继上一篇文章介绍了autoreleasepool的AutoreleasepoolPage，这篇文章我们就来主要讲讲AutoreleasepoolPage::Push跟AutoreleasepoolPage::Pop 还有一个autorelease
AutoreleasepoolPage::Push
static inline void *push()
{
id *dest;
if (DebugPoolAllocation) {
// Each autorelease pool starts on a new pool page.
// 每个autoreleasepool从一个新的page开始
dest = autoreleaseNewPage(POOL_BOUNDARY);
} else {
// 已经有线程的页
dest = autoreleaseFast(POOL_BOUNDARY);
}
assert(dest == EMPTY_POOL_PLACEHOLDER || *dest == POOL_BOUNDARY);
return dest;
}
这里分开两条路，我们先看第一个
autoreleaseNewPage
先说一下大概流程
1.获取一个活跃的页
2.判断这页是不是满了，满的就新建一个page
3.如果没获取到page，也是新建一个页
4.把要push的obj，push到对应的页里
1、
id *autoreleaseNewPage(id obj)
{
//获取一个活跃的页
AutoreleasePoolPage *page = hotPage();
//如果有就返回，并且判断是否是满的，满的就会新建一个page
if (page) return autoreleaseFullPage(obj, page);
//如果没获取到page
else return autoreleaseNoPage(obj);
}
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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);
// 如果满了就循环，这一页有child的话 就指向child，再继续判断是否满
// 直到没有child 就创建一个新的页
do {
if (page->child) page = page->child;
else page = new AutoreleasePoolPage(page);
} while (page->full());
// 设置成活跃的页
setHotPage(page);
// 将Obj 添加到这页里
return page->add(obj);
}
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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());
// 翻译：pushExtraBoundary 推入一个额外边界
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.
//当push一个新页或者第一页的时候，在push之前需要先push一个poolboundary(边界)
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);
}
到此，第一条路就走完了。
autoreleaseFast
1、
static inline id *autoreleaseFast(id obj)
{
//同样获取一个活页
AutoreleasePoolPage *page = hotPage();
if (page && !page->full()) {//如果page没满的话
return page->add(obj);
} else if (page) {//如果page满了
return autoreleaseFullPage(obj, page);
} else {//没有获取到活页
return autoreleaseNoPage(obj);
}
}
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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);
// 如果满了就循环，这一页有child的话 就指向child，再继续判断是否满
// 直到没有child 就创建一个新的页
do {
if (page->child) page = page->child;
else page = new AutoreleasePoolPage(page);
} while (page->full());
// 设置成活跃的页
setHotPage(page);
// 将Obj 添加到这页里
return page->add(obj);
}
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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());
// 翻译：pushExtraBoundary 推入一个额外边界
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.
//当push一个新页或者第一页的时候，在push之前需要先push一个poolboundary(边界)
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);
}
其实这两条路是差不多的，在底层中都走了这两个的方法
autoreleaseFullPage
autoreleaseNoPage
感觉完全可以合起来啊- -，不知道苹果这么处理是为了什么。
page->add()
push的最终操作也就是这个add，看看这里都干了什么
id *add(id obj)
{
assert(!full());
unprotect(); //取消保护
id *ret = next; // faster than `return next-1` because of aliasing
*next++ = obj;
protect(); // 保护
return ret;
}
这里做了什么呢
- 这里面有unprotect() protect() 这两个可以理解为加锁类似NSLock
- 创建一个临时变量ret 来存储next
- next ++ 内存地址移动。类似 0x0001 -> 0x0002
- 将obj存储到新移动出来的内存地址上
那么push就先到这里，接下来说说autorelease
autorelease
在了解过push之后，我们可以很容易理解这个autorelease。看下面的代码跳转，
autorelease最终走到了一个上文出现过的方法autoreleaseFast()，那么其实autorelease做的操作也跟push一样
- (id)autorelease {
return ((id)self)->rootAutorelease();
}
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objc_object::rootAutorelease()
{
if (isTaggedPointer()) return (id)this;
if (prepareOptimizedReturn(ReturnAtPlus1)) return (id)this;
return rootAutorelease2();
}
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objc_object::rootAutorelease2()
{
assert(!isTaggedPointer());
return AutoreleasePoolPage::autorelease((id)this);
}
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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;
}
AutoreleasepoolPage::Pop
先简单说一下都做了什么
- 判断当前pop对象是否是空池占位符，是的话就推出去，按道理一般很少进去
- 通过栈顶的地址找到对应的page
- 记录最高水位标记
- 开始pop，objc_release(obj);
- kill空白页
static inline void pop(void *token)
{
AutoreleasePoolPage *page;
id *stop;
//只有在push的时候 push了一个边界符， 才有可能出现EMPTY_POOL_PLACEHOLDER
if (token == (void*)EMPTY_POOL_PLACEHOLDER) {
// Popping the top-level placeholder pool.
// 推出最顶上的占位池
if (hotPage()) {
// Pool was used. Pop its contents normally.
// Pool pages remain allocated for re-use as usual.
// 递归 coldPage调用begin
pop(coldPage()->begin());
} else {
// Pool was never used. Clear the placeholder.
setHotPage(nil);
}
return;
}
// 通过栈顶的地址找到对应的page
page = pageForPointer(token);
//token赋值给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);
}
}
// 记录最高水位标记
if (PrintPoolHiwat) printHiwat();
// 从栈顶开始操作出栈，并向栈中的对象发送release消息，直到遇到第一个边界符
page->releaseUntil(stop);
// memory: delete empty children
// 删除空的child页
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();
}
}
}
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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);
}
//类似push时候的操作
page->unprotect();
id obj = *--page->next;
memset((void*)page->next, SCRIBBLE, sizeof(*page->next));
page->protect();
if (obj != POOL_BOUNDARY) {
//如果当前对象不是边界符，就释放对象
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
}
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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节点 就不断循环直到不再有child
while (page->child) page = page->child;
//创建一个deathptr临时变量来存储page
AutoreleasePoolPage *deathptr;
//如果page不等于this(可以理解成self)
//就不断的循环把child节点一层一层的置为nil
do {
deathptr = page;
page = page->parent;
if (page) {
page->unprotect();
page->child = nil;
page->protect();
}
delete deathptr;
} while (deathptr != this);
}
