在app启动到main之前这一阶段，先是利用dyld进行加载动态库，各种映射。这部分的内容，可以阅读文章应用加载。
接着就进入到runtime层中的_objc_init方法。通过该方法，主要完成类的加载等重要工作。
先放出源码。
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(); //【读取环境变量】
tls_init(); //【线程key的绑定】
static_init(); //【初始化系统内置的C++静态构造函数】
lock_init(); //【空方法，说明直接使用C++的锁机制】
exception_init(); //【初始化libobjc的异常处理机制，由map_images调用】
//！！【重点在这里】
_dyld_objc_notify_register(&map_images, load_images, unmap_image);
}
开始重点解析_dyld_objc_notify_register。
map_images
map_images，用于加载资源模块。贴上源码。
/***********************************************************************
* map_images
* Process the given images which are being mapped in by dyld.
* Calls ABI-agnostic code after taking ABI-specific locks.
*
* Locking: write-locks runtimeLock
**********************************************************************/
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);
}
处理已经由dyld镜像mapped的资源模块。进入到map_images_nolock。
void
map_images_nolock(unsigned mhCount, const char * const mhPaths[],
const struct mach_header * const mhdrs[])
{
.....//读取文件信息 并统计
//！！【重点在这里，读取资源模块】
if (hCount > 0) {
_read_images(hList, hCount, totalClasses, unoptimizedTotalClasses);
}
firstTime = NO;
}
重点来到_read_images。代码太多，按照顺序和功能拆分。
doneOnce
if (!doneOnce) { //【👇只会执行一次】
doneOnce = YES;
#if SUPPORT_NONPOINTER_ISA
// Disable non-pointer isa under some conditions.
//【如果支持内存优化的isa，判断哪些场景下是不能使用的】
# if SUPPORT_INDEXED_ISA
// Disable nonpointer isa if any image contains old Swift code
//【Swift 3.0之前不能使用nonpointer isa】
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 //【判断mac os上】
// 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.
//【已经优化过的类，不会在这】
// 4/3 is NXMapTable's load factor
int namedClassesSize = (isPreoptimized() ? unoptimizedTotalClasses : totalClasses) * 4 / 3;
gdb_objc_realized_classes = NXCreateMapTable(NXStrValueMapPrototype, namedClassesSize);
allocatedClasses = NXCreateHashTable(NXPtrPrototype, 0, nil);
ts.log("IMAGE TIMES: first time tasks");
}
这里有两张表，gdb_objc_realized_classes 和 allocatedClasses。
// This is a misnomer(误称): gdb_objc_realized_classes is actually a list of
// named classes not in the dyld shared cache, whether realized or not.
NXMapTable *gdb_objc_realized_classes; // exported for debuggers in objc-gdb.h
gdb_objc_realized_classes中实际存放的是不在动态分享缓存库中命名的类，不管该类有没有实现。相当于类的总表。
/***********************************************************************
* allocatedClasses
* A table of all classes (and metaclasses) which have been allocated
* with objc_allocateClassPair.
**********************************************************************/
static NXHashTable *allocatedClasses = nil;
allocatedClasses中存放已经通过objc_allocateClassPair开辟内存的类和元类。
gdb_objc_realized_classes包含allocatedClasses。
discover classes
// Discover classes. Fix up unresolved future classes. Mark bundle classes.
//【发现类。修正没有解析的future类。标记bundle类】
for (EACH_HEADER) {
//【取出所有类】
classref_t *classlist = _getObjc2ClassList(hi, &count);
if (! mustReadClasses(hi)) { //【已经优化过的资源，不需要再read】
// Image is sufficiently optimized that we need not call readClass()
continue;
}
//【是否是bundle、是否已经优化过】
bool headerIsBundle = hi->isBundle();
bool headerIsPreoptimized = hi->isPreoptimized();
//【遍历读取类信息】
for (i = 0; i < count; i++) {
Class cls = (Class)classlist[i];
Class newCls = readClass(cls, headerIsBundle, headerIsPreoptimized);
//【cls不相等，且newCls不为空，重新开辟空间，放到resolvedFutureClasses中】
if (newCls != cls && newCls) {
// Class was moved but not deleted. Currently this occurs
// only when the new class resolved a future class.
// Non-lazily realize the class below.
