在上一篇文章 objc_init 分析 中，最后有三个函数 map_images、load_images、unmap_image。
一、map_images
/***********************************************************************
* 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);
}
1.1 map_images_nolock
void
map_images_nolock(unsigned mhCount, const char * const mhPaths[],
const struct mach_header * const mhdrs[])
{
// 省略准备逻辑...
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]);
}
}
}
因为代码比较多，省略了很多代码，浏览代码后，会发现重要的是 _read_images(hList, hCount, totalClasses, unoptimizedTotalClasses); 这一行代码。
二、_read_images
因为源码比较长，这里就不贴了。
void _read_images(header_info **hList, uint32_t hCount, int totalClasses, int unoptimizedTotalClasses)
{
// 省略代码...
// 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++) {
// 断点1
Class cls = (Class)classlist[i];
Class newCls = readClass(cls, headerIsBundle, headerIsPreoptimized); // 读取类
}
}
// 省略 protocol 处理...
// Realize non-lazy classes (for +load methods and static instances)
// 实现非惰性类（用于+ load方法和静态实例）
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);
}
}
}
主要流程如下：
- 条件控制进行一次的加载
- 修复预编译阶段的
@selector的混乱问题 - 错误混乱的类处理
- 修复重映射⼀些没有被镜像⽂件加载进来的类
- 修复⼀些消息
- 当我们类⾥⾯有协议的时候 :
readProtocol - 修复没有被加载的协议
- 分类处理
- 类的加载处理
- 没有被处理的类 优化那些被侵犯的类
在上面代码中 断点1 打个断点，运行
(lldb) po cls
0x000000010048dfe8
--- 走过断点1和readClass之后再打印
(lldb) po newCls
OS_dispatch_io
在没有 readClass 之前，classlist 里面存的还是内存地址，readClass 之后，就有名字了。下面看下 readClass 具体做了什么?
2.1 readClass
读取编译器编写的类和元类。返回新的类指针。
Class readClass(Class cls, bool headerIsBundle, bool headerIsPreoptimized)
{
const char *mangledName = cls->mangledName();
if (missingWeakSuperclass(cls)) {
// 省略...
}
cls->fixupBackwardDeployingStableSwift();
Class replacing = nil;
if (Class newCls = popFutureNamedClass(mangledName)) {
// 省略...
}
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 {
// 重点
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;
}
在 mangleName 打个断点
mangledName断点.png
发现是系统的类。我们还是研究自定义类 GLPerson，在mangleName 下面添加如下代码：
// godlong test begin
const char *GLPersonName = "GLPerson";
if (strcmp(mangledName, GLPersonName) == 0) {
printf("%s 来到了自定义类 %s", __func__, mangledName);
}
// godlong test end
断点到 if 条件里面，运行工程，成功进入断点
进入自定义类.png
2.2 addNamedClass
走到 addNamedClass 断点查看
addNamedClass.png
可以发现这时 cls 还是地址，而 mangledName 就是我们的类名。
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.
addNonMetaClass(cls);
} else {
NXMapInsert(gdb_objc_realized_classes, name, cls);
}
ASSERT(!(cls->data()->flags & RO_META));
}
解释：将类的名称添加到非元类的类映射表中。警告有关重复的类名，并保留旧的映射。
2.3 addClassTableEntry
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);
}
解释 : 将一个类添加到所有类的表中。如果 addMeta 为 true，也自动添加该类的元类。
三、realizeClassWithoutSwift
先重点关注类的加载，在 read_images 的源码中，往下看的时候，发现注释
Realize non-lazy classes (for
+loadmethods and static instances)
【译】实现非懒加载类（用于+ load方法和静态实例）
那什么是 non-lazy classes 呢？
3.1 non-lazy classes 非懒加载类
如果一个类实现了 + load 方法，那这个类就是 non-lazy class(非懒加载类)。
反之，没实现 + load 方法，就是懒加载类。
3.2 进入自定义类的 realizeClassWithoutSwift
给 GLPerson 类添加 + load 方法，然后把 for(EACH_HEADER) 循环里面的代码改成如下:
// 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;
// loong test begin
const char *mangledName = cls->mangledName();
const char *GLPersonName = "GLPerson";
if (strcmp(mangledName, GLPersonName) == 0) {
printf("%s 来到了自定义类 %s", __func__, mangledName); // 断点2
}
// loong test end
addClassTableEntry(cls);
// class is a Swift class from the stable Swift ABI 所以不会走这里
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);
}
}
断点到上面代码的 断点2，确认来到了自定义类 GLPerson。
断点2.png
3.3【重点】realizeClassWithoutSwift 源码
realizeClassWithoutSwift 虽然很长，但是都比较重要，所以也没有省略，全贴出来了。
方法注释：
- 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.
