objc庫中初始化方法如下
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();//初始化運行環(huán)境
tls_init();
static_init();//初始化靜態(tài)變量方法
runtime_init();//初始化運行時環(huán)境
exception_init();//初始化異常
cache_init();//初始化緩存
_imp_implementationWithBlock_init();
_dyld_objc_notify_register(&map_images, load_images, unmap_image);
#if __OBJC2__
didCallDyldNotifyRegister = true;
#endif
}
其中包含了一些列的初始化，以及image的映射和image的加載。
其中 _dyld_objc_notify_register(&map_images, load_images, unmap_image);是供dyld做回調(diào)使用，其中&map_images是方法是處理image的映射的，
map_images函數(shù)最最終會調(diào)用 runtime-new.mm中_read_images的函數(shù)，
其中_read_images函數(shù)的調(diào)用邏輯如下
_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;//控制{}里面的代碼只執(zhí)行一次
launchTime = YES;
#if SUPPORT_NONPOINTER_ISA
// Disable non-pointer isa under some conditions.
# if SUPPORT_INDEXED_ISA //如果庫中有舊版本swift代碼 則不支持isa指針優(yōu)化
// 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 //如果是 OS X 10.11之前的系統(tǒng)，不支持isa指針優(yōu)化
// 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) {
//經(jīng)過前面的判斷，如果不支持指針優(yōu)化，
disableTaggedPointers();
}
//tagged pointer混淆，讓指針地址對開發(fā)者看起來像是正常的內(nèi)存地址
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
//創(chuàng)建存儲已識別類的哈希表
//只會執(zhí)行一次
int namedClassesSize =
(isPreoptimized() ? unoptimizedTotalClasses : totalClasses) * 4 / 3;
gdb_objc_realized_classes =
NXCreateMapTable(NXStrValueMapPrototype, namedClassesSize);
ts.log("IMAGE TIMES: first time tasks");
}
// Fix up @selector references
#pragma mark // 注冊方法
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);//獲取可執(zhí)行文件中待注冊的方法
for (i = 0; i < count; i++) {
const char *name = sel_cname(sels[i]);
SEL sel = sel_registerNameNoLock(name, isBundle);//注冊方法
if (sels[i] != sel) {
sels[i] = sel;
}
}
}
}
ts.log("IMAGE TIMES: fix up selector references");
#pragma mark 類
// Discover classes. Fix up unresolved future classes. Mark bundle classes.
bool hasDyldRoots = dyld_shared_cache_some_image_overridden();
for (EACH_HEADER) {
if (! mustReadClasses(hi, hasDyldRoots)) {
// Image is sufficiently optimized that we need not call readClass() 判斷是否需要執(zhí)行readclass()方法
continue;
}
classref_t const *classlist = _getObjc2ClassList(hi, &count);
bool headerIsBundle = hi->isBundle();
bool headerIsPreoptimized = hi->hasPreoptimizedClasses();
for (i = 0; i < count; i++) {
Class cls = (Class)classlist[i];
Class newCls = readClass(cls, headerIsBundle, headerIsPreoptimized);
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.
//如果readClass返回的結(jié)果不是原來的class，則將class放入帶解決的class列表
resolvedFutureClasses = (Class *)
realloc(resolvedFutureClasses,
(resolvedFutureClassCount+1) * sizeof(Class));
resolvedFutureClasses[resolvedFutureClassCount++] = newCls;//已解決的futurec class
}
}
}
ts.log("IMAGE TIMES: discover classes");
// Fix up remapped classes
// Class list and nonlazy class list remain unremapped.
// Class refs and super refs are remapped for message dispatching.
// ref是源文件，才用了ASLR (Address Space Layout Randomization) 地址空間布局隨機化技術(shù)
// 所以要映射類的地址，才可以找到真實地址
if (!noClassesRemapped()) {//如果存在類沒有被影射到，則重新影射
for (EACH_HEADER) {
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?
classrefs = _getObjc2SuperRefs(hi, &count);
for (i = 0; i < count; i++) {
remapClassRef(&classrefs[i]);
}
}
}
ts.log("IMAGE TIMES: remap classes");
#if SUPPORT_FIXUP
#pragma mark // Fix up old objc_msgSend_fixup call sites 修正引用計數(shù)和消息發(fā)送相關(guān)的方法
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
#pragma mark 協(xié)議
bool cacheSupportsProtocolRoots = sharedCacheSupportsProtocolRoots();
// Discover protocols. Fix up protocol refs. 影射協(xié)議和影射協(xié)議的修正
for (EACH_HEADER) {
extern objc_class OBJC_CLASS_$_Protocol;
Class cls = (Class)&OBJC_CLASS_$_Protocol;
ASSERT(cls);
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) { //如果是在運行過程中，而且是動態(tài)庫的協(xié)議，則跳過，不加載該類的協(xié)議
if (PrintProtocols) {
_objc_inform("PROTOCOLS: Skipping reading protocols in image: %s",
hi->fname());
}
continue;
}
bool isBundle = hi->isBundle();
//
protocol_t * const *protolist = _getObjc2ProtocolList(hi, &count);
for (i = 0; i < count; i++) {
readProtocol(protolist[i], cls, protocol_map,
isPreoptimized, isBundle);//這一步會把符合條件的協(xié)議（之前不存在同名的協(xié)議）添加到協(xié)議列表
}
}
ts.log("IMAGE TIMES: discover protocols");
// Fix up @protocol references
// Preoptimized images may have the right
// answer already but we don't know for sure.
