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上周寫了篇關(guān)于iOS 分類的文章，我們可以在分類中定義屬性，但只是定義了屬性，編譯器既沒有生成相應(yīng)的setter和getter方法，也沒有生成對(duì)應(yīng)的成員屬性。如果想給對(duì)象動(dòng)態(tài)添加屬性就需要用到runtime庫中的API了

我們給一個(gè)類動(dòng)態(tài)添加屬性的做法如下：

@interface Person (Test1)
@property (nonatomic, strong) NSString *name;
@end

@implementation Person (Test1)
- (void)setName:(NSString *)name {
    objc_setAssociatedObject(self, @selector(name), name, OBJC_ASSOCIATION_COPY_NONATOMIC);
}
- (NSString *)name {
    return objc_getAssociatedObject(self, @selector(name));
}
@end

嘗試給Person對(duì)象設(shè)置name屬性，再打印Person對(duì)象的name屬性看看是不是我們剛剛設(shè)置的值

設(shè)置屬性

打印結(jié)果表明，我們給Person實(shí)例對(duì)象關(guān)聯(lián)的name屬性真的設(shè)置了，并且不同對(duì)象之間關(guān)聯(lián)的屬性也互不影響。

我們都很肯定關(guān)聯(lián)的屬性值不是存放在實(shí)例對(duì)象結(jié)構(gòu)中的，因?yàn)閷?shí)例對(duì)象的結(jié)構(gòu)在編譯時(shí)就確定的，而屬性關(guān)聯(lián)卻是在運(yùn)行時(shí)動(dòng)態(tài)關(guān)聯(lián)的。objc_setAssociatedObject(...)和objc_getAssociatedObject(...)這兩個(gè)方法都需要2個(gè)參數(shù)，分別是需要關(guān)聯(lián)屬性的對(duì)象self和屬性對(duì)應(yīng)的key。兩個(gè)方法都需要傳key過去，而且這兩個(gè)方法的key要一樣，因此猜測(cè)底層存儲(chǔ)的方式為map(runtime庫是C/C++)

課前小菜

在閱讀源碼之前，我們首先要理解幾個(gè)關(guān)鍵的C++類

1. AssociationsManager

spinlock_t AssociationsManagerLock;

class AssociationsManager {
    // associative references: object pointer -> PtrPtrHashMap.
    static AssociationsHashMap *_map;
public:
    AssociationsManager()   { AssociationsManagerLock.lock(); }
    ~AssociationsManager()  { AssociationsManagerLock.unlock(); }
    
    AssociationsHashMap &associations() {
        if (_map == NULL)
            _map = new AssociationsHashMap();
        return *_map;
    }
};

AssociationsHashMap *AssociationsManager::_map = NULL;

我將AssociationsManager稱之為管理對(duì)象，其內(nèi)部有一個(gè)AssociationsHashMap類型的靜態(tài)指針變量_map，這個(gè)變量在程序運(yùn)行期間只有一份。而從AssociationsManager的構(gòu)造函數(shù)和析構(gòu)函數(shù)中猜測(cè)該類存在的意義是維持一個(gè)原子鎖，確保在程序運(yùn)行期間，內(nèi)存中只存在一個(gè)AssociationsManager類型的對(duì)象。

2. AssociationsHashMap

typedef ObjcAllocator<std::pair<const disguised_ptr_t, ObjectAssociationMap*> > AssociationsHashMapAllocator;
class AssociationsHashMap : public unordered_map<disguised_ptr_t, ObjectAssociationMap *, DisguisedPointerHash, DisguisedPointerEqual, AssociationsHashMapAllocator> {
public:
    void *operator new(size_t n) { return ::malloc(n); }
    void operator delete(void *ptr) { ::free(ptr); }
};

在objc-references.mm文件中對(duì)AssociationsHashMap類的定義分成了win32架構(gòu)和其他架構(gòu)，由于iOS目前僅可發(fā)布arm64架構(gòu)的，因此這里只截取了64架構(gòu)的源碼，有興趣了解win32位的實(shí)現(xiàn)的同學(xué)可自行查閱相關(guān)源碼。

