1. 分类的加载
Objective-C的初始化方法_objc_init：
void _objc_init(void)
{
static bool initialized = false;
if (initialized) return;
initialized = true;
// fixme defer initialization until an objc-using image is found?
environ_init();
tls_init();
static_init();
runtime_init();
exception_init();
cache_init();
_imp_implementationWithBlock_init();
// dyld映射、加载images
_dyld_objc_notify_register(&map_images, load_images, unmap_image);
}
我们首先看map_images：
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);
}
//Objective-C的元数据中找到所有images
hCount = 0;
// 统计class的数量
int totalClasses = 0;
int unoptimizedTotalClasses = 0;
{
uint32_t i = mhCount;
while (i--) {
const headerType *mhdr = (const headerType *)mhdrs[i];
auto hi = addHeader(mhdr, mhPaths[i], totalClasses, unoptimizedTotalClasses);
if (!hi) {
// no objc data in this entry
continue;
}
if (mhdr->filetype == MH_EXECUTE) {
// Size some data structures based on main executable's size
#if __OBJC2__
size_t count;
_getObjc2SelectorRefs(hi, &count);
selrefCount += count;
_getObjc2MessageRefs(hi, &count);
selrefCount += count;
#else
_getObjcSelectorRefs(hi, &selrefCount);
#endif
#if SUPPORT_GC_COMPAT
// Halt if this is a GC app.
if (shouldRejectGCApp(hi)) {
_objc_fatal_with_reason
(OBJC_EXIT_REASON_GC_NOT_SUPPORTED,
OS_REASON_FLAG_CONSISTENT_FAILURE,
"Objective-C garbage collection "
"is no longer supported.");
}
#endif
}
hList[hCount++] = hi;
if (PrintImages) {
_objc_inform("IMAGES: loading image for %s%s%s%s%s\n",
hi->fname(),
mhdr->filetype == MH_BUNDLE ? " (bundle)" : "",
hi->info()->isReplacement() ? " (replacement)" : "",
hi->info()->hasCategoryClassProperties() ? " (has class properties)" : "",
hi->info()->optimizedByDyld()?" (preoptimized)":"");
}
}
}
// 执行一次性运行时初始化，该初始化必须延迟到找到可执行文件本身为止。这需要在进一步初始化之前完成。
// 如果可执行文件不包含Objective-C代码，但Objective-C稍后会动态加载。
if (firstTime) {
sel_init(selrefCount);
arr_init();
#if SUPPORT_GC_COMPAT
// Reject any GC images linked to the main executable.
// We already rejected the app itself above.
// Images loaded after launch will be rejected by dyld.
for (uint32_t i = 0; i < hCount; i++) {
auto hi = hList[i];
auto mh = hi->mhdr();
if (mh->filetype != MH_EXECUTE && shouldRejectGCImage(mh)) {
_objc_fatal_with_reason
(OBJC_EXIT_REASON_GC_NOT_SUPPORTED,
OS_REASON_FLAG_CONSISTENT_FAILURE,
"%s requires Objective-C garbage collection "
"which is no longer supported.", hi->fname());
}
}
#endif
#if TARGET_OS_OSX
// Disable +initialize fork safety if the app is too old (< 10.13).
// Disable +initialize fork safety if the app has a
// __DATA,__objc_fork_ok section.
if (dyld_get_program_sdk_version() < DYLD_MACOSX_VERSION_10_13) {
DisableInitializeForkSafety = true;
if (PrintInitializing) {
_objc_inform("INITIALIZE: disabling +initialize fork "
"safety enforcement because the app is "
"too old (SDK version " SDK_FORMAT ")",
FORMAT_SDK(dyld_get_program_sdk_version()));
}
}
for (uint32_t i = 0; i < hCount; i++) {
auto hi = hList[i];
auto mh = hi->mhdr();
if (mh->filetype != MH_EXECUTE) continue;
unsigned long size;
if (getsectiondata(hi->mhdr(), "__DATA", "__objc_fork_ok", &size)) {
DisableInitializeForkSafety = true;
if (PrintInitializing) {
_objc_inform("INITIALIZE: disabling +initialize fork "
"safety enforcement because the app has "
"a __DATA,__objc_fork_ok section");
}
}
break; // assume only one MH_EXECUTE image
}
#endif
}
// 如果找到了images
if (hCount > 0) {
// 加载image
_read_images(hList, hCount, totalClasses, unoptimizedTotalClasses);
}
firstTime = NO;
// Call image load funcs after everything is set up.
for (auto func : loadImageFuncs) {
for (uint32_t i = 0; i < mhCount; i++) {
func(mhdrs[i]);
}
}
}
我们看到找到images之后就会进行加载_read_images：
void _read_images(header_info **hList, uint32_t hCount, int totalClasses, int unoptimizedTotalClasses)
{
header_info *hi;
uint32_t hIndex;
size_t count;
size_t i;
Class *resolvedFutureClasses = nil;
size_t resolvedFutureClassCount = 0;
static bool doneOnce;
bool launchTime = NO;
TimeLogger ts(PrintImageTimes);
runtimeLock.assertLocked();
#define EACH_HEADER \
hIndex = 0; \
hIndex < hCount && (hi = hList[hIndex]); \
hIndex++
if (!doneOnce) {
doneOnce = YES;
launchTime = YES;
#if SUPPORT_NONPOINTER_ISA
// Disable non-pointer isa under some conditions.
