一、weak的作用

這里的weak包括屬性關(guān)鍵字weak和__weak兩種：__weak用于修飾變量（variable），weak用于修飾屬性（property）。

1. weak是弱引用，用weak描述修飾或者所引用對(duì)象的計(jì)數(shù)不會(huì)加1
2. weak修飾的指針在引用的對(duì)象被釋放的時(shí)候自動(dòng)被設(shè)置為nil，避免野指針
3. weak可以解決使用block引起的循環(huán)引用。

看到這里又想起了block和assign，復(fù)習(xí)一下這幾個(gè)的區(qū)別：

1.__block不管是ARC還是MRC模式下都可以使用，可以修飾對(duì)象，還可以修飾基本數(shù)據(jù)類型。
2.__weak只能在ARC模式下使用，也只能修飾對(duì)象（NSString），不能修飾基本數(shù)據(jù)類型（int）。assign用來修飾基本數(shù)據(jù)類型
3.__block對(duì)象可以在block中被重新賦值，__weak不可以。
4.__block對(duì)象在ARC下可能會(huì)導(dǎo)致循環(huán)引用，非ARC下會(huì)避免循環(huán)引用，__weak只在ARC下使用，可以避免循環(huán)引用。

OK，以上說的都沒有錯(cuò)，大家也都會(huì)用，但是你知道是怎么實(shí)現(xiàn)的嗎

二、weak的源碼實(shí)現(xiàn)

1、先看下objc-weak.h文件里面的API

typedef DisguisedPtr<objc_object *> weak_referrer_t;
struct weak_entry_t { ... };
struct weak_table_t { ... };
/// Adds an (object, weak pointer) pair to the weak table.
id weak_register_no_lock(weak_table_t *weak_table, id referent, 
                         id *referrer, bool crashIfDeallocating);
/// Removes an (object, weak pointer) pair from the weak table.
void weak_unregister_no_lock(weak_table_t *weak_table, id referent, id *referrer);
#if DEBUG
/// Returns true if an object is weakly referenced somewhere.
bool weak_is_registered_no_lock(weak_table_t *weak_table, id referent);
#endif
/// Called on object destruction. Sets all remaining weak pointers to nil.
void weak_clear_no_lock(weak_table_t *weak_table, id referent);

看起來很簡(jiǎn)單的結(jié)構(gòu)體和幾個(gè)方法：

• weak_table_t：翻譯一下注釋上的說明，弱引用的哈希表table，對(duì)象id作為key，weak_entry_t結(jié)構(gòu)體作為value
• weak_register_no_lock：向weak table里面添加一個(gè)object和它的weak引用指針
• weak_unregister_no_lock：從weak table移除一個(gè)object和它的weak引用指針
• weak_is_registered_no_lock：如果object有弱引用返回true
• weak_clear_no_lock：object對(duì)象被銷毀，把所有指向它的弱引用指針全部置為nil

2、根據(jù)API看下.m里面的實(shí)現(xiàn)

2.1 weak_register_no_lock添加弱引用
源碼實(shí)現(xiàn)：

id 
weak_register_no_lock(weak_table_t *weak_table, id referent_id, 
                      id *referrer_id, bool crashIfDeallocating)
{
    objc_object *referent = (objc_object *)referent_id;
    objc_object **referrer = (objc_object **)referrer_id;
    // ensure that the referenced object is viable
    。。。這里省略判斷被引用的對(duì)象是否存在的代碼，只看關(guān)鍵部分
    weak_entry_t *entry;
    if ((entry = weak_entry_for_referent(weak_table, referent))) {
        append_referrer(entry, referrer);
    } 
    else {
        weak_entry_t new_entry(referent, referrer);
        weak_grow_maybe(weak_table);
        weak_entry_insert(weak_table, &new_entry);
    }
    return referent_id;
}

解釋一下關(guān)鍵地方：

1. if ((entry = weak_entry_for_referent(weak_table, referent)))：判斷weak_table是否存在這個(gè)對(duì)象referent
2. 如果table表中已經(jīng)存在對(duì)象referent，那么執(zhí)行append_referrer(entry, referrer);把referrer這個(gè)新的引用加入到referent已經(jīng)存在的引用列表中
3. 如果不存在，執(zhí)行
   weak_entry_t new_entry(referent, referrer)：給對(duì)象referent創(chuàng)建一個(gè)新的引用列表
   weak_grow_maybe(weak_table)：weak_table增加內(nèi)存
   weak_entry_insert(weak_table, &new_entry)：把referent的引用列表加入到weak_table中