【译】* 对类cls进行首次初始化，包括分配其读写数据。不执行任何Swift端初始化。返回该类的真实类结构。
static Class realizeClassWithoutSwift(Class cls, Class previously)
{
runtimeLock.assertLocked();
class_rw_t *rw; // 读写数据
Class supercls; // 父类
Class metacls; // 元类
if (!cls) return nil; // 如果为空，返回nil
if (cls->isRealized()) return cls; // 如果已经实现，直接返回
ASSERT(cls == remapClass(cls));
// fixme verify class is not in an un-dlopened part of the shared cache?
auto ro = (const class_ro_t *)cls->data(); // 读取类的数据
auto isMeta = ro->flags & RO_META; // 是否是元类
if (ro->flags & RO_FUTURE) { // rw已经有值的话走这里
// This was a future class. rw data is already allocated.
rw = cls->data();
ro = cls->data()->ro();
ASSERT(!isMeta);
cls->changeInfo(RW_REALIZED|RW_REALIZING, RW_FUTURE);
} else { // 正常的类走这里
// Normal class. Allocate writeable class data.
rw = objc::zalloc<class_rw_t>(); // 开辟rw
rw->set_ro(ro); // 把cls的数据ro赋值给rw
rw->flags = RW_REALIZED|RW_REALIZING|isMeta; // 更新flags
cls->setData(rw); // 再把rw设置为cls的data数据
}
#if FAST_CACHE_META
if (isMeta) cls->cache.setBit(FAST_CACHE_META);
#endif
// 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.
//实现超类和元类（如果尚未实现）。
// This needs to be done after RW_REALIZED is set above, for root classes.
//对于根类，需要在上面设置了RW_REALIZED之后执行此操作。
// This needs to be done after class index is chosen, for root metaclasses.
//对于根元类，需要在选择类索引之后执行此操作。
// 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.
// 递归调用 realizeClassWithoutSwift ，实现父类和元类
supercls = realizeClassWithoutSwift(remapClass(cls->superclass), nil);
metacls = realizeClassWithoutSwift(remapClass(cls->ISA()), nil);
#if SUPPORT_NONPOINTER_ISA
if (isMeta) { // 如果是元类，对isa处理
// Metaclasses do not need any features from non pointer ISA
// This allows for a faspath for classes in objc_retain/objc_release.
cls->setInstancesRequireRawIsa();
} else { // 不是元类，也是对isa处理
// 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 && 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->setInstancesRequireRawIsaRecursively(rawIsaIsInherited);
}
}
// SUPPORT_NONPOINTER_ISA
#endif
// Update superclass and metaclass in case of remapping
// 确定继承链，赋值父类和元类
cls->superclass = supercls;
cls->initClassIsa(metacls);
// Reconcile instance variable offsets / layout.
// 协调实例变量的偏移量/布局。
// 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.
// 经过上一步，再次协调属性对齐后，设置实例大小
cls->setInstanceSize(ro->instanceSize);
// Copy some flags from ro to rw
// 赋值一些 ro 中的 flags标识位 到 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.
