AutoReleasePool 工作机制
背景：
相比很多人都有这个困惑，@autoreleasepool 到底做了什么，今天我们就从源码的基础上去分析，彻底揭开它神秘面纱
源码路径：
https://opensource.apple.com/tarballs/objc4/
得益于objc 开源，我们可以一窥其本质，在接下来分析中我会尽可能引入源码
@autoreleasepool 转成c++到底是什么样的
| Objective-C
@autoreleasepool
{
NSObject *pObjecdt = nil;
pObjecdt = [[Test alloc] init];
[pObjecdt autorelease];
NSLog(@"refcount = %d", CFGetRetainCount(pObjecdt));
} |
OC 转 C++
使用命令
xcrun -sdk iphoneos clang -arch arm64 -rewrite-objc main.m -o main-arm64.cpp
就可以把对对应m 转成c++
| Objective-C
{
__AtAutoreleasePool __autoreleasepool;
NSObject *pObjecdt = __null;
pObjecdt = ((Test *(*)(id, SEL))(void *)objc_msgSend)((id)((Test *(*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("Test"), sel_registerName("alloc")), sel_registerName("init"));
((NSObject *(*)(id, SEL))(void *)objc_msgSend)((id)pObjecdt, sel_registerName("autorelease"));
NSLog((NSString *)&__NSConstantStringImpl__var_folders_x4_q4k0pbd17n90b9x6_v_h_rb40000gn_T_main_092efe_mi_0, CFGetRetainCount(pObjecdt));
} |
也就是说@autoreleasepool 转换成了__AtAutoreleasePool __autoreleasepool;
下边就介绍__AtAutoreleasePool的定义
__AtAutoreleasePool
| C++
struct __AtAutoreleasePool {
__AtAutoreleasePool() {atautoreleasepoolobj = objc_autoreleasePoolPush();}
~__AtAutoreleasePool() {objc_autoreleasePoolPop(atautoreleasepoolobj);}
void * atautoreleasepoolobj;
}; |
通过上边可以看的出来 这是很简单的结构图，遵循的RAII的原则，也就是说构造的时候生成atautoreleasepoolobj 指针，而 西沟的时候把atautoreleasepoolobj 传入到objc_autoreleasePoolPop中去，所以要搞清楚AutoReleasePool 就要搞清楚objc_autoreleasePoolPush 与 objc_autoreleasePoolPop 是干啥的。
objc_autoreleasePoolPush
| C++
struct AutoreleasePoolPageData
{
magic_t const magic;
__unsafe_unretained id *next;
pthread_t const thread;
AutoreleasePoolPage * const parent;
AutoreleasePoolPage *child;
uint32_t const depth;
uint32_t hiwat;
};
class AutoreleasePoolPage : private AutoreleasePoolPageData
{
}
void *
objc_autoreleasePoolPush(void)
{
return AutoreleasePoolPage::push();
}
# define POOL_BOUNDARY nil
static inline void *push()
{
id *dest;
dest = autoreleaseFast(POOL_BOUNDARY);
return dest;
}
//obj = POOL_BOUNDARY
static inline id *autoreleaseFast(id obj)
{
AutoreleasePoolPage *page = hotPage();
if (page && !page->full()) {
return page->add(obj);
} else if (page) {
return autoreleaseFullPage(obj, page);
} else {
return autoreleaseNoPage(obj);
}
}
static inline AutoreleasePoolPage *hotPage()
{
AutoreleasePoolPage *result = (AutoreleasePoolPage *)
tls_get_direct(key);
if ((id *)result == EMPTY_POOL_PLACEHOLDER) return nil;
if (result) result->fastcheck();
return result;
}
class AutoreleasePoolPage
{
//obj = POOL_BOUNDARY
id *add(id obj)
{
id *ret;
ret = next; // faster than `return next-1` because of aliasing
*next++ = obj;
return ret;
}
}
|
通过上边调用关系可以看的出来，AutoreleasePoolPage 是跟线程相关的，push首先从tls 中取出当前线程相关连的AutoreleasePoolPage指针，类似于opengl context的原理，把POOL_BOUNDARY 放入到AutoreleasePoolPage的next位置的地方
objc_autoreleasePoolPop
| C++
void