//
resolvedFutureClasses = (Class *)realloc(resolvedFutureClasses, (resolvedFutureClassCount+1) * sizeof(Class));
resolvedFutureClasses[resolvedFutureClassCount++] = newCls;
}
}
}
Fix up remapped classes
修复重映射类
// Fix up remapped classes
// Class list and nonlazy class list remain unremapped.
// Class refs and super refs are remapped for message dispatching.
if (!noClassesRemapped()) {
for (EACH_HEADER) {
//【重新映射 类和非懒加载类】
Class *classrefs = _getObjc2ClassRefs(hi, &count);
for (i = 0; i < count; i++) {
remapClassRef(&classrefs[i]);
}
// fixme why doesn't test future1 catch the absence of this?
//【由于消息转发，类引用 和 父类引用 也会被重新映射 】
classrefs = _getObjc2SuperRefs(hi, &count);
for (i = 0; i < count; i++) {
remapClassRef(&classrefs[i]);
}
}
}
Fix up @selector references
修复@selector引用
// Fix up @selector references
static size_t UnfixedSelectors;
{
mutex_locker_t lock(selLock); //【加锁】
for (EACH_HEADER) {
if (hi->isPreoptimized()) 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]);
//【注册sel】
sels[i] = sel_registerNameNoLock(name, isBundle);
}
}
}
继续看下方法是怎么注册的。sel_registerNameNoLock -> __sel_registerName
static SEL __sel_registerName(const char *name, bool shouldLock, bool copy)
{
SEL result = 0;
if (shouldLock) selLock.assertUnlocked();
else selLock.assertLocked();
if (!name) return (SEL)0;
result = search_builtins(name); //【先根据name从builtins中查找】
if (result) return result;
conditional_mutex_locker_t lock(selLock, shouldLock);
//【从namedSelectors中查找】
if (namedSelectors) {
result = (SEL)NXMapGet(namedSelectors, name);
}
if (result) return result;
// No match. Insert.
//【找不到，注册】
if (!namedSelectors) {
namedSelectors = NXCreateMapTable(NXStrValueMapPrototype,
(unsigned)SelrefCount);
}
if (!result) {
result = sel_alloc(name, copy);
// fixme choose a better container (hash not map for starters)
NXMapInsert(namedSelectors, sel_getName(result), result);
}
return result;
}
Fix up old objc_msgSend_fixup
如果支持FIXUP，则修复消息引用。
#if SUPPORT_FIXUP
// Fix up old objc_msgSend_fixup call sites
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++) {
fixupMessageRef(refs+i);
}
}
ts.log("IMAGE TIMES: fix up objc_msgSend_fixup");
#endif
Discover protocols
发现协议，修复协议引用，主要是读取协议。
// Discover protocols. Fix up protocol refs.
for (EACH_HEADER) {
extern objc_class OBJC_CLASS_$_Protocol;
Class cls = (Class)&OBJC_CLASS_$_Protocol;
assert(cls);
NXMapTable *protocol_map = protocols();
bool isPreoptimized = hi->isPreoptimized();
bool isBundle = hi->isBundle();
protocol_t **protolist = _getObjc2ProtocolList(hi, &count);
for (i = 0; i < count; i++) {
readProtocol(protolist[i], cls, protocol_map, isPreoptimized, isBundle);
}
}
Fix up @protocol references
重映射协议引用。
// Fix up @protocol references
// Preoptimized images may have the right
// answer already but we don't know for sure.
for (EACH_HEADER) {
protocol_t **protolist = _getObjc2ProtocolRefs(hi, &count);
for (i = 0; i < count; i++) {
remapProtocolRef(&protolist[i]);
}
}
Realize non-lazy classes
初始化非懒加载的类。非懒加载的类指的是实现了load方法或者带有静态实例的类。后面将讲述一下非懒加载的类和懒加载的类。
// Realize non-lazy classes (for +load methods and static instances)
for (EACH_HEADER) {
classref_t *classlist = _getObjc2NonlazyClassList(hi, &count);
for (i = 0; i < count; i++) {
Class cls = remapClass(classlist[i]);
if (!cls) continue;
// hack for class __ARCLite__, which didn't get this above
#if TARGET_OS_SIMULATOR //【os模拟器】
if (cls->cache._buckets == (void*)&_objc_empty_cache &&
(cls->cache._mask || cls->cache._occupied))
{
cls->cache._mask = 0;
cls->cache._occupied = 0;
}
if (cls->ISA()->cache._buckets == (void*)&_objc_empty_cache &&
(cls->ISA()->cache._mask || cls->ISA()->cache._occupied))
{
cls->ISA()->cache._mask = 0;
cls->ISA()->cache._occupied = 0;
}
#endif
addClassTableEntry(cls);
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); //【重点，涉及到初始化，后面讲述】
}
}
Realize newly-resolved future classes
初始化新解析的类。内容来源于 discover classes。
// Realize newly-resolved future classes, in case CF manipulates them
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); //【此处同上】
cls->setInstancesRequireRawIsa(false/*inherited*/);
}
free(resolvedFutureClasses);
}
Discover categories
发现分类。
// Discover categories.
