上一篇中，我们分析了dyld，最后说到会调用_objc_init函数，那_objc_init是干嘛的呢？
一、_objc_init
直接源码中搜索_objc_init，我们可以找到_objc_init的实现：
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_objc_notify_register(&map_images, load_images, unmap_image);
#if __OBJC2__
didCallDyldNotifyRegister = true;
#endif
}
可以看到，在_dyld_objc_notify_register之前调用了很多初始化函数，这里我们稍微介绍下：
1 environ_init
void environ_init(void)
{
if (issetugid()) {
// All environment variables are silently ignored when setuid or setgid
// This includes OBJC_HELP and OBJC_PRINT_OPTIONS themselves.
return;
}
bool PrintHelp = false;
bool PrintOptions = false;
bool maybeMallocDebugging = false;
// Scan environ[] directly instead of calling getenv() a lot.
// This optimizes the case where none are set.
for (char **p = *_NSGetEnviron(); *p != nil; p++) {
if (0 == strncmp(*p, "Malloc", 6) || 0 == strncmp(*p, "DYLD", 4) ||
0 == strncmp(*p, "NSZombiesEnabled", 16))
{
maybeMallocDebugging = true;
}
if (0 != strncmp(*p, "OBJC_", 5)) continue;
if (0 == strncmp(*p, "OBJC_HELP=", 10)) {
PrintHelp = true;
continue;
}
if (0 == strncmp(*p, "OBJC_PRINT_OPTIONS=", 19)) {
PrintOptions = true;
continue;
}
const char *value = strchr(*p, '=');
if (!*value) continue;
value++;
for (size_t i = 0; i < sizeof(Settings)/sizeof(Settings[0]); i++) {
const option_t *opt = &Settings[i];
if ((size_t)(value - *p) == 1+opt->envlen &&
0 == strncmp(*p, opt->env, opt->envlen))
{
*opt->var = (0 == strcmp(value, "YES"));
break;
}
}
}
// Special case: enable some autorelease pool debugging
// when some malloc debugging is enabled
// and OBJC_DEBUG_POOL_ALLOCATION is not set to something other than NO.
if (maybeMallocDebugging) {
const char *insert = getenv("DYLD_INSERT_LIBRARIES");
const char *zombie = getenv("NSZombiesEnabled");
const char *pooldebug = getenv("OBJC_DEBUG_POOL_ALLOCATION");
if ((getenv("MallocStackLogging")
|| getenv("MallocStackLoggingNoCompact")
|| (zombie && (*zombie == 'Y' || *zombie == 'y'))
|| (insert && strstr(insert, "libgmalloc")))
&&
(!pooldebug || 0 == strcmp(pooldebug, "YES")))
{
DebugPoolAllocation = true;
}
}
// Print OBJC_HELP and OBJC_PRINT_OPTIONS output.
if (PrintHelp || PrintOptions) {
if (PrintHelp) {
_objc_inform("Objective-C runtime debugging. Set variable=YES to enable.");
_objc_inform("OBJC_HELP: describe available environment variables");
if (PrintOptions) {
_objc_inform("OBJC_HELP is set");
}
_objc_inform("OBJC_PRINT_OPTIONS: list which options are set");
}
if (PrintOptions) {
_objc_inform("OBJC_PRINT_OPTIONS is set");
}
for (size_t i = 0; i < sizeof(Settings)/sizeof(Settings[0]); i++) {
const option_t *opt = &Settings[i];
if (PrintHelp) _objc_inform("%s: %s", opt->env, opt->help);
if (PrintOptions && *opt->var) _objc_inform("%s is set", opt->env);
}
}
}
这个函数就是进行环境变量操作的
image.png
其中这里是执行打印操作，我们可以将这个
for放到前面来，并且去掉前面的条件判断：
image.png
我们运行下工程：
image.png
可以看到，这里打印了所有的环境变量
2 tls_init
void tls_init(void)
{
#if SUPPORT_DIRECT_THREAD_KEYS
pthread_key_init_np(TLS_DIRECT_KEY, &_objc_pthread_destroyspecific);
#else
_objc_pthread_key = tls_create(&_objc_pthread_destroyspecific);
#endif
}
tls_init这个函数是对线程池进行初始化的
3 static_init
static void static_init()
{
size_t count;
auto inits = getLibobjcInitializers(&_mh_dylib_header, &count);
for (size_t i = 0; i < count; i++) {
inits[i]();
}
}
static_init函数对系统级别的C++构造函数进行调用，而且它的调用会在dyld调用构造函数之前
4 runtime_init
void runtime_init(void)
{
objc::unattachedCategories.init(32);
objc::allocatedClasses.init();
}
runtime_init这个函数作用就是进行分类和类的初始化容器工作。
5 exception_init
/***********************************************************************
* exception_init
* Initialize libobjc's exception handling system.
