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内存问题研究
定时器内存泄漏
NSTimer、CADisplayLink 的 基础 API [NSTimer scheduledTimersWithTimeInterval:1 repeat:YES block:nil] 和当前的 VC 都会互相持有，造成环，会存在内存泄漏问题。
定时器内存泄漏原因，解决方案以及高精度定时器，具体可以看这篇 NSTimer 中的内存泄露 。
iOS 内存布局
栈、堆、BSS、数据段、代码段
栈（stack）：又称作堆栈，用来存储程序的局部变量（但不包括static声明的变量，static修饰的数据存放于数据段中）。除此之外，在函数被调用时，栈用来传递参数和返回值。栈内存地址越来越少
func a {
变量 1 地址最大
变量 2 地址第二大
// ...
变量n 地址最小
}
堆（heap）：用于存储程序运行中被动态分配的内存段，它的大小并不固定，可动态的扩张和缩减。操作函数(malloc／free)。分配的内存空间地址越来越大。
BSS段（bss segment）：通常用来存储程序中未被初始化的全局变量和静态变量的一块内存区域。BSS是英文Block Started by Symbol的简称。BSS段输入静态内存分配
数据段（data segment）：通常用来存储程序中已被初始化的全局变量和静态变量和字符串的一块内存区域。数据段包含3部分：
-
字符串常量。比如
NSString *str = @"杭城小刘"; -
已初始化数据：已经初始化的全局变量、静态变量等
-
未初始化数据：未初始化的全局变量、静态变量等
代码段（code segment）：编译之后的代码。通常是指用来存储程序可执行代码的一块内存区域。这部分区域的大小在程序运行前就已经确定，并且内存区域通常属于只读，某些架构也允许代码段为可写，即允许修改程序。
上 Demo 验证
int a = 10;
static int b;
int main () {
NSString *name = @"杭城小刘";
int age = 27;
int height = 177;
NSObject *obj = [[NSObject alloc] init];
NSLog(@"\na: %p\nb: %p\n name: %p\nage: %p\n height: %p\nobj:%p", &a, &b, &name, &age, &height, obj);
}
a: 0x107b09b80
b: 0x107b09c48
name: 0x7ff7b83fdbc0
age: 0x7ff7b83fdbbc
height: 0x7ff7b83fdbb8
obj:0x6000012780e0
我们按照内存地址由低到高排个序(如下)，发现和我们总结的规律一致。
// 字符串常量
name: 0x7ff7b83fdbc0
// 已初始化的全局变量、静态变量
a: 0x107b09b80
// 未初始化的全局变量、静态变量
b: 0x107b09c48
// 堆
obj: 0x6000012780e0
// 栈
height: 0x7ff7b83fdbb8
age: 0x7ff7b83fdbbc
NSObject *obj = [[NSObject alloc] init];
NSLog(@"%p %p %@", obj, &obj, obj);
分别打印 obj指针指向的堆上的内存地址、obj 指针在栈上的地址、obj 内容
Tagged Pointer
先来一个 Demo 开启本部分内容（画外音：代码很短，但让我产生了一个大大的问号）
- (bool)isTaggedPointer:(const void *)ptr
{
return ((uintptr_t)ptr & (1UL<<63)) == (1UL<<63);
}
NSNumber *number = [NSNumber numberWithInt:10]; // 0xb0000000000000a2 b:12 1100
NSLog(@"%p %d %@", number, [self isTaggedPointer:(__bridge const void *)number], number.class);
NSString *name1 = [NSString stringWithFormat:@"ss"]; // 0xa000000000073732 a:11 1011
NSLog(@"%p %d %@", name1, [self isTaggedPointer:(__bridge const void *)name1], name1.class);
前提说明：
创建一个 NSNumer 类型的变量 number，NSString 类型的 name1，代码打印地址、类型。产生一个问题：为什么 NSNumber 是 TaggedPointer，但是 class 却显示 __NSCFNumber ？
static inline bool _objc_isTaggedPointer(const void * _Nullable ptr)
{
return ((uintptr_t)ptr & _OBJC_TAG_MASK) == _OBJC_TAG_MASK;
}
-
通过 objc4 源码研究写了个判断对象是否是 Tagged Pointer 类型的方法。通过系统源码参考写了判断方法
isTaggedPointer。调用方法得到 number 对象是 Tagged Pointer 类型 -
根据 iOS 平台特性，根据内存地址高位分析确实是 TaggedPointer 类型
-
同样的 NSString 指针指向的字符串内容比较少，占用内存没必要开创新的内存时，name1 就是 NSTaggedPointerString，打印出 class 也是 NSTaggedPointerString。调用
isTaggedPointer得到也是 Tageed Pointer 类型
带着问题开始吧
什么是 Tagged Pointer
