繼上一篇文章介紹了autoreleasepool的AutoreleasepoolPage，這篇文章我們就來主要講講AutoreleasepoolPage::Push跟AutoreleasepoolPage::Pop 還有一個autorelease

AutoreleasepoolPage::Push

static inline void *push() 
    {
        id *dest;
        if (DebugPoolAllocation) {
            // Each autorelease pool starts on a new pool page.
            // 每個autoreleasepool從一個新的page開始
            dest = autoreleaseNewPage(POOL_BOUNDARY);
        } else {
            // 已經有執行緒的頁
            dest = autoreleaseFast(POOL_BOUNDARY);
        }
        assert(dest == EMPTY_POOL_PLACEHOLDER || *dest == POOL_BOUNDARY);
        return dest;
    }
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這裡分開兩條路，我們先看第一個

autoreleaseNewPage

先說一下大概流程

1.獲取一個活躍的頁 2.判斷這頁是不是滿了，滿的就新建一個page 3.如果沒獲取到page，也是新建一個頁 4.把要push的obj，push到對應的頁裡

1、
   id *autoreleaseNewPage(id obj)
    {
        //獲取一個活躍的頁
        AutoreleasePoolPage *page = hotPage();
        //如果有就返回，並且判斷是否是滿的，滿的就會新建一個page
        if (page) return autoreleaseFullPage(obj,page);
        //如果沒獲取到page
        else return autoreleaseNoPage(obj);
    }
---------------------------------------- 華麗的分割線 ---------------------------------------------------------------------
2、
    id *autoreleaseFullPage(id obj,AutoreleasePoolPage *page)
    {
        // The hot page is full. 
        // Step to the next non-full page,adding a new page if necessary.
        // Then add the object to that page.
        assert(page == hotPage());
        assert(page->full()  ||  DebugPoolAllocation);

        // 如果滿了就迴圈，這一頁有child的話 就指向child，再繼續判斷是否滿
        // 直到沒有child 就建立一個新的頁
        do {
            if (page->child) page = page->child;
            else page = new AutoreleasePoolPage(page);
        } while (page->full());
        // 設定成活躍的頁
        setHotPage(page);
        // 將Obj 新增到這頁裡
        return page->add(obj);
    }
---------------------------------------- 華麗的分割線 ---------------------------------------------------------------------
3、
    id *autoreleaseNoPage(id obj)
    {
        // "No page" could mean no pool has been pushed
        // or an empty placeholder pool has been pushed and has no contents yet
        assert(!hotPage());

        // 翻譯：pushExtraBoundary 推入一個額外邊界
        bool pushExtraBoundary = false;
        if (haveEmptyPoolPlaceholder()) {
            // We are pushing a second pool over the empty placeholder pool
            // or pushing the first object into the empty placeholder pool.
            // Before doing that,push a pool boundary on behalf of the pool 
            // that is currently represented by the empty placeholder.
            //當push一個新頁或者第一頁的時候，在push之前需要先push一個poolboundary(邊界)
            pushExtraBoundary = true;
        }
        else if (obj != POOL_BOUNDARY  &&  DebugMissingPools) {
            // We are pushing an object with no pool in place,// and no-pool debugging was requested by environment.
            _objc_inform("MISSING POOLS: (%p) Object %p of class %s "
                         "autoreleased with no pool in place - "
                         "just leaking - break on "
                         "objc_autoreleaseNoPool() to debug",pthread_self(),(void*)obj,object_getClassName(obj));
            objc_autoreleaseNoPool(obj);
            return nil;
        }
        else if (obj == POOL_BOUNDARY  &&  !DebugPoolAllocation) {
            // We are pushing a pool with no pool in place,// and alloc-per-pool debugging was not requested.
            // Install and return the empty pool placeholder.
            return setEmptyPoolPlaceholder();
        }

        // We are pushing an object or a non-placeholder'd pool.