* Called by map_images().
**********************************************************************/
void exception_init(void)
{
old_terminate = std::set_terminate(&_objc_terminate);
}
exception_init这个函数就是注册监听异常回调，系统方法在执行的过程中，出现异常触发中断，就会报出异常，如果我们在上层对这个方法处理，我们就能捕获这次异常。注意：是系统方法执行异常。我们可以在这里去监听系统异常，我们看下怎么处理。 我们看下_objc_terminate方法
/***********************************************************************
* _objc_terminate
* Custom std::terminate handler.
*
* The uncaught exception callback is implemented as a std::terminate handler.
* 1. Check if there's an active exception
* 2. If so, check if it's an Objective-C exception
* 3. If so, call our registered callback with the object.
* 4. Finally, call the previous terminate handler.
**********************************************************************/
static void (*old_terminate)(void) = nil;
static void _objc_terminate(void)
{
if (PrintExceptions) {
_objc_inform("EXCEPTIONS: terminating");
}
if (! __cxa_current_exception_type()) {
// No current exception.
(*old_terminate)();
}
else {
// There is a current exception. Check if it's an objc exception.
@try {
__cxa_rethrow();
} @catch (id e) {
// It's an objc object. Call Foundation's handler, if any.
(*uncaught_handler)((id)e);
(*old_terminate)();
} @catch (...) {
// It's not an objc object. Continue to C++ terminate.
(*old_terminate)();
}
}
}
这里的关键代码就是(*uncaught_handler)((id)e);这一行，这里用了一个回调将我捕获到的异常抛出去，我们可以看下uncaught_handler：
static void _objc_default_uncaught_exception_handler(id exception)
{
}
static objc_uncaught_exception_handler uncaught_handler = _objc_default_uncaught_exception_handler;
这里看到uncaught_handler是给了一个默认实现，我们再搜索下uncaught_handler：
image.png
objc_uncaught_exception_handler
objc_setUncaughtExceptionHandler(objc_uncaught_exception_handler fn)
{
objc_uncaught_exception_handler result = uncaught_handler;
uncaught_handler = fn;
return result;
}
可以看到这里传入了一个fn，并赋值给uncaught_handler，所以我们可以再外部调用objc_setUncaughtExceptionHandler传入一个实现fn，就可以在内部捕获到异常的时候抛出到外面去，方便我们可以处理异常。
我们可以写一个简单的demo验证下:
// 创建新类：UncaughtExceptionHandle，在.m文件写如下代码
@implementation UncaughtExceptionHandle
void LPExceptionHandlers(NSException *exception) {
NSLog(@"---->%@---->%@", exception.name, exception.reason);
}
+ (void)installUncaughtSignalExceptionHandler {
NSSetUncaughtExceptionHandler(&TestExceptionHandlers);
}
@end
// 在ViewController.m做如下代码
@implementation ViewController
- (void)viewDidLoad {
[super viewDidLoad];
self.dataArray = @[@"1",@"2",@"3",@"4",@"5",@"6"];
}
- (IBAction)exceptionAction:(id)sender {
NSLog(@"%@",self.dataArray[100]);
}
@end