iOS 从 64bit 开始引入了Tagged Pointer 技术，用于优化 NSNumber、NSDate、NSString等小对象的存储。
在此之前，创建对象需要动态分配内存、维护引用计数等，对象指针存储的是堆中对象的地址值创建一个对象的流程。先在堆上申请一块内存，然后再在栈上增加一个指针类型，指针指向堆上这块内存。假如是 NSNumber *value = [NSNumber numberWithInt:2] value 是指针长度为8字节，堆上内存16字节。加起来24字节就存一个int 2。
此外还需要维护引用技术，沿用一个真正对象那一套，太大材小用了。
Tagged Pointer 格式，对象指针里面存储的数据变成了：Tag + Data，将数据直接存储在了指针中。当指针不够存储数据时，才会使用动态分配内存的方式来存储数据
objc_msgSend 能识别 Tagged Pointer，比如 NSNumber 的 intValue 方法，直接从指针提取数据，节省了调用开销。
经典问题
Demo1
- (void)test {
dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
for (NSInteger i = 0; i<1000; i++) {
dispatch_async(queue, ^{
self.name = [NSString stringWithFormat:@"和好多好多好多好多事看看上课上课上课"];
});
}
}
运行该代码会 Crash，报错信息如下
说明：一开始的报错信息只说坏内存访问，但是并没有显示具体的方法调用堆。想知道具体 Crash 原因还是需要看看堆栈比较方便。输入 bt 查看最后是由于 objc_release 方法造成 crash。
小窍门：利用 LLDB 模式下输入 bt，可以查看堆栈。也就是 backtrace 的缩写。
不仔细想可能发现不了问题，看到 objc_release 就会想到是在多线程情况下 NSString 的 setter 方法内，ARC 代码经过编译器最后会按照 MRC 去运行。所以 Setter 类似下面代码。
-(void)setName:(NSString *)name
{
if (_name!=name) {
[_name release];
_name = [name retain];
}
}
Demo
- (void)test {
dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
for (NSInteger i = 0; i<1000; i++) {
dispatch_async(queue, ^{
self.name = [NSString stringWithFormat:@"ss"];
if (i == 100) {
NSLog(@"%p %@", self.name, self.name.class);
}
});
}
}
// 0xa000000000073732 NSTaggedPointerString
同样的代码字符串变短居然不 crash 了？因为命中 Tagged Pointer 逻辑了，查看类型是 NSTaggedPointerString
本问题本质是
-
ARC 代码在编译后真正运行阶段是走 MRC 的，strong、copy 内部都会 release 旧的，copy/retain 新的
-
多线程情况下访问 setter 需要加锁
-
字符串在 NSTaggedPointerString 情况下不存在像 OC 对象的 setter 方法内的 release、copy 操作
如何判断一个指针是否为Tagged Pointer
查看 objc4 源码
#if TARGET_OS_OSX && __x86_64__
// 64-bit Mac - tag bit is LSB
# define OBJC_MSB_TAGGED_POINTERS 0
#else
// Everything else - tag bit is MSB
# define OBJC_MSB_TAGGED_POINTERS 1
#endif
#if OBJC_MSB_TAGGED_POINTERS
# define _OBJC_TAG_MASK (1UL<<63)
# define _OBJC_TAG_INDEX_SHIFT 60
# define _OBJC_TAG_SLOT_SHIFT 60
# define _OBJC_TAG_PAYLOAD_LSHIFT 4
# define _OBJC_TAG_PAYLOAD_RSHIFT 4
# define _OBJC_TAG_EXT_MASK (0xfUL<<60)
# define _OBJC_TAG_EXT_INDEX_SHIFT 52
# define _OBJC_TAG_EXT_SLOT_SHIFT 52
# define _OBJC_TAG_EXT_PAYLOAD_LSHIFT 12
# define _OBJC_TAG_EXT_PAYLOAD_RSHIFT 12
#else
# define _OBJC_TAG_MASK 1UL
# define _OBJC_TAG_INDEX_SHIFT 1
# define _OBJC_TAG_SLOT_SHIFT 0
# define _OBJC_TAG_PAYLOAD_LSHIFT 0
# define _OBJC_TAG_PAYLOAD_RSHIFT 4
# define _OBJC_TAG_EXT_MASK 0xfUL
# define _O BJC_TAG_EXT_INDEX_SHIFT 4
# define _OBJC_TAG_EXT_SLOT_SHIFT 4
# define _OBJC_TAG_EXT_PAYLOAD_LSHIFT 0
# define _OBJC_TAG_EXT_PAYLOAD_RSHIFT 12
#endif
static inline bool _objc_isTaggedPointer(const void * _Nullable ptr)
{
