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# 类别（Category）、拓展（Extension）、load、initialize
> 很多人都知道类别、分类的用法，但是对于一些细节就不是很清楚了，本文主要梳理下这3个概念的细节
## 类别（Category）
### 文件特征
- 类别文件有2个，分别为 .h 和 .m
- 命名为： “类名+类别名.h”和“类名+类别名.m”
### 文件内容格式
.h 文件格式
```
#import "类名.h"
@interface 类名 (类别名)
// 在此处声明方法
@end
```
.m 文件格式
```
#import "类名+类别名.h"
@implementation 类名 (类别名)
// 在此处实现声明的方法
@end
```
### 类别的作用
拓展当前类，为类添加方法
### 类别的局限性
- 无法向现有的类添加实例变量（编译器报“instance variables may not be placed in categories”）。Category 一般只为类提供方法的拓展，不提供属性的拓展。但是利用 Runtime 可以在 Category 中添加属性
- 方法名称冲突的情况下，如果 Category 中的方法与当前类的方法名称重名，Category 具有更高的优先级，类别中的方法将完全取代现有类中的方法（调用方法的时候不会去调用现有类里面的方法实现）。
- 当现有类具有多个 Category 的时候，如果每个 Category 都有同名的方法，那么在调用方法的时候肯定不会调用现有类的方法实现。系统根据编译顺序决定调用哪个 Category 下的方法实现。（可以在 Targets -> Build phases -> Compile Sources 下给多个 Category 更换顺序看看到底在执行哪个方法）
### Category 的使用和注意
1. Category 中的方法如果和现有类方法一致，工程中任何调用当前类的方法的时候都会去调用 Category 里面的方法（比如：UIViewCtroller、UITableView这些）的方法时要慎重。因为用Category重写类中的方法会对子类造成很大的影响。比如：用Category 重写了 UIViewCtroller 的方法 A，那么如果你在工程中用到的所有继承自 UIViewCtroller 的子类，去调用方法 A 时，执行的都是 Category 中重写的方法 A,如果不幸的是，你写的方法 A 有 Bug，那么会造成整个工程中调用该方法的所有 UIViewCtroller 子类的不正常。除非你在子类中重写了父类的方法 A，这样子类调用方法 A 时是调用的自己重写的方法 A，消除了父类 Category 中重写方法对自己的影响
2. Category拓展方法按照有没有重写当前类中的方法，分为未重写的拓展方法和重写拓展方法。且类引用自己的 Category 时，只能在 .m 文件中引用（.h 文件引用自己的类别会报错）。子类引用父类的 Category 在 .h 或 .m 都可以。如果类调用 Category 中重写的方法，不用引入 Category 头文件，系统会自动调用 Category 中的重写方法
3. Category 中如果重写了 A 类从父类继承来的某方法，不会影响与 A 同层级的 B 类
4. 子类会不会继承父类的 Category： Category 中重写的方法会对子类造成影响，但是子类不会继承非重写的方法（现有类中没有的方法）。但是在子类中引入父类 Category 的声明文件后，子类就会继承 Category 的非重写方法。继承的表现是：当子类的方法和父类 Category 中的方法名完全相同，那么子类里的方法会覆盖掉父类 Category，相当于子类重写了继承自父类的方法
5. Category 的作用是向下有效的。即只会影响到该类的所有子类。比如 A 类和 B 类是继承自 Super 类的2个子类，当给 A 类添加一个 Category sayHello 方法，仅有A 类的子类才可以使用 sayHello 方法
## Category 底层原理
### Category 是 category_t 结构体
Demo
```objectivec
@interface Person : NSObject
@property (nonatomic, strong) NSString *name;
- (void)sayHi;
- (void)sleep;
@end
@implementation Person
- (void)sayHi{
NSLog(@"Person sayHi");
}
- (void)sleep{
NSLog(@"人生无常，抓紧睡觉");
}
@end
// Person+Study.h
@interface Person (Study)<NSCopying>
@property (nonatomic, strong) NSString *no;
- (void)study;
+ (void)sleep;
@end
#import "Person+Study.h"
@implementation Person (Study)
- (void)study{
}
+ (void)sleep{
}
- (void)setNo:(NSString *)no{
}
- (NSString *)no{
return nil;
}
- (id)copyWithZone:(NSZone *)zone{
return self;
}
@end
```
clang 转为 c++ 代码
查看到 Category 本质是一个结构体
```c
struct _category_t {
const char *name;
struct _class_t *cls;
const struct _method_list_t *instance_methods;
const struct _method_list_t *class_methods;
const struct _protocol_list_t *protocols;
const struct _prop_list_t *properties;
};
struct _class_t {
struct _class_t *isa;
struct _class_t *superclass;
void *cache;
void *vtable;
struct _class_ro_t *ro;
};
static struct /*_method_list_t*/ {
unsigned int entsize; // sizeof(struct _objc_method)
unsigned int method_count;
struct _objc_method method_list[1];
} _OBJC_$_CATEGORY_CLASS_METHODS_Person_$_Study __attribute__ ((used, section ("__DATA,__objc_const"))) = {
sizeof(_objc_method),
1,
{{(struct objc_selector *)"sleep", "v16@0:8", (void *)_C_Person_Study_sleep}}
