NSObject Lifecycle 探索

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NSObject

声明

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@interface NSObject <NSObject> {
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wobjc-interface-ivars"
    Class isa  OBJC_ISA_AVAILABILITY;
#pragma clang diagnostic pop
}

NSObject 包含Class isa,我们来看下Class的定义

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typedef struct objc_class *Class;

struct objc_class : objc_object {
    // Class ISA;
    Class superclass;
    cache_t cache;             // formerly cache pointer and vtable
    class_data_bits_t bits;    // class_rw_t * plus custom rr/alloc flags

    ...
};

struct cache_t {
    struct bucket_t *_buckets; 
    mask_t _mask;
    mask_t _occupied;

public:
    ...

    /* cache 扩容相关 */
    mask_t capacity();
    ...
    void expand();

    ...
};

/* method cache */
struct bucket_t { 
private:
    cache_key_t _key;
    IMP _imp;

    ...
};

struct objc_object {
private:
    isa_t isa;

    ...

    /* 初始化isa指针 */

    void initInstanceIsa(Class cls, bool hasCxxDtor);

    // changeIsa() should be used to change the isa of existing objects.
    // If this is a new object, use initIsa() for performance.

    /* KVO 实现通过该方法指向新的实现类的isa */

    Class changeIsa(Class newCls);

    ...
};

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

    Class cls;
    uintptr_t bits;

    ...
};

创建

初始化Class (Runtime自动创建)
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static Class 
alloc_class_for_subclass(Class supercls, size_t extraBytes) {
    ...

    return (Class) calloc(1, sizeof(objc_class) + extraBytes)
}
调用alloc
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+ (id)alloc {
    return return callAlloc(self, false/*checkNil*/, true/*allocWithZone*/);;
}

static ALWAYS_INLINE id
callAlloc(Class cls, bool checkNil, bool allocWithZone=false)
{
    ...

    id obj = (id)calloc(1, size);
    obj->initInstanceIsa(cls, hasCxxDtor);
    ...
}

+ (id)allocWithZone:(struct _NSZone *)zone {

}
/* 最终的实现跟alloc一样，有兴趣的自己去查看吧 */
调用init
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- (id)init {
    return _objc_rootInit(self);
}

+ (id)new {
    return [callAlloc(self, false/*checkNil*/) init];
}

从上面的代码可以看出new就是[[Class alloc] init]

retain

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- (id)retain {
    return ((id)self)->rootRetain();
}

ALWAYS_INLINE id 
objc_object::rootRetain(bool tryRetain, bool handleOverflow)
{
    if (isTaggedPointer()) return (id)this;

    bool sideTableLocked = false;
    bool transcribeToSideTable = false;

    isa_t oldisa;
    isa_t newisa;

    do {
        transcribeToSideTable = false;
        oldisa = LoadExclusive(&isa.bits);
        newisa = oldisa;

        /* 不支持 nonpointer 会走这里 引用计数加1 */

        if (slowpath(!newisa.nonpointer)) {
            ClearExclusive(&isa.bits);
            if (!tryRetain && sideTableLocked) sidetable_unlock();
            if (tryRetain) return sidetable_tryRetain() ? (id)this : nil;
            else return sidetable_retain();
        }
        // don't check newisa.fast_rr; we already called any RR overrides

        /* NSObject dealloc */

        if (slowpath(tryRetain && newisa.deallocating)) {
            ClearExclusive(&isa.bits);
            if (!tryRetain && sideTableLocked) sidetable_unlock();
            return nil;
        }
        uintptr_t carry;
        newisa.bits = addc(newisa.bits, RC_ONE, 0, &carry);  // extra_rc++

        if (slowpath(carry)) {
            // newisa.extra_rc++ overflowed
            if (!handleOverflow) {
                ClearExclusive(&isa.bits);
                return rootRetain_overflow(tryRetain);
            }
            // Leave half of the retain counts inline and 
            // prepare to copy the other half to the side table.
            if (!tryRetain && !sideTableLocked) sidetable_lock();
            sideTableLocked = true;
            transcribeToSideTable = true;
            newisa.extra_rc = RC_HALF;
            newisa.has_sidetable_rc = true;
        }
    } while (slowpath(!StoreExclusive(&isa.bits, oldisa.bits, newisa.bits)));

    if (slowpath(transcribeToSideTable)) {
        // Copy the other half of the retain counts to the side table.

