *cls, SEL name, IMP imp, const char *types, BOOL replace)
{
    IMP result = NULL;

    rwlock_assert_writing(&runtimeLock);

    assert(types);
    assert(isRealized(cls));

    method_t *m;
    // 1. 在自己的类的方法列表里找这个方法
    if ((m = getMethodNoSuper_nolock(cls, name))) {
        // already exists
        if (!replace) {
            // 不取代, 返回 m->imp
            result = _method_getImplementation(m);
        } else {
            // 取代, 设置 cls 的 m 方法实现为 imp
            result = _method_setImplementation(cls, m, imp);
        }
    } else {
        // fixme optimize
        // 2. 建立一个method_list_t节点
        method_list_t *newlist;
        newlist = (method_list_t *)_calloc_internal(sizeof(*newlist), 1);
        newlist->entsize_NEVER_USE = (uint32_t)sizeof(method_t) | fixed_up_method_list;
        newlist->count = 1;
        newlist->first.name = name;
        newlist->first.types = strdup(types);
        if (!ignoreSelector(name)) {
            newlist->first.imp = imp;
        } else {
            newlist->first.imp = (IMP)&_objc_ignored_method;
        }

        // 3. 把newlist加到cls的方法列表里
        BOOL vtablesAffected = NO;
        attachMethodLists(cls, &newlist, 1, NO, &vtablesAffected);
        // 4. 刷新cls缓存
        flushCaches(cls);
        if (vtablesAffected) flushVtables(cls);

        result = NULL;
    }

    return result;
}

我们用class_addMethod时, replace == NO, 所以cls已经存在这个方法的时候添加是失败的

IMP class_replaceMethod(Class cls, SEL name, IMP imp, const char *types)

替换cls的name方法的指针

void method_exchangeImplementations(Method m1_gen, Method m2_gen)
{
    method_t *m1 = newmethod(m1_gen);
    method_t *m2 = newmethod(m2_gen);
    if (!m1  ||  !m2) return;

    rwlock_write(&runtimeLock);

    if (ignoreSelector(m1->name)  ||  ignoreSelector(m2->name)) {
        // Ignored methods stay ignored. Now they're both ignored.
        m1->imp = (IMP)&_objc_ignored_method;
        m2->imp = (IMP)&_objc_ignored_method;
        rwlock_unlock_write(&runtimeLock);
        return;
    }

    // 交换2个方法的实现指针
    IMP m1_imp = m1->imp;
    m1->imp = m2->imp;
    m2->imp = m1_imp;

    if (vtable_containsSelector(m1->name)  ||  
        vtable_containsSelector(m2->name)) 
    {
        // Don't know the class - will be slow if vtables are affected
        // fixme build list of classes whose Methods are known externally?
        flushVtables(NULL);
    }

    // fixme catch NSObject changing to custom RR
    // cls->setCustomRR();

    // fixme update monomorphism if necessary

    rwlock_unlock_write(&runtimeLock);
}

其实这里有个坑, Method是怎么来的呢, 通过class_getInstanceMethod,如果子类没有的话,会返回父类的方法, 如果这个时候在用method_exchangeImplementations替换,会把父类替的方法替换掉,这显然不是我们想要的.所以呢,我们的移魂大发通常是这么写

IMP method_getImplementation(Method m)
{
    return _method_getImplementation(newmethod(m));
}

static IMP _method_getImplementation(method_t *m)
{
    if (!m) return NULL;
    return m->imp;
}

IMP method_setImplementation(Method method, IMP imp)

设置方法的新的实现指针, 返回旧的实现指针

const char *method_getTypeEncoding(Method m)
{
    if (!m) return NULL;
    return newmethod(m)->types;
}

method及相关方法分析