# if SUPPORT_INDEXED_ISA
// Disable nonpointer isa if any image contains old Swift code
for (EACH_HEADER) {
if (hi->info()->containsSwift() &&
hi->info()->swiftUnstableVersion() < objc_image_info::SwiftVersion3)
{
DisableNonpointerIsa = true;
if (PrintRawIsa) {
_objc_inform("RAW ISA: disabling non-pointer isa because "
"the app or a framework contains Swift code "
"older than Swift 3.0");
}
break;
}
}
# endif
# if TARGET_OS_OSX
// Disable non-pointer isa if the app is too old
// (linked before OS X 10.11)
if (dyld_get_program_sdk_version() < DYLD_MACOSX_VERSION_10_11) {
DisableNonpointerIsa = true;
if (PrintRawIsa) {
_objc_inform("RAW ISA: disabling non-pointer isa because "
"the app is too old (SDK version " SDK_FORMAT ")",
FORMAT_SDK(dyld_get_program_sdk_version()));
}
}
// Disable non-pointer isa if the app has a __DATA,__objc_rawisa section
// New apps that load old extensions may need this.
for (EACH_HEADER) {
if (hi->mhdr()->filetype != MH_EXECUTE) continue;
unsigned long size;
if (getsectiondata(hi->mhdr(), "__DATA", "__objc_rawisa", &size)) {
DisableNonpointerIsa = true;
if (PrintRawIsa) {
_objc_inform("RAW ISA: disabling non-pointer isa because "
"the app has a __DATA,__objc_rawisa section");
}
}
break; // assume only one MH_EXECUTE image
}
# endif
#endif
if (DisableTaggedPointers) {
disableTaggedPointers();
}
initializeTaggedPointerObfuscator();
if (PrintConnecting) {
_objc_inform("CLASS: found %d classes during launch", totalClasses);
}
// namedClasses
// Preoptimized classes don't go in this table.
// 4/3 is NXMapTable's load factor
// 实例化存储类的哈希表，并且根据当前类数量做动态扩容
int namedClassesSize =
(isPreoptimized() ? unoptimizedTotalClasses : totalClasses) * 4 / 3;
gdb_objc_realized_classes =
NXCreateMapTable(NXStrValueMapPrototype, namedClassesSize);
ts.log("IMAGE TIMES: first time tasks");
}
// 将所有SEL都注册到哈希表中，是另外一张哈希表
static size_t UnfixedSelectors;
{
mutex_locker_t lock(selLock);
for (EACH_HEADER) {
if (hi->hasPreoptimizedSelectors()) continue;
bool isBundle = hi->isBundle();
SEL *sels = _getObjc2SelectorRefs(hi, &count);
UnfixedSelectors += count;
for (i = 0; i < count; i++) {
const char *name = sel_cname(sels[i]);
SEL sel = sel_registerNameNoLock(name, isBundle);
if (sels[i] != sel) {
sels[i] = sel;
}
}
}
}
ts.log("IMAGE TIMES: fix up selector references");
// Discover classes. Fix up unresolved future classes. Mark bundle classes.