};
static const char *_OBJC_PROTOCOL_METHOD_TYPES_NSCopying [] __attribute__ ((used, section ("__DATA,__objc_const"))) =
{
"@24@0:8^{_NSZone=}16"
};
static struct /*_method_list_t*/ {
unsigned int entsize; // sizeof(struct _objc_method)
unsigned int method_count;
struct _objc_method method_list[1];
} _OBJC_PROTOCOL_INSTANCE_METHODS_NSCopying __attribute__ ((used, section ("__DATA,__objc_const"))) = {
sizeof(_objc_method),
1,
{{(struct objc_selector *)"copyWithZone:", "@24@0:8^{_NSZone=}16", 0}}
};
```
可以看到 Person+Study 的Category 底层赋值代码如下，就是结构体对象的初始化。
```c
static struct _category_t _OBJC_$_CATEGORY_Person_$_Study __attribute__ ((used, section ("__DATA,__objc_const"))) =
{
"Person",
0, // &OBJC_CLASS_$_Person,
(const struct _method_list_t *)&_OBJC_$_CATEGORY_INSTANCE_METHODS_Person_$_Study,
(const struct _method_list_t *)&_OBJC_$_CATEGORY_CLASS_METHODS_Person_$_Study,
(const struct _protocol_list_t *)&_OBJC_CATEGORY_PROTOCOLS_$_Person_$_Study,
(const struct _prop_list_t *)&_OBJC_$_PROP_LIST_Person_$_Study,
};
```
`_OBJC_CATEGORY_INSTANCE_METHODS_Person__Study` 结构体存放的是对象方法信息，如下
```c
static struct /*_method_list_t*/ {
unsigned int entsize; // sizeof(struct _objc_method)
unsigned int method_count;
struct _objc_method method_list[4];
} _OBJC_$_CATEGORY_INSTANCE_METHODS_Person_$_Study __attribute__ ((used, section ("__DATA,__objc_const"))) = {
sizeof(_objc_method),
4,
{{(struct objc_selector *)"study", "v16@0:8", (void *)_I_Person_Study_study},
{(struct objc_selector *)"setNo:", "v24@0:8@16", (void *)_I_Person_Study_setNo_},
{(struct objc_selector *)"no", "@16@0:8", (void *)_I_Person_Study_no},
{(struct objc_selector *)"copyWithZone:", "@24@0:8^{_NSZone=}16", (void *)_I_Person_Study_copyWithZone_}}
};
```
`_OBJC_CATEGORY_CLASS_METHODS_Person__Study` 结构体存放的是类方法信息，如下
```c
static struct /*_method_list_t*/ {
unsigned int entsize; // sizeof(struct _objc_method)
unsigned int method_count;
struct _objc_method method_list[1];
} _OBJC_$_CATEGORY_CLASS_METHODS_Person_$_Study __attribute__ ((used, section ("__DATA,__objc_const"))) = {
sizeof(_objc_method),
1,
{{(struct objc_selector *)"sleep", "v16@0:8", (void *)_C_Person_Study_sleep}}
};
```
`_OBJC_CATEGORY_PROTOCOLS*_Person__Study` 结构体存放的是遵循的协议信息，如下
```c
static struct /*_protocol_list_t*/ {
long protocol_count; // Note, this is 32/64 bit
struct _protocol_t *super_protocols[1];
} _OBJC_CATEGORY_PROTOCOLS_$_Person_$_Study __attribute__ ((used, section ("__DATA,__objc_const"))) = {
1,
&_OBJC_PROTOCOL_NSCopying
};
```
`_OBJC_PROP_LIST_Person__Study` 存放的是 Category 中的属性信息，如下
```c
static struct /*_prop_list_t*/ {
unsigned int entsize; // sizeof(struct _prop_t)
unsigned int count_of_properties;
struct _prop_t prop_list[1];
} _OBJC_$_PROP_LIST_Person_$_Study __attribute__ ((used, section ("__DATA,__objc_const"))) = {
sizeof(_prop_t),
1,
{{"no","T@\"NSString\",&,N"}}
};
```
查看 Objc 4 源代码，Category 定义如下
```c
struct category_t {
const char *name;
classref_t cls;
struct method_list_t *instanceMethods;
struct method_list_t *classMethods;
struct protocol_list_t *protocols;
struct property_list_t *instanceProperties;
// Fields below this point are not always present on disk.