        /* 引用计数加1 */

        sidetable_addExtraRC_nolock(RC_HALF) = {
            assert(isa.nonpointer);
            SideTable& table = SideTables()[this];

            size_t& refcntStorage = table.refcnts[this];
            size_t oldRefcnt = refcntStorage;
            // isa-side bits should not be set here
            assert((oldRefcnt & SIDE_TABLE_DEALLOCATING) == 0);
            assert((oldRefcnt & SIDE_TABLE_WEAKLY_REFERENCED) == 0);

            if (oldRefcnt & SIDE_TABLE_RC_PINNED) return true;

            uintptr_t carry;
            size_t newRefcnt = 
                addc(oldRefcnt, delta_rc << SIDE_TABLE_RC_SHIFT, 0, &carry);
            if (carry) {
                refcntStorage =
                    SIDE_TABLE_RC_PINNED | (oldRefcnt & SIDE_TABLE_FLAG_MASK);
                return true;
            }
            else {
                refcntStorage = newRefcnt;
                return false;
            }
        };
    }

    if (slowpath(!tryRetain && sideTableLocked)) sidetable_unlock();
    return (id)this;
}

bool
objc_object::sidetable_tryRetain()
{
    SideTable& table = SideTables()[this];

    bool result = true;

    /* 
    找到NSObject isa对应的引用计数 
    1.引用计数为0时，新引用计数为1
    2.对象正在被释放，返回retain失败
    3.引用计数>0时，新引用计数加1
    */

    RefcountMap::iterator it = table.refcnts.find(this);
    if (it == table.refcnts.end()) {
        table.refcnts[this] = SIDE_TABLE_RC_ONE;
    } else if (it->second & SIDE_TABLE_DEALLOCATING) {
        result = false;
    } else if (! (it->second & SIDE_TABLE_RC_PINNED)) {
        it->second += SIDE_TABLE_RC_ONE;
    }
    
    return result;

release

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- (oneway void)release {
    ((id)self)->rootRelease();
}

ALWAYS_INLINE bool 
objc_object::rootRelease(bool performDealloc, bool handleUnderflow)
{
   ...

         /* 引用计数减一 */

        size_t borrowed = sidetable_subExtraRC_nolock(RC_HALF) = {
            assert(isa.nonpointer);
            SideTable& table = SideTables()[this];

            RefcountMap::iterator it = table.refcnts.find(this);
            if (it == table.refcnts.end()  ||  it->second == 0) {
                // Side table retain count is zero. Can't borrow.
                return 0;
            }
            size_t oldRefcnt = it->second;

            // isa-side bits should not be set here
            assert((oldRefcnt & SIDE_TABLE_DEALLOCATING) == 0);
            assert((oldRefcnt & SIDE_TABLE_WEAKLY_REFERENCED) == 0);

            size_t newRefcnt = oldRefcnt - (delta_rc << SIDE_TABLE_RC_SHIFT);
            assert(oldRefcnt > newRefcnt);  // shouldn't underflow
            it->second = newRefcnt;
            return delta_rc;
        };

    ...

    // Really deallocate.
    /* 如果正在dealloc，清除isa */

    if (slowpath(newisa.deallocating)) {
        ClearExclusive(&isa.bits);
        if (sideTableLocked) sidetable_unlock();
        return overrelease_error();
        // does not actually return
    }
    newisa.deallocating = true;
    if (!StoreExclusive(&isa.bits, oldisa.bits, newisa.bits)) goto retry;

    if (slowpath(sideTableLocked)) sidetable_unlock();