AssociationsHashMap類繼承自unordered_map，unordered_map和map都是存儲(chǔ)鍵值對(duì)的容器，但map底層是紅黑樹存儲(chǔ)方式，存儲(chǔ)順序?yàn)閗ey有序序列。而unordered_map則是無序，哈希表存儲(chǔ)方式。

unordered_map

AssociationsHashMap中的key類型為disguised_ptr_t，value類型為ObjectAssociationMap類型的指針，key的哈希算法為DisguisedPointerHash，key判斷函數(shù)為DisguisedPointerEqual，鍵值對(duì)內(nèi)存分配器為AssociationsHashMapAllocator

3. ObjectAssociationMap

typedef ObjcAllocator<std::pair<void * const, ObjcAssociation> > ObjectAssociationMapAllocator;
class ObjectAssociationMap : public std::map<void *, ObjcAssociation, ObjectPointerLess, ObjectAssociationMapAllocator> {
public:
    void *operator new(size_t n) { return ::malloc(n); }
    void operator delete(void *ptr) { ::free(ptr); }
};

map

ObjectAssociationMap繼承自map，和AssociationsHashMap一樣都是鍵值對(duì)存儲(chǔ)。key的類型對(duì)指針，value為ObjcAssociation，key的判斷函數(shù)為ObjectPointerLess，鍵值對(duì)內(nèi)存分配器為ObjectAssociationMapAllocator。

4. ObjectAssociationMap

class ObjcAssociation {
    uintptr_t _policy;
    id _value;
public:
    ObjcAssociation(uintptr_t policy, id value) : _policy(policy), _value(value) {}
    ObjcAssociation() : _policy(0), _value(nil) {}

    uintptr_t policy() const { return _policy; }
    id value() const { return _value; }
        
    bool hasValue() { return _value != nil; }
};

ObjcAssociation類中只有2個(gè)成員屬性，_policy和_value。通過名字我們可以猜到這個(gè)類主要存儲(chǔ)的是關(guān)聯(lián)的值和關(guān)聯(lián)策略。

正菜

objc_setAssociatedObject

void objc_setAssociatedObject(id object, const void *key, id value, objc_AssociationPolicy policy) {
    _object_set_associative_reference(object, (void *)key, value, policy);
}

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);
    id new_value = value ? acquireValue(value, policy) : nil;
    {
        AssociationsManager manager;    // 獲取
        AssociationsHashMap &associations(manager.associations());
        disguised_ptr_t disguised_object = DISGUISE(object);
        if (new_value) {
            // break any existing association.
            AssociationsHashMap::iterator i = associations.find(disguised_object);
            if (i != associations.end()) {
                // secondary table exists
                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).
                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;
                    refs->erase(j);
                }
            }
        }
    }
    // release the old value (outside of the lock).
    if (old_association.hasValue()) ReleaseValue()(old_association);
}

首先根據(jù)關(guān)聯(lián)策略決定對(duì)關(guān)聯(lián)值還是引用計(jì)數(shù)器+1，亦或是拷貝關(guān)聯(lián)值
初始化AssociationsManager對(duì)象，若此時(shí)正有另一個(gè)對(duì)象在做關(guān)聯(lián)操作，則當(dāng)前AssociationsManager對(duì)象的初始化方法會(huì)被阻塞。在同一時(shí)間中，有且僅有一個(gè)AssociationsManager實(shí)例對(duì)象存在在內(nèi)存中，即同一時(shí)間只有一個(gè)對(duì)象在進(jìn)行關(guān)聯(lián)操作
獲取AssociationsManager中的靜態(tài)變量AssociationsHashMap
調(diào)用DISGUISE()函數(shù)獲取對(duì)象地址的反碼disguised_object
將對(duì)象地址的反碼作為key，在AssociationsHashMap哈希表中查找key為disguised_object的ObjectAssociationMap類型的對(duì)象
如果找到ObjectAssociationMap，則繼續(xù)根據(jù)我們傳入的關(guān)聯(lián)key繼續(xù)在ObjectAssociationMap表中查找對(duì)應(yīng)的ObjcAssociation對(duì)象，并替換成新的ObjcAssociation對(duì)象
如果在第6步中沒找到ObjcAssociation，則新建一個(gè)ObjcAssociation對(duì)象，并和關(guān)聯(lián)key綁定
如果在第5步中沒有找到ObjectAssociationMap ，則創(chuàng)建一個(gè)ObjectAssociationMap，把對(duì)應(yīng)的ObjcAssociation和key存儲(chǔ)起來
如果我們?cè)O(shè)置的value為nil，對(duì)應(yīng)的操作是從對(duì)象的關(guān)聯(lián)哈希表ObjectAssociationMap中刪除key對(duì)應(yīng)的ObjcAssociation對(duì)象