// 在AppDelegate调用installUncaughtSignalExceptionHandler。
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
[UncaughtExceptionHandle installUncaughtSignalExceptionHandler];
return YES;
}
LPExceptionHandlers就是上层封装的一个异常处理函数，然后我们在didFinishLaunchingWithOptions中调用UncaughtExceptionHandle，利用objc_setUncaughtExceptionHandler的上层函数NSSetUncaughtExceptionHandler来传入内部中，我们看现在的代码肯定是会因为数据越界而崩溃的，但是我们观察下控制台输出：
image.png
可以看到，虽然崩溃了，但是异常我们是成功捕获到了。
6.cache_init
void cache_init()
{
#if HAVE_TASK_RESTARTABLE_RANGES
mach_msg_type_number_t count = 0;
kern_return_t kr;
while (objc_restartableRanges[count].location) {
count++;
}
kr = task_restartable_ranges_register(mach_task_self(),
objc_restartableRanges, count);
if (kr == KERN_SUCCESS) return;
_objc_fatal("task_restartable_ranges_register failed (result 0x%x: %s)",
kr, mach_error_string(kr));
#endif // HAVE_TASK_RESTARTABLE_RANGES
}
cache_init这个函数主要是负责cache的初始化
7 _imp_implementationWithBlock_init
void
_imp_implementationWithBlock_init(void)
{
#if TARGET_OS_OSX
// Eagerly load libobjc-trampolines.dylib in certain processes. Some
// programs (most notably QtWebEngineProcess used by older versions of
// embedded Chromium) enable a highly restrictive sandbox profile which
// blocks access to that dylib. If anything calls
// imp_implementationWithBlock (as AppKit has started doing) then we'll
// crash trying to load it. Loading it here sets it up before the sandbox
// profile is enabled and blocks it.
//
// This fixes EA Origin (rdar://problem/50813789)
// and Steam (rdar://problem/55286131)
if (__progname &&
(strcmp(__progname, "QtWebEngineProcess") == 0 ||
strcmp(__progname, "Steam Helper") == 0)) {
Trampolines.Initialize();
}
#endif
}
这个函数是在OS下执行，这个方法就是对imp的Block标记进行初始化。
8. _dyld_objc_notify_register
接下来就是我们本文研究的重点_dyld_objc_notify_register，在上篇文章中我们已经了解到了_dyld_objc_notify_register，再次看下它的源码：
// Note: only for use by objc runtime
// Register handlers to be called when objc images are mapped, unmapped, and initialized.
// Dyld will call back the "mapped" function with an array of images that contain an objc-image-info section.
// Those images that are dylibs will have the ref-counts automatically bumped, so objc will no longer need to
// call dlopen() on them to keep them from being unloaded. During the call to _dyld_objc_notify_register(),
// dyld will call the "mapped" function with already loaded objc images. During any later dlopen() call,
// dyld will also call the "mapped" function. Dyld will call the "init" function when dyld would be called
// initializers in that image. This is when objc calls any +load methods in that image.