for (EACH_HEADER) {
//【分类列表】
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);
if (!cls) { //【分类的原有类，没有被重映射】
// 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. 【开始处理分类】
// First, register the category with its target class.
// Then, rebuild the class's method lists (etc) if
// the class is realized.
bool classExists = NO;
//【分类中的实例方法、协议和属性】
if (cat->instanceMethods || cat->protocols || cat->instanceProperties)
{
addUnattachedCategoryForClass(cat, cls, hi);//【查看代码，见后文，只是用于记录一下没有被附加的分类】
if (cls->isRealized()) {
remethodizeClass(cls);//【处理分类和类对象】
classExists = YES;
}
if (PrintConnecting) {
_objc_inform("CLASS: found category -%s(%s) %s", cls->nameForLogging(), cat->name, classExists ? "on existing class" : "");
}
}
//【分类中的类方法、协议等】
if (cat->classMethods || cat->protocols
|| (hasClassProperties && cat->_classProperties))
{
addUnattachedCategoryForClass(cat, cls->ISA(), hi);//【添加到元类中】
if (cls->ISA()->isRealized()) {
remethodizeClass(cls->ISA());//【处理分类和元类】
}
if (PrintConnecting) {
_objc_inform("CLASS: found category +%s(%s)",
cls->nameForLogging(), cat->name);
}
}
}
}
后面有一段注释，也值得一看，讲述的是category的加载必须放在最后，避免出现可能的线程资源竞争问题。
// Category discovery MUST BE LAST to avoid potential races
// when other threads call the new category code before
// this thread finishes its fixups.
// +load handled by prepare_load_methods()
重点方法
realizeClassWithoutSwift
真正的初始化类结构，蛀牙是rw和ro
/***********************************************************************
* realizeClassWithoutSwift
* Performs first-time initialization on class cls,
* including allocating its read-write data.
* Does not perform any Swift-side initialization.
* Returns the real class structure for the class.
* Locking: runtimeLock must be write-locked by the caller
**********************************************************************/
static Class realizeClassWithoutSwift(Class cls)
{
runtimeLock.assertLocked();
const class_ro_t *ro;
class_rw_t *rw;
Class supercls;
Class metacls;
bool isMeta;
if (!cls) return nil;
if (cls->isRealized()) return cls;
assert(cls == remapClass(cls));
// fixme verify class is not in an un-dlopened part of the shared cache?
ro = (const class_ro_t *)cls->data();
if (ro->flags & RO_FUTURE) {
// This was a future class. rw data is already allocated.
rw = cls->data();
ro = cls->data()->ro;
cls->changeInfo(RW_REALIZED|RW_REALIZING, RW_FUTURE);
} else {
// Normal class. Allocate writeable class data.
rw = (class_rw_t *)calloc(sizeof(class_rw_t), 1);
rw->ro = ro;
rw->flags = RW_REALIZED|RW_REALIZING;
cls->setData(rw);
}
isMeta = ro->flags & RO_META;
rw->version = isMeta ? 7 : 0; // old runtime went up to 6
// Choose an index for this class.
// Sets cls->instancesRequireRawIsa if indexes no more indexes are available
cls->chooseClassArrayIndex();
if (PrintConnecting) {
_objc_inform("CLASS: realizing class '%s'%s %p %p #%u %s%s",
cls->nameForLogging(), isMeta ? " (meta)" : "",
(void*)cls, ro, cls->classArrayIndex(),
cls->isSwiftStable() ? "(swift)" : "",
cls->isSwiftLegacy() ? "(pre-stable swift)" : "");
}
// Realize superclass and metaclass, if they aren't already.
