自动释放池的主要底层数据结构是：__AtAutoreleasePool、AutoreleasePoolPage
调用了autorelease的对象最终都是通过AutoreleasePoolPage对象来管理的
源码分析
clang重写@autoreleasepool
objc4源码：NSObject.mm
autoreleasepool
开始调用objc_autoreleasePoolPush
结束调用objc_autoreleasePoolPop
int main(int argc, const char * argv[]) {
@autoreleasepool {
// atautoreleasepoolobj = objc_autoreleasePoolPush();
// objc_autoreleasePoolPop(atautoreleasepoolobj);
}
return 0;
}
AutoreleasePoolPage
每个AutoreleasePoolPage对象占用4096字节内存，除了用来存放它内部的成员变量，剩下的空间用来存放autorelease对象的地址
所有的AutoreleasePoolPage对象通过双向链表的形式连接在一起
image.png
AutoreleasePoolPage的结构
调用push方法会将一个POOL_BOUNDARY入栈，并且返回其存放的内存地址
调用pop方法时传入一个POOL_BOUNDARY的内存地址，会从最后一个入栈的对象开始发送release消息，直到遇到这个POOL_BOUNDARY
id *next指向了下一个能存放autorelease对象地址的区域
/*
typedef CF_OPTIONS(CFOptionFlags, CFRunLoopActivity) {
kCFRunLoopEntry = (1UL << 0), 1
kCFRunLoopBeforeTimers = (1UL << 1), 2
kCFRunLoopBeforeSources = (1UL << 2), 4
kCFRunLoopBeforeWaiting = (1UL << 5), 32
kCFRunLoopAfterWaiting = (1UL << 6), 64
kCFRunLoopExit = (1UL << 7), 128
kCFRunLoopAllActivities = 0x0FFFFFFFU
};
*/
/*
kCFRunLoopEntry push
{valid = Yes, activities = 0x1, repeats = Yes, order = -2147483647, callout = _wrapRunLoopWithAutoreleasePoolHandler (0x103376df2), context = {type = mutable-small, count = 1, values = (\n\t0 : <0x7fd0bf802048>\n)}}
kCFRunLoopBeforeWaiting | kCFRunLoopExit
kCFRunLoopBeforeWaiting pop、push
kCFRunLoopExit pop
{valid = Yes, activities = 0xa0, repeats = Yes, order = 2147483647, callout = _wrapRunLoopWithAutoreleasePoolHandler (0x103376df2), context = {type = mutable-small, count = 1, values = (\n\t0 : <0x7fd0bf802048>\n)}}
*/
Runloop和Autorelease
iOS在主线程的Runloop中注册了2个Observer
第1个Observer监听了kCFRunLoopEntry事件，会调用objc_autoreleasePoolPush()
第2个Observer
监听了kCFRunLoopBeforeWaiting事件，会调用objc_autoreleasePoolPop()、objc_autoreleasePoolPush()
监听了kCFRunLoopBeforeExit事件，会调用objc_autoreleasePoolPop()