for (EACH_HEADER) {
// At launch time, we know preoptimized image refs are pointing at the
// shared cache definition of a protocol. We can skip the check on
// launch, but have to visit @protocol refs for shared cache images
// loaded later.
if (launchTime && cacheSupportsProtocolRoots && hi->isPreoptimized())
continue;
protocol_t **protolist = _getObjc2ProtocolRefs(hi, &count);
for (i = 0; i < count; i++) {
remapProtocolRef(&protolist[i]);//重新影射一次協(xié)議。如果協(xié)議中存在不符合條件的協(xié)議，統(tǒng)計出其中的數(shù)量
}
}
ts.log("IMAGE TIMES: fix up @protocol references");
// Discover categories. Only do this after the initial category
// attachment has been done. For categories present at startup,
// discovery is deferred until the first load_images call after
// the call to _dyld_objc_notify_register completes. rdar://problem/53119145
if (didInitialAttachCategories) {//等所有的分類綁定完以后才影射
for (EACH_HEADER) {
load_categories_nolock(hi);//把分類里面的內(nèi)容添加到類里面
}
}
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()
// Realize non-lazy classes (for +load methods and static instances)
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);
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");
// Realize newly-resolved future classes, in case CF manipulates them 對readclass()以后那些待處理的類，重新創(chuàng)建。以防CoreFoundate去調(diào)用
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);//重新創(chuàng)建類
cls->setInstancesRequireRawIsaRecursively(false/*inherited*/);//
}
free(resolvedFutureClasses);
}
ts.log("IMAGE TIMES: realize future classes");
//如果不存在帶修正的變量，則realize所有類
//什么是帶修正的變量？帶修正的變量是指類繼承了別的類，別的類有變量，則子類的結(jié)構(gòu)要修改
if (DebugNonFragileIvars) {
realizeAllClasses();
}
// Print preoptimization statistics 輸出動態(tài)庫中由dyld進行了預(yù)先優(yōu)化的類
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()) {//如果由dyld進行了優(yōu)化，則不用執(zhí)行
_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
}
_read_images的調(diào)用邏輯如下
_read_images執(zhí)行流程.jpg
最主要做了下面幾個事情
1 初始化
2 修正方法 Fix up @selector references
3 發(fā)現(xiàn)類Discover classes
4 修正消息發(fā)送相關(guān)fixupMessageRef
5 發(fā)現(xiàn)協(xié)議discover protocols
6 加載分類到類中
7 realize實現(xiàn)了load方法的類
8 輸出動態(tài)庫優(yōu)化的內(nèi)容
為什么要fix up呢？
在加載所有的動態(tài)鏈接庫之后，它們只是處在相互獨立的狀態(tài)，需要將它們綁定起來，這就是 Fix-ups。代碼簽名使得我們不能修改指令，那樣就不能讓一個 dylib 的調(diào)用另一個 dylib。這時需要加很多間接層。
現(xiàn)代 code-gen 被叫做動態(tài) PIC（Position Independent Code），意味著代碼可以被加載到間接的地址上。當調(diào)用發(fā)生時，code-gen 實際上會在 __DATA 段中創(chuàng)建一個指向被調(diào)用者的指針，然后加載指針并跳轉(zhuǎn)過去。所以 dyld 做的事情就是修正（fix-up）指針和數(shù)據(jù)。Fix-up 有兩種類型，rebasing 和 binding。
Rebasing：在鏡像內(nèi)部調(diào)整指針的指向
Binding：將指針指向鏡像外部的內(nèi)容
簡而言之，fix up就是為了找到調(diào)整指針的指向，找到我們所需要的內(nèi)容。
discover class
#pragma mark 類
// Discover classes. Fix up unresolved future classes. Mark bundle classes.
bool hasDyldRoots = dyld_shared_cache_some_image_overridden();
for (EACH_HEADER) {
if (! mustReadClasses(hi, hasDyldRoots)) {
// Image is sufficiently optimized that we need not call readClass() 判斷是否需要執(zhí)行readclass()方法
continue;
}
classref_t const *classlist = _getObjc2ClassList(hi, &count);
bool headerIsBundle = hi->isBundle();
bool headerIsPreoptimized = hi->hasPreoptimizedClasses();
for (i = 0; i < count; i++) {
Class cls = (Class)classlist[i];
Class newCls = readClass(cls, headerIsBundle, headerIsPreoptimized);
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.
