目录
一，作用
二，本质
三，同名方法调用优先级
一，作用
1，给系统类或第三方类添加属性
- 因为无法修改系统类或第三方类的代码，所以只能利用分类进行添加
- 因为在分类中不能添加成员变量，所以系统不会自动生成带下划线的成员变量和
get/set方法 - 如果想正常使用分类中的属性，那必须利用
runtime的关联函数来手动实现get/set方法
@interface UIView (Add)
@property (nonatomic, copy) NSString *name;
@end
@implementation UIView (Add)
- (NSString *)name {
return objc_getAssociatedObject(self, @"name");
}
- (void)setName:(NSString *)name {
objc_setAssociatedObject(self, @"name", name, OBJC_ASSOCIATION_COPY_NONATOMIC);
}
@end
2，给系统类或第三方类添加方法
- 因为无法修改系统类或第三方类的代码，所以只能利用分类进行添加
@interface UIView (Add)
- (void)removeAllSubviews;
@end
@implementation UIView (Add)
- (void)removeAllSubviews {
[self.subviews enumerateObjectsUsingBlock:^(__kindof UIView * _Nonnull obj, NSUInteger idx, BOOL * _Nonnull stop) {
[obj removeFromSuperview];
}];
}
@end
3，拆分自定义类的代码
- 如果某类的代码量比较大，可以将代码按照功能拆分到各个分类中
@interface ApiRequest : NSObject
@end
@interface ApiRequest (Login)
@end
@interface ApiRequest (Home)
@end
@interface ApiRequest (My)
@end
二，本质
@interface Person (Add) <NSCopying>
@property (nonatomic, assign) NSInteger age;
- (void)eat;
+ (void)run;
@end
@implementation Person (Add)
- (void)eat {
NSLog(@"eat");
}
+ (void)run {
NSLog(@"run");
}
@end
将上述代码用clang转为C++代码，Person (Add)的底层代码如下，可以看到分类的本质是结构体
// 底层结构
struct _category_t {
const char *name; // 类名
struct _class_t *cls; // 指向类的指针
const struct _method_list_t *instance_methods; // 实例方法列表
const struct _method_list_t *class_methods; // 类方法列表
const struct _protocol_list_t *protocols; // 协议列表
const struct _prop_list_t *properties; // 属性列表
};
// 用_category_t实例化Person (Add)
static struct _category_t _OBJC_$_CATEGORY_Person_$_Add __attribute__ ((used, section ("__DATA,__objc_const"))) =
{
"Person",
0, // &OBJC_CLASS_$_Person,
(const struct _method_list_t *)&_OBJC_$_CATEGORY_INSTANCE_METHODS_Person_$_Add,
(const struct _method_list_t *)&_OBJC_$_CATEGORY_CLASS_METHODS_Person_$_Add,
(const struct _protocol_list_t *)&_OBJC_CATEGORY_PROTOCOLS_$_Person_$_Add,
(const struct _prop_list_t *)&_OBJC_$_PROP_LIST_Person_$_Add,
};
// Person (Add)的实例方法列表
static struct /*_method_list_t*/ {
unsigned int entsize; // sizeof(struct _objc_method)
unsigned int method_count;
struct _objc_method method_list[1];
} _OBJC_$_CATEGORY_INSTANCE_METHODS_Person_$_Add __attribute__ ((used, section ("__DATA,__objc_const"))) = {
sizeof(_objc_method),
1,
{{(struct objc_selector *)"eat", "v16@0:8", (void *)_I_Person_Add_eat}}
};
// Person (Add)的类方法列表
static struct /*_method_list_t*/ {
unsigned int entsize; // sizeof(struct _objc_method)
unsigned int method_count;
struct _objc_method method_list[1];
} _OBJC_$_CATEGORY_CLASS_METHODS_Person_$_Add __attribute__ ((used, section ("__DATA,__objc_const"))) = {
sizeof(_objc_method),
1,
{{(struct objc_selector *)"run", "v16@0:8", (void *)_C_Person_Add_run}}
};
// Person (Add)的协议列表
static struct /*_protocol_list_t*/ {
long protocol_count; // Note, this is 32/64 bit
struct _protocol_t *super_protocols[1];
} _OBJC_CATEGORY_PROTOCOLS_$_Person_$_Add __attribute__ ((used, section ("__DATA,__objc_const"))) = {
