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标题: AutoreleasePool从入门到放弃（3） [打印本页]

作者: look_w 时间: 2019-3-8 19:33 标题: AutoreleasePool从入门到放弃（3）

3. AutoreleasePool源码解析

AutoreleasePool源码可以从Runtime源码中获得。下载好我们可以在NSObject.mm中看到AutoreleasePool的实现原理和结构说明：

Autorelease pool implementation
A thread's autorelease pool is a stack of pointers. Each pointer is either an object to release, or POOL_BOUNDARY which is an autorelease pool boundary.
A pool token is a pointer to the POOL_BOUNDARY for that pool. When the pool is popped, every object hotter than the sentinel is released.
The stack is divided into a doubly-linked list of pages. Pages are added and deleted as necessary.
Thread-local storage points to the hot page, where newly autoreleased objects are stored.

AutoreleasePool其实是一个存贮指针的栈，内部指针代表延迟释放的对象或者是表示不同AutoreleasePool之间的边界对象。
AutoreleasePool的token指针名为POOL_BOUNDARY，当AutoreleasePool执行Pop操作时后于POOL_BOUNDARY入栈的对象将被释放。
AutoreleasePool是由多页的双向列表页组成，列表页在必要时可添加与删除。
TLS(Thread-local storage)指向autoreleased最近存储的AutoreleasePool

class AutoreleasePoolPage
{
      ...
      magic_t const magic;                //校验AutoreleasePoolPage完整性
      id *next;                            //存放下一个autoreleased对象地址
      pthread_t const thread;             //对应线程
      AutoreleasePoolPage * const parent; //父AutoreleasePoolPage
      AutoreleasePoolPage *child;          //子AutoreleasePoolPage
      uint32_t const depth;                //当前AutoreleasePoolPage索引index
      uint32_t hiwat;                      //存储autoreleased对象峰值数
      ...
}

接下来我们分析在使用@autoreleasepool block时代码的具体执行。我们先从每个Cocoa Touch工程都包含的Main.m文件入手，cd到Main.m路径下再通过命令行来将代码转成c++形式

clang -rewrite-objc -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator11.3.sdk main.m

我们可以看到原本的代码被转换成：

int main(int argc, char * argv[]) {
      /* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool;
         return UIApplicationMain(argc, argv, __null, NSStringFromClass(((Class (*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("AppDelegate"), sel_registerName("class"))));
      }
}

而结构体__AtAutoreleasePool的定义我们也可以在转换代码中找到：

struct __AtAutoreleasePool {
   __AtAutoreleasePool() {atautoreleasepoolobj = objc_autoreleasePoolPush();}
   ~__AtAutoreleasePool() {objc_autoreleasePoolPop(atautoreleasepoolobj);}
   void * atautoreleasepoolobj;
};

由此我们可以知道__AtAutoreleasePool结构体在初始化和析构时分别执行了objc_autoreleasePoolPush和objc_autoreleasePoolPop(atautoreleasepoolobj)函数。我们先从这两个函数开始分析：(函数源码可以在之前的NSObject.mm文件中查找到)
objc_autoreleasePoolPush(void)

void *objc_autoreleasePoolPush(void)
{
      return AutoreleasePoolPage::push();
}

push()

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;
      }

Debug模式
      autoreleaseNewPage(POOL_BOUNDARY);
非Debug模式下
      autoreleaseFast(POOL_BOUNDARY);

返回空池标志位或者AutoreleasePool边界对象

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