bool hasDyldRoots = dyld_shared_cache_some_image_overridden();
// 读取类列表，并将所有类添加到类的哈希表中，并且标记懒加载的类并初始化内存空间
for (EACH_HEADER) {
if (! mustReadClasses(hi, hasDyldRoots)) {
// Image is sufficiently optimized that we need not call readClass()
continue;
}
classref_t const *classlist = _getObjc2ClassList(hi, &count);
bool headerIsBundle = hi->isBundle();
bool headerIsPreoptimized = hi->hasPreoptimizedClasses();
// 从编译后的类列表中取出所有类，获取到的是一个classref_t类型的指针
for (i = 0; i < count; i++) {
Class cls = (Class)classlist[i];
// 通过readClass函数获取处理后的新类，内部主要操作ro和rw结构体
Class newCls = readClass(cls, headerIsBundle, headerIsPreoptimized);
// 初始化所有懒加载的类需要的内存空间
if (newCls != cls && newCls) {
// 将懒加载的类添加到数组中
resolvedFutureClasses = (Class *)
realloc(resolvedFutureClasses,
(resolvedFutureClassCount+1) * sizeof(Class));
resolvedFutureClasses[resolvedFutureClassCount++] = newCls;
}
}
}
ts.log("IMAGE TIMES: discover classes");
// 将未映射Class和Super Class重映射，被remap的类都是非懒加载的类
if (!noClassesRemapped()) {
for (EACH_HEADER) {
// 重映射Class，注意是从_getObjc2ClassRefs函数中取出类的引用
Class *classrefs = _getObjc2ClassRefs(hi, &count);
for (i = 0; i < count; i++) {
remapClassRef(&classrefs[i]);
}
// 重映射父类
classrefs = _getObjc2SuperRefs(hi, &count);
for (i = 0; i < count; i++) {
remapClassRef(&classrefs[i]);
}
}
}
ts.log("IMAGE TIMES: remap classes");
#if SUPPORT_FIXUP
// 修复旧的函数指针调用
for (EACH_HEADER) {
message_ref_t *refs = _getObjc2MessageRefs(hi, &count);
if (count == 0) continue;
if (PrintVtables) {
_objc_inform("VTABLES: repairing %zu unsupported vtable dispatch "
"call sites in %s", count, hi->fname());
}
for (i = 0; i < count; i++) {
// 内部将常用的alloc、objc_msgSend等函数指针进行注册，并fix为新的函数指针
fixupMessageRef(refs+i);
}
}
ts.log("IMAGE TIMES: fix up objc_msgSend_fixup");
#endif
bool cacheSupportsProtocolRoots = sharedCacheSupportsProtocolRoots();
// 遍历所有协议列表，并且将协议列表加载到Protocol的哈希表中
for (EACH_HEADER) {
extern objc_class OBJC_CLASS_$_Protocol;
// cls = Protocol类，所有协议和对象的结构体都类似，isa都对应Protocol类
Class cls = (Class)&OBJC_CLASS_$_Protocol;
ASSERT(cls);
// 获取protocol哈希表
NXMapTable *protocol_map = protocols();
bool isPreoptimized = hi->hasPreoptimizedProtocols();
// Skip reading protocols if this is an image from the shared cache
// and we support roots
// Note, after launch we do need to walk the protocol as the protocol
// in the shared cache is marked with isCanonical() and that may not
// be true if some non-shared cache binary was chosen as the canonical
// definition
if (launchTime && isPreoptimized && cacheSupportsProtocolRoots) {
if (PrintProtocols) {
_objc_inform("PROTOCOLS: Skipping reading protocols in image: %s",
hi->fname());
}
continue;
}
bool isBundle = hi->isBundle();
// 从编译器中读取并初始化Protocol
protocol_t * const *protolist = _getObjc2ProtocolList(hi, &count);
for (i = 0; i < count; i++) {
readProtocol(protolist[i], cls, protocol_map,
isPreoptimized, isBundle);
}
}
ts.log("IMAGE TIMES: discover protocols");
// 修复协议列表引用，优化后的images可能是正确的，但是并不确定
for (EACH_HEADER) {
// 需要注意到是，下面的函数是_getObjc2ProtocolRefs，和上面的_getObjc2ProtocolList不一样
if (launchTime && cacheSupportsProtocolRoots && hi->isPreoptimized())
continue;
protocol_t **protolist = _getObjc2ProtocolRefs(hi, &count);
for (i = 0; i < count; i++) {
remapProtocolRef(&protolist[i]);
}
}
ts.log("IMAGE TIMES: fix up @protocol references");
// 处理所有Category
for (EACH_HEADER) {
bool hasClassProperties = hi->info()->hasCategoryClassProperties();
// 外部循环遍历找到当前类，查找类对应的Category数组
auto processCatlist = [&](category_t * const *catlist) {
// 内部循环遍历当前类的所有Category
for (i = 0; i < count; i++) {
category_t *cat = catlist[i];
Class cls = remapClass(cat->cls);
locstamped_category_t lc{cat, hi};
if (!cls) {
// Category's target class is missing (probably weak-linked).
// Ignore the category.
if (PrintConnecting) {
_objc_inform("CLASS: IGNORING category \?\?\?(%s) %p with "
"missing weak-linked target class",
cat->name, cat);
}
continue;
}
// Process this category.