// 从ro或父类传播关联的对象禁止标志。
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
// 添加当前类到父类的子类列表中，如果没有父类，设置自己就是根类
if (supercls) {
addSubclass(supercls, cls);
} else {
addRootClass(cls);
}
// Attach categories
// 附加分类
methodizeClass(cls, previously);
return cls;
}
确认是 GLPerson 的时候进入 realizeClassWithoutSwift
，在 auto ro = (const class_ro_t *)cls->data(); 这行代码打一个端点，这一步是 cls 读取 data 数据赋值给 ro。
然后往下走一步，看看读取到了什么数据？
类的ro数据查看.png
3.4 递归调用 realizeClassWithoutSwift
supercls = realizeClassWithoutSwift(remapClass(cls->superclass), nil);
metacls = realizeClassWithoutSwift(remapClass(cls->ISA()), nil);
// Update superclass and metaclass in case of remapping
cls->superclass = supercls;
cls->initClassIsa(metacls);
realizeClassWithoutSwift 的源码中，递归调用了 realizeClassWithoutSwift，分别传入的是父类和元类（ISA()）。
这是为了确定继承链的关系。
3.5 懒加载类实现时机
知道了非懒加载类在 map_images 时，调用 realizeClassWithoutSwift 实现的，那懒加载类是什么时候实现的呢？
还是上面的代码，把 GLPerson 中的 + load 方法去掉，然后再realizeClassWithoutSwift 方法里面添加如下代码并断点在 断点3
static Class realizeClassWithoutSwift(Class cls, Class previously)
{
runtimeLock.assertLocked();
class_rw_t *rw;
Class supercls;
Class metacls;
// loong test begin
if (cls) {
const char *mangledName = cls->mangledName();
const char *GLPersonName = "GLPerson";
if (strcmp(mangledName, GLPersonName) == 0) {
printf("%s 来到了自定义类 %s", __func__, mangledName); // 断点3
}
}
// loong test end
// 省略...
}
在 main.m 中添加，并添加断点
GLPerson *p = [GLPerson alloc];
运行工程，首先断点在 GLPerson *p = [GLPerson alloc];
继续运行，发现走到了 断点3。查看堆栈
懒加载类加载堆栈.png
可知：懒加载类 是在第一次发送消息的时候，调用 realizeClassWithoutSwift 实现类的。
3.6 懒加载类的优点
因为正常的工程中，实现 + load 方法的类很少，大部分类都是懒加载类。
如果所有的类都在启动的时候实现完成，就会非常慢，懒加载类等到调用的时候再去实现，这样会加快启动速度。
再就是每个类都有很多的代码，包括变量，方法等，会占用很多内存，而如果你这个类在工程中就没调用，或者在很深的页面才会调用，正常情况下很少人会使用到，如果在启动加载了，就会造成内存浪费。
懒加载类优点：
- 加快启动速度
- 节省应用初始内存
四、methodizeClass 附加分类
作用：修复 cls 的方法列表，协议列表和属性列表。附加任何未解决的分类。
源码：
static void methodizeClass(Class cls, Class previously)
{
runtimeLock.assertLocked();
bool isMeta = cls->isMetaClass();
auto rw = cls->data();
auto ro = rw->ro(); // 读取ro数据
auto rwe = rw->ext(); // 读取ext，赋值给rwe
// 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(); // 获取ro中的方法列表
if (list) {
// 对方法列表list重新排序
prepareMethodLists(cls, &list, 1, YES, isBundleClass(cls));
if (rwe) rwe->methods.attachLists(&list, 1); // 如果有rwe，添加方法列表list到rwe的methodsList
}
property_list_t *proplist = ro->baseProperties;
if (rwe && proplist) {
rwe->properties.attachLists(&proplist, 1);
}
protocol_list_t *protolist = ro->baseProtocols;
if (rwe && protolist) {
rwe->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 根元类添加initialize方法
addMethod(cls, @selector(initialize), (IMP)&objc_noop_imp, "", NO);
}
// Attach categories. 附加分类
if (previously) {
if (isMeta) {
objc::unattachedCategories.attachToClass(cls, previously,
ATTACH_METACLASS);
} else {
// When a class relocates, categories with class methods
// may be registered on the class itself rather than on
// the metaclass. Tell attachToClass to look for those.
objc::unattachedCategories.attachToClass(cls, previously,
ATTACH_CLASS_AND_METACLASS);
}
}
objc::unattachedCategories.attachToClass(cls, cls,
isMeta ? ATTACH_METACLASS : ATTACH_CLASS);
#if DEBUG
// Debug: sanity-check all SELs; log method list contents
for (const auto& meth : rw->methods()) {
if (PrintConnecting) {
_objc_inform("METHOD %c[%s %s]", isMeta ? '+' : '-',
cls->nameForLogging(), sel_getName(meth.name));
}
ASSERT(sel_registerName(sel_getName(meth.name)) == meth.name);
}
#endif
}