//
void _dyld_objc_notify_register(_dyld_objc_notify_mapped mapped,
_dyld_objc_notify_init init,
_dyld_objc_notify_unmapped unmapped);
从注释中，可以得出：
仅供
objc运行时使用注册处理程序，以便在映射、取消映射和初始化
objc图像时调用dyld将会通过一个包含objc-image-info的镜像文件的数组回调mapped函数
方法中的三个参数分别表示的含义如下：
map_images：dyld将image（镜像文件）加载进内存时，会触发该函数load_image：dyld初始化image会触发该函数unmap_image：dyld将image移除时，会触发该函数
这里在梳理一下dyld的关键流程：
- 在
recursiveInitialization方法中调用bool hasInitializers = this->doInitialization(context);这个方法是来判断image是否已加载 -
doInitialization这个方法会调用doModInitFunctions(context)这个方法就会进入libSystem框架里调用libSystem_initializer方法，最后就会调用_objc_init方法 -
_objc_init会调用_dyld_objc_notify_register将map_images、load_images、unmap_image传入dyld方法registerObjCNotifiers。 - 在
registerObjCNotifiers方法中，我们把_dyld_objc_notify_register传入的map_images赋值给sNotifyObjCMapped，将load_images赋值给sNotifyObjCInit，将unmap_image赋值给sNotifyObjCUnmapped。 - 在
registerObjCNotifiers方法中，我们将传参复制后就开始调用notifyBatchPartial()。 -
notifyBatchPartial方法中会调用(*sNotifyObjCMapped)(objcImageCount, paths, mhs)；触发map_images方法。 -
dyld的recursiveInitialization方法在调用完bool hasInitializers = this->doInitialization(context)方法后，会调用notifySingle()方法 - 在
notifySingle()中会调用(*sNotifyObjCInit)(image->getRealPath(),image->machHeader();上面我们将load_images赋值给了sNotifyObjCInit，所以此时就会触发load_images方法。 -
sNotifyObjCUnmapped会在removeImage方法里触发，字面理解就是删除Image（映射的镜像文件）。
下面我们看下map_images、load_images、unmap_image都做了什么
二、map_images、load_images和unmap_image
1 map_images
老样子，先看下源码：
/***********************************************************************
* 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);
}
这个函数是处理镜像的，主要是调用了map_images_nolock，所以我们再看下map_images_nolock的源码：
/***********************************************************************
* map_images_nolock
* Process the given images which are being mapped in by dyld.
* All class registration and fixups are performed (or deferred pending
* discovery of missing superclasses etc), and +load methods are called.
*
* info[] is in bottom-up order i.e. libobjc will be earlier in the
* array than any library that links to libobjc.
*
* Locking: loadMethodLock(old) or runtimeLock(new) acquired by map_images.
**********************************************************************/
#if __OBJC2__
#include "objc-file.h"
#else
#include "objc-file-old.h"
#endif
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.
hCount = 0;
// Count classes. Size various table based on the total.
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)":"");
}
}
}
// Perform one-time runtime initialization that must be deferred until
// the executable itself is found. This needs to be done before
// further initialization.
// (The executable may not be present in this infoList if the
// executable does not contain Objective-C code but Objective-C
// is dynamically loaded later.
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
}
if (hCount > 0) {
_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]);
}
}
}
我们将一些不太重要的代码收起来后，可以看到，关键代码就是_read_images函数，所以我们继续研究_read_images的源码：
/***********************************************************************
* _read_images
* Perform initial processing of the headers in the linked
* list beginning with headerList.
*
* Called by: map_images_nolock
*
* Locking: runtimeLock acquired by map_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++
///判断是否是第一次，如果是会进入if
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
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
// 简单的字符串 -- 地址 字符串
// 修正sel
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();
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();
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.
resolvedFutureClasses = (Class *)
realloc(resolvedFutureClasses,
(resolvedFutureClassCount+1) * sizeof(Class));
resolvedFutureClasses[resolvedFutureClassCount++] = newCls;
}
}
}
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.