struct property_list_t *_classProperties;
method_list_t *methodsForMeta(bool isMeta) {
if (isMeta) return classMethods;
else return instanceMethods;
}
property_list_t *propertiesForMeta(bool isMeta, struct header_info *hi);
};
```
### category 中定义的方法，存储在哪？
查看 objc4 的源代码 `objc-os.mm` 文件中的 `_objc_init` 方法
```c
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();
lock_init();
exception_init();
_dyld_objc_notify_register(&map_images, load_images, unmap_image);
}
```
`_objc_init` 内部会调用 `map_images` 方法，其内部如下
```c
void map_images(unsigned count, const char * const paths[],
const struct mach_header * const mhdrs[]) {
rwlock_writer_t lock(runtimeLock);
return map_images_nolock(count, paths, mhdrs);
}
void map_images_nolock(unsigned mhCount, const char * const mhPaths[],
const struct mach_header * const mhdrs[]) {
static bool firstTime = YES;
header_info *hList[mhCount];
uint32_t hCount;
size_t selrefCount = 0;
// Perform first-time initialization if necessary.
// This function is called before ordinary library initializers.
// fixme defer initialization until an objc-using image is found?
if (firstTime) {
preopt_init();
}
if (PrintImages) {
_objc_inform("IMAGES: processing %u newly-mapped images...\n", mhCount);
}
// 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;
}
```
`map_images` 内部会调用 `map_images_nolock`， `map_images_nolock` 会调用 `_read_images`
```c
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;
TimeLogger ts(PrintImageTimes);
runtimeLock.assertWriting();
#define EACH_HEADER \
hIndex = 0; \
hIndex < hCount && (hi = hList[hIndex]); \
hIndex++
if (!doneOnce) {
doneOnce = 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()->swiftVersion() < 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();
}
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");
}
// Discover classes. Fix up unresolved future classes. Mark bundle classes.
for (EACH_HEADER) {
if (! mustReadClasses(hi)) {
// Image is sufficiently optimized that we need not call readClass()
continue;
}
bool headerIsBundle = hi->isBundle();
bool headerIsPreoptimized = hi->isPreoptimized();
classref_t *classlist = _getObjc2ClassList(hi, &count);
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");
// Fix up @selector references
static size_t UnfixedSelectors;
sel_lock();
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]);
sels[i] = sel_registerNameNoLock(name, isBundle);
}
}
sel_unlock();
ts.log("IMAGE TIMES: fix up selector references");
#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. 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);
}
}
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) {
protocol_t **protolist = _getObjc2ProtocolRefs(hi, &count);
for (i = 0; i < count; i++) {
remapProtocolRef(&protolist[i]);
}
}
ts.log("IMAGE TIMES: fix up @protocol references");
// 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
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
realizeClass(cls);
}
}
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++) {
realizeClass(resolvedFutureClasses[i]);
resolvedFutureClasses[i]->setInstancesRequireRawIsa(false/*inherited*/);
}
free(resolvedFutureClasses);
}
ts.log("IMAGE TIMES: realize future classes");
// Discover categories.
for (EACH_HEADER) {
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) {
// 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.
// First, register the category with its target class.
