源码版本:objc-runtime-750
1.category源码入口
void
map_images(unsigned count, const char * const paths[],
const struct mach_header * const mhdrs[])
{
mutex_locker_t lock(runtimeLock);
return map_images_nolock(count, paths, mhdrs);
}
void
map_images_nolock(unsigned mhCount, const char * const mhPaths[],
const struct mach_header * const mhdrs[])
{
static bool firstTime = YES;
header_info *hList[mhCount];
uint32_t hCount;
size_t selrefCount = 0;
// Perform first-time initialization if necessary.
// This function is called before ordinary library initializers.
// fixme defer initialization until an objc-using image is found?
if (firstTime) {
preopt_init();
}
if (PrintImages) {
_objc_inform("IMAGES: processing %u newly-mapped images...\n", mhCount);
}
// Find all images with Objective-C metadata.(查找所有带有Objective-C元数据的镜像。)
hCount = 0;
// Count classes. Size various table based on the total.
int totalClasses = 0;
int unoptimizedTotalClasses = 0;
{
...
if (hCount > 0) {
/*
hList:
*/
_read_images(hList, hCount, totalClasses, unoptimizedTotalClasses);//入口
}
firstTime = NO;
}
void _read_images(header_info **hList, uint32_t hCount, int totalClasses, int unoptimizedTotalClasses)
{
...
// Discover categories. :读取分类信息
for (EACH_HEADER) {
category_t **catlist =
_getObjc2CategoryList(hi, &count);
bool hasClassProperties = hi->info()->hasCategoryClassProperties();
for (i = 0; i < count; i++) {
category_t *cat = catlist[i];
Class cls = remapClass(cat->cls);//在realizeClass中已经有添加分类，这时候直接map查找
if (!cls) {
// Category's target class is missing (probably weak-linked).
// Disavow any knowledge of this category.
catlist[i] = nil;
if (PrintConnecting) {
_objc_inform("CLASS: IGNORING category \?\?\?(%s) %p with "
"missing weak-linked target class",
cat->name, cat);
}
continue;
}
// Process this category.
// First, register the category with its target class.
// Then, rebuild the class's method lists (etc) if
// the class is realized.
bool classExists = NO;
if (cat->instanceMethods || cat->protocols
|| cat->instanceProperties)
{
addUnattachedCategoryForClass(cat, cls, hi);
if (cls->isRealized()) {
remethodizeClass(cls);
classExists = YES;
}
if (PrintConnecting) {
_objc_inform("CLASS: found category -%s(%s) %s",
cls->nameForLogging(), cat->name,
classExists ? "on existing class" : "");
}
}
if (cat->classMethods || cat->protocols
|| (hasClassProperties && cat->_classProperties))
{
addUnattachedCategoryForClass(cat, cls->ISA(), hi);
if (cls->ISA()->isRealized()) {
remethodizeClass(cls->ISA());
}
if (PrintConnecting) {
_objc_inform("CLASS: found category +%s(%s)",
cls->nameForLogging(), cat->name);
}
}
}
}
...
}
map_images_nolock方法：会解析Mach-o文件读取到header_info *hList
_read_images：从hList读取读取信息,包括:class、protocol、category、sel等。category是在最后加载的。
1.2原理
1.2.1 remethodizeClass
static void remethodizeClass(Class cls)
{
category_list *cats;
bool isMeta;
runtimeLock.assertWriting();
isMeta = cls->isMetaClass();
// 获取cls中未完成整合的所有分类
if ((cats = unattachedCategoriesForClass(cls, false/*not realizing*/))) {
if (PrintConnecting) {
_objc_inform("CLASS: attaching categories to class '%s' %s",
cls->nameForLogging(), isMeta ? "(meta)" : "");
}
//将分类cats拼接到cls上
attachCategories(cls, cats, true /*flush caches*/);
free(cats);
}
}
static category_list *
unattachedCategoriesForClass(Class cls, bool realizing)
{
runtimeLock.assertLocked();
//疑问:NXMapRemove返回的是void *，可以直接强转成category_list?