if (!noClassesRemapped()) {
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
// 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
///修正协议
bool cacheSupportsProtocolRoots = sharedCacheSupportsProtocolRoots();
// 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->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();
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");
// 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]);
}
}
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) {
for (EACH_HEADER) {
load_categories_nolock(hi);
}
}
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()
///实现非懒加载的类，即重新了load方法的
// 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
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
}
代码仍然很多，我将部分代码收起来了，接下来针对其中比较重要的代码我们提出来，单独做下分析：
1.1 初始化缓存表
///判断是否是第一次，如果是会进入if
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
int namedClassesSize =
(isPreoptimized() ? unoptimizedTotalClasses : totalClasses) * 4 / 3;
gdb_objc_realized_classes =
NXCreateMapTable(NXStrValueMapPrototype, namedClassesSize);
ts.log("IMAGE TIMES: first time tasks");
}
这个if是只有第一次才会进入，主要就是为了第一次进来对TaggedPointer进行优化处理，通过NXCreateMapTable创建缓存表。表中主要是存储我们的类、协议、sel以及分类。
1.2 修正sel
// Fix up @selector references
// 简单的字符串 -- 地址 字符串
// 修正sel
static size_t UnfixedSelectors;
{
mutex_locker_t lock(selLock);
for (EACH_HEADER) {
if (hi->hasPreoptimizedSelectors()) continue;
bool isBundle = hi->isBundle();
///获取所有的sel
SEL *sels = _getObjc2SelectorRefs(hi, &count);
UnfixedSelectors += count;
for (i = 0; i < count; i++) {
////获取sel的name
const char *name = sel_cname(sels[i]);
///通过name注册sel
SEL sel = sel_registerNameNoLock(name, isBundle);
if (sels[i] != sel) {
sels[i] = sel;
}
}
}
}
ts.log("IMAGE TIMES: fix up selector references");
因为直接获取的sel，和使用name注册的sel地址可能会不一样，所以需要判断处理。将sel修正为sel_registerNameNoLock注册后的sel。
1.3 读取classes
for (EACH_HEADER) {
if (! mustReadClasses(hi, hasDyldRoots)) {
// Image is sufficiently optimized that we need not call readClass()
continue;
}
///获取所有的class，得到一个classref_t指针
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.
resolvedFutureClasses = (Class *)
realloc(resolvedFutureClasses,
(resolvedFutureClassCount+1) * sizeof(Class));
resolvedFutureClasses[resolvedFutureClassCount++] = newCls;
}
}
}
这里主要是读取class，关键函数就是readClass，我们后面会单独研究它。
1.4 类的重映射
// Fix up remapped classes
// Class list and nonlazy class list remain unremapped.
// Class refs and super refs are remapped for message dispatching.
//未映射的class和非懒加载的类建立映射关系，分为当前类和当前类的父类两步
if (!noClassesRemapped()) {
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]);
}
}
}
1.5 修正协议
///修正协议
bool cacheSupportsProtocolRoots = sharedCacheSupportsProtocolRoots();
// 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->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();
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");
// 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]);
}
}
和 sel类似
1.6 加载分类
// 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) {
for (EACH_HEADER) {
load_categories_nolock(hi);
}
}
1.7 实现非懒加载的类
// 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()
///实现非懒加载的类，即重写了load方法的
// 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);
- 懒加载类：类没有实现
load方法，在使用的第一次才会加载，当我们再给这个类的发送消息，如果是第一次，在消息查找的过程中就会判断这个类是否加载，没有加载就会加载这个类。 - 非懒加载类：类的内部实现了
load方法，类的加载就会提前
懒加载类在首次调用方法的时候，才会调用realizeClassWithoutSwift()方法去初始化加载。
1.8 readClass
/***********************************************************************
* readClass
* Read a class and metaclass as written by a compiler.
* Returns the new class pointer. This could be:
* - cls
* - nil (cls has a missing weak-linked superclass)
* - something else (space for this class was reserved by a future class)
*
* Note that all work performed by this function is preflighted by
* mustReadClasses(). Do not change this function without updating that one.
*
* Locking: runtimeLock acquired by map_images or objc_readClassPair
**********************************************************************/
Class readClass(Class cls, bool headerIsBundle, bool headerIsPreoptimized)
{
const char *mangledName = cls->mangledName();
if (missingWeakSuperclass(cls)) {
// No superclass (probably weak-linked).
// Disavow any knowledge of this subclass.
if (PrintConnecting) {
_objc_inform("CLASS: IGNORING class '%s' with "
"missing weak-linked superclass",
cls->nameForLogging());
}
addRemappedClass(cls, nil);
cls->superclass = nil;
return nil;
}
cls->fixupBackwardDeployingStableSwift();
Class replacing = nil;
if (Class newCls = popFutureNamedClass(mangledName)) {
// This name was previously allocated as a future class.
// Copy objc_class to future class's struct.