// Then, rebuild the class's method lists (etc) if
// the class is realized.
bool classExists = NO;
if (cat->instanceMethods || cat->protocols
|| cat->instanceProperties)
{
addUnattachedCategoryForClass(cat, cls, hi);
if (cls->isRealized()) {
remethodizeClass(cls);
classExists = YES;
}
if (PrintConnecting) {
_objc_inform("CLASS: found category -%s(%s) %s",
cls->nameForLogging(), cat->name,
classExists ? "on existing class" : "");
}
}
if (cat->classMethods || cat->protocols
|| (hasClassProperties && cat->_classProperties))
{
addUnattachedCategoryForClass(cat, cls->ISA(), hi);
if (cls->ISA()->isRealized()) {
remethodizeClass(cls->ISA());
}
if (PrintConnecting) {
_objc_inform("CLASS: found category +%s(%s)",
cls->nameForLogging(), cat->name);
}
}
}
}
ts.log("IMAGE TIMES: discover categories");
// 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()
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->isPreoptimized()) {
_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 *classlist = _getObjc2ClassList(hi, &count);
for (i = 0; i < count; i++) {
Class cls = remapClass(classlist[i]);
if (!cls) continue;
PreoptTotalClasses++;
if (hi->isPreoptimized()) {
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
}
```
可以看到内部有 `Discover categories` 相关逻辑，里面和 category 方法相关的有 `remethodizeClass`，其实现如下
```c
static void remethodizeClass(Class cls){
category_list *cats;
bool isMeta;
runtimeLock.assertWriting();
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);
}
}
```
可以看到内部调用 `attachCategories` 方法。 `attachCategories` 方法传入 3个参数，第一个是类对象（Person），第二个参数是 Category 数组。内部实现如下
```c
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
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;
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);
}
```
可以看到通过传入的类对象 `cls` 调用其 `cls->data()` 方法，找到对应的类 `class_rw_t` 信息，里面存放方法、属性、协议信息。
```c
// 类对象结构体
struct objc_class : objc_object {
// Class ISA;
Class superclass;
}cache_t cache; // formerly cache pointer and vtable
class_data_bits_t bits; // class_rw_t * plus custom rr/alloc flags
class_rw_t *data() {
return bits.data();
}
}
struct class_rw_t {
uint32_t flags;
uint32_t version;
const class_ro_t *ro;
method_array_t methods;
property_array_t properties;
protocol_array_t protocols;
class_rw_t* data() {
return (class_rw_t *)(bits & FAST_DATA_MASK);
}
}
```
可以看到最后调用 `attachLists`，内部实现如下
```c
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;
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]));
}
}
```
其中关键函数 `memmove` 代表将 __src 中的前 __len 个字节长度移动到 __dst 中去。
```c
memmove(array()->lists + addedCount, array()->lists,
oldCount * sizeof(array()->lists[0]));
memcpy(array()->lists, addedLists,
addedCount * sizeof(array()->lists[0]));
```
其中，`array()->lists` 代表类对象原来的方法列表、`oldCount * sizeof(array()->lists[0])` 代表类对象原来方法列表长度，`addedCount` 代表 category 方法列表长度。
c 数组指针 `array()->lists + addedCount` 可以代表其中的位置。
`memmove` 效果为将类原方法列表移动到第 n个（n为 category 方法列表长度位置，前面空出n个坑位。
`memcopy` 效果将 category 方法列表拷贝到类原方法列表的前面去。位置刚好是 `memmove` 留出的坑位。
过程如下
![](https://github.com/FantasticLBP/knowledge-kit/raw/master/assets/runtime-categoryattachLists.png)
QA：
分类中可以写属性吗？
不可以，查看分类的 category_t 结构体可以看到没有 `**const** ivar_list_t * ivars;` ，所以 category 声明属性底层只会生成 setter、getter 方法声明，没有实现。需要程序员利用 runtime 关联属性自己实现
