iOS底层原理 Category与关联对象本质--(4)

今天我们再看一下Category的底层原理。
先看一下Category的简单使用,首先新增一个类的Category,然后添加需要的函数,然后在使用的文件中导入就可以直接使用了。代码如下:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
@interface FYPerson : NSObject
- (void)run;
@end
@implementation FYPerson
-(void)run{
NSLog(@"run is run");
}
@end
//类别
@interface FYPerson (test)
- (void)test;
@end
@implementation FYPerson (test)
- (void)test{
NSLog(@"test is run");
}
@end
//使用
#import "FYPerson.h"
#import "FYPerson+test.h"
FYPerson *person=[[FYPerson alloc]init];
[person test];
[person run];

类别使用就是这么简单。
那么类别的本质是什么呢?类的方法是存储在什么地方呢?
第一篇类的本质已经讲过了,运行时中,类对象是有一份,方法都存储在类对象结构体fy_objc_class中的class_data_bits_t->data()->method_list_t中的,那么类别方法也是存储在method_list_t和取元类对象的method_list_t中的。编译的时候类别编译成结构体_category_t,然后runtime在运行时动态将方法添加到method_list_t中。运行xcrun -sdk iphoneos clang -arch arm64 -rewrite-objc FYPerson+test.m -o FYPerson+test.cpp进入到FYPerson+test.cpp内部查看编译之后的代码

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
struct _category_t {
const char *name; //"FYPerson"
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;
};
//存储 test方法
static struct /*_method_list_t*/ {
unsigned int entsize; // sizeof(struct _objc_method)
unsigned int method_count;
struct _objc_method method_list[1];
} _OBJC_$_CATEGORY_INSTANCE_METHODS_FYPerson_$_test __attribute__ ((used, section ("__DATA,__objc_const"))) = {
sizeof(_objc_method),
1,
{{(struct objc_selector *)"test", "v16@0:8", (void *)_I_FYPerson_test_test}}
};
extern "C" __declspec(dllimport) struct _class_t OBJC_CLASS_$_FYPerson;
//_category_t 存储FYPerson的分类的数据
static struct _category_t _OBJC_$_CATEGORY_FYPerson_$_test __attribute__ ((used, section ("__DATA,__objc_const"))) =
{
"FYPerson",
0, // &OBJC_CLASS_$_FYPerson,
(const struct _method_list_t *)&_OBJC_$_CATEGORY_INSTANCE_METHODS_FYPerson_$_test,//instace方法
0,//类方法
0,//协议方法
0,//属性
};

存储在_category_t中的数据是什么时间加载到FYPersonclass_data_bits_t.data呢?我们探究一下,打开源码下载打开工程阅读源码找到objc-os.mm,通过查找函数运行顺序得到_objec_init->map_images->map_images_noljock->_read_images->remethodizeClass(cls)->attachCategories(cls, cats, true /*flush caches*/),最终进入到attachCategories关键函数内部:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
// 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
//方法数组[[1,2,3],[4,5,6],[7,8,9]]
method_list_t **mlists = (method_list_t **)
malloc(cats->count * sizeof(*mlists));
//属性数组
property_list_t **proplists = (property_list_t **)
malloc(cats->count * sizeof(*proplists));
//协议数组
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;
//最后的编译文件放到最前边
while (i--) {
//取出某个分类
auto& entry = cats->list[i];
//取出分类 的 instance方法或者class方法
method_list_t *mlist = entry.cat->methodsForMeta(isMeta);
if (mlist) {
mlists[mcount++] = mlist; //mlists 接受所有分类方法
fromBundle |= entry.hi->isBundle();
}
//proplist 接受所有分类属性
property_list_t *proplist =
entry.cat->propertiesForMeta(isMeta, entry.hi);
if (proplist) {
proplists[propcount++] = proplist;
}
//proplist 接受所有协议方法
protocol_list_t *protolist = entry.cat->protocols;
if (protolist) {
protolists[protocount++] = protolist;
}
}
//收集了所有协议 分类方法
auto rw = cls->data();
prepareMethodLists(cls, mlists, mcount, NO, fromBundle);
//追加所有分类方法
rw->methods.attachLists(mlists, mcount);
//释放数组
free(mlists);
//刷新该类的缓存
if (flush_caches && mcount > 0) flushCaches(cls);
//追加所有分类属性
rw->properties.attachLists(proplists, propcount);
free(proplists);//释放数组
//追加所有分类协议
rw->protocols.attachLists(protolists, protocount);
free(protolists);//释放数组
}