    /* 发SEL_dealloc消息，自动释放池Check引用计数，当引用计数为0时，释放该对象 */

    __sync_synchronize();
    if (performDealloc) {
        ((void(*)(objc_object *, SEL))objc_msgSend)(this, SEL_dealloc);
    }
    return true;
}

autorelease

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- (id)autorelease {
    return ((id)self)->rootAutorelease();
}

/* 以Page的方式加入自动释放池中 */
inline id 
objc_object::rootAutorelease()
{
    return AutoreleasePoolPage::autorelease((id)this);
}

autoreleasePool

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static inline void *push() {
    id *dest;
    if (DebugPoolAllocation) {
        // Each autorelease pool starts on a new pool page.
        dest = autoreleaseNewPage(POOL_BOUNDARY);
    } else {
        dest = autoreleaseFast(POOL_BOUNDARY);
    }
    assert(dest == EMPTY_POOL_PLACEHOLDER || *dest == POOL_BOUNDARY);
    return dest;
}

static inline void pop(void *token) {
    ...
    /* 
    从hotPage()中找到不为空的Page页，置空
    若为对象，调用其release方法
    找到下一个不为空的Page页 
    */

    page->releaseUntil(stop) = {
        // Not recursive: we don't want to blow out the stack 
        // if a thread accumulates a stupendous amount of garbage
        
        while (this->next != stop) {
            // Restart from hotPage() every time, in case -release 
            // autoreleased more objects
            AutoreleasePoolPage *page = hotPage();

            // fixme I think this `while` can be `if`, but I can't prove it
            while (page->empty()) {
                page = page->parent;
                setHotPage(page);
            }

            page->unprotect();
            id obj = *--page->next;
            memset((void*)page->next, SCRIBBLE, sizeof(*page->next));
            page->protect();

            if (obj != POOL_BOUNDARY) {
                objc_release(obj);
            }
        }

        setHotPage(this);

#if DEBUG
        // we expect any children to be completely empty
        for (AutoreleasePoolPage *page = child; page; page = page->child) {
            assert(page->empty());
        }
#endif
    };

    // memory: 删除空的AutoreleasePage
    if (DebugPoolAllocation  &&  page->empty()) {
        // special case: delete everything during page-per-pool debugging
        AutoreleasePoolPage *parent = page->parent;
        page->kill();
        setHotPage(parent);
    } else if (DebugMissingPools  &&  page->empty()  &&  !page->parent) {
        // special case: delete everything for pop(top) 
        // when debugging missing autorelease pools
        page->kill();
        setHotPage(nil);
    } 
    else if (page->child) {
        // hysteresis: keep one empty child if page is more than half full
        if (page->lessThanHalfFull()) {
            page->child->kill();
        }
        else if (page->child->child) {
            page->child->child->kill();
        }
    }
}

上面的retain/release都提到了SideTable结构体，其实例是全局的。SideTable管理引用计数的增加与减少和weak指针。

SideTable

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struct SideTable {
    spinlock_t slock; //自旋锁，SideTable的方法是线程安全的
    RefcountMap refcnts; //引用计数管理
    weak_table_t weak_table; //weak指针管理

    ...
};

/**
 * The global weak references table. Stores object ids as keys,
 * and weak_entry_t structs as their values.
 */
struct weak_table_t {
    weak_entry_t *weak_entries;
    size_t    num_entries;
    uintptr_t mask;
    uintptr_t max_hash_displacement;
};

struct weak_entry_t {
    DisguisedPtr<objc_object> referent;
    union {
        struct {
            weak_referrer_t *referrers;
            uintptr_t        out_of_line_ness : 2;
            uintptr_t        num_refs : PTR_MINUS_2;
            uintptr_t        mask;
            uintptr_t        max_hash_displacement;
        };
        struct {
            // out_of_line_ness field is low bits of inline_referrers[1]
            weak_referrer_t  inline_referrers[WEAK_INLINE_COUNT];
        };
    };

    ...
};