1,
&_OBJC_PROTOCOL_NSCopying
};
// Person (Add)的属性列表
static struct /*_prop_list_t*/ {
unsigned int entsize; // sizeof(struct _prop_t)
unsigned int count_of_properties;
struct _prop_t prop_list[1];
} _OBJC_$_PROP_LIST_Person_$_Add __attribute__ ((used, section ("__DATA,__objc_const"))) = {
sizeof(_prop_t),
1,
{{"age","Tq,N"}}
};
注意：因为_category_t中没有成员变量列表，所以在分类中不能添加成员变量
三，同名方法调用优先级
- 原类与分类：优先调用分类的
- 多个分类：优先调用后编译的（编译顺序从上往下）
1， 代码验证
编译顺序
// Person
@interface Person : NSObject
- (void)eat;
@end
@implementation Person
- (void)eat {
NSLog(@"Person eat");
}
@end
// Person (Add1)
@interface Person (Add1)
- (void)eat;
@end
@implementation Person (Add1)
- (void)eat {
NSLog(@"Person (Add1) eat");
}
@end
// Person (Add2)
@interface Person (Add2)
- (void)eat;
@end
@implementation Person (Add2)
- (void)eat {
NSLog(@"Person (Add2) eat");
}
@end
// 使用
Person *person = [Person new];
[person eat];
// 打印
Person (Add1) eat
2，源码分析（源码下载地址）
- 方法一：重新组织类中的方法
static void remethodizeClass(Class cls)
{
category_list *cats;
bool isMeta;
runtimeLock.assertLocked();
isMeta = cls->isMetaClass();
// Re-methodizing: check for more categories
// 获取所有的分类
if ((cats = unattachedCategoriesForClass(cls, false/*not realizing*/))) {
if (PrintConnecting) {
_objc_inform("CLASS: attaching categories to class '%s' %s",
cls->nameForLogging(), isMeta ? "(meta)" : "");
}
// 方法二
attachCategories(cls, cats, true /*flush caches*/);
free(cats);
}
}
- 方法二：将分类中的方法附加到原类中
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
// 创建方法列表数组
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;
// 倒序遍历分类列表
while (i--) {
// 取出分类
auto& entry = cats->list[i];
// 取出分类中的方法列表
method_list_t *mlist = entry.cat->methodsForMeta(isMeta);
if (mlist) {
// 将方法列表正序放入方法列表数组中
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);
// 方法三
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);
}
- 方法三：将分类的方法列表数组附加到原类的方法列表数组中
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;
// 扩大原类方法列表数组的容量
setArray((array_t *)realloc(array(), array_t::byteSize(newCount)));
array()->count = newCount;
// 将原类的方法列表移动到数组末位
memmove(array()->lists + addedCount, array()->lists,
oldCount * sizeof(array()->lists[0]));
// 将分类的方法列表数组复制到原类方法列表数组的首位
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]));
}
}
- 上述三个方法的逻辑图
步骤一
步骤二
步骤三
3，注意点
分类是在运行期合并到原类中的
分类方法并没有覆盖原类方法，只是放在原类方法的前面，而方法调用是顺序查找，所以优先调用分类方法
- (void)viewDidLoad {
[super viewDidLoad];
[self logMethodNamesWithClass:[Person class]];
}
// 打印类对象或元类对象中的方法名
- (void)logMethodNamesWithClass:(Class)cls {
unsigned int count;
Method *methodList = class_copyMethodList(cls, &count);
NSMutableString *methodNames = [NSMutableString string];
for (int i = 0; i < count; i++) {
Method method = methodList[i];
SEL selector = method_getName(method);
NSString *methodName = NSStringFromSelector(selector);
[methodNames appendString:methodName];
[methodNames appendString:@", "];
}
free(methodList);
NSLog(@"%@---%@", cls, methodNames);
}
// 打印
Person---eat, eat, eat,