同理，分类中也不可以添加成员变量，下面代码会报错。
```
@interface Person (Study)<NSCopying>
{
int _age;
}
@end
```
总结：
Category 编译之后 底层结构为 struct category_t，里面存储着分类的对象方法、类方法、属性、协议信息
程序运行的时候，runtime 会将 Category 中的数据，合并到类自身信息中（类对象、元类对象）
## 拓展（Extension）
### 文件特征
- 只存在一个文件
- 命名方式：“类名_拓展名.h”
```
#import "类名.h"
@interface 类名 ()
// 在此添加私有成员变量、属性、声明方法
@end
```
### 拓展的作用
1. 为类增加额外的属性、成员变量、方法声明
2. 一般将类拓展直接写到当前类的 .m 文件中。不单独创建
3. 一般私有的属性和方法写到类拓展中
4. 和 Category 类似，但是小括号里面没有拓展的名字
5. 拓展里面的属性和方法，会在编译阶段将相关数据和类本身合并
### 拓展的局限性
1. Extension 中添加的属性、成员变量、方法属于私有（只可以在本类的 .m 文件中访问、调用。在其他类里面是无法访问的，同时子类也是无法继承的）。假如我们有这样一个需求，一个属性对外是只读的，对内是可以读写的，那么我们可以通过 Extension 实现。
2. 通常 Extension 都写在 .m 文件中，不会单独建立一个 Extension 文件。而且 Extension 必须写到 @implementation 上方，否则编译报错
3. 类拓展定义的方法和属性必须在类的实现文件中实现。如果单独定义类扩展的文件并且只定义属性的话，也需要将类实现文件中包含进类扩展文件，否则会找不到属性的 setter 和 getter 方法。
```objectivec
//Web.h
#import "Person.h"
NS_ASSUME_NONNULL_BEGIN
@interface Web : Person
@end
NS_ASSUME_NONNULL_END
//Web.m
#import "Web.h"
#import "Web+H5.h"
@interface Web ()
@property (nonatomic, strong) NSString *skillStacks;
@end
@implementation Web
- (void)test {
self.skills = @"iOS && Web && Node && Hybrid";
self.skillStacks = @"iOS && Web && Node && Hybrid";
}
- (void)show {
NSLog(@"%@",self.skillStacks);
}
@end
```
## 总结
1. Category 只能拓充方法，不能拓展属性和成员变量（包含成员变量会报错。属性虽然不可以直接拓展，利用 Runtime 可以实现）
2. 如果 Category 中声明了1个属性，那么 Category 只会生成 setter 和 getter 的声明，不会有实现
3. Extension 也被成为匿名的 Category
4. 分类的方法本质是追加在当前类方法列表后，所以分类的方法会覆盖当前类的方法。
关于第4点，我们可以查看源代码印证下。去 opensource 下载 objc4
OC 入口函数`_objc_init`
```objectivec
void _objc_init(void)
{
    // ...
_dyld_objc_notify_register(&map_images, load_images, unmap_image);
}
```
之后注册各种镜像，那么 map_images 哪里来的？
```objectivec
void
map_images_nolock(unsigned mhCount, const char * const mhPaths[],
const struct mach_header * const mhdrs[])
{
// ...
if (hCount > 0) {
_read_images(hList, hCount, totalClasses, unoptimizedTotalClasses);
}
firstTime = NO;
}
```
_read_images 方法内部会调用 remethodizeClass
```objectivec
void _read_images(header_info **hList, uint32_t hCount, int totalClasses, int unoptimizedTotalClasses)
{
// ...
    if (cls->isRealized()) {
        remethodizeClass(cls);
// ...
}
```
remethodizeClass 内部会调用 attachCategories
```objectivec
static void remethodizeClass(Class cls)
{
category_list *cats;
bool isMeta;
runtimeLock.assertWriting();
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 会调用 attachLists
```objectivec
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
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;
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 内部会调用 realloc、memmove、memmcpy
```objectivec
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;
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]));
}
}
```
最后会把类对象、元类对象、分类二元数组整体处理，结果为最后编译的分类在整合后数组的最前面，也就是为什么说分类和原类存在同名方法时，会被覆盖，且最后编译的分类的方法实现是会被调用的原因。
- 通过 Runtime 加载某个类所有的 Category
- 所有的 Category 方法、属性、协议数据，合并到一个大数组中，后面参与编译的 Category 数据，会放在数组前面
- 合并后的 Category 数据(属性、方法、协议)插入到类原来数据的前面(比如class_rw_t->methods)
## 小插曲：为 Category 实现属性的 Setter 和 Getter
```objectivec
#import "Person.h"
NS_ASSUME_NONNULL_BEGIN
@interface Person (Student)
/**< 学号*/
@property (nonatomic, strong) NSString *studyNumber;
@end
NS_ASSUME_NONNULL_END
#import "Person+Student.h"
#import <objc/message.h>
@implementation Person (Student)
- (void)sayHi {
