extension（類擴展）
簡單來說，extension在.m文件中添加，所以其權限爲private，所以只能拿到源碼的類添加extension。另外extension是編譯時決議，和interface和implement裏的代碼融合在一塊了一般。
category（類別）
category能在不繼承類的情況下給類動態添加方法。
1、創建category
關於@dynamic的特性及用法可參考：
https://blog.csdn.net/qq_28446287/article/details/79094491
2、category的優缺點
可以將類的實現代碼分散到多個不同的文件或框架中
創建對私有方法的前向引用
OC語法中，你不能對一個類的方法定義爲private，只有+、-,對實例變量可以private、public。
具體可參考此文檔http://www.cnblogs.com/stevenwuzheng/p/5457487.html向對象添加非正式協議
對NSObject進行一個類別叫做非正式協議，可以只實現想要的方法
3、category在runtime中的源碼
typedef struct objc_category *Category;struct objc_category
{
//類別的名字
char * _Nonnull category_name OBJC2_UNAVAILABLE;
//該類的名字
char * _Nonnull class_name OBJC2_UNAVAILABLE;
//實例方法列表
struct objc_method_list * _Nullable instance_methods OBJC2_UNAVAILABLE;
//類方法列表
struct objc_method_list * _Nullable class_methods OBJC2_UNAVAILABLE;
//協議列表
struct objc_protocol_list * _Nullable protocols OBJC2_UNAVAILABLE;
} OBJC2_UNAVAILABLE;明顯看出，Category沒有容納變量的地方。
4、category的原理
objective-c的運行依賴runtime，runtime依賴於dyld動態加載，查看objc-os.mm文件發現其調用棧如下：
void _objc_init(void)
{
static bool initialized = false;
if (initialized) return;
initialized = true;
// fixme defer initialization until an objc-using image is found?
environ_init();
tls_init();
static_init();
lock_init();
exception_init();
// Register for unmap first, in case some +load unmaps something
_dyld_register_func_for_remove_image(&unmap_image);
dyld_register_image_state_change_handler(dyld_image_state_bound,
1/*batch*/, &map_2_images);
dyld_register_image_state_change_handler(dyld_image_state_dependents_initialized, 0/*not batch*/, &load_images);
}
category被附加到類上是在map_images的時候發生的。在new-ABI的標準下，_objc_init函數裏調用的map_iamges最終會調用objc-runtime-new.mm中的_read_images函數。_read_images中部分代碼：
// Discover categories.
for (EACH_HEADER) {
category_t **catlist =
_getObjc2CategoryList(hi, &count);
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
/* || 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");從代碼可以看出：
將category和它的主類(或元類)註冊到哈希表中.
如果主類(或元類)已經實現，那麼重建它的方列表。
category中的實例方法和屬性被整合到主類中，而類方法被整合到元類中。而協議被同時整合到了主類和元類中。
/***********************************************************************
* remethodizeClass
* Attach outstanding categories to an existing class.
* Fixes up cls's method list, protocol list, and property list.
* Updates method caches for cls and its subclasses.
* Locking: runtimeLock must be held by the caller
**********************************************************************/
static void remethodizeClass(Class cls)
{
category_list *cats;
bool isMeta;
runtimeLock.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);
}
}如註釋所說，此函數將未處理的category整合到主類(或元類)，整合cls的方法、協議、屬性列表，更新cls及其子類的方法緩存。
查看其中的attachCategories函數：
// Attach method lists and properties and protocols from categories to a class.
// Assumes the categories in cats are all loaded and sorted by load order,
// oldest categories first.
static void
attachCategories(Class cls, category_list *cats, bool flush_caches)
{
if (!cats) return;
if (PrintReplacedMethods) printReplacements(cls, cats);
bool isMeta = cls->isMetaClass();
// fixme rearrange to remove these intermediate allocations
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);
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);
}通過while循環，遍歷所有的category，得每個category的方法列表mlist、proplist和protolist並存入主類(或元類)的mlists、proplists和protolists中。從而更新類的數據字段data()中mlist、proplist和protolist的值。
category沒有替換掉原來類的方法，也就是說如果category和原來類有method1，那麼在將category整合到原來類之後，類的method_list會有兩個method1
category中的method1會被放在類的method_list前面，而原來類的method1被放 到了method_list後面，在調用時候會先調用method_list前面的，所以看起來是將原來類的method1覆蓋了，實際上並不是那麼回事。
5、category的兩個面試題
3.1 一個類和它的category同時擁有一個方法，在調用時候調用哪一個？答案參考“2、category的原理”
3.2 一個類有多個category並都擁有同一個方法，在調用時候調用哪一個?答案參考“2、category的原理”
舉個例子：
//#import "type.m"
- (void)test
{
NSLog(@"type class！！");
}
//#import "type+xxxx.m"
- (void)test
{
NSLog(@"category xxxx");
}
//#import "type+xxxx1.m"
- (void)test
{
NSLog(@"category xxxx1");
}
//#import "type+xxxx2.m"
- (void)test
{
NSLog(@"category xxxx2");
}
可以知道，輸出的結果跟compile source文件中的category的.m文件放置順序有關。且放最底部的時候輸出（主類.m文件的放置不影響，理由參考”2、category的原理”）
6、category動態添加變量
@interface type (xxxx)
@property (nonatomic, assign) NSInteger number;
@end
static void *numberKey = &numberKey;
@implementation type (xxxx)
- (void)setNumber:(NSInteger)number
{
objc_setAssociatedObject(self, &numberKey, [NSString stringWithFormat:@"%ld",number], OBJC_ASSOCIATION_ASSIGN);
}
- (NSInteger)number
{
return [objc_getAssociatedObject(self, &numberKey) integerValue];
}
@end
objc_setAssociatedObject和objc_getAssociatedObject的描述可參考：https://www.cnblogs.com/liuting-1204/p/6526342.html