if (cls->isStubClass()) {
// 存根类永远不会实现。存根类在初始化之前不知道它们的元类，因此我们必须将带有类方法或属性的类别添加到存根本身。
// methodizeClass()将找到它们并酌情将它们添加到元类中。
if (cat->instanceMethods ||
cat->protocols ||
cat->instanceProperties ||
cat->classMethods ||
cat->protocols ||
(hasClassProperties && cat->_classProperties))
{
objc::unattachedCategories.addForClass(lc, cls);
}
} else {
// 首先，将类别注册到其目标类。
// 然后，如果类已实现，则重新生成该类的方法列表。
if (cat->instanceMethods || cat->protocols
|| cat->instanceProperties)
{
if (cls->isRealized()) {
attachCategories(cls, &lc, 1, ATTACH_EXISTING);
} else {
objc::unattachedCategories.addForClass(lc, cls);
}
}
// 这块和上面逻辑一样，区别在于这块是对Meta Class做操作，而上面则是对Class做操作
if (cat->classMethods || cat->protocols
|| (hasClassProperties && cat->_classProperties))
{
if (cls->ISA()->isRealized()) {
attachCategories(cls->ISA(), &lc, 1, ATTACH_EXISTING | ATTACH_METACLASS);
} else {
objc::unattachedCategories.addForClass(lc, cls->ISA());
}
}
}
}
};
processCatlist(_getObjc2CategoryList(hi, &count));
processCatlist(_getObjc2CategoryList2(hi, &count));
}
ts.log("IMAGE TIMES: discover categories");
// Category discovery MUST BE Late to avoid potential races
// when other threads call the new category code before
// this thread finishes its fixups.
// +load handled by prepare_load_methods()
// 初始化所有非懒加载的类
for (EACH_HEADER) {
classref_t const *classlist =
_getObjc2NonlazyClassList(hi, &count);
for (i = 0; i < count; i++) {
Class cls = remapClass(classlist[i]);
if (!cls) continue;
addClassTableEntry(cls);
// 初始化所有非懒加载的类(实例化类对象的一些信息，例如rw)
if (cls->isSwiftStable()) {
if (cls->swiftMetadataInitializer()) {
_objc_fatal("Swift class %s with a metadata initializer "
"is not allowed to be non-lazy",
cls->nameForLogging());
}
// fixme also disallow relocatable classes
// We can't disallow all Swift classes because of
// classes like Swift.__EmptyArrayStorage
}
realizeClassWithoutSwift(cls, nil);
}
}
ts.log("IMAGE TIMES: realize non-lazy classes");
// 解析的未来要用的类，以防CF操纵它们
if (resolvedFutureClasses) {
for (i = 0; i < resolvedFutureClassCount; i++) {
Class cls = resolvedFutureClasses[i];
if (cls->isSwiftStable()) {
_objc_fatal("Swift class is not allowed to be future");
}
realizeClassWithoutSwift(cls, nil);
cls->setInstancesRequireRawIsaRecursively(false/*inherited*/);
}
free(resolvedFutureClasses);
}
ts.log("IMAGE TIMES: realize future classes");
if (DebugNonFragileIvars) {
realizeAllClasses();
}
// Print preoptimization statistics
if (PrintPreopt) {
static unsigned int PreoptTotalMethodLists;
static unsigned int PreoptOptimizedMethodLists;
static unsigned int PreoptTotalClasses;
static unsigned int PreoptOptimizedClasses;
for (EACH_HEADER) {
if (hi->hasPreoptimizedSelectors()) {
_objc_inform("PREOPTIMIZATION: honoring preoptimized selectors "
"in %s", hi->fname());
}
else if (hi->info()->optimizedByDyld()) {
_objc_inform("PREOPTIMIZATION: IGNORING preoptimized selectors "
"in %s", hi->fname());
}
classref_t const *classlist = _getObjc2ClassList(hi, &count);
for (i = 0; i < count; i++) {
Class cls = remapClass(classlist[i]);
if (!cls) continue;
PreoptTotalClasses++;
if (hi->hasPreoptimizedClasses()) {
PreoptOptimizedClasses++;
}
const method_list_t *mlist;
if ((mlist = ((class_ro_t *)cls->data())->baseMethods())) {
PreoptTotalMethodLists++;
if (mlist->isFixedUp()) {
PreoptOptimizedMethodLists++;
}
}
if ((mlist=((class_ro_t *)cls->ISA()->data())->baseMethods())) {
PreoptTotalMethodLists++;
if (mlist->isFixedUp()) {
PreoptOptimizedMethodLists++;
}
}
}
}
_objc_inform("PREOPTIMIZATION: %zu selector references not "
"pre-optimized", UnfixedSelectors);
_objc_inform("PREOPTIMIZATION: %u/%u (%.3g%%) method lists pre-sorted",
PreoptOptimizedMethodLists, PreoptTotalMethodLists,
PreoptTotalMethodLists
? 100.0*PreoptOptimizedMethodLists/PreoptTotalMethodLists
: 0.0);
_objc_inform("PREOPTIMIZATION: %u/%u (%.3g%%) classes pre-registered",
PreoptOptimizedClasses, PreoptTotalClasses,
PreoptTotalClasses
? 100.0*PreoptOptimizedClasses/PreoptTotalClasses
: 0.0);
_objc_inform("PREOPTIMIZATION: %zu protocol references not "
"pre-optimized", UnfixedProtocolReferences);
}
#undef EACH_HEADER
}
核心的就是attachCategories：
static void
attachCategories(Class cls, const locstamped_category_t *cats_list, uint32_t cats_count,
int flags)
{
if (slowpath(PrintReplacedMethods)) {
printReplacements(cls, cats_list, cats_count);
}
if (slowpath(PrintConnecting)) {
_objc_inform("CLASS: attaching %d categories to%s class '%s'%s",
cats_count, (flags & ATTACH_EXISTING) ? " existing" : "",
cls->nameForLogging(), (flags & ATTACH_METACLASS) ? " (meta)" : "");
}
/*
* Only a few classes have more than 64 categories during launch.