NSLog(@"大家好，我叫%@，我今年%zd岁了",self.name,self.age);
}
/*
* 传统的做法是在 setter 里面这样写
_studyNumber = studyNumber;
ARC 自动管理内存
MRC
[_studyNumber release];
[studyNumber retain];
_studyNumber = studyNumber;
但是在 Category里面不会生成对应的实例变量，因此我们可以利用 Runtime 为我们的 category 关联属性的值
setter:objc_setAssociatedObject(self, @selector(firstView), firstView, OBJC_ASSOCIATION_RETAIN);
getter:objc_getAssociatedObject(self, @selector(firstView));
}
*/
- (void)setStudyNumber:(NSString *)studyNumber {
objc_setAssociatedObject(self, @selector(studyNumber), studyNumber
, OBJC_ASSOCIATION_RETAIN);
}
//@selector(studyNumber)
- (NSString *)studyNumber {
return objc_getAssociatedObject(self, @selector(studyNumber));
}
@end
```
说明： `objc_setAssociatedObject` 的第二个参数是`const void * _Nonnull key` 所以可以用 "studyNumber" 或者利用 `@selector()` 的特性返回的数据类型也满足，所以示例代码选用第二种方式
给分类添加属性的时候，为了避免多人开发对于属性添加造成的覆盖，我们需要为属性起一个独特的名字。比如我们的工程是组件化、模块化开展的工程，那么我们可以为属性命名的时候在前面添加当前模块的前缀。
比如我们在 Login-Register-Module 模块为 NSURL 的 Category 添加一个 title 的属性的时候，可以这样命名 LR_Title。请查看下面的代码
```Objective-c
#import <Foundation/Foundation.h>
NS_ASSUME_NONNULL_BEGIN
@interface NSURL (Title)
@property (nonatomic, copy) NSString *LR_title;
@end
NS_ASSUME_NONNULL_END
#import "NSURL+Title.h"
#import <objc/runtime.h>
@implementation NSURL (Title)
- (void)setLR_title:(NSString *)LR_title
{
objc_setAssociatedObject(self, @selector(LR_title), LR_title
, OBJC_ASSOCIATION_RETAIN);
}
- (NSString *)LR_title
{
return objc_getAssociatedObject(self, @selector(LR_title));
}
@end
```
## 底层窥探 load 方法
Demo 验证。
```objectivec
@interface Person : NSObject
@end
@interface Student : Person
@end
@interface Student (Good)
@end
@interface Student (Bad)
@end
// 其中每个类都存在3个方法
+ (void)load{
NSLog(@"%s", __func__);
}
+ (void)initialize{
NSLog(@"%s", __func__);
}
- (void)test{
NSLog(@"%s", __func__);
}
// Test
Student *st = [[Student alloc] init];
2022-04-16 01:35:22.237692+0800 Main[8752:2908124] +[Person load]
2022-04-16 01:35:22.238305+0800 Main[8752:2908124] +[Student load]
2022-04-16 01:35:22.238450+0800 Main[8752:2908124] +[Student(Good) load]
2022-04-16 01:35:22.238562+0800 Main[8752:2908124] +[Student(Bad) load]
2022-04-16 01:35:22.238664+0800 Main[8752:2908124] +[Person initialize]
2022-04-16 01:35:22.238733+0800 Main[8752:2908124] +[Student(Bad) initialize]
2022-04-16 01:35:22.238794+0800 Main[8752:2908124] -[Student(Bad) test]
```
QA:
- 为什么 load 方法打印顺序是这样的？
因为调用 student alloc，相当于发送了消息。则肯定先执行 load 方法。类在 Runtime 启动阶段会调用 `schedule_class_load` 方法。方法内部递归调用，如果当前类存在父类则递归调用，否则将当前类加载到 loadable_classes 最后面。load 方法在本质上是执行 `call_load_methods`，方法地址是确定的。不走 objc_msgSend 这套流程。所以先打印父类 load、再打印子类 load、最后打印分类 load。如果存在多个分类，则按照编译顺序打印 load。
- 为什么 load 方法不是按照 Category 编译顺序倒序调用 load 方法？
看源代码 Objc4
```c
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();
lock_init();
exception_init();
_dyld_objc_notify_register(&map_images, load_images, unmap_image);
}
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
{
rwlock_writer_t lock2(runtimeLock);
prepare_load_methods((const headerType *)mh);
}
// Call +load methods (without runtimeLock - re-entrant)
call_load_methods();
}
void call_load_methods(void){
static bool loading = NO;
bool more_categories;
loadMethodLock.assertLocked();
// Re-entrant calls do nothing; the outermost call will finish the job.