// This needs to be done after RW_REALIZED is set above, for root classes.
// This needs to be done after class index is chosen, for root metaclasses.
// This assumes that none of those classes have Swift contents,
// or that Swift's initializers have already been called.
// fixme that assumption will be wrong if we add support
// for ObjC subclasses of Swift classes.
// ！！！【父类和元类】
supercls = realizeClassWithoutSwift(remapClass(cls->superclass));
metacls = realizeClassWithoutSwift(remapClass(cls->ISA()));
#if SUPPORT_NONPOINTER_ISA
// Disable non-pointer isa for some classes and/or platforms.
// Set instancesRequireRawIsa.
bool instancesRequireRawIsa = cls->instancesRequireRawIsa();
bool rawIsaIsInherited = false;
static bool hackedDispatch = false;
if (DisableNonpointerIsa) {
// Non-pointer isa disabled by environment or app SDK version
instancesRequireRawIsa = true;
}
else if (!hackedDispatch && !(ro->flags & RO_META) &&
0 == strcmp(ro->name, "OS_object"))
{
// hack for libdispatch et al - isa also acts as vtable pointer
hackedDispatch = true;
instancesRequireRawIsa = true;
}
else if (supercls && supercls->superclass &&
supercls->instancesRequireRawIsa())
{
// This is also propagated by addSubclass()
// but nonpointer isa setup needs it earlier.
// Special case: instancesRequireRawIsa does not propagate
// from root class to root metaclass
instancesRequireRawIsa = true;
rawIsaIsInherited = true;
}
if (instancesRequireRawIsa) {
cls->setInstancesRequireRawIsa(rawIsaIsInherited);
}
// SUPPORT_NONPOINTER_ISA
#endif
// Update superclass and metaclass in case of remapping
cls->superclass = supercls;
cls->initClassIsa(metacls);
// Reconcile instance variable offsets / layout.
// This may reallocate class_ro_t, updating our ro variable.
if (supercls && !isMeta) reconcileInstanceVariables(cls, supercls, ro);
// Set fastInstanceSize if it wasn't set already.
cls->setInstanceSize(ro->instanceSize);
// Copy some flags from ro to rw
if (ro->flags & RO_HAS_CXX_STRUCTORS) {
cls->setHasCxxDtor();
if (! (ro->flags & RO_HAS_CXX_DTOR_ONLY)) {
cls->setHasCxxCtor();
}
}
// Propagate the associated objects forbidden flag from ro or from
// the superclass.
if ((ro->flags & RO_FORBIDS_ASSOCIATED_OBJECTS) ||
(supercls && supercls->forbidsAssociatedObjects()))
{
rw->flags |= RW_FORBIDS_ASSOCIATED_OBJECTS;
}
// Connect this class to its superclass's subclass lists
if (supercls) {
addSubclass(supercls, cls);
} else {
addRootClass(cls);
}
// Attach categories
methodizeClass(cls);
return cls;
}
methodizeClass
主要是修复类本身的方法列表、协议列表和属性列表。
/***********************************************************************
* methodizeClass
* Fixes up cls's method list, protocol list, and property list.
* Attaches any outstanding categories.
* Locking: runtimeLock must be held by the caller
**********************************************************************/
static void methodizeClass(Class cls)
{
runtimeLock.assertLocked();
bool isMeta = cls->isMetaClass();
auto rw = cls->data();
auto ro = rw->ro;
// Methodizing for the first time
if (PrintConnecting) {
_objc_inform("CLASS: methodizing class '%s' %s",
cls->nameForLogging(), isMeta ? "(meta)" : "");
}
// Install methods and properties that the class implements itself.
method_list_t *list = ro->baseMethods();
if (list) {
prepareMethodLists(cls, &list, 1, YES, isBundleClass(cls));
rw->methods.attachLists(&list, 1);
}
property_list_t *proplist = ro->baseProperties;
if (proplist) {
rw->properties.attachLists(&proplist, 1);
}
protocol_list_t *protolist = ro->baseProtocols;
if (protolist) {
rw->protocols.attachLists(&protolist, 1);
}
// Root classes get bonus method implementations if they don't have
// them already. These apply before category replacements.
if (cls->isRootMetaclass()) {
// root metaclass
addMethod(cls, SEL_initialize, (IMP)&objc_noop_imp, "", NO);
}
// Attach categories.
category_list *cats = unattachedCategoriesForClass(cls, true /*realizing*/);
attachCategories(cls, cats, false /*don't flush caches*/);
if (cats) free(cats);
}
addUnattachedCategoryForClass
主要是记录category。真正的附加操作在remethodizeClass。
/***********************************************************************
* addUnattachedCategoryForClass
* Records an unattached category.