//如果readClass返回的結(jié)果不是原來的class，則將class放入帶解決的class列表
resolvedFutureClasses = (Class *)
realloc(resolvedFutureClasses,
(resolvedFutureClassCount+1) * sizeof(Class));
resolvedFutureClasses[resolvedFutureClassCount++] = newCls;//已解決的futurec class
}
}
}
判斷是否是動態(tài)庫的內(nèi)容，如果不是動態(tài)庫。則遍歷這個可執(zhí)行文件里的類。
load_images
load_images的實現(xiàn)如下
load_images(const char *path __unused, const struct mach_header *mh)
{
recursive_mutex_locker_t lock(loadMethodLock);
// Discover +load methods
prepare_load_methods((const headerType *)mh);
// Call +load methods (without classLock - re-entrant)
call_load_methods();
}
做了兩個事情
1 準備load方法prepare_load_methods()
2 執(zhí)行l(wèi)oad方法call_load_methods()
準備load方法prepare_load_methods
void prepare_load_methods(const headerType *mhdr)
{
size_t count, i;
runtimeLock.assertLocked();
classref_t const *classlist =
_getObjc2NonlazyClassList(mhdr, &count);//獲取有l(wèi)oad方法的類
for (i = 0; i < count; i++) {
//將實現(xiàn)了load方法的類放到loadable_classes數(shù)組中
schedule_class_load(remapClass(classlist[i]));
}
//將實現(xiàn)了load方法的分類放到loadable_categories數(shù)組中
category_t * const *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//忽略弱連接的類
if (cls->isSwiftStable()) {//swfit 的extensions 和 catrogry不支持load方法
_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);
}
}
//將實現(xiàn)了load方法的類放到loadable_classes數(shù)組中
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 遞歸調(diào)用方法，確保父類的load優(yōu)先執(zhí)行
schedule_class_load(cls->superclass);
add_class_to_loadable_list(cls);
cls->setInfo(RW_LOADED);
}
prepare_load_methods方法主要是執(zhí)行了兩個事情
1 將實現(xiàn)了load方法的類放到loadable_classes數(shù)組中
2 將實現(xiàn)了load方法的分類放到loadable_categories數(shù)組中
_getObjc2NonlazyClassList返回的是image中帶有l(wèi)oad方法的class的數(shù)組，會按照編譯的順序添加到數(shù)組loadable_classes當中。
_getObjc2NonlazyCategoryList返回的是image中帶有l(wèi)oad方法的category的數(shù)組，會按照編譯的順序添加到數(shù)組loadable_categories當中。
一般來說，先編譯的class和category，其的load方法會先執(zhí)行
注意在prepare_load_methods()中調(diào)用schedule_class_load()方法，schedule_class_load()方法是一個遞歸調(diào)用，如果存在父類，則調(diào)用schedule_class_load()方法，這樣保證了實現(xiàn)了load的方法可以放在loadable_classes數(shù)組的前面，這樣也就保證了父類的load方法比子類的load方法優(yōu)先執(zhí)行
執(zhí)行l(wèi)oad方法call_load_methods()
call_load_methods()實現(xiàn)如下
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; //保證call_load_methods只執(zhí)行一次
loading = YES;
void *pool = objc_autoreleasePoolPush();//自動釋放池
do {
// 1. Repeatedly call class +loads until there aren't any more
//循環(huán)執(zhí)行類的load的方法
while (loadable_classes_used > 0) {
call_class_loads();
}
// 2. Call category +loads ONCE
//執(zhí)行完類的load方法后才cagetgory中的方法
//檢測是否有未執(zhí)行l(wèi)oad方法的category,如果有，則循環(huán)執(zhí)行
//為什么會有未執(zhí)行l(wèi)oad方法的category？因為category可能會引入別的有l(wèi)oad方法class，別的class的load要先執(zhí)行，category中的load要等待執(zhí)行
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;
}
可以看到，通過do while循環(huán)中確保所有的類和分類的load方法都被調(diào)用。
do while中優(yōu)先call_class_loads(),再執(zhí)行call_category_loads(),也就是說類的load方法比分類的load方法先執(zhí)行。
call_class_loads方法會從loadable_classes數(shù)組中找到對應(yīng)的類，并通過直接調(diào)用的方法，調(diào)用類的loadable_classes方法。
call_category_loads()方法會從loadable_categories中找到相應(yīng)的分類load方法。并調(diào)用load方法.
在前面可知，在類和分類的加載過程中，有三個結(jié)果
- 先編譯的class和category，其的load方法會先執(zhí)行
- 父類的load方法比子類的load方法優(yōu)先執(zhí)行
- 類的load方法比分類的load方法先執(zhí)行。