objc_autoreleasePoolPop(void *ctxt)
{
AutoreleasePoolPage::pop(ctxt);
}
void AutoreleasePoolPage::pop(void *token)
{
AutoreleasePoolPage *page;
id *stop;
return popPage<false>(token, page, stop);
}
template<bool allowDebug>
static void
popPage(void *token, AutoreleasePoolPage *page, id *stop)
{
page->releaseUntil(stop);
}
//stop 最接近的一个POOL_BOUNDARY 的地址
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();
id obj = *--page->next;
if (obj != POOL_BOUNDARY) {
objc_release(obj);
}
}
}
void
objc_release(id obj)
{
if (obj->isTaggedPointerOrNil()) return;
return obj->release();
}
inline void
objc_object::release()
{
ASSERT(!isTaggedPointer());
rootRelease(true, RRVariant::FastOrMsgSend);
}
ALWAYS_INLINE bool
objc_object::rootRelease()
{
return rootRelease(true, RRVariant::Fast);
}
bool
objc_object::rootRelease(bool performDealloc, objc_object::RRVariant variant)
{
// don't check newisa.fast_rr; we already called any RR overrides
uintptr_t carry;
newisa.bits = subc(newisa.bits, RC_ONE, 0, &carry); // extra_rc--
//#define slowpath(x) (__builtin_expect(bool(x), 0))
// __builtin_expect((x),1)表示 x 的值为真的可能性更大；
//bool isDeallocating() {
// return extra_rc == 0;
if (slowpath(newisa.isDeallocating()))
goto deallocate;
return false;
deallocate:
if (performDealloc) {
((void(*)(objc_object *, SEL))objc_msgSend)(this, @selector(dealloc));
}
} |
Autorelease 源码解析
[pObjecdt autorelease];
也就是看下这个里边到底在干什么
| C++
// Replaced by ObjectAlloc
- (id)autorelease {
return _objc_rootAutorelease(self);
}
id _objc_rootAutorelease(id obj)
{
ASSERT(obj);
return obj->rootAutorelease();
}
inline id objc_object::rootAutorelease()
{
if (isTaggedPointer()) return (id)this;
return rootAutorelease2();
}
id objc_object::rootAutorelease2()
{
ASSERT(!isTaggedPointer());
return AutoreleasePoolPage::autorelease((id)this);
}
static inline id AutoreleasePoolPage::autorelease(id obj)
{
id *dest __unused = autoreleaseFast(obj);
return obj;
}
static inline id *AutoreleasePoolPage::autoreleaseFast(id obj)
{
//这里就跟PoolPush 方法调用逻辑是一样的了，就把这个指针入到page页面里面
} |
经过上边分析，我们看下边代码情景会生发生什么
Case 1:
| Objective-C
NSObject *pObjecdt = nil;
@autoreleasepool
{
pObjecdt = [[Test alloc] init];
[pObjecdt autorelease];
NSLog(@"refcount = %d", CFGetRetainCount(pObjecdt));
}
NSLog(@"refcount = %d", CFGetRetainCount(pObjecdt));
|
Case 2:
| Objective-C
@autoreleasepool {
NSObject *pObjecdt1 = [[[Test alloc] init] retain];
@autoreleasepool
{
NSObject *pObjecdt = [[Test alloc] init];
[pObjecdt autorelease];
_objc_autoreleasePoolPrint();
}
} |
相关源码
| Objective-C
#ifndef Test_h
#define Test_h
@interface Test : NSObject
{
}
-(void)dealloc;
+ (NSString *)description;
@end
#endif /* Test_h */
|
| Objective-C
#import
#import "Test.h"
@implementation Test
-(void)dealloc
{
NSLog(@"Test Destory!");
}
+ (NSString *)description
{
return @"Test";
}
@end |
总结：
也就是auto release pool 只有调用对象调用 auto release才会起作用，如果没有，可以忽略这个auto release pool