* This uses a little stack, and avoids malloc.
*
* Categories must be added in the proper order, which is back
* to front. To do that with the chunking, we iterate cats_list
* from front to back, build up the local buffers backwards,
* and call attachLists on the chunks. attachLists prepends the
* lists, so the final result is in the expected order.
*/
constexpr uint32_t ATTACH_BUFSIZ = 64;
method_list_t *mlists[ATTACH_BUFSIZ];
property_list_t *proplists[ATTACH_BUFSIZ];
protocol_list_t *protolists[ATTACH_BUFSIZ];
uint32_t mcount = 0;
uint32_t propcount = 0;
uint32_t protocount = 0;
bool fromBundle = NO;
bool isMeta = (flags & ATTACH_METACLASS);
auto rw = cls->data();
// 遍历分类
for (uint32_t i = 0; i < cats_count; i++) {
auto& entry = cats_list[i];
// 获取每个分类的方法
method_list_t *mlist = entry.cat->methodsForMeta(isMeta);
if (mlist) {
if (mcount == ATTACH_BUFSIZ) {
prepareMethodLists(cls, mlists, mcount, NO, fromBundle);
rw->methods.attachLists(mlists, mcount);
mcount = 0;
}
mlists[ATTACH_BUFSIZ - ++mcount] = mlist;
fromBundle |= entry.hi->isBundle();
}
// 获取每个分类的属性
property_list_t *proplist =
entry.cat->propertiesForMeta(isMeta, entry.hi);
if (proplist) {
if (propcount == ATTACH_BUFSIZ) {
rw->properties.attachLists(proplists, propcount);
propcount = 0;
}
proplists[ATTACH_BUFSIZ - ++propcount] = proplist;
}
// 获取每个分类的协议
protocol_list_t *protolist = entry.cat->protocolsForMeta(isMeta);
if (protolist) {
if (protocount == ATTACH_BUFSIZ) {
rw->protocols.attachLists(protolists, protocount);
protocount = 0;
}
protolists[ATTACH_BUFSIZ - ++protocount] = protolist;
}
}
// 遍历完还有方法、属性、分类，再扫个尾
if (mcount > 0) {
prepareMethodLists(cls, mlists + ATTACH_BUFSIZ - mcount, mcount, NO, fromBundle);
rw->methods.attachLists(mlists + ATTACH_BUFSIZ - mcount, mcount);
if (flags & ATTACH_EXISTING) flushCaches(cls);
}
rw->properties.attachLists(proplists + ATTACH_BUFSIZ - propcount, propcount);
rw->protocols.attachLists(protolists + ATTACH_BUFSIZ - protocount, protocount);
}
可以看到分类中最核心的是attachLists：
void attachLists(List* const * addedLists, uint32_t addedCount) {
if (addedCount == 0) return;
if (hasArray()) {
// many lists -> many lists
// 要扩充多列表
uint32_t oldCount = array()->count;
uint32_t newCount = oldCount + addedCount;
// 重新分配内存空间
setArray((array_t *)realloc(array(), array_t::byteSize(newCount)));
// 更新总数
array()->count = newCount;
// 把老的列表放在后移，放在addedCount之后
memmove(array()->lists + addedCount, array()->lists,
oldCount * sizeof(array()->lists[0]));
// 新列表放在头部
memcpy(array()->lists, addedLists,
addedCount * sizeof(array()->lists[0]));
}
else if (!list && addedCount == 1) {
// 0 lists -> 1 list
// 只有一个列表直接赋值
list = addedLists[0];
}
else {
// 1 list -> many lists
// 只有1个列表，要扩充为多列表
List* oldList = list;
uint32_t oldCount = oldList ? 1 : 0;
uint32_t newCount = oldCount + addedCount;
// 分配内存空间
setArray((array_t *)malloc(array_t::byteSize(newCount)));
// 更新总数
array()->count = newCount;
// 老列表接在尾部
if (oldList) array()->lists[addedCount] = oldList;
// 新列表放在头部
memcpy(array()->lists, addedLists,
addedCount * sizeof(array()->lists[0]));
}
}
我们看到，分类在加载方法的时候，分类的方法是放在方法列表的头部的。
所以在查找方法列表时，一个分类的方法会被先找到，则会执行分类的方法，而不是类本身的方法。此外，最后加载的分类，会最先被找到。