return ((uintptr_t)ptr & _OBJC_TAG_MASK) == _OBJC_TAG_MASK;
}
可以看到源码通过 _objc_isTaggedPointer 方法判断是否是 Tagged Pointer 类型。传入对象地址，内部通过 _OBJC_TAG_MASK 按位与运算。
其中 _OBJC_TAG_MASK 是一个宏，宏定义外部有个 if 判读，判断是 Mac OS 并且是 x86_64 架构则为0，否则为1。也就是 Mac OS 并且是 x86_64 架构情况下则与 1UL 按位与，否则与 1UL<<63 按位与。
-
iOS平台 ｜ Mac 非 x86 平台： 最高有效位是1（第64bit）
1UL<<63 -
Mac 且 x86平台： 最低有效位是1
1UL
比如 iOS 平台下
0xb0000000000000a2 b:12 1100
1100
& 1000
-------
1000
OC 对象内存管理
iOS 中使用引用计数来管理 OC 对象的内存。一个新创建的 OC 对象引用计数默认是1，当引用计数减为 0，OC 对象就会销毁，释放其占用的内存空间
调用 retain/copy 会让 OC 对象的引用计数 +1，调用 release 会让 OC 对象的引用计数 -1。
内存管理的经验总结
-
当调用 alloc、new、copy、mutableCopy 方法返回了一个对象，在不需要这个对象时，要调用 release 或者 autorelease 来释放它
-
想拥有某个对象，就让它的引用计数 +1；不想再拥有某个对象，就让它的引用计数 -1
-
可以通过以下私有函数来查看自动释放池的情况
extern void _objc_autoreleasePoolPrint(void);
僵尸对象：重复释放内存造成的。一个典型场景是多次 setter。setter 内部实现不合理，比如下面 setter。
Person *p = [[Person aloc] init]; // 1
Cat *cat = [[Cat alloc] init]; // 1
[p setCat:cat]; // 2
[cat release]; // 1
[p setCat:cat]; // 0
[p setCat:cat]; // badAccess
- (void)setCat:(Cat *)cat
{
[_cat release];
_cat = [cat retain];
}
改进
- (void)setCat:(Cat *)cat
{
if (_cat != cat) {
[_cat release];
_cat = [cat retain];
}
}
早期在 MRC 时代，在 .h 文件中 @property 只会属性的 getter、setter 声明，@synthesize 会自动生成成员变量和属性的 setter、getter 的实现。随着编译器进步，现在 @property 会做完全部的事情。
早期 VC 中使用属性
@property (nonatomic, strong) NSMutableDictionary *dict;
NSMutableDictionary *dict = [[NSMutableDictionary alloc] init];
self.dict = dict;
[dict release];
通过 Foundation 框架中类方法创建出来的对象，会自动调用 autorelease 方法。
简写为 self.dict = [NSMutableDictionary dictionary];
上述可以查看 GUNStep 源码 NSDictionary.m
#define AUTORELEASE(object) [(id)(object) autorelease]
+ (id) dictionary {
return AUTORELEASE([[self allocWithZone: NSDefaultMallocZone()] init]);
}
QA：ARC 做了什么
ARC 其实是 LLVM + Runtime 共同作用的结果。LLVM 编译器自动插入 retain、release 内存管理代码。Runtime 运行时帮我们处理类似 __weak 程序运行过程中弱引用清除掉。
copy/mutableCopy
OC 有2个拷贝方法
-
copy 不可变拷贝，产生新不可变对象
-
mutableCopy 可变拷贝，产生新可变对象
上个 Demo1
NSArray *array1 = [[NSArray alloc] initWithObjects:@"1", @"2", nil];
NSLog(@"array1 --- %zd", array1.retainCount);
NSArray *array2 = [array1 copy];
NSLog(@"array1 --- %zd", array1.retainCount);
NSLog(@"array2 --- %zd", array2.retainCount);
NSMutableArray *array3 = [array1 mutableCopy];
NSLog(@"array1 --- %zd", array1.retainCount);
NSLog(@"array2 --- %zd", array2.retainCount);
NSLog(@"array3 --- %zd" array3.retainCount);
[array3 release];
NSLog(@"array3 --- %zd", array3.retainCount);
[array2 release];
NSLog(@"array2 --- %zd", array2.retainCount);
NSLog(@"array1 --- %zd", array1.retainCount);
[array1 release];
NSLog(@"array1 --- %zd", array1.retainCount);
2022-04-12 20:50:43.639296+0800 Main[4408:60897] array1 --- 1