引用计数上面已经介绍过了，下面再说说weak references table

weak references table

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/** 
 * Initialize a fresh weak pointer to some object location. 
 * It would be used for code like: 
 *
 * (The nil case) 
 * __weak id weakPtr;
 * (The non-nil case) 
 * NSObject *o = ...;
 * __weak id weakPtr = o;
 * 
 * This function IS NOT thread-safe with respect to concurrent 
 * modifications to the weak variable. (Concurrent weak clear is safe.)
 *
 * @param location Address of __weak ptr. 
 * @param newObj Object ptr. 
 */

 id objc_initWeak(id *location, id newObj) {
    if (!newObj) {
        *location = nil;
        return nil;
    }

    return storeWeak<DontHaveOld, DoHaveNew, DoCrashIfDeallocating>
        (location, (objc_object*)newObj);
}

// If HaveOld is true, the variable has an existing value 
//   that needs to be cleaned up. This value might be nil.
// If HaveNew is true, there is a new value that needs to be 
//   assigned into the variable. This value might be nil.
// If CrashIfDeallocating is true, the process is halted if newObj is 
//   deallocating or newObj's class does not support weak references. 
//   If CrashIfDeallocating is false, nil is stored instead.
/*
如果HavOld为真，说明有旧的weak对象要清理
如果HavNew为真，说明有新的weak对象要加入
CrashIfDeallocating为真时，不能加入正在被释放的对象;为假时，清理该对象的weak references table
*/
static id storeWeak(id *location, objc_object *newObj) {
    assert(haveOld  ||  haveNew);
    if (!haveNew) assert(newObj == nil);

    Class previouslyInitializedClass = nil;
    id oldObj;
    SideTable *oldTable;
    SideTable *newTable;

    // Acquire locks for old and new values.
    // Order by lock address to prevent lock ordering problems. 
    // Retry if the old value changes underneath us.
 retry:
    if (haveOld) {
        oldObj = *location;
        oldTable = &SideTables()[oldObj];
    } else {
        oldTable = nil;
    }
    if (haveNew) {
        newTable = &SideTables()[newObj];
    } else {
        newTable = nil;
    }

    SideTable::lockTwo<haveOld, haveNew>(oldTable, newTable);

    if (haveOld  &&  *location != oldObj) {
        SideTable::unlockTwo<haveOld, haveNew>(oldTable, newTable);
        goto retry;
    }

    // Prevent a deadlock between the weak reference machinery
    // and the +initialize machinery by ensuring that no 
    // weakly-referenced object has an un-+initialized isa.
    if (haveNew  &&  newObj) {
        Class cls = newObj->getIsa();
        if (cls != previouslyInitializedClass  &&  
            !((objc_class *)cls)->isInitialized()) 
        {
            SideTable::unlockTwo<haveOld, haveNew>(oldTable, newTable);
            _class_initialize(_class_getNonMetaClass(cls, (id)newObj));

            // If this class is finished with +initialize then we're good.
            // If this class is still running +initialize on this thread 
            // (i.e. +initialize called storeWeak on an instance of itself)
            // then we may proceed but it will appear initializing and 
            // not yet initialized to the check above.
            // Instead set previouslyInitializedClass to recognize it on retry.
            previouslyInitializedClass = cls;

            goto retry;
        }
    }

    // Clean up old value, if any.
    if (haveOld) {
        weak_unregister_no_lock(&oldTable->weak_table, oldObj, location);
    }

    // Assign new value, if any.
    if (haveNew) {
        newObj = (objc_object *)
            weak_register_no_lock(&newTable->weak_table, (id)newObj, location, 
                                  crashIfDeallocating);
        // weak_register_no_lock returns nil if weak store should be rejected

        // Set is-weakly-referenced bit in refcount table.
        if (newObj  &&  !newObj->isTaggedPointer()) {
            newObj->setWeaklyReferenced_nolock();
        }

        // Do not set *location anywhere else. That would introduce a race.
        *location = (id)newObj;
    }
    else {
        // No new value. The storage is not changed.
    }
    
    SideTable::unlockTwo<haveOld, haveNew>(oldTable, newTable);

    return (id)newObj;
}
weak_register_no_lock
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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;

    if (!referent  ||  referent->isTaggedPointer()) return referent_id;