2. +load
看了map_images，下面我们来看load_images：
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);
// 进行数据的准备
prepare_load_methods((const headerType *)mh);
}
// Call +load methods (without runtimeLock - re-entrant)
// 调用load方法
call_load_methods();
}
我们先看数据准备prepare_load_methods：
struct loadable_class {
Class cls; // may be nil
IMP method;
};
struct loadable_category {
Category cat; // may be nil
IMP method;
};
void prepare_load_methods(const headerType *mhdr)
{
size_t count, i;
runtimeLock.assertLocked();
// 拿到类列表
classref_t const *classlist =
_getObjc2NonlazyClassList(mhdr, &count);
// 遍历类，把没有加载的类和相关父类转换为loadable_class结构，添加到loadable_classes中
// 父类相比于子类会先被添加到列表中
for (i = 0; i < count; i++) {
schedule_class_load(remapClass(classlist[i]));
}
// 拿到分类列表
category_t * const *categorylist = _getObjc2NonlazyCategoryList(mhdr, &count);
// 遍历分类，把没有加载的分类转换为loadable_category结构，添加到loadable_categories中
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
if (cls->isSwiftStable()) {
_objc_fatal("Swift class extensions and categories on Swift "
"classes are not allowed to have +load methods");
}
realizeClassWithoutSwift(cls, nil);
ASSERT(cls->ISA()->isRealized());
add_category_to_loadable_list(cat);
}
}
下面是调用部分call_load_methods()：
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.
// 防止重复进入
if (loading) return;
loading = YES;
// 自动释放池push
void *pool = objc_autoreleasePoolPush();
do {
// 1. Repeatedly call class +loads until there aren't any more
// 首先调用类方法中的+load方法
while (loadable_classes_used > 0) {
call_class_loads();
}
// 2. Call category +loads ONCE
// 调用分类中的+load方法
more_categories = call_category_loads();
// 3. Run more +loads if there are classes OR more untried categories
// 扫尾工作，看还有没有没有调用的+load
} while (loadable_classes_used > 0 || more_categories);
// 自动释放池pop
objc_autoreleasePoolPop(pool);
loading = NO;
}
我们先看call_class_loads：
typedef void(*load_method_t)(id, SEL);
static void call_class_loads(void)
{
int i;
// Detach current loadable list.
struct loadable_class *classes = loadable_classes;
int used = loadable_classes_used;
loadable_classes = nil;
loadable_classes_allocated = 0;
loadable_classes_used = 0;
// Call all +loads for the detached list.
// 遍历loadable_class，获取到+load的IMP，然后直接调用
for (i = 0; i < used; i++) {
Class cls = classes[i].cls;
load_method_t load_method = (load_method_t)classes[i].method;
if (!cls) continue;
if (PrintLoading) {
_objc_inform("LOAD: +[%s load]\n", cls->nameForLogging());
}
(*load_method)(cls, @selector(load));
}
// Destroy the detached list.
if (classes) free(classes);
}
注意：
+load方法的调用是通过直接使用函数内存地址的方式实现的，而不是objc_msgSend来发送消息。这就意味着，类、父类与分类之间+load方法的调用是互不影响的。
子类不会主动调用父类的
+load方法，如果类与分类都实现了+load，那么两个+load方法都会被调用。
分类的加载call_category_loads：
static bool call_category_loads(void)
{
int i, shift;
bool new_categories_added = NO;
// Detach current loadable list.
struct loadable_category *cats = loadable_categories;
int used = loadable_categories_used;
int allocated = loadable_categories_allocated;
loadable_categories = nil;
loadable_categories_allocated = 0;
loadable_categories_used = 0;
// Call all +loads for the detached list.