// Preserve future's rw data block.
if (newCls->isAnySwift()) {
_objc_fatal("Can't complete future class request for '%s' "
"because the real class is too big.",
cls->nameForLogging());
}
class_rw_t *rw = newCls->data();
const class_ro_t *old_ro = rw->ro();
memcpy(newCls, cls, sizeof(objc_class));
rw->set_ro((class_ro_t *)newCls->data());
newCls->setData(rw);
freeIfMutable((char *)old_ro->name);
free((void *)old_ro);
addRemappedClass(cls, newCls);
replacing = cls;
cls = newCls;
}
if (headerIsPreoptimized && !replacing) {
// class list built in shared cache
// fixme strict assert doesn't work because of duplicates
// ASSERT(cls == getClass(name));
ASSERT(getClassExceptSomeSwift(mangledName));
} else {
addNamedClass(cls, mangledName, replacing);
addClassTableEntry(cls);
}
// for future reference: shared cache never contains MH_BUNDLEs
if (headerIsBundle) {
cls->data()->flags |= RO_FROM_BUNDLE;
cls->ISA()->data()->flags |= RO_FROM_BUNDLE;
}
return cls;
}
readClass函数是读取一个类和它的元类，并返回一个class类型的指针，这个指针可能为nil，整个代码比较多，但是重点就在：
addNamedClass(cls, mangledName, replacing);
addClassTableEntry(cls);
1.8.1 addNamedClass
/***********************************************************************
* addNamedClass
* Adds name => cls to the named non-meta class map.
* Warns about duplicate class names and keeps the old mapping.
* Locking: runtimeLock must be held by the caller
**********************************************************************/
static void addNamedClass(Class cls, const char *name, Class replacing = nil)
{
runtimeLock.assertLocked();
Class old;
if ((old = getClassExceptSomeSwift(name)) && old != replacing) {
inform_duplicate(name, old, cls);
// getMaybeUnrealizedNonMetaClass uses name lookups.
// Classes not found by name lookup must be in the
// secondary meta->nonmeta table.
// 不能通过name查找到class时
addNonMetaClass(cls);
} else {
NXMapInsert(gdb_objc_realized_classes, name, cls);
}
ASSERT(!(cls->data()->flags & RO_META));
// wrong: constructed classes are already realized when they get here
// ASSERT(!cls->isRealized());
}
从源码中可以看出，这个函数是将name => cls添加到命名的非元类映射。只有不能通过name查找到class时才会执行addNonMetaClass，但是通常情况下肯定是可以通过name获取到class的，所以我们就看else中的代码即NXMapInsert函数：
NXMapInsert有三个参数：
-
gdb_objc_realized_classes：还记得_read_image中第一次执行的时候的代码吗？其中gdb_objc_realized_classes就是在那个时候创建的。
-name：名字 -
cls：类
我们再看下NXMapInsert的源码：
void *NXMapInsert(NXMapTable *table, const void *key, const void *value) {
MapPair *pairs = (MapPair *)table->buckets;
unsigned index = bucketOf(table, key);
MapPair *pair = pairs + index;
if (key == NX_MAPNOTAKEY) {
_objc_inform("*** NXMapInsert: invalid key: -1\n");
return NULL;
}
unsigned numBuckets = table->nbBucketsMinusOne + 1;
if (pair->key == NX_MAPNOTAKEY) {
pair->key = key; pair->value = value;
table->count++;
if (table->count * 4 > numBuckets * 3) _NXMapRehash(table);
return NULL;
}
if (isEqual(table, pair->key, key)) {
const void *old = pair->value;
if (old != value) pair->value = value;/* avoid writing unless needed! */
return (void *)old;
} else if (table->count == numBuckets) {
/* no room: rehash and retry */
_NXMapRehash(table);
return NXMapInsert(table, key, value);
} else {
unsigned index2 = index;
while ((index2 = nextIndex(table, index2)) != index) {