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(); // 先调用类的 load 方法
}
// 2. Call category +loads ONCE
more_categories = call_category_loads(); // 再调用 category 的 load 方法
// 3. Run more +loads if there are classes OR more untried categories
} while (loadable_classes_used > 0 || more_categories);
objc_autoreleasePoolPop(pool);
loading = NO;
}
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);
}
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;
}
```
会发现源码中先调用类的 load 方法，再调用 category 的 load 方法。
再看看 `call_class_loads`、`call_category_loads` 方法内部实现，是直接找到 `load_method_t load_method = (load_method_t)classes[i].method;` 类对象的 load 方法地址。最后直接调用 `(*load_method)(cls, SEL_load);` 方法本身。
test 方法是走消息发送流程 `objc_msgSend()` 所以会走 isa、superclass 这一套流程，test 是对象方法，所以需要根据 isa 找到类对象，从类对象的对象方法列表找到 test 方法，找不到则根据 superclass 找到当前类对象的父类对象，继续查找方法列表。直到 NSObject、nil 对象为止，然后走消息起死回生的阶段。
看2个结构体
```c
struct loadable_class {
Class cls; // may be nil
IMP method; // 指向类的 load 方法
};
struct loadable_category {
Category cat; // may be nil
IMP method; // 指向分类的 load 方法
};
```
类、分类的 load 方法调用顺序？
1. 调用类的 +load 方法顺序
- 调用类的 +load
- 根据编译先后顺序调用 +load（先编译先调用）
- 存在继承关系的类，会先调用父类的 +load
2. 调用分类的 +load 方法顺序
- 按照编译顺序调用分类的 +load（先编译先调用）
源代码印证
```objectivec
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
{
rwlock_writer_t lock2(runtimeLock);
prepare_load_methods((const headerType *)mh);
}
// Call +load methods (without runtimeLock - re-entrant)
call_load_methods();
}
void prepare_load_methods(const headerType *mhdr){
size_t count, i;
runtimeLock.assertWriting();
classref_t *classlist =
_getObjc2NonlazyClassList(mhdr, &count);
for (i = 0; i < count; i++) {
schedule_class_load(remapClass(classlist[i]));
}
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
realizeClass(cls);
assert(cls->ISA()->isRealized());
add_category_to_loadable_list(cat);
}
}
```
我们看看 `schedule_class_load` 方法。方法内部递归调用，如果当前类存在父类则递归调用，否则将当前类加载到 loadable_classes 最后面
```objectivec
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
schedule_class_load(cls->superclass);
add_class_to_loadable_list(cls);
cls->setInfo(RW_LOADED);
}
```
```objectivec
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; // 加载到最后
loadable_classes_used++;
}
```
`prepare_load_methods` 处理完再执行 `call_load_methods`
```objectivec
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();
}
// 2. Call category +loads ONCE
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;
}
这里
```
这里的代码已经看过了，也就先加载类的 +load 方法，加载顺序按照 loadable_classes 中的类顺序进行访问 +load。之后再加载 Catetory 的 +load 方法。
在 `prepare_load_methods` 方法内部先给普通类按照编译顺序（谁先编译谁先添加，遇到存在父类的类，递归调用父类对象）添加类信息到 loadable_classes 中，之后给分类按照编译顺序添加到（谁先编译谁先添加） loadable_categories 中。
+load 方法在 Runtime 加载类、分类的时候调用。
Extension 在编译阶段，数据已经包含在类信息中。
Category 是在运行阶段，才会将数据合并到类信息中。
## 底层窥探 Initialize 方法
上 Demo
```objectivec
@interface Person : NSObject
@end
@interface Student : Person
@end
@interface Student (Good)
@end
```
`Person *p1 = [[Person alloc] init];` 这句代码输出什么? 这个比较简单，initialize 方法在类第一次收到消息的时候调用。所以输出 `+[Person initialize]`
`Student *st = [[Student alloc] init];` 输出什么?