* Locking: runtimeLock must be held by the caller.
**********************************************************************/
static void addUnattachedCategoryForClass(category_t *cat, Class cls,
header_info *catHeader)
{
runtimeLock.assertLocked();
// DO NOT use cat->cls! cls may be cat->cls->isa instead
NXMapTable *cats = unattachedCategories();
category_list *list;
list = (category_list *)NXMapGet(cats, cls);
if (!list) {
list = (category_list *)
calloc(sizeof(*list) + sizeof(list->list[0]), 1);
} else {
list = (category_list *)
realloc(list, sizeof(*list) + sizeof(list->list[0]) * (list->count + 1));
}
list->list[list->count++] = (locstamped_category_t){cat, catHeader};
NXMapInsert(cats, cls, list);
}
remethodizeClass
将categories附加到已经存在的类上，并修复方法列表、协议列表和属性列表。这个方法在发现category时调用。而methodizeClass这个方法，是在类初始化的时候调用。
/***********************************************************************
* remethodizeClass
* Attach outstanding categories to an existing class.
* Fixes up cls's method list, protocol list, and property list.
* Updates method caches for cls and its subclasses.
* Locking: runtimeLock must be held by the caller
**********************************************************************/
static void remethodizeClass(Class cls)
{
category_list *cats;
bool isMeta;
runtimeLock.assertLocked();
isMeta = cls->isMetaClass();
// Re-methodizing: check for more categories
if ((cats = unattachedCategoriesForClass(cls, false/*not realizing*/))) {
if (PrintConnecting) {
_objc_inform("CLASS: attaching categories to class '%s' %s",
cls->nameForLogging(), isMeta ? "(meta)" : "");
}
attachCategories(cls, cats, true /*flush caches*/);
free(cats);
}
}
attachCategories
将category中的方法列表、属性和协议附加到类中。
// Attach method lists and properties and protocols from categories to a class.
// Assumes the categories in cats are all loaded and sorted by load order,
// oldest categories first.
static void
attachCategories(Class cls, category_list *cats, bool flush_caches)
{
if (!cats) return;
if (PrintReplacedMethods) printReplacements(cls, cats);
bool isMeta = cls->isMetaClass();
// fixme rearrange to remove these intermediate allocations
//【初始化二维数组】
method_list_t **mlists = (method_list_t **)
malloc(cats->count * sizeof(*mlists));
property_list_t **proplists = (property_list_t **)
malloc(cats->count * sizeof(*proplists));
protocol_list_t **protolists = (protocol_list_t **)
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;//【分类倒序遍历，涉及到的问题是同样的a方法，分类1和分类2哪个先调用】
bool fromBundle = NO;
while (i--) {
auto& entry = cats->list[i];
method_list_t *mlist = entry.cat->methodsForMeta(isMeta);
if (mlist) {
mlists[mcount++] = mlist;
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();
prepareMethodLists(cls, mlists, mcount, NO, fromBundle);
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);
}
attachLists
兼容多种场景。
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)));
array()->count = newCount;
//【重点位置，先后移原有类，然后copy分类中的内容】
memmove(array()->lists + addedCount, array()->lists,
oldCount * sizeof(array()->lists[0]));
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]));
}
}
这段代码就涉及到category的常用问题，为什么调用的是category中的方法。
讲完类加载，继续看_dyld_objc_notify_register中的第二个参数load_images。
load_images
直接贴源码。
根据注释可以看到，这个方法主要是处理已经被映射过的带有+load方法的资源。
/***********************************************************************
* 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);
//！！！【重点位置】
prepare_load_methods((const headerType *)mh);
}
// Call +load methods (without runtimeLock - re-entrant)
call_load_methods();//【调用】
}
prepare_load_methods
void prepare_load_methods(const headerType *mhdr)
{
size_t count, i;
runtimeLock.assertLocked();
//【先获取非懒加载的类列表】
classref_t *classlist =
_getObjc2NonlazyClassList(mhdr, &count);
for (i = 0; i < count; i++) {
//【先加入类的load】
schedule_class_load(remapClass(classlist[i]));
}
//【获取非懒加载的分类】
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
if (cls->isSwiftStable()) {
_objc_fatal("Swift class extensions and categories on Swift "
"classes are not allowed to have +load methods");
}
realizeClassWithoutSwift(cls);
assert(cls->ISA()->isRealized());
add_category_to_loadable_list(cat);
}
}
schedule_class_load
/***********************************************************************
* prepare_load_methods
* Schedule +load for classes in this image, any un-+load-ed
* superclasses in other images, and any categories in this image.