    // 确保正在被加入的对象不能是正在被释放的

    if (!referent->ISA()->hasCustomRR()) {
        deallocating = referent->rootIsDeallocating();
    }
    else {
        BOOL (*allowsWeakReference)(objc_object *, SEL) = 
            (BOOL(*)(objc_object *, SEL))
            object_getMethodImplementation((id)referent, 
                                           SEL_allowsWeakReference);
        if ((IMP)allowsWeakReference == _objc_msgForward) {
            return nil;
        }
        deallocating =
            ! (*allowsWeakReference)(referent, SEL_allowsWeakReference);
    }

    if (deallocating) {
        if (crashIfDeallocating) {
            _objc_fatal("Cannot form weak reference to instance (%p) of "
                        "class %s. It is possible that this object was "
                        "over-released, or is in the process of deallocation.",
                        (void*)referent, object_getClassName((id)referent));
        } else {
            return nil;
        }
    }

    /* 
    加入到weak引用列表里
    1.如果已经有了weak引用，追加到weak_entry_t的referrers里
    2.如果没有weak引用，生成新的weak_entry_t，加入到weak引用列表里
    */
    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_table扩容策略，当为原来table大小的3/4时，创建一个新的（扩大一倍，初始值是64），旧数据加入到新的table里 */

        weak_grow_maybe(weak_table);
        weak_entry_insert(weak_table, &new_entry);
    }

    // Do not set *referrer. objc_storeWeak() requires that the 
    // value not change.

    return referent_id;
}
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static void append_referrer(weak_entry_t *entry, objc_object **new_referrer)
{
    if (! entry->out_of_line()) {
        // Try to insert inline.
        for (size_t i = 0; i < WEAK_INLINE_COUNT; i++) {
            if (entry->inline_referrers[i] == nil) {
                entry->inline_referrers[i] = new_referrer;
                return;
            }
        }

        // Couldn't insert inline. Allocate out of line.
        weak_referrer_t *new_referrers = (weak_referrer_t *)
            calloc(WEAK_INLINE_COUNT, sizeof(weak_referrer_t));
        // This constructed table is invalid, but grow_refs_and_insert
        // will fix it and rehash it.
        for (size_t i = 0; i < WEAK_INLINE_COUNT; i++) {
            new_referrers[i] = entry->inline_referrers[i];
        }
        entry->referrers = new_referrers;
        entry->num_refs = WEAK_INLINE_COUNT;
        entry->out_of_line_ness = REFERRERS_OUT_OF_LINE;
        entry->mask = WEAK_INLINE_COUNT-1;
        entry->max_hash_displacement = 0;
    }

    assert(entry->out_of_line());

    if (entry->num_refs >= TABLE_SIZE(entry) * 3/4) {
        return grow_refs_and_insert(entry, new_referrer);
    }
    size_t begin = w_hash_pointer(new_referrer) & (entry->mask);
    size_t index = begin;
    size_t hash_displacement = 0;
    while (entry->referrers[index] != nil) {
        hash_displacement++;
        index = (index+1) & entry->mask;
        if (index == begin) bad_weak_table(entry);
    }
    if (hash_displacement > entry->max_hash_displacement) {
        entry->max_hash_displacement = hash_displacement;
    }
    weak_referrer_t &ref = entry->referrers[index];
    ref = new_referrer;
    entry->num_refs++;
}


static void weak_entry_insert(weak_table_t *weak_table, weak_entry_t *new_entry)
{
    weak_entry_t *weak_entries = weak_table->weak_entries;
    assert(weak_entries != nil);

    size_t begin = hash_pointer(new_entry->referent) & (weak_table->mask);
    size_t index = begin;
    size_t hash_displacement = 0;
    while (weak_entries[index].referent != nil) {
        index = (index+1) & weak_table->mask;
        if (index == begin) bad_weak_table(weak_entries);
        hash_displacement++;
    }

    weak_entries[index] = *new_entry;
    weak_table->num_entries++;

    if (hash_displacement > weak_table->max_hash_displacement) {
        weak_table->max_hash_displacement = hash_displacement;
    }
}

static inline uintptr_t hash_pointer(objc_object *key) {
    return ptr_hash((uintptr_t)key);
}

static inline uint32_t ptr_hash(uint64_t key)
{
    key ^= key >> 4;
    key *= 0x8a970be7488fda55;
    key ^= __builtin_bswap64(key);
    return (uint32_t)key;
}