objc_getAssociatedObject

id objc_getAssociatedObject(id object, const void *key) {
    return _object_get_associative_reference(object, (void *)key);
}
id _object_get_associative_reference(id object, void *key) {
    id value = nil;
    uintptr_t policy = OBJC_ASSOCIATION_ASSIGN;
    {
        AssociationsManager manager;
        AssociationsHashMap &associations(manager.associations());
        disguised_ptr_t disguised_object = DISGUISE(object);
        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()) {
                ObjcAssociation &entry = j->second;
                value = entry.value();
                policy = entry.policy();
                if (policy & OBJC_ASSOCIATION_GETTER_RETAIN) {
                    objc_retain(value);
                }
            }
        }
    }
    if (value && (policy & OBJC_ASSOCIATION_GETTER_AUTORELEASE)) {
        objc_autorelease(value);
    }
    return value;
}

初始化AssociationsManager對(duì)象，若此時(shí)正有另一個(gè)對(duì)象在做關(guān)聯(lián)操作，則同樣**AssociationsManager****對(duì)象的初始化方法會(huì)被阻塞
獲取AssociationsManager中的靜態(tài)變量AssociationsHashMap
調(diào)用DISGUISE函數(shù)獲取對(duì)象地址的反碼disguised_object
將對(duì)象地址的反碼作為key，在AssociationsHashMap哈希表中查找key為disguised_object的ObjectAssociationMap類型的對(duì)象
如果在第4步中找到ObjectAssociationMap，則繼續(xù)根據(jù)我們傳入的關(guān)聯(lián)key繼續(xù)在ObjectAssociationMap表中查找對(duì)應(yīng)的ObjcAssociation對(duì)象，如果找到了，取出其中的value
如果在第5步中沒找到ObjcAssociation，則該key不存在關(guān)聯(lián)的值
如果在第4步中沒有找到ObjectAssociationMap ，則該對(duì)象沒有設(shè)置過關(guān)聯(lián)屬性
根據(jù)關(guān)聯(lián)策略決定是否對(duì)查詢到的value進(jìn)行retain操作或則autorelease

沙拉

文筆不是很好，??的這段話應(yīng)該很難理解了，沒關(guān)系，把關(guān)系圖畫出來就容易理解了。

關(guān)系圖

AssociationsManager可重復(fù)創(chuàng)建，但是AssociationsHashMap是AssociationsManager內(nèi)的靜態(tài)變量，全局只有一個(gè)，負(fù)責(zé)管理所有對(duì)象的關(guān)聯(lián)表。通過對(duì)象地址的反碼取出該對(duì)象的屬性關(guān)聯(lián)表ObjectAssociationMap。然后再通過關(guān)聯(lián)key從對(duì)象關(guān)聯(lián)表中獲取對(duì)應(yīng)的ObjectAssociation，ObjectAssociation內(nèi)存儲(chǔ)的就是真正需要的存儲(chǔ)的value值和內(nèi)存關(guān)聯(lián)策略policy了。

如果想刪除對(duì)象關(guān)聯(lián)的某一個(gè)value值，可傳nil空值即可刪除。

// 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;
        refs->erase(j);
    }
}

甜點(diǎn)

在對(duì)象釋放的時(shí)候，會(huì)判斷對(duì)象是否關(guān)聯(lián)過屬性，如果關(guān)聯(lián)過屬性，則會(huì)調(diào)用_object_remove_assocations刪除其在AssociationsHashMap中對(duì)應(yīng)的值ObjectAssociationMap和ObjectAssociationMap中的每個(gè)ObjectAssociation。