**********************************************************************/
// Recursively schedule +load for cls and any un-+load-ed superclasses.
// cls must already be connected.
static void schedule_class_load(Class cls)
{
if (!cls) return;
assert(cls->isRealized()); // _read_images should realize
if (cls->data()->flags & RW_LOADED) return;
// Ensure superclass-first ordering
//【确保父类先调用，常见面试问题】
schedule_class_load(cls->superclass);
add_class_to_loadable_list(cls);
cls->setInfo(RW_LOADED);
}
add_class_to_loadable_list
将可以执行load方法的cls，加入到loadable_classes。
/***********************************************************************
* add_class_to_loadable_list
* Class cls has just become connected. Schedule it for +load if
* it implements a +load method.
**********************************************************************/
void add_class_to_loadable_list(Class cls)
{
IMP method;
loadMethodLock.assertLocked();
method = cls->getLoadMethod();
if (!method) return; // Don't bother if cls has no +load method
if (PrintLoading) {
_objc_inform("LOAD: class '%s' scheduled for +load",
cls->nameForLogging());
}
if (loadable_classes_used == loadable_classes_allocated) {
loadable_classes_allocated = loadable_classes_allocated*2 + 16;
loadable_classes = (struct loadable_class *)
realloc(loadable_classes,
loadable_classes_allocated *
sizeof(struct loadable_class));
}
loadable_classes[loadable_classes_used].cls = cls;
loadable_classes[loadable_classes_used].method = method;
loadable_classes_used++;
}
add_category_to_loadable_list
将可以执行load方法的category加入到loadable_categories。
/***********************************************************************
* add_category_to_loadable_list
* Category cat's parent class exists and the category has been attached
* to its class. Schedule this category for +load after its parent class
* becomes connected and has its own +load method called.
**********************************************************************/
void add_category_to_loadable_list(Category cat)
{
IMP method;
loadMethodLock.assertLocked();
method = _category_getLoadMethod(cat);
// Don't bother if cat has no +load method
if (!method) return;
if (PrintLoading) {
_objc_inform("LOAD: category '%s(%s)' scheduled for +load",
_category_getClassName(cat), _category_getName(cat));
}
if (loadable_categories_used == loadable_categories_allocated) {
loadable_categories_allocated = loadable_categories_allocated*2 + 16;
loadable_categories = (struct loadable_category *)
realloc(loadable_categories,
loadable_categories_allocated *
sizeof(struct loadable_category));
}
loadable_categories[loadable_categories_used].cat = cat;
loadable_categories[loadable_categories_used].method = method;
loadable_categories_used++;
}
通过prepare_load_methods以及内部的方法，按照父类 -> 类 -> 分类的顺序，将load方法进行收集。接下来就是调用call_load_methods。
call_load_methods
/***********************************************************************
* 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(); //【先是class】
}
// 2. Call category +loads ONCE
more_categories = call_category_loads(); //【然后是category】
// 3. Run more +loads if there are classes OR more untried categories
} while (loadable_classes_used > 0 || more_categories);
objc_autoreleasePoolPop(pool);
loading = NO;
}
call_class_loads
/***********************************************************************
* call_class_loads
* Call all pending class +load methods.
* If new classes become loadable, +load is NOT called for them.
*
* Called only by call_load_methods().
**********************************************************************/
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.
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, SEL_load);
}
// Destroy the detached list.
if (classes) free(classes);
}
call_category_loads
/***********************************************************************
* call_category_loads
* Call some pending category +load methods.
* The parent class of the +load-implementing categories has all of
* its categories attached, in case some are lazily waiting for +initalize.
* Don't call +load unless the parent class is connected.
