extension（类扩展）和 category（类别）

extension（类扩展）

简单来说，extension在.m文件中添加，所以其权限为private，所以只能拿到源码的类添加extension。另外extension是编译时决议，和interface和implement里的代码融合在一块了一般。

category（类别）

category能在不继承类的情况下给类动态添加方法。

1、创建category

关于@dynamic的特性及用法可参考：
https://blog.csdn.net/qq_28446287/article/details/79094491

2、category的优缺点

可以将类的实现代码分散到多个不同的文件或框架中
创建对私有方法的前向引用
OC语法中，你不能对一个类的方法定义为private，只有+、-,对实例变量可以private、public。
具体可参考此文档http://www.cnblogs.com/stevenwuzheng/p/5457487.html
向对象添加非正式协议
对NSObject进行一个类别叫做非正式协议，可以只实现想要的方法

3、category在runtime中的源码

typedef struct objc_category *Category;

struct objc_category
{//类别的名字char * _Nonnull category_name                            OBJC2_UNAVAILABLE;//该类的名字char * _Nonnull class_name                               OBJC2_UNAVAILABLE;//实例方法列表struct objc_method_list * _Nullable instance_methods     OBJC2_UNAVAILABLE;//类方法列表struct objc_method_list * _Nullable class_methods        OBJC2_UNAVAILABLE;//协议列表struct objc_protocol_list * _Nullable protocols          OBJC2_UNAVAILABLE;
}                                                            OBJC2_UNAVAILABLE;

明显看出，Category没有容纳变量的地方。

4、category的原理

objective-c的运行依赖runtime，runtime依赖于dyld动态加载，查看objc-os.mm文件发现其调用栈如下：

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被附加到类上是在map_images的时候发生的。在new-ABI的标准下，_objc_init函数里调用的map_iamges最终会调用objc-runtime-new.mm中的_read_images函数。_read_images中部分代码：

 // 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");

从代码可以看出：

将category和它的主类(或元类)注册到哈希表中.
如果主类(或元类)已经实现，那么重建它的方列表。

category中的实例方法和属性被整合到主类中，而类方法被整合到元类中。而协议被同时整合到了主类和元类中。

/***********************************************************************
* 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);
    }
}

如注释所说，此函数将未处理的category整合到主类(或元类)，整合cls的方法、协议、属性列表，更新cls及其子类的方法缓存。

查看其中的attachCategories函数：

// 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 allocationsmethod_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 firstint 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);
}

通过while循环，遍历所有的category，得每个category的方法列表mlist、proplist和protolist并存入主类(或元类)的mlists、proplists和protolists中。从而更新类的数据字段data()中mlist、proplist和protolist的值。

category没有替换掉原来类的方法，也就是说如果category和原来类有method1，那么在将category整合到原来类之后，类的method_list会有两个method1

category中的method1会被放在类的method_list前面，而原来类的method1被放到了method_list后面，在调用时候会先调用method_list前面的，所以看起来是将原来类的method1覆盖了，实际上并不是那么回事。

5、category的两个面试题

3.1 一个类和它的category同时拥有一个方法，在调用时候调用哪一个？答案参考“2、category的原理”

3.2 一个类有多个category并都拥有同一个方法，在调用时候调用哪一个?答案参考“2、category的原理”

举个例子：

//#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，输出的结果跟compile source文件中的category的.m文件放置顺序有关。且放最底部的时候输出（主类.m文件的放置不影响，理由参考”2、category的原理”）

6、category动态添加变量

@interface type (xxxx)@property (nonatomic, assign)  NSInteger number;@endstatic 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的描述可参考：https://www.cnblogs.com/liuting-1204/p/6526342.html