        // Install the first page.
        AutoreleasePoolPage *page = new AutoreleasePoolPage(nil);
        setHotPage(page);
        
        // Push a boundary on behalf of the previously-placeholder'd pool.
        if (pushExtraBoundary) {
            page->add(POOL_BOUNDARY);
        }
        
        // Push the requested object or pool.
        return page->add(obj);
    }
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到此，第一條路就走完了。

autoreleaseFast

1、
    static inline id *autoreleaseFast(id obj)
    {
        //同樣獲取一個活頁
        AutoreleasePoolPage *page = hotPage();
        if (page && !page->full()) {//如果page沒滿的話
            return page->add(obj);
        } else if (page) {//如果page滿了
            return autoreleaseFullPage(obj,page);
        } else {//沒有獲取到活頁
            return autoreleaseNoPage(obj);
        }
    }
---------------------------------------- 華麗的分割線 ---------------------------------------------------------------------
2、
    id *autoreleaseFullPage(id obj,adding a new page if necessary.
        // Then add the object to that page.
        assert(page == hotPage());
        assert(page->full()  ||  DebugPoolAllocation);

        // 如果滿了就迴圈，這一頁有child的話 就指向child，再繼續判斷是否滿
        // 直到沒有child 就建立一個新的頁
        do {
            if (page->child) page = page->child;
            else page = new AutoreleasePoolPage(page);
        } while (page->full());
        // 設定成活躍的頁
        setHotPage(page);
        // 將Obj 新增到這頁裡
        return page->add(obj);
    }
---------------------------------------- 華麗的分割線 ---------------------------------------------------------------------
3、 
    id *autoreleaseNoPage(id obj)
    {
        // "No page" could mean no pool has been pushed
        // or an empty placeholder pool has been pushed and has no contents yet
        assert(!hotPage());

        // 翻譯：pushExtraBoundary 推入一個額外邊界
        bool pushExtraBoundary = false;
        if (haveEmptyPoolPlaceholder()) {
            // We are pushing a second pool over the empty placeholder pool
            // or pushing the first object into the empty placeholder pool.
            // Before doing that,// and alloc-per-pool debugging was not requested.
            // Install and return the empty pool placeholder.
            return setEmptyPoolPlaceholder();
        }

        // We are pushing an object or a non-placeholder'd pool.

        // Install the first page.
        AutoreleasePoolPage *page = new AutoreleasePoolPage(nil);
        setHotPage(page);
        
        // Push a boundary on behalf of the previously-placeholder'd pool.
        if (pushExtraBoundary) {
            page->add(POOL_BOUNDARY);
        }
        
        // Push the requested object or pool.
        return page->add(obj);
    }
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其實這兩條路是差不多的，在底層中都走了這兩個的方法
autoreleaseFullPage
autoreleaseNoPage
感覺完全可以合起來啊- -，不知道蘋果這麼處理是為了什麼。
#####page->add()
push的最終操作也就是這個add，看看這裡都幹了什麼

    id *add(id obj)
    {
        assert(!full());
        unprotect(); //取消保護
        id *ret = next;  // faster than `return next-1` because of aliasing
        *next++ = obj;
        protect(); // 保護
        return ret;
    }
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這裡做了什麼呢

1. 這裡面有unprotect() protect() 這兩個可以理解為加鎖類似NSLock
2. 建立一個臨時變數ret 來儲存next
3. next ++ 記憶體地址移動。類似 0x0001 -> 0x0002
4. 將obj儲存到新移動出來的記憶體地址上

那麼push就先到這裡，接下來說說autorelease

autorelease

在瞭解過push之後，我們可以很容易理解這個autorelease。看下面的程式碼跳轉， autorelease最終走到了一個上文出現過的方法autoreleaseFast()，那麼其實autorelease做的操作也跟push一樣