* If new categories become loadable, +load is NOT called, and they
* are added to the end of the loadable list, and we return TRUE.
* Return FALSE if no new categories became loadable.
*
* Called only by call_load_methods().
**********************************************************************/
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.
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, SEL_load);
cats[i].cat = nil;
}
}
// Compact detached list (order-preserving)
shift = 0;
for (i = 0; i < used; i++) {
if (cats[i].cat) {
cats[i-shift] = cats[i];
} else {
shift++;
}
}
used -= shift;
// Copy any new +load candidates from the new list to the detached list.
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];
}
// Destroy the new list.
if (loadable_categories) free(loadable_categories);
// Reattach the (now augmented) detached list.
// But if there's nothing left to load, destroy the list.
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;
}
提到了load方法，不得不提一下initialize方法，见👇。
unmap_image
处理没有被映射的资源。
/***********************************************************************
* unmap_image
* Process the given image which is about to be unmapped by dyld.
*
* Locking: write-locks runtimeLock and loadMethodLock
**********************************************************************/
void
unmap_image(const char *path __unused, const struct mach_header *mh)
{
recursive_mutex_locker_t lock(loadMethodLock);
mutex_locker_t lock2(runtimeLock);
unmap_image_nolock(mh);
}
unmap_image_nolock
/***********************************************************************
* unmap_image_nolock
* Process the given image which is about to be unmapped by dyld.
* mh is mach_header instead of headerType because that's what
* dyld_priv.h says even for 64-bit.
*
* Locking: loadMethodLock(both) and runtimeLock(new) acquired by unmap_image.
**********************************************************************/
void
unmap_image_nolock(const struct mach_header *mh)
{
if (PrintImages) {
_objc_inform("IMAGES: processing 1 newly-unmapped image...\n");
}
header_info *hi;
// Find the runtime's header_info struct for the image
for (hi = FirstHeader; hi != NULL; hi = hi->getNext()) {
if (hi->mhdr() == (const headerType *)mh) {
break;
}
}
if (!hi) return;
if (PrintImages) {
_objc_inform("IMAGES: unloading image for %s%s%s\n",
hi->fname(),
hi->mhdr()->filetype == MH_BUNDLE ? " (bundle)" : "",
hi->info()->isReplacement() ? " (replacement)" : "");
}
_unload_image(hi);
// Remove header_info from header list
removeHeader(hi);
free(hi);
}
其他
懒加载类和非懒加载类
实现load方法和带有静态实例的类。这个类因为带有load方法，而load方法要在main之前调用，所以肯定要提前初始化，所以称为非懒加载类。相应的，懒加载类只有当第一次发送消息的时候，才会去初始化。 关于懒加载和非懒加载对应的类和分类中的rw和ro的情况，参考带你了解分类的加载流程。
+initialize
什么时候会触发这个方法呢？当这个类被发送第一条消息的时候，会先调用initialize。所以，查看【标准的IMP查找流程】代码。
//【直接看关键点】
if (initialize && !cls->isInitialized()) {
cls = initializeAndLeaveLocked(cls, inst, runtimeLock);
// runtimeLock may have been dropped but is now locked again
// If sel == initialize, class_initialize will send +initialize and
// then the messenger will send +initialize again after this
// procedure finishes. Of course, if this is not being called
// from the messenger then it won't happen. 2778172
}
initializeAndLeaveLocked -> initializeAndMaybeRelock -> initializeNonMetaClass。
/***********************************************************************
* class_initialize. Send the '+initialize' message on demand to any
* uninitialized class. Force initialization of superclasses first.
**********************************************************************/
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.
//！！！！【优先父类】
supercls = cls->superclass;
if (supercls && !supercls->isInitialized()) {
initializeNonMetaClass(supercls);
}
...//省略，直接看重点语句
callInitialize(cls);
...//
}
void callInitialize(Class cls)
{
((void(*)(Class, SEL))objc_msgSend)(cls, SEL_initialize);
asm("");
}
load和initialize
load方法的调用顺序：父类、子类、分类.
多个分类中load方法的执行顺序取决于编译顺序
每个类的load方法只会调用一次。
不会出现子类覆盖父类的场景。
因为在调用时，直接通过函数内存地址`(*load_method)(cls, SEL_load)`。
相应的，如果子类没有+load方法，也不会出现打印两次父类+load方法内容的场景，只会打印父类的load。