pair = pairs + index2;
if (pair->key == NX_MAPNOTAKEY) {
pair->key = key; pair->value = value;
table->count++;
if (table->count * 4 > numBuckets * 3) _NXMapRehash(table);
return NULL;
}
if (isEqual(table, pair->key, key)) {
const void *old = pair->value;
if (old != value) pair->value = value;/* avoid writing unless needed! */
return (void *)old;
}
}
/* no room: can't happen! */
_objc_inform("**** NXMapInsert: bug\n");
return NULL;
}
}
这里不太重要的代码已经收起来了，这个函数主要就是使用name为key，class为value，将类插入到table中，具体代码可以参考下面:
- 314-316行：1.取出第一个
MapPair地址 2.通过bucketOf找到key在table中的位置 3.将地址偏移取出，该位置下的MapPair。 - 340行：将获取的地址给
index2 - 341-353行：是个循环。
- 341行：将
index2+1和table的nbBucketsMinusOne进行与运算得到的值重新给index2，如果值不等于最开始算的index就进来 - 342行：让
pair在等于pairs偏移新的位置- 343-348行：如果
pair的key地址不存在则进去，所以这个判断不会进。 - 349行：如果
pair->key和key相等，就进入，不等就继续循环 - 350行：将
pair的value取出给old - 351行：如果
old值不等于传的value就将value值重新赋值给pair的value。 - 352行：返回
old值
- 343-348行：如果
1.8.2 addClassTableEntry
/***********************************************************************
* addClassTableEntry
* Add a class to the table of all classes. If addMeta is true,
* automatically adds the metaclass of the class as well.
* Locking: runtimeLock must be held by the caller.
**********************************************************************/
static void
addClassTableEntry(Class cls, bool addMeta = true)
{
runtimeLock.assertLocked();
// This class is allowed to be a known class via the shared cache or via
// data segments, but it is not allowed to be in the dynamic table already.
auto &set = objc::allocatedClasses.get();
ASSERT(set.find(cls) == set.end());
if (!isKnownClass(cls))
set.insert(cls);
if (addMeta)
addClassTableEntry(cls->ISA(), false);
}
这个函数注释说的比较清楚，就是将这个类添加到类表中（类表中存放所有的初始化类），allocatedClasses容器在runtime_init初始化过了。
最后readClass方法结束，将cls返回。 我们验证一下，因为系统的类非常多，不利于我们探索，为了看到我们自己创建的LPPerson`类,我们需要在源码中添加一点代码。
// **测试代码**
const char *mangledName = cls->mangledName();
const char *className = "LPPerson";
if (strcmp(mangledName, className)==0) {
printf("%s 他来了!- %s \n",__func__,mangledName);
}
// **测试代码**
并且在readClass前后分别打上两个断点：
image.png
运行工程，来到第一个断点时，我们po一下当前的cls：
(lldb) po cls
objc[70712]: mutex incorrectly locked
objc[70712]: mutex incorrectly locked
0x00000001000084a8
然后放到下一点断点：
_read_images 他来了!- LPPerson
(lldb) po cls
objc[70712]: mutex incorrectly locked
objc[70712]: mutex incorrectly locked
LPPerson
我们发现进行readClass后，cls就打印出类名，不再是地址，下面我们在打印下cls地址：
(lldb) p/x cls
(Class) $2 = 0x00000001000084a8 LPPerson
我们发现LPPerson类的首地址就是我们在调用readClass时候的cls地址，这也验证了我们上面说的内容。
2 load_images
老规矩，看码说话：
void
load_images(const char *path __unused, const struct mach_header *mh)
{
if (!didInitialAttachCategories && didCallDyldNotifyRegister) {
didInitialAttachCategories = true;
loadAllCategories();
}
// 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();
}
其中最重要的代码就是
-
prepare_load_methods:准备load方法 -
call_load_methods：调用load方法
2.1 prepare_load_methods
void prepare_load_methods(const headerType *mhdr)
{
size_t count, i;
runtimeLock.assertLocked();
///获取所有的非懒加载的类
classref_t const *classlist =
_getObjc2NonlazyClassList(mhdr, &count);
for (i = 0; i < count; i++) {
//遍历获取load方法，并保存，优先获取父类的load方法
schedule_class_load(remapClass(classlist[i]));
}
///获取所有的非懒加载的分类