- (id)autorelease {
    return ((id)self)->rootAutorelease();
}
---------------------------------------- 華麗的分割線 ---------------------------------------------------------------------
objc_object::rootAutorelease()
{
    if (isTaggedPointer()) return (id)this;
    if (prepareOptimizedReturn(ReturnAtPlus1)) return (id)this;

    return rootAutorelease2();
}
---------------------------------------- 華麗的分割線 ---------------------------------------------------------------------
objc_object::rootAutorelease2()
{
    assert(!isTaggedPointer());
    return AutoreleasePoolPage::autorelease((id)this);
}
---------------------------------------- 華麗的分割線 ---------------------------------------------------------------------
static inline id autorelease(id obj)
{
    assert(obj);
    assert(!obj->isTaggedPointer());
    id *dest __unused = autoreleaseFast(obj);
    assert(!dest  ||  dest == EMPTY_POOL_PLACEHOLDER  ||  *dest == obj);
        return obj;
}
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AutoreleasepoolPage::Pop

先簡單說一下都做了什麼

1. 判斷當前pop物件是否是空池佔位符，是的話就推出去，按道理一般很少進去
2. 通過棧頂的地址找到對應的page
3. 記錄最高水位標記
4. 開始pop，objc_release(obj);
5. kill空白頁

    static inline void pop(void *token) 
    {
        AutoreleasePoolPage *page;
        id *stop;
        //只有在push的時候 push了一個邊界符， 才有可能出現EMPTY_POOL_PLACEHOLDER
        if (token == (void*)EMPTY_POOL_PLACEHOLDER) {
            // Popping the top-level placeholder pool.
            // 推出最頂上的佔位池
            if (hotPage()) {
                // Pool was used. Pop its contents normally.
                // Pool pages remain allocated for re-use as usual.
                // 遞迴 coldPage呼叫begin
                pop(coldPage()->begin());
            } else {
                // Pool was never used. Clear the placeholder.
                setHotPage(nil);
            }
            return;
        }

        // 通過棧頂的地址找到對應的page
        page = pageForPointer(token);
        //token賦值給stop
        stop = (id *)token;
        if (*stop != POOL_BOUNDARY) {
            if (stop == page->begin()  &&  !page->parent) {
                // Start of coldest page may correctly not be POOL_BOUNDARY:
                // 1. top-level pool is popped,leaving the cold page in place
                // 2. an object is autoreleased with no pool
            } else {
                // Error. For bincompat purposes this is not 
                // fatal in executables built with old SDKs.
                return badPop(token);
            }
        }

        // 記錄最高水位標記
        if (PrintPoolHiwat) printHiwat();
        // 從棧頂開始操作出棧，並向棧中的物件傳送release訊息，直到遇到第一個邊界符
        page->releaseUntil(stop);

        // memory: delete empty children
        // 刪除空的child頁
        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();
            }
        }
    }

---------------------------------------- 華麗的分割線 ---------------------------------------------------------------------
    void releaseUntil(id *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);
            }

            //類似push時候的操作
            page->unprotect();
            id obj = *--page->next;
            memset((void*)page->next,SCRIBBLE,sizeof(*page->next));
            page->protect();

            if (obj != POOL_BOUNDARY) {
                //如果當前物件不是邊界符，就釋放物件
                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
    }
---------------------------------------- 華麗的分割線 ---------------------------------------------------------------------
    void kill() 
    {
        // Not recursive: we don't want to blow out the stack 
        // if a thread accumulates a stupendous amount of garbage
        AutoreleasePoolPage *page = this;
        //如果page有child節點 就不斷迴圈直到不再有child
        while (page->child) page = page->child;

        //建立一個deathptr臨時變數來儲存page
        AutoreleasePoolPage *deathptr;
        //如果page不等於this(可以理解成self)
        //就不斷的迴圈把child節點一層一層的置為nil
        do {
            deathptr = page;
            page = page->parent;
            if (page) {
                page->unprotect();
                page->child = nil;
                page->protect();
            }
            delete deathptr;
        } while (deathptr != this);
    }
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