从上面的代码可以看出：weak_table_t使用对象的hash_pointer值作为index，weak_entry_t作为value。

weak_unregister_no_lock
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/* 清理已注册的weak引用列表 */

void weak_unregister_no_lock(weak_table_t *weak_table, id referent_id, 
                        id *referrer_id) {
    objc_object *referent = (objc_object *)referent_id;
    objc_object **referrer = (objc_object **)referrer_id;

    weak_entry_t *entry;

    if (!referent) return;

    if ((entry = weak_entry_for_referent(weak_table, referent))) {
        remove_referrer(entry, referrer);
        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) {
            weak_entry_remove(weak_table, entry);
        }
    }

    // Do not set *referrer = nil. objc_storeWeak() requires that the 
    // value not change.
}

dealloc

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- (void)dealloc {
    self->rootDealloc();
}

inline void objc_object::rootDealloc() {
    if (isTaggedPointer()) return;  // fixme necessary?

    if (fastpath(isa.nonpointer  &&  
                 !isa.weakly_referenced  &&  
                 !isa.has_assoc  &&  
                 !isa.has_cxx_dtor  &&  
                 !isa.has_sidetable_rc))
    {
        assert(!sidetable_present());
        free(this);
    } 
    else {
        object_dispose((id)this) = {
            if (!obj) return nil;

            objc_destructInstance(obj) = {
                if (obj) {
                    // Read all of the flags at once for performance.
                    bool cxx = obj->hasCxxDtor();
                    bool assoc = obj->hasAssociatedObjects();

                    // This order is important.
                    /*
                    如果有C++成员，释放C++成员
                    如果有关联对象，释放关联对象
                    */
                    if (cxx) object_cxxDestruct(obj);
                    if (assoc) _object_remove_assocations(obj);
                    obj->clearDeallocating() = {
                        if (slowpath(!isa.nonpointer)) {
                            // Slow path for raw pointer isa.
                            sidetable_clearDeallocating();
                        }
                        else if (slowpath(isa.weakly_referenced  ||  isa.has_sidetable_rc)) {
                            // Slow path for non-pointer isa with weak refs and/or side table data.
                            clearDeallocating_slow();
                        }

                        assert(!sidetable_present());
                    };
                }

                return obj;
            };    
            free(obj);

            return nil;
        };
    }
}

最终都会调用对象的SideTable的weak_clear_no_lock方法清理掉该对象的weak_entry_t以及weak_entry_t里的所有weak引用置nil。

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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);
    if (entry == nil) {
        /// XXX shouldn't happen, but does with mismatched CF/objc
        //printf("XXX no entry for clear deallocating %p\n", referent);
        return;
    }

    // zero out references
    weak_referrer_t *referrers;
    size_t count;
    
    if (entry->out_of_line()) {
        referrers = entry->referrers;
        count = TABLE_SIZE(entry);
    } 
    else {
        referrers = entry->inline_referrers;
        count = WEAK_INLINE_COUNT;
    }
    
    for (size_t i = 0; i < count; ++i) {
        objc_object **referrer = referrers[i];
        if (referrer) {
            if (*referrer == referent) {
                *referrer = nil;
            }
            else if (*referrer) {
                _objc_inform("__weak variable at %p holds %p instead of %p. "
                             "This is probably incorrect use of "
                             "objc_storeWeak() and objc_loadWeak(). "
                             "Break on objc_weak_error to debug.\n", 
                             referrer, (void*)*referrer, (void*)referent);
                objc_weak_error();
            }
        }
    }
    
    weak_entry_remove(weak_table, entry);
}
-------------本文结束感谢您的阅读-------------