category_t * const *categorylist = _getObjc2NonlazyCategoryList(mhdr, &count);
for (i = 0; i < count; i++) {///遍历所有的分类
category_t *cat = categorylist[i];
Class cls = remapClass(cat->cls);
///判断分类所属的类是否存在，不存在则继续下次循环
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");
}
///如果分类所属的类未实现，需要去实现
realizeClassWithoutSwift(cls, nil);
ASSERT(cls->ISA()->isRealized());
///添加分类的load方法到数组中
add_category_to_loadable_list(cat);
}
}
可以看到prepare_load_methods函数就是获取当前类和分类的load方法。不太理解的同学可以看下上面的注释，我们可以再研究下schedule_class_load函数：
/***********************************************************************
* 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
///已经load过，直接return
if (cls->data()->flags & RW_LOADED) return;
// Ensure superclass-first ordering
//递归遍历继承链
schedule_class_load(cls->superclass);
///添加load方法
add_class_to_loadable_list(cls);
cls->setInfo(RW_LOADED);
}
schedule_class_load函数里面使用递归来遍历所有的类获取load方法，并且是先添加class的，再添加class的父类的，知道继承链遍历完毕。
2.2 add_category_to_loadable_list
/***********************************************************************
* 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();
///获取分类的load方法
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));
}
///主要是用来进行扩容处理
///loadable_categories_used和loadable_categories_allocated初始值都为0
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));
}
///保存分类
loadable_categories[loadable_categories_used].cat = cat;
///保存分类的load方法的地址
loadable_categories[loadable_categories_used].method = method;
///loadable_categories使用状态处理
loadable_categories_used++;
}
2.3 call_load_methods
{
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;
///创建autoreleasePool
void *pool = objc_autoreleasePoolPush();
do {
// 1. Repeatedly call class +loads until there aren't any more
///遍历loadable_classes调用load方法
while (loadable_classes_used > 0) {
call_class_loads();
}
///遍历分类，并调用其load方法
// 2. Call category +loads ONCE
more_categories = call_category_loads();
///只要loadable_classes_used不为0，继续遍历
///一般情况到这里loadable_classes_used已经在第一步置为0了，除非当前的cls和其继承链的父类没有load方法，但是其分类有
// 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_load_methods函数就是去调用所有的load方法，我们可以再看下call_class_loads和call_category_loads函数：
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.
//遍历所有的类
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);
}
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;
///遍历所有的分类
// 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, @selector(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;
}
3 unmap_image
/***********************************************************************
* 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函数主要就是两件事情，卸载和移除镜像
三、简单总结：
-
mapImage:主要是调用了map_images_nolock，map_images_nolock又调用了_read_images。_read_images主要流程如下：- 1.判断是否使用
non-pointer对isa进行优化 - 2.对
TaggedPointer的优化处理 - 3.创建保存类的哈希表
- 4.注册修正sel
- 5.获取所有类，读取类并将类存储到3创建的表中
- 6.修复需要重映射的类
- 7.获取并修正协议
- 8.对非懒加载类进行处理
- 9.对懒加载类进行处理
- 1.判断是否使用
-
load_images：主要是获取当前类和分类的所有load方法并调用，具体流程如需爱：- 1.获取所有非懒加载类，将
+load方法保存到loadable_classes中 - 2.获取所有分类的
+load方法，保存到loadable_categories中 - 3.先从
loadable_categories拿类，父类的load方法进行调用 - 4.再从
loadable_categories拿分类的load方法进行调用
- 1.获取所有非懒加载类，将
-
unmap_image：卸载image中的数据，然后移除image
其实也可以看到，dyld在这个过程中的作用就是完成类的加载，不过这并不是所有，后面我们还是继续研究类的加载。