// 遍历所有loadable_category，获取到+load的IMP，然后直接调用
for (i = 0; i < used; i++) {
Category cat = cats[i].cat;
load_method_t load_method = (load_method_t)cats[i].method;
Class cls;
if (!cat) continue;
cls = _category_getClass(cat);
if (cls && cls->isLoadable()) {
if (PrintLoading) {
_objc_inform("LOAD: +[%s(%s) load]\n",
cls->nameForLogging(),
_category_getName(cat));
}
(*load_method)(cls, @selector(load));
cats[i].cat = nil;
}
}
// 压缩分离列表（保序），去掉调用过+load的
shift = 0;
for (i = 0; i < used; i++) {
if (cats[i].cat) {
cats[i-shift] = cats[i];
} else {
shift++;
}
}
used -= shift;
// 将任何新的+load候选项从新列表复制到分离列表
new_categories_added = (loadable_categories_used > 0);
for (i = 0; i < loadable_categories_used; i++) {
if (used == allocated) {
allocated = allocated*2 + 16;
cats = (struct loadable_category *)
realloc(cats, allocated *
sizeof(struct loadable_category));
}
cats[used++] = loadable_categories[i];
}
// 销毁新列表
if (loadable_categories) free(loadable_categories);
// 重新附加（现在已扩展）分离的列表。
// 但如果没什么东西可以装，就把名单销毁。
if (used) {
loadable_categories = cats;
loadable_categories_used = used;
loadable_categories_allocated = allocated;
} else {
if (cats) free(cats);
loadable_categories = nil;
loadable_categories_used = 0;
loadable_categories_allocated = 0;
}
if (PrintLoading) {
if (loadable_categories_used != 0) {
_objc_inform("LOAD: %d categories still waiting for +load\n",
loadable_categories_used);
}
}
return new_categories_added;
}
总结
-
+load方法什么时候调用？
通知
dyld，由dyld进行调用。dyld通过load_images中的call_load_methods进行调用。
-
+load方法调用原理？
找到类和分类中的
+load方法的IMP，直接调用。
-
+load方法调用顺序？
我们发现：
-
+load方法是根据方法地址直接调用，并不是通过objc_msgSend函数调用。类、父类、分类+load方法相互独立。 - 在准备阶段，父类会先于子类被加载到列表中，所以会先被调用。
- 当调用
+load方法时是先调用本类中的+load方法，再调用分类中的+load方法。 -
+load方法只走一次。
例如我们有父类
Father、子类Son、分类Father (Test1)、Father (Test2)。+ (void)load { NSLog(@"%s", __FUNCTION__); } // 输出 Father +load Son +load Father (Test1) +load Father (Test2) +load可以看出当存在继承关系时默认首先调用父类的
+load方法，再调用子类的+load方法。在Build Settings中的Compile Sources，调整编译顺序会发现：
- 类：
+load方法按照编译先后顺序调用。即，先编译，先调用。 - 分类：
+load方法也是按照编译先后顺序调用。
所以总的调用顺序为：
先类的
+load，从最上层的父类开始一次调用+load方法，直到调用到本类的+load。下一个类的
+load，按照1中的方式调用。整体顺序按照编译先后顺序调用。直到所有类调用完。调用分类的
+load。按照编译先后顺序调用。
-
-
+load方法调用次数？
1次。
3. +initialize
这个问题其实之前在《Runtime消息、消息转发深入源码》已经瞥见过一些端倪了。
我们在调用objc_msgSend方法后，查找方法IMP的过程中会来到lookUpImpOrForward方法。我们这次的目的不是IMP，把代码简化一下：
IMP lookUpImpOrForward(Class cls, SEL sel, id inst,
bool initialize, bool cache, bool resolver)
{
...
// 判断类是否已经被创建，如果没有被创建，则将类实例化
if (slowpath(!cls->isRealized())) {
cls = realizeClassMaybeSwiftAndLeaveLocked(cls, runtimeLock);
// runtimeLock may have been dropped but is now locked again
}
// 第一次调用当前类的话，执行initialize的代码
if (slowpath((behavior & LOOKUP_INITIALIZE) && !cls->isInitialized())) {
// 对类进行初始化，并开辟内存空间
cls = initializeAndLeaveLocked(cls, inst, runtimeLock);
// runtimeLock may have been dropped but is now locked again
}
...
return imp;
}
我们看到，查找过程中会先查看类是否被实例化，如果没有则会实例化。然后判断是不是第一次调用、类是否调用过+initialize，如果没有则会对类进行初始化。
我们看一下它是怎么初始化的：
static Class initializeAndLeaveLocked(Class cls, id obj, mutex_t& lock)
{
return initializeAndMaybeRelock(cls, obj, lock, true);
}
static Class initializeAndMaybeRelock(Class cls, id inst,
mutex_t& lock, bool leaveLocked)
{
lock.assertLocked();
ASSERT(cls->isRealized());
if (cls->isInitialized()) {
if (!leaveLocked) lock.unlock();
return cls;
}
// Find the non-meta class for cls, if it is not already one.
// The +initialize message is sent to the non-meta class object.
Class nonmeta = getMaybeUnrealizedNonMetaClass(cls, inst);
// Realize the non-meta class if necessary.
if (nonmeta->isRealized()) {
// nonmeta is cls, which was already realized
// OR nonmeta is distinct, but is already realized
// - nothing else to do
lock.unlock();
} else {
nonmeta = realizeClassMaybeSwiftAndUnlock(nonmeta, lock);
// runtimeLock is now unlocked
// fixme Swift can't relocate the class today,
// but someday it will:
cls = object_getClass(nonmeta);
}
// runtimeLock is now unlocked, for +initialize dispatch
ASSERT(nonmeta->isRealized());
initializeNonMetaClass(nonmeta);
if (leaveLocked) runtimeLock.lock();
return cls;
}
由于+initialize是类方法，存在元类中需要保证元类已经实例化。我们再看看initializeNonMetaClass：
void initializeNonMetaClass(Class cls)
{
ASSERT(!cls->isMetaClass());
Class supercls;
bool reallyInitialize = NO;
// Make sure super is done initializing BEFORE beginning to initialize cls.