2.2 weak_unregister_no_lock：移除弱引用
當(dāng)我們使用weak指針指向一個(gè)對(duì)象__weak typeof(self) = self的時(shí)候,會(huì)調(diào)weak_register_no_lock在weak table表中添加這個(gè)引用，但是移除引用什么時(shí)候會(huì)用呢，還是看一下源碼和注釋:

/** 
 * Unregister an already-registered weak reference.
 * This is used when referrer's storage is about to go away, but referent
 * isn't dead yet. (Otherwise, zeroing referrer later would be a
 * bad memory access.)
 * Does nothing if referent/referrer is not a currently active weak reference.
 * Does not zero referrer.
 */
void
weak_unregister_no_lock(weak_table_t *weak_table, id referent_id, 
                        id *referrer_id)
{
    // referent是對(duì)象也就是weak table中的key
    objc_object *referent = (objc_object *)referent_id;
    //*referrer是指向這個(gè)對(duì)象的弱引用指針
    objc_object **referrer = (objc_object **)referrer_id;

    weak_entry_t *entry;

    if (!referent) return;
    ///從weak_table中根據(jù)key（referent）找到entry（指向?qū)ο笕跻昧斜淼闹羔槪?    if ((entry = weak_entry_for_referent(weak_table, referent))) {
        /// 從引用列表中移除這個(gè)引用referrer, remove_referrer這個(gè)方法會(huì)把這個(gè)referrer置為nil
        remove_referrer(entry, referrer);
        /// 之后遍歷對(duì)象referent的引用列表是不是空了
        bool empty = true;
        if (entry->out_of_line()  &&  entry->num_refs != 0) {
            empty = false;
        }
        else {
            for (size_t i = 0; i < WEAK_INLINE_COUNT; i++) {
                if (entry->inline_referrers[i]) {
                    empty = false; 
                    break;
                }
            }
        }
        if (empty) {
            /// 如果該對(duì)象的弱引用列表已經(jīng)empty，就把該對(duì)象從weak_table中移除
            weak_entry_remove(weak_table, entry);
        }
    }
}

這個(gè)應(yīng)該是這種情況下使用的，比如我們?cè)谝粋€(gè)方法里面這樣定義：

- (void)testWeakReferrer {
    __weak typeof(self) weakSelf = self;
    ...
    return;
}

weakSelf是方法的局部變量，當(dāng)方法執(zhí)行完的時(shí)候，局部變量weakSelf會(huì)被銷毀，這個(gè)時(shí)候系統(tǒng)應(yīng)該就會(huì)自動(dòng)執(zhí)行weak_unregister_no_lock方法，把weakSelf從self對(duì)象的弱引用列表中移除，并且把weakSelf置為nil。

2.3 weak_clear_no_lock清空所有的weak指針

/** 
 * Called by dealloc; nils out all weak pointers that point to the 
 * provided object so that they can no longer be used.
 * 
 * @param weak_table 
 * @param referent The object being deallocated. 
 */
void 
weak_clear_no_lock(weak_table_t *weak_table, id referent_id) 
{
    objc_object *referent = (objc_object *)referent_id;

    weak_entry_t *entry = weak_entry_for_referent(weak_table, referent);
    ...
    // zero out references
    weak_referrer_t *referrers;
    size_t count;
    ...//獲取列表中的數(shù)量
    for (size_t i = 0; i < count; ++i) {
        objc_object **referrer = referrers[i];
        if (referrer) {
            if (*referrer == referent) {
                *referrer = nil;
            }
            ...
        }
    }
    
    weak_entry_remove(weak_table, entry);
}

Called by dealloc，當(dāng)對(duì)象被銷毀的時(shí)候調(diào)用這個(gè)方法，在這個(gè)方法里面把所有指向這個(gè)對(duì)象的弱引用指針全部置為nil，最后從weak table中移除該對(duì)象和它的弱引用列表。

ok，這也就解釋了為什么weak指針會(huì)被自動(dòng)置為nil了