attachCategories是将所有的分类方法和协议,属性倒序添加到类中,具体添加的优先级是怎么操作的?进入到rw->protocols.attachLists内部:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
void attachLists(List* const * addedLists, uint32_t addedCount) {
if (addedCount == 0) return;
if (hasArray()) {
// many lists -> many lists
uint32_t oldCount = array()->count;
//一共需要的数量
uint32_t newCount = oldCount + addedCount;
//分配内存 内存不够用了,需要扩容
setArray((array_t *)realloc(array(), array_t::byteSize(newCount)));
//赋值count
array()->count = newCount;
// array()->lists:原来的方法列表向后移动 oldCount * sizeof(array()->lists[0]个长度
memmove(array()->lists + addedCount/*指针移动到数组末尾*/, array()->lists/*数组*/,
oldCount * sizeof(array()->lists[0])/*移动数据的大小*/);
//空出来的 内存使用addedLists拷贝过去 大小是:addedCount * sizeof(array()->lists[0])
memcpy(array()->lists, addedLists,
addedCount * sizeof(array()->lists[0]));
/*
图示讲解:
array()->lists:A->B->C->D->E
addedCount:3
addedLists:P->L->V
memmove之后:nil->nil->nil->A->B->C->D->E
然后再讲addedLists插入到数组前边,最终array()->lists的值是:
P->L->V->A->B->C->D->E
*/
}
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]));
}
}

可以看出来:

  1. 首先通过runtime加载某个类的所有Category数据
  2. 把所有Category的方法,属性,协议数据合并到一个大数组中,后面参与编译的数组会出现在数组前边
  3. 将合并后的分类数组(方法,属性,协议)插入到类原来的数据的前面。

具体的编译顺序是project文件中->Build Phases->Complile Sources的顺序。

调用顺序

+load加载顺序

每个类和分类都会加载的时候调用+load方法,具体是怎么调用呢?我们查看源码_objc_init->load_images->call_load_methods

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
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
//执行class+load直到完成
while (loadable_classes_used > 0) {
call_class_loads();
}
//执行Category +load 一次
// 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;
}

+loadCategory+load前边执行,当类的+load执行完毕然后再去执行Category+load,而且只有一次。
当class有子类的时候加载顺序呢?其实所有类都是基于NSObject,那么我们假设按照编译顺序加载Class+load,就有一个问题是父类+load执行的操作岂不是在子类执行的时候还没有执行吗?这个假设明显不对,基类+load中的操作是第一个执行的,其他子类是按照superclass->class->sonclass的顺序执行的。
查看源码_objc_init->load_images->prepare_load_methods((const headerType *)mh)->schedule_class_loadobjc-runtime-new.mm2856行

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
/***********************************************************************
* Schedule +load for classes in this image, any un-+load-ed
* superclasses in other images, and any categories in this image.
**********************************************************************/
// Recursively schedule +load for cls and any un-+load-ed superclasses.
// cls must already be connected.
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
//递归调用自己直到调用clas->self
schedule_class_load(cls->superclass);
//添加class
add_class_to_loadable_list(cls);
cls->setInfo(RW_LOADED);
}

可以了解到该函数递归调用自己,直到+load方法已经调用过为止,所以不管编译顺序是高低,+load的加载顺序始终是NSObject->FYPrson->FYStudent。多个类平行关系的话,按照编译顺序加载。
下边是稍微复杂点的类关系:

1
2
3
4
5
6
7
8
NSObject
Person
Student
NSObjet
Car
BigCar
BigOrSmallCar

编译顺序是

1
2
3
4
Person
Student
Car
BigOrSmallCar

那么他们+load的加载顺序是:

1
2
NSobject->Person->Student->Car->BigCar->BigOrSmallCar

看着不是很明白的 可以再看一下刚才的schedule_class_load函数。
加载成功之后,是按照objc_msgsend()流程发送的吗?我们进入到call_class_loads内部

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
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);
}
if (classes) free(classes);
}

可以找到(*load_method)(cls, SEL_load);该函数,该函数是直接使用IMP执行的,IMP就是函数地址,可以直接访问函数而不用消息的转发流程。

+initialize调用

  • +initialize方法会在类第一次接收到消息时调用
  • 先调用父类的+initialize,再调用子类的+initialize
  • 先初始化父类,再初始化子类,每个类只会初始化1次

objc源码解读过程objc-msg-arm64.x->objc_msgSend->objc->runtime-new->class_getinstanceMethod->lookUpImpOrNil->lookUpImpOrForward->_clas_initialize->callInitialize->objc_msgSend(cls,SEL_Initialize)
runtime-new.h4819行

1
2
3
4
5
6
7
8
9
10
Method class_getInstanceMethod(Class cls, SEL sel)
{
if (!cls || !sel) return nil;
lookUpImpOrNil(cls, sel, nil,
NO/*initialize*/, NO/*cache*/, YES/*resolver*/);
return _class_getMethod(cls, sel);
}

根据lookUpImpOrNil查看4916行

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
IMP lookUpImpOrForward(Class cls, SEL sel, id inst,
bool initialize, bool cache, bool resolver)
{
IMP imp = nil;
bool triedResolver = NO;
runtimeLock.assertUnlocked();
// Optimistic cache lookup
if (cache) {
imp = cache_getImp(cls, sel);
if (imp) return imp;
}
runtimeLock.lock();
checkIsKnownClass(cls);
if (!cls->isRealized()) {
realizeClass(cls);
}
if (initialize && !cls->isInitialized()) {
runtimeLock.unlock();
_class_initialize (_class_getNonMetaClass(cls, inst));
runtimeLock.lock();
//当第一次收到消息,cls没有初始化,则调用_class_initialize进行初始化
}
retry:
runtimeLock.assertLocked();
imp = cache_getImp(cls, sel);
if (imp) goto done;
// Try this class's method lists.
//在本类中查找method
{
Method meth = getMethodNoSuper_nolock(cls, sel);
if (meth) {
log_and_fill_cache(cls, meth->imp, sel, inst, cls);
imp = meth->imp;
goto done;
}
}
// Try superclass caches and method lists.
{
unsigned attempts = unreasonableClassCount();
for (Class curClass = cls->superclass;
curClass != nil;
curClass = curClass->superclass)
{
// Halt if there is a cycle in the superclass chain.
if (--attempts == 0) {
_objc_fatal("Memory corruption in class list.");
}
// Superclass cache.
imp = cache_getImp(curClass, sel);
if (imp) {
if (imp != (IMP)_objc_msgForward_impcache) {
// Found the method in a superclass. Cache it in this class.
log_and_fill_cache(cls, imp, sel, inst, curClass);
goto done;
}
else {
// Found a forward:: entry in a superclass.
// Stop searching, but don't cache yet; call method
// resolver for this class first.
break;
}
}
// Superclass method list.
Method meth = getMethodNoSuper_nolock(curClass, sel);
if (meth) {
log_and_fill_cache(cls, meth->imp, sel, inst, curClass);
imp = meth->imp;
goto done;
}
}
}
// No implementation found. Try method resolver once.
if (resolver && !triedResolver) {
runtimeLock.unlock();
_class_resolveMethod(cls, sel, inst);
runtimeLock.lock();
// Don't cache the result; we don't hold the lock so it may have
// changed already. Re-do the search from scratch instead.
triedResolver = YES;
goto retry;
}
// No implementation found, and method resolver didn't help.
// Use forwarding.
imp = (IMP)_objc_msgForward_impcache;
cache_fill(cls, sel, imp, inst);
done:
runtimeLock.unlock();
return imp;
}

