在swizzling方法中使用dispatch_once

Question

在NSHipter关于方法swizzling的文章中，它说"swizzling应该总是在dispatch_once中完成。"既然每个类只发生一次+load，为什么这是必要的？

Answer 1

这不是必需的。保证了+load是线程安全的和可重入的。参见objc-runtime-new.mm:中的load_images

/***********************************************************************
* load_images
* Process +load in the given images which are being mapped in by dyld.
* Calls ABI-agnostic code after taking ABI-specific locks.
*
* Locking: write-locks runtimeLock and loadMethodLock
**********************************************************************/
const char *
load_images(enum dyld_image_states state, uint32_t infoCount,
            const struct dyld_image_info infoList[])
{
    BOOL found;
    recursive_mutex_lock(&loadMethodLock);
    // Discover load methods
    rwlock_write(&runtimeLock);
    found = load_images_nolock(state, infoCount, infoList);
    rwlock_unlock_write(&runtimeLock);
    // Call +load methods (without runtimeLock - re-entrant)
    if (found) {
        call_load_methods();
    }
    recursive_mutex_unlock(&loadMethodLock);
    return nil;
}

请注意递归互斥，它保证在阻塞时完成所有加载，而call_load_methods()将确保每次+load实现只调用+load一次。

请注意，+load的特殊之处在于，它可以在每个类的基础上有多个实现，这也是为什么它更适合swizzling的原因之一——您的+load以及原始的+load都保证被调用。

额外的好处：以下是关于call_load_methods()的相关文档，它直接说明了为什么这是线程安全的：

/***********************************************************************
* call_load_methods
* Call all pending class and category +load methods.
* Class +load methods are called superclass-first. 
* Category +load methods are not called until after the parent class's +load.
* 
* This method must be RE-ENTRANT, because a +load could trigger 
* more image mapping. In addition, the superclass-first ordering 
* must be preserved in the face of re-entrant calls. Therefore, 
* only the OUTERMOST call of this function will do anything, and 
* that call will handle all loadable classes, even those generated 
* while it was running.
*
* The sequence below preserves +load ordering in the face of 
* image loading during a +load, and make sure that no 
* +load method is forgotten because it was added during 
* a +load call.
* Sequence:
* 1. Repeatedly call class +loads until there aren't any more
* 2. Call category +loads ONCE.
* 3. Run more +loads if:
*    (a) there are more classes to load, OR
*    (b) there are some potential category +loads that have 
*        still never been attempted.
* Category +loads are only run once to ensure "parent class first" 
* ordering, even if a category +load triggers a new loadable class 
* and a new loadable category attached to that class. 
*
* Locking: loadMethodLock must be held by the caller 
*   All other locks must not be held.
**********************************************************************/
void call_load_methods(void)

Answer 2

我认为这篇文章是建议"Swizzling应该在+load中完成"。但你仍然可以在其他地方喝。在这种情况下，您应该在dispatch_once中对原子性进行swizzling操作。我认为也没有必要将swizzing包装在+加载中的dispatch_once中。