// See note about deadlock above.
// 确保父类的+initialize已经调用过了，否则先走父类
supercls = cls->superclass;
if (supercls && !supercls->isInitialized()) {
initializeNonMetaClass(supercls);
}
// Try to atomically set CLS_INITIALIZING.
SmallVector<_objc_willInitializeClassCallback, 1> localWillInitializeFuncs;
{
monitor_locker_t lock(classInitLock);
if (!cls->isInitialized() && !cls->isInitializing()) {
cls->setInitializing();
reallyInitialize = YES;
// Grab a copy of the will-initialize funcs with the lock held.
localWillInitializeFuncs.initFrom(willInitializeFuncs);
}
}
if (reallyInitialize) {
// We successfully set the CLS_INITIALIZING bit. Initialize the class.
// Record that we're initializing this class so we can message it.
_setThisThreadIsInitializingClass(cls);
if (MultithreadedForkChild) {
// LOL JK we don't really call +initialize methods after fork().
performForkChildInitialize(cls, supercls);
return;
}
for (auto callback : localWillInitializeFuncs)
callback.f(callback.context, cls);
// Send the +initialize message.
// Note that +initialize is sent to the superclass (again) if
// this class doesn't implement +initialize. 2157218
if (PrintInitializing) {
_objc_inform("INITIALIZE: thread %p: calling +[%s initialize]",
objc_thread_self(), cls->nameForLogging());
}
// Exceptions: A +initialize call that throws an exception
// is deemed to be a complete and successful +initialize.
//
// Only __OBJC2__ adds these handlers. !__OBJC2__ has a
// bootstrapping problem of this versus CF's call to
// objc_exception_set_functions().
#if __OBJC2__
@try
#endif
{
// 核心重点
callInitialize(cls);
if (PrintInitializing) {
_objc_inform("INITIALIZE: thread %p: finished +[%s initialize]",
objc_thread_self(), cls->nameForLogging());
}
}
#if __OBJC2__
@catch (...) {
if (PrintInitializing) {
_objc_inform("INITIALIZE: thread %p: +[%s initialize] "
"threw an exception",
objc_thread_self(), cls->nameForLogging());
}
@throw;
}
@finally
#endif
{
// Done initializing.
lockAndFinishInitializing(cls, supercls);
}
return;
}
else if (cls->isInitializing()) {
// We couldn't set INITIALIZING because INITIALIZING was already set.
// If this thread set it earlier, continue normally.
// If some other thread set it, block until initialize is done.
// It's ok if INITIALIZING changes to INITIALIZED while we're here,
// because we safely check for INITIALIZED inside the lock
// before blocking.
if (_thisThreadIsInitializingClass(cls)) {
return;
} else if (!MultithreadedForkChild) {
waitForInitializeToComplete(cls);
return;
} else {
// We're on the child side of fork(), facing a class that
// was initializing by some other thread when fork() was called.
_setThisThreadIsInitializingClass(cls);
performForkChildInitialize(cls, supercls);
}
}
else if (cls->isInitialized()) {
// Set CLS_INITIALIZING failed because someone else already
// initialized the class. Continue normally.
// NOTE this check must come AFTER the ISINITIALIZING case.
// Otherwise: Another thread is initializing this class. ISINITIALIZED
// is false. Skip this clause. Then the other thread finishes
// initialization and sets INITIALIZING=no and INITIALIZED=yes.
// Skip the ISINITIALIZING clause. Die horribly.
return;
}
else {
// We shouldn't be here.
_objc_fatal("thread-safe class init in objc runtime is buggy!");
}
}
我们看到首先确保父类已经执行过+initialize，之后看到我们的核心方法callInitialize：
void callInitialize(Class cls)
{
((void(*)(Class, SEL))objc_msgSend)(cls, @selector(initialize));
asm("");
}
通过objc_msgSend给类发送@selector(initialize)消息。
总结
-
+initialize方法什么时候调用？
第一次收到消息的时候调用。
-
+initialize方法调用原理？
第一次收到消息的时候，确保类和元类都实例化，且没有调用过
+initialize方法。通过objc_msgSend发送@selector(initialize)消息，进行调用。
-
+initialize方法调用顺序？
先调用父类的
+initialize方法，再调用子类的+initialize方法。
-
+initialize方法调用次数？
1次。