当第一次收到消息,cls没有初始化,则调用_class_initialize进行初始化
我们进入到_class_initialize内部objc-initialize.mm484行

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
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.
//递归调用父类是否有初始化和是否有父类
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!");
}
}

可以看出来,和+load方法一样,先父类后子类。然后赋值reallyInitialize = YES;,后边使用try主动调用callInitialize(cls);,来到callInitialize(cls);内部:

1
2
3
4
5
void callInitialize(Class cls)
{
((void(*)(Class, SEL))objc_msgSend)(cls, SEL_initialize);
asm("");
}

可以看到最终还是使用((void(*)(Class, SEL))objc_msgSend)(cls, SEL_initialize)主动调用了该函数。

区别

+initialize+load的很大区别是,+initialize是通过objc_msgSend进行调用的,所以有以下特点
如果子类没有实现+initialize,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,就覆盖类本身的+initialize调用

用伪代码实现以下思路:

1
2
3
4
5
6
if(class 没有初始化){
父类初始化
子类初始化
调用initialize
}
如果子类没有实现initialize,则去调用父类initialize。

至于子类没有实现的话是直接调用父类的initialize,是使用objc-msgsend的原因。

验证

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
@interface FYPerson : NSObject
@end
+(void)initialize{
printf("\n%s",__func__);
}
+(void)load{
printf("\n%s",__func__);
}
@interface FYPerson (test1)
@end
+(void)initialize{
printf("\n%s",__func__);
}
+(void)load{
printf("\n%s",__func__);
}
//输出
+[FYPerson load]
+[FYPerson(test2) load]
+[FYPerson(test1) load]

总结

  • +load是根据函数地址直接调用,initialize是通过objc_msgSend调用
  • +load是runtime加载类、分类时候调用(只会调用一次)
  • initialize是第一次接受消息的时候调用,每个类只会调用一次(子类没实现,父类可能被调用多次)
  • +load调用优先于initialize,子类调用+load之前会调用父类的+load,再调用分类的+load,分类之间先编译,先调用。
  • initialize先初始化父类,再初始化子类(可能最终调用父类的initialize

关联对象本质

关联对象的本质-结构体

继承NSObject是可以可以直接使用@property (nonatomic,assign) int age;,但是在Category中会报错,那么怎么实现和继承基类一样的效果呢?
我们查看Category结构体

1
2
3
4
5
6
7
8
struct _category_t {
const char *name; //"FYPerson"
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;
};

其中const struct _prop_list_t *properties;是存储属性的,但是缺少成员变量,而我们也不能主动在_category_t插入ivar,那么我们可以使用objc_setAssociatedObject将属性的值存储全局的AssociationsHashMap中,使用的时候objc_getAssociatedObject(id object, const void *key),不使用的时候删除使用objc_removeAssociatedObjects删除。

