AutoreleasePool的原理

本文主要是介绍AutoreleasePool的原理，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

Tips:利用_objc_autoreleasePoolPrint()可以查看注册到autoreleasepool的对象，方便我们调试。

autoreleasepool的结构是一个双向链表，分别指向parent和child表，一个page表相当于一个栈结构，遵循先进后出

结构分析

AutoreleasePoolPage

打开NSObject源码我们发现autoreleasepool接口围绕AutoreleasePoolPage进行调用，源码如下：

class AutoreleasePoolPage 
{// EMPTY_POOL_PLACEHOLDER is stored in TLS when exactly one pool is // pushed and it has never contained any objects. This saves memory // when the top level (i.e. libdispatch) pushes and pops pools but // never uses them.
#   define EMPTY_POOL_PLACEHOLDER ((id*)1)#   define POOL_BOUNDARY nilstatic pthread_key_t const key = AUTORELEASE_POOL_KEY;static uint8_t const SCRIBBLE = 0xA3;  // 0xA3A3A3A3 after releasingstatic size_t const SIZE = 
#if PROTECT_AUTORELEASEPOOLPAGE_MAX_SIZE;  // must be multiple of vm page size
#elsePAGE_MAX_SIZE;  // size and alignment, power of 2
#endifstatic size_t const COUNT = SIZE / sizeof(id);magic_t const magic;id *next;pthread_t const thread;AutoreleasePoolPage * const parent;AutoreleasePoolPage *child;uint32_t const depth;uint32_t hiwat;// SIZE-sizeof(*this) bytes of contents followstatic void * operator new(size_t size) {return malloc_zone_memalign(malloc_default_zone(), SIZE, SIZE);}static void operator delete(void * p) {return free(p);}...... //此处略去代码id * begin() {return (id *) ((uint8_t *)this+sizeof(*this));}id * end() {return (id *) ((uint8_t *)this+SIZE);}bool empty() {return next == begin();}bool full() { return next == end();}bool lessThanHalfFull() {return (next - begin() < (end() - begin()) / 2);}id *add(id obj){assert(!full());unprotect();id *ret = next;  // faster than `return next-1` because of aliasing*next++ = obj;protect();return ret;}......//此处略去代码public: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;}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;}

• magic 用来校验 AutoreleasePoolPage 的结构是否完整；
• next 指向最新添加的 autoreleased 对象的下一个位置，初始化时指向 begin()；
• thread 指向当前线程；
• parent 指向父结点，第一个结点的 parent 值为 nil；
• child 指向子结点，最后一个结点的 child 值为 nil ；
• depth 代表深度，从 0 开始，往后递增 1；
• hiwat 代表 high water mark 。
• 分配的内存大小PAGE_MAX_SIZE为4096，即虚拟内存一页大小
• obj->isTaggedPointer()表示Tagged Pointer对象不能加入autoreleasepool
  另外，当next == begin() 时，表示 AutoreleasePoolPage 为空；当 next == end() 时，表示 AutoreleasePoolPage 已满。

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创建函数

由源码可知，由push操作创建一个新的AutoreleasePoolPage，看下创建函数

AutoreleasePoolPage(AutoreleasePoolPage *newParent) : magic(), next(begin()), thread

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