return (category_list *)NXMapRemove(unattachedCategories(), cls);//从当前未分配类别的NXMapTable移除cls
}
从为未加载类的分类中找到category_list，通过attachCategories拼接到原来类的方法列表中。
1.2.2 attachCategories
static void
attachCategories(Class cls, category_list *cats, bool flush_caches)
{
if (!cats) return;
if (PrintReplacedMethods) printReplacements(cls, cats);
bool isMeta = cls->isMetaClass();//是否是元类
// 按照Category个数，分配对应的内存空间
// fixme rearrange to remove these intermediate allocations
method_list_t **mlists = (method_list_t **)//方法列表
malloc(cats->count * sizeof(*mlists));
property_list_t **proplists = (property_list_t **)//属性列表
malloc(cats->count * sizeof(*proplists));
protocol_list_t **protolists = (protocol_list_t **)//协议列表
malloc(cats->count * sizeof(*protolists));
// Count backwards through cats to get newest categories first
int mcount = 0;
int propcount = 0;
int protocount = 0;
int i = cats->count;//宿主类分类的总数
bool fromBundle = NO;
// 循环查找出Protocol list、Property list、Method list
while (i--) {//倒序遍历，最先访问的最后编译的分类
auto& entry = cats->list[i];//获取一个分类
method_list_t *mlist = entry.cat->methodsForMeta(isMeta);//获取该分类的方法列表
if (mlist) {
mlists[mcount++] = mlist;//最后编译的分类最先添加到分类数组中
fromBundle |= entry.hi->isBundle();
}
property_list_t *proplist =
entry.cat->propertiesForMeta(isMeta, entry.hi);
if (proplist) {
proplists[propcount++] = proplist;
}
protocol_list_t *protolist = entry.cat->protocols;
if (protolist) {
protolists[protocount++] = protolist;
}
}
auto rw = cls->data();//获取宿主类中的rw数据，其中包含宿主类的方法列表s信息
// 主要是针对 分类中有关于内存管理相关方法情况下的 一些特殊处理
prepareMethodLists(cls, mlists, mcount, NO, fromBundle);
/*
rw代表类
methods代表类的方法类别
attachLists 将含有mcount个元素的mlists拼接到rw的methods上
*/
rw->methods.attachLists(mlists, mcount);
free(mlists);
if (flush_caches && mcount > 0) flushCaches(cls);
rw->properties.attachLists(proplists, propcount);
free(proplists);
rw->protocols.attachLists(protolists, protocount);
free(protolists);
}
- 按照Category个数，分配对应的数组mlists、proplists、protolists内存空间
- while循环把cats->list中的list赋给数组
- 把上面三个数组添加到对应的方法列表上
1.2.3 attachLists
void attachLists(List* const * addedLists, uint32_t addedCount) {
if (addedCount == 0) return;
if (hasArray()) {
// many lists -> many lists
uint32_t oldCount = array()->count;
uint32_t newCount = oldCount + addedCount;
setArray((array_t *)realloc(array(), array_t::byteSize(newCount)));//根据新总数重新分配内存
array()->count = newCount;//重新设置元素总数
//内存移动:把oldArray整体往后挪addedCount位置
memmove(array()->lists + addedCount, array()->lists,
oldCount * sizeof(array()->lists[0]));
//内存拷贝:把addedLists放到0-(addedCount-1)的位置
memcpy(array()->lists, addedLists,
addedCount * sizeof(array()->lists[0]));
}
else if (!list && addedCount == 1) {
// 0 lists -> 1 list
list = addedLists[0];
}
else {
// 1 list -> many lists
List* oldList = list;
uint32_t oldCount = oldList ? 1 : 0;
uint32_t newCount = oldCount + addedCount;
setArray((array_t *)malloc(array_t::byteSize(newCount)));
array()->count = newCount;
if (oldList) array()->lists[addedCount] = oldList;
memcpy(array()->lists, addedLists,
addedCount * sizeof(array()->lists[0]));
}
}
- 根据新总数重新分配内存
- 重新设置元素总数
- 内存移动:把oldArray整体往后挪addedCount位置
- 内存拷贝:把addedLists放到0-(addedCount-1)的位置
结果导致分类的方法插在了类的方法列表前面，方法调用时是找到即停止。这也是为什么分类方法会优先与类的方法调用，真正意义上并不是覆盖。
总结:
1.分类添加的方法可以"覆盖"原类方法(效果是覆盖的，但原宿主类的方法仍然存在)
2.同名分类方法谁能生效取决于编译顺序(最后被编译的分类会优先生效)
3.名字相同的分类会引起编译报错。
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