load

问题一：Category中有load方法吗？

答：有的，但是和其他方法不同，Category 中的load和类中的load方法，并不是简单的覆盖或者继承。用一个简单的工程可以看出：

LoadClass.m：

#import "LoadClass.h"

@implementation LoadClass

+(void)load{
    
    NSLog(@"Class Load Execute");
}
@end

LoadClass+LoadClassCategory.m：

#import "LoadClass+LoadClassCategory.h"

@implementation LoadClass (LoadClassCategory)

+(void)load{
    NSLog(@"Category Load Execute");
}
@end

输出：

2019-11-10 20:57:22.986038+0800 LoadDemo[88431:1086125] Class Load Execute
2019-11-10 20:57:22.986486+0800 LoadDemo[88431:1086125] Category Load Execute

从输出可以看出，并没有出现方法覆盖的情况，两个load方法都被执行了。

问题二：load方法是什么时候被调到的？

要探究这个问题其实很简单，只需要在上述代码中的load方法中下一个断点，查看调用堆栈即可：

image.png
程序运行并命中断点之后，查看输出：
image.png
这里我们主要看load的上层调用load_images。显然这个时候，我们需要借助runtime源码去分析。
首先找到这个函数：

/***********************************************************************
* load_images
* Process +load in the given images which are being mapped in by dyld.
*
* Locking: write-locks runtimeLock and loadMethodLock
**********************************************************************/
extern bool hasLoadMethods(const headerType *mhdr);
extern void prepare_load_methods(const headerType *mhdr);

void
load_images(const char *path __unused, const struct mach_header *mh)
{
    // Return without taking locks if there are no +load methods here.
    if (!hasLoadMethods((const headerType *)mh)) return;

    recursive_mutex_locker_t lock(loadMethodLock);

    // Discover load methods
    {
        mutex_locker_t lock2(runtimeLock);
        prepare_load_methods((const headerType *)mh);
    }

    // Call +load methods (without runtimeLock - re-entrant)
    call_load_methods();
}

函数内部其实很简单，就是几个函数的调用，这里，我们主要关注两个函数-----prepare_load_methods、call_load_methods。
先看prepare_load_methods：

void prepare_load_methods(const headerType *mhdr)
{
    size_t count, i;

    runtimeLock.assertLocked();

    classref_t *classlist = 
        _getObjc2NonlazyClassList(mhdr, &count);
    for (i = 0; i < count; i++) {
        schedule_class_load(remapClass(classlist[i]));
    }

    category_t **categorylist = _getObjc2NonlazyCategoryList(mhdr, &count);
    for (i = 0; i < count; i++) {
        category_t *cat = categorylist[i];
        Class cls = remapClass(cat->cls);
        if (!cls) continue;  // category for ignored weak-linked class
        realizeClass(cls);
        assert(cls->ISA()->isRealized());
        add_category_to_loadable_list(cat);
    }
}

看函数的内部流程和调用，其实不难看出，prepare_load_methods函数内部遍历所有类，并调用schedule_class_load函数，将所有类中的load方法加入列表中，之后有遍历所有的category并调用add_category_to_loadable_list函数将category中的load方法加入列表中。大致的流程就是这样子的，如果想更深入的研究load方法添加到列表中的细节，可以详细分析一下这两个函数。
接下来看看call_load_methods是怎么调用load方法的。

/***********************************************************************
* 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)
{
    static bool loading = NO;
    bool more_categories;

    loadMethodLock.assertLocked();

    // Re-entrant calls do nothing; the outermost call will finish the job.
    if (loading) return;
    loading = YES;

    void *pool = objc_autoreleasePoolPush();

    do {
        // 1. Repeatedly call class +loads until there aren't any more
        while (loadable_classes_used > 0) {
            call_class_loads();
        }

        // 2. Call category +loads ONCE
        more_categories = call_category_loads();

        // 3. Run more +loads if there are classes OR more untried categories
    } while (loadable_classes_used > 0  ||  more_categories);

    objc_autoreleasePoolPop(pool);

    loading = NO;
}

根据代码和注释，可以清楚的看出，load方法的调用流程为：先调用类中的load方法，再调用类别中的load方法。这其实可以说明一个问题：类中的load方法比类别中的load方法先执行。
到这里，其实咱们就可以回答刚刚的那个问题了：
答：load方法是在类或者类别添加到runtime的时候，被调用的，而且其调用顺序是：先类中的load，再类别中的load。