2022-04-12 20:50:43.639715+0800 Main[4408:60897] array1 --- 2
2022-04-12 20:50:43.639772+0800 Main[4408:60897] array2 --- 2
2022-04-12 20:50:43.639846+0800 Main[4408:60897] array1 --- 2
2022-04-12 20:50:43.639899+0800 Main[4408:60897] array2 --- 2
2022-04-12 20:50:43.639957+0800 Main[4408:60897] array3 --- 1
2022-04-12 20:50:43.640013+0800 Main[4408:60897] array3 --- 0
2022-04-12 20:50:43.640059+0800 Main[4408:60897] array2 --- 1
2022-04-12 20:50:43.640105+0800 Main[4408:60897] array1 --- 1
2022-04-12 20:50:43.640159+0800 Main[4408:60897] array1 --- 0
疑问1: 为什么在 array2 创建之后 array2、array1 的引用技术都是2.
因为 array1 指针指向堆上一块内存（NSArray 类型），创建好后 array1 引用计数为1。在创建 array2 的时候发现是对 array1 的浅拷贝，系统为了内存的节省优化，array2 的指针也指向堆上的这一块内存，copy 本身会对 array1 引用技术 +1，变为2。所以这时候 array2 指针指向的内存，引用计数也是2.
基于此，我们稍微修改下，看看 Demo2
NSArray *array1 = [[NSArray alloc] initWithObjects:@"1", @"2", nil];
NSLog(@"array1 --- %zd", array1.retainCount);
NSArray *array2 = [array1 mutableCopy];
NSLog(@"array1 --- %zd", array1.retainCount);
NSLog(@"array2 --- %zd", array2.retainCount);
2022-04-12 20:55:36.539060+0800 Main[4576:65031] array1 --- 1
2022-04-12 20:55:36.539514+0800 Main[4576:65031] array1 --- 1
2022-04-12 20:55:36.539631+0800 Main[4576:65031] array2 --- 1
因为 array1 指针指向堆上一块内存（NSArray 类型），创建好后 array1 引用计数为1。在创建 array2 的时候发现是对 array1 的深拷贝，要产生不可变对象，所以堆上申请内存空间，array2 指针指向这块内存，引用技术为1。
此外 mutableCopy 是 Foundation 针对集合类提供的。如果自定义对象需要支持 copy 方法，需遵循对应的NSCopyint 协议，实现协议方法 -(id)copyWithZone:(NSZone *)zone
总结：
| NSString | NSMutableString | NSArray | NSMutableArray | NSDictionary | NSMutableDictionary | |
|---|---|---|---|---|---|---|
| copy | NSString 浅拷贝 | NSString 深拷贝 | NSArray 浅拷贝 | NSArray 深拷贝 | NSDictionary 浅拷贝 | NSDictionary 深拷贝 |
| mutableCopy | NSMutableString 深拷贝 | NSMutableString 深拷贝 | NSMutableArray 深拷贝 | NSMutableArray 深拷贝 | NSMutableDictionary 深拷贝 | NSMutableDictionary 深拷贝 |
引用计数
union isa_t {
Class cls;
uintptr_t bits;
struct {
uintptr_t nonpointer : 1;
uintptr_t has_assoc : 1;
uintptr_t has_cxx_dtor : 1;
uintptr_t shiftcls : 33; // MACH_VM_MAX_ADDRESS 0x1000000000
uintptr_t magic : 6;
uintptr_t weakly_referenced : 1;
uintptr_t deallocating : 1;
uintptr_t has_sidetable_rc : 1;
uintptr_t extra_rc : 19;
};
}
iOS 从 64 位开始开始，对 isa 进行了优化，信息存放于 union 结构中
-
extra_rc存储引用计数信息-1，可以看到是 19位。存储引用计数器 -1 -
has_sidetable_rc引用计数是否过大无法存储在 isa。当过大无法存储与 isa 中时，has_sidetable_rc这位会变为1，引用计数存储在 SideTable 的类的属性中
也就是说，iOS 从64位开始，引用计数存放于 isa 结构体的一个 union 中，字段为 extra_rc，值为对象引用计数值 -1。当引用计数过大无法存放的时候 union 中 has_sidetable_rc 为 1，则引用计数存放于 SideTable 结构体中。
SideTable 结构如下
struct SideTable {
spinlock_t slock;
RefcountMap refcnts;
weak_table_t weak_table;
};
其中 refcnts 是一个存放着对象引用计数的散列表