```objectivec
+[Person initialize]
+[Student(Good) initialize]
```
查看分类在 Runtime 加载类信息时候的调用原理可以知道，分类中的类方法、对象方法都会被加载原始类的前面去（initialize 是类方法）如下图：
![](https://github.com/FantasticLBP/knowledge-kit/raw/master/assets/runtime-categoryattachLists.png)
+initialize 和 +load 最大区别是 +initialize 是通过 objc_msgSend 进行调用的
- 调用方式：load 根据函数地址直接调用，initialize 是根据 objc_msgSend 调用的
- 调用时刻：load 是 runtime 加载类、分类的时候调用的。initialize 是在类第一次接收消息的时候调用的。每个类只会 initialize 一次，但是父类的 initialize 可能会调用多次
- 调用顺序：
- load：先调用类的 load（先编译的类优先调用 load、调用子类的 load，会先调用父类的 load）、再调用分类的 load（先编译的分类，优先调用 load ）
- 如果子类没有实现 +initialize 则会调用父类的 +initialize（所以父类的 +initialize 可能会被调用多次）
- 如果分类实现了 +initialize，就会覆盖类本身的 +initialize 调用
查看源码，伪代码如下：
```
if (自己没有初始化) {
    if (父类没有初始化) {
    objc_msgSend(父类，@selector(initializ))
    }
objc_msgSend(子类，@selector(initializ))    
}
```
```objectivec
void _class_initialize(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.
supercls = cls->superclass;
if (supercls && !supercls->isInitialized()) {
_class_initialize(supercls);
}
// Try to atomically set CLS_INITIALIZING.
{
monitor_locker_t lock(classInitLock);
if (!cls->isInitialized() && !cls->isInitializing()) {
cls->setInitializing();
reallyInitialize = YES;
}
}
if (reallyInitialize) {
// We successfully set the CLS_INITIALIZING bit. Initialize the class.
// Record that we're initializing this class so we can message it.
_setThisThreadIsInitializingClass(cls);
if (MultithreadedForkChild) {
// LOL JK we don't really call +initialize methods after fork().
performForkChildInitialize(cls, supercls);
return;
}
// Send the +initialize message.
// Note that +initialize is sent to the superclass (again) if
// this class doesn't implement +initialize. 2157218
if (PrintInitializing) {
_objc_inform("INITIALIZE: thread %p: calling +[%s initialize]",
pthread_self(), cls->nameForLogging());
}
// Exceptions: A +initialize call that throws an exception
// is deemed to be a complete and successful +initialize.
//
// Only __OBJC2__ adds these handlers. !__OBJC2__ has a
// bootstrapping problem of this versus CF's call to
// objc_exception_set_functions().
#if __OBJC2__
@try
#endif
{
callInitialize(cls);
if (PrintInitializing) {
_objc_inform("INITIALIZE: thread %p: finished +[%s initialize]",
pthread_self(), cls->nameForLogging());
}
}
#if __OBJC2__
@catch (...) {
if (PrintInitializing) {
_objc_inform("INITIALIZE: thread %p: +[%s initialize] "
"threw an exception",
pthread_self(), cls->nameForLogging());
}
@throw;
}
@finally
#endif
{
// Done initializing.
lockAndFinishInitializing(cls, supercls);
}
return;
}
else if (cls->isInitializing()) {
// We couldn't set INITIALIZING because INITIALIZING was already set.
// If this thread set it earlier, continue normally.
// If some other thread set it, block until initialize is done.
// It's ok if INITIALIZING changes to INITIALIZED while we're here,
// because we safely check for INITIALIZED inside the lock
// before blocking.
if (_thisThreadIsInitializingClass(cls)) {
return;
} else if (!MultithreadedForkChild) {
waitForInitializeToComplete(cls);
return;
} else {
// We're on the child side of fork(), facing a class that
// was initializing by some other thread when fork() was called.
_setThisThreadIsInitializingClass(cls);
performForkChildInitialize(cls, supercls);
}
}
else if (cls->isInitialized()) {
// Set CLS_INITIALIZING failed because someone else already
// initialized the class. Continue normally.
// NOTE this check must come AFTER the ISINITIALIZING case.
// Otherwise: Another thread is initializing this class. ISINITIALIZED
// is false. Skip this clause. Then the other thread finishes
// initialization and sets INITIALIZING=no and INITIALIZED=yes.
// Skip the ISINITIALIZING clause. Die horribly.
return;
}
else {
// We shouldn't be here.
_objc_fatal("thread-safe class init in objc runtime is buggy!");
}
}
void callInitialize(Class cls) {
((void(*)(Class, SEL))objc_msgSend)(cls, SEL_initialize);
asm("");
}
```
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