我们进入到objc_setAssociatedObject内部,objc-references.mm275行

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
void _object_set_associative_reference(id object, void *key, id value, uintptr_t policy) {
// retain the new value (if any) outside the lock.
ObjcAssociation old_association(0, nil);
//根据key value 处理
id new_value = value ? acquireValue(value, policy) : nil;
{
AssociationsManager manager;
//生成一个全局的 HashMap
AssociationsHashMap &associations(manager.associations());
disguised_ptr_t disguised_object = DISGUISE(object);
//有value 就处理
if (new_value) {
// break any existing association.
// 遍历 hashMap是否有该obj
AssociationsHashMap::iterator i = associations.find(disguised_object);
if (i != associations.end()) {
// secondary table exists
//有的话 更新其 value
ObjectAssociationMap *refs = i->second;
ObjectAssociationMap::iterator j = refs->find(key);
if (j != refs->end()) {
old_association = j->second;
j->second = ObjcAssociation(policy, new_value);
} else {
(*refs)[key] = ObjcAssociation(policy, new_value);
}
} else {
// create the new association (first time).
//没有的话 赋值给 refs
ObjectAssociationMap *refs = new ObjectAssociationMap;
associations[disguised_object] = refs;
(*refs)[key] = ObjcAssociation(policy, new_value);
object->setHasAssociatedObjects();
}
} else {
// setting the association to nil breaks the association.
AssociationsHashMap::iterator i = associations.find(disguised_object);
if (i != associations.end()) {
ObjectAssociationMap *refs = i->second;
ObjectAssociationMap::iterator j = refs->find(key);
if (j != refs->end()) {
old_association = j->second;
//删除refs
refs->erase(j);
}
}
}
}
// release the old value (outside of the lock).
if (old_association.hasValue()) ReleaseValue()(old_association);
}

通过该函数我们了解到

  • 关联对象并不是存储在关联对象的本身内存中
  • 关联对象是存储在全局统一的AssociationsManager管理的AssociationsHashMap
  • 传入value =nil,会移除该关联对线
    AssociationsManager其实是管理了已key为id object对应的AssociationsHashMapAssociationsHashMap存储了key对应的ObjcAssociationObjcAssociation是存储了valuepolicyObjcAssociation的数据结构如下:
1
2
3
4
5
class ObjcAssociation {
uintptr_t _policy;
id _value;
*****
}

具体抽象关系见下图

1
2
AssociationsManager --> AssociationsHashMap --> ObjectAssociationMap
-->void * ObjectAssociation -->uintprt_t _policy ,id _value;

简单来讲就是一个全局变量保存了以classkey对应的AssociationsHashMap,这个AssociationsHashMap存储了一个key对应的ObjectAssociationObjectAssociation包含了value_policy。通过2层map保存了数据。

关联对象的使用

objc_setAssociatedObject obj,key,value,policy
objc_getAssociatedObject 根据 obj 和 key获取值
void objc_removeAssociatedObjects(id object) 根据obj 删除关联函数

objc_AssociationPolicy的类型:

OBJC_ASSOCIATION_ASSIGN weak 引用
OBJC_ASSOCIATION_RETAIN_NONATOMIC 非原子强引用
OBJC_ASSOCIATION_COPY_NONATOMIC 非原子相当于copy
OBJC_ASSOCIATION_RETAIN 强引用
OBJC_ASSOCIATION_COPY 原子操作,相当于copy

代码示例

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
@interface NSObject (test)
@property (nonatomic,assign) NSString * name;
@end
#import "NSObject+test.h"
#import "objc/runtime.h"
@implementation NSObject (test)
-(void)setName:(NSString *)name{
objc_setAssociatedObject(self, @selector(name), name, OBJC_ASSOCIATION_COPY);
}
- (NSString *)name{
return objc_getAssociatedObject(self, @selector(name));
}
@end
NSObject *obj =[[NSObject alloc]init];
obj.name = @"老弟来了";
printf("%s",obj.name.UTF8String);
//老弟来了

这段代码我们实现了给基类添加一个成员变量name,然后又成功取出了值,标示我们做新增的保存成员变量的值是对的。

总结

  • Category +load在冷启动时候执行,执行顺序和编译顺序成弱相关,先父类,后子类,而且每个类执行一次,执行是直接调用函数地址。
  • Category +initialize在第一次接受消息执行,先父类,后子类,子类没实现,会调用父类,利用objc-msgsend机制调用。
  • Category 可以利用Associative添加和读取属性的值

资料下载