Runtime源码下载

1. API

/** 
 * Adds a new method to a class with a given name and implementation.
 * 
 * @param cls The class to which to add a method.
 * @param name A selector that specifies the name of the method being added.
 * @param imp A function which is the implementation of the new method. The function must take at least two arguments—self and _cmd.
 * @param types An array of characters that describe the types of the arguments to the method. 
 * 
 * @return YES if the method was added successfully, otherwise NO 
 *  (for example, the class already contains a method implementation with that name).
 *
 * @note class_addMethod will add an override of a superclass's implementation, 
 *  but will not replace an existing implementation in this class. 
 *  To change an existing implementation, use method_setImplementation.
 */
OBJC_EXPORT BOOL
class_addMethod(Class _Nullable cls, SEL _Nonnull name, IMP _Nonnull imp, 
                const char * _Nullable types) 
    OBJC_AVAILABLE(10.5, 2.0, 9.0, 1.0, 2.0);

由API知道，当方法添加成功的时返回YES，方法添加失败时返回NO（当方法已经存在的情况下会返回NO）
该方法会添加父类实现的方法
不会添加本类中已经存在的IMP，如要更改本类中已经存在的IMP。需要用请使用method_setImplementation

2. Runtime 源码

objc-runtime-new.mm

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

    mutex_locker_t lock(runtimeLock);
    return ! addMethod(cls, name, imp, types ?: "", NO);
}

/**********************************************************************
* addMethod
* fixme
* Locking: runtimeLock must be held by the caller
**********************************************************************/
static IMP 
addMethod(Class cls, SEL name, IMP imp, const char *types, bool replace)
{
    IMP result = nil;

    runtimeLock.assertLocked();

    checkIsKnownClass(cls);
    
    ASSERT(types);
    ASSERT(cls->isRealized());

    method_t *m;
    if ((m = getMethodNoSuper_nolock(cls, name))) {
        // already exists
        if (!replace) {
            result = m->imp(false);
        } else {
            result = _method_setImplementation(cls, m, imp);
        }
    } else {
        // fixme optimize
        method_list_t *newlist;
        newlist = (method_list_t *)calloc(method_list_t::byteSize(method_t::bigSize, 1), 1);
        newlist->entsizeAndFlags = 
            (uint32_t)sizeof(struct method_t::big) | fixed_up_method_list;
        newlist->count = 1;
        auto &first = newlist->begin()->big();
        first.name = name;
        first.types = strdupIfMutable(types);
        first.imp = imp;

        addMethods_finish(cls, newlist);
        result = nil;
    }

    return result;
}

由源码可以看到，Runtime先查找该类在父类中是否存在，若存在，生成一个新方法(重写) 若不存在，生成新的method_list,并将新方法插入到新生成的method_list中，然后替换原来的method_list

iOS - Runtime class_addMethod源码

1. API

2. Runtime 源码