// - (void)dealloc;                             (NSObject.mm)
// void_objc_rootDealloc(id obj);               (NSObject.mm)
// inline void objc_object::rootDealloc();      (objc-object.h)
// id object_dispose(id obj);                   (objc-runtime-new.mm)
// void *objc_destructInstance(id obj);         (objc-runtime-new.mm)
// void _object_remove_assocations(id object)   (objc-references.mm)
void _object_remove_assocations(id object) {
    vector< ObjcAssociation,ObjcAllocator<ObjcAssociation> > elements;
    {
        AssociationsManager manager;
        AssociationsHashMap &associations(manager.associations());
        if (associations.size() == 0) return;
        disguised_ptr_t disguised_object = DISGUISE(object);
        AssociationsHashMap::iterator i = associations.find(disguised_object);
        if (i != associations.end()) {
            // copy all of the associations that need to be removed.
            ObjectAssociationMap *refs = i->second;
            for (ObjectAssociationMap::iterator j = refs->begin(), end = refs->end(); j != end; ++j) {
                elements.push_back(j->second);
            }
            // remove the secondary table.
            delete refs;
            associations.erase(i);
        }
    }
    // the calls to releaseValue() happen outside of the lock.
    for_each(elements.begin(), elements.end(), ReleaseValue());
}

在_object_set_associative_reference()函數(shù)中，當(dāng)為對(duì)象創(chuàng)建ObjectAssociationMap哈希表的時(shí)候，也會(huì)調(diào)用objc_object::setHasAssociatedObjects()將對(duì)象ISA指針的第二位has_assoc設(shè)置為1。這一步只要設(shè)置了之后，直到被釋放，第二位has_assoc都為1。

Tips：在iOS arm64位系統(tǒng)中，ISA指針是經(jīng)過優(yōu)化的，采用了共同體，可存儲(chǔ)更多的信息。而ISA指針的第2位0和1分別表示該對(duì)象是否關(guān)聯(lián)過對(duì)象

union isa_t 
{
    isa_t() { }
    isa_t(uintptr_t value) : bits(value) { }

    Class cls;
    uintptr_t bits;

# if __arm64__
#   define ISA_MASK        0x0000000ffffffff8ULL
#   define ISA_MAGIC_MASK  0x000003f000000001ULL
#   define ISA_MAGIC_VALUE 0x000001a000000001ULL
    struct {
        uintptr_t nonpointer        : 1;
        uintptr_t has_assoc         : 1;
        uintptr_t has_cxx_dtor      : 1;
        uintptr_t shiftcls          : 33; // MACH_VM_MAX_ADDRESS 0x1000000000
        uintptr_t magic             : 6;
        uintptr_t weakly_referenced : 1;
        uintptr_t deallocating      : 1;
        uintptr_t has_sidetable_rc  : 1;
        uintptr_t extra_rc          : 19;
#       define RC_ONE   (1ULL<<45)
#       define RC_HALF  (1ULL<<18)
    };

# elif __x86_64__
    ...
#endif
};

驗(yàn)證一下關(guān)聯(lián)屬性前后ISA指針第二位has-assoc的變化

test-isa

在沒關(guān)聯(lián)之前，ISA指針的第二位has_assoc的為0，在關(guān)聯(lián)屬性之后，ISA指針的第二位has_assoc變?yōu)榱?。

isa

這次寫的不是很好，理解起來會(huì)有點(diǎn)困難，本人也還在繼續(xù)提升寫作能力，望諒解

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iOS - 關(guān)聯(lián)對(duì)象Association

iOS - 關(guān)聯(lián)對(duì)象Association

課前小菜

正菜

沙拉

甜點(diǎn)

相關(guān)閱讀更多精彩內(nèi)容

友情鏈接更多精彩內(nèi)容

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iOS - 關(guān)聯(lián)對(duì)象Association

課前小菜

正菜

沙拉

甜點(diǎn)

相關(guān)閱讀更多精彩內(nèi)容

友情鏈接更多精彩內(nèi)容

色偷偷精品伊人,欧洲久久精品,欧美综合婷婷骚逼,国产AV主播,国产最新探花在线,九色在线视频一区,伊人大交九欧美,1769亚洲,黄色成人av