查看 objc4 关于引用计数的实现
uintptr_t _objc_rootRetainCount(id obj) {
assert(obj);
return obj->rootRetainCount();
}
inline uintptr_t objc_object::rootRetainCount() {
if (isTaggedPointer()) return (uintptr_t)this;
sidetable_lock();
isa_t bits = LoadExclusive(&isa.bits);
ClearExclusive(&isa.bits);
if (bits.nonpointer) { // 优化过的 isa
uintptr_t rc = 1 + bits.extra_rc;
if (bits.has_sidetable_rc) { // 引用计数不是存储在 isa 中，而是 SideTable
rc += sidetable_getExtraRC_nolock();
}
sidetable_unlock();
return rc;
}
sidetable_unlock();
return sidetable_retainCount();
}
size_t objc_object::sidetable_getExtraRC_nolock() {
assert(isa.nonpointer);
SideTable& table = SideTables()[this];
RefcountMap::iterator it = table.refcnts.find(this); // key 拿值
if (it == table.refcnts.end()) return 0;
else return it->second >> SIDE_TABLE_RC_SHIFT;
}
__unsafe_unretained 不安全如何体现？上 Demo
__weak Person *p2;
__unsafe_unretained Person *p3;
{
Person *p = [[Person alloc] init];
p2 = p;
}
NSLog(@"%@", p2);
2022-04-12 21:39:30.308917+0800 Main[5307:98296] -[Person dealloc]
2022-04-12 21:39:30.309413+0800 Main[5307:98296] (null)
可以看到出了代码块，之后 p2 虽然指向 p，但是 p 没有强指针指向，所以回收了，此时打印 p2，是 null。
__unsafe_unretained Person *p3;
{
Person *p = [[Person alloc] init];
p3 = p;
}
NSLog(@"%@", p3);
2022-04-12 21:40:47.558581+0800 Main[5342:99598] -[Person dealloc]
2022-04-12 21:40:47.559330+0800 Main[5342:99598] <Person: 0x101206130>
当用 __unsafe_unretained 修饰后，虽然释放了，但是内存还没回收，这时候去使用很容易出错。
dealloc
查看 objc4 源码
- (void)dealloc {
_objc_rootDealloc(self);
}
void _objc_rootDealloc(id obj) {
assert(obj);
obj->rootDealloc();
}
inline void objc_object::rootDealloc() {
if (isTaggedPointer()) return; // fixme necessary?
// fastpath 判断当前对象是否满足条件。
if (fastpath(isa.nonpointer && // nonpointer
!isa.weakly_referenced && // 是否有弱引用
!isa.has_assoc && // 关联对象
!isa.has_cxx_dtor && // c++ 析构函数
!isa.has_sidetable_rc)) // 是否有 SideTable
{
assert(!sidetable_present());
free(this);
} else {
object_dispose((id)this);
}
}
id object_dispose(id obj){
if (!obj) return nil;
objc_destructInstance(obj);
free(obj);
return nil;
}
void *objc_destructInstance(id obj) {
if (obj) {
// Read all of the flags at once for performance.
bool cxx = obj->hasCxxDtor();
bool assoc = obj->hasAssociatedObjects();
// This order is important.
if (cxx) object_cxxDestruct(obj); // 清除成员变量
if (assoc) _object_remove_assocations(obj);
obj->clearDeallocating(); // 将指向当前对象的弱指针置为 nil
}
return obj;
}
inline void objc_object::clearDeallocating() {
if (slowpath(!isa.nonpointer)) {
// Slow path for raw pointer isa.
sidetable_clearDeallocating();
}
else if (slowpath(isa.weakly_referenced || isa.has_sidetable_rc)) {
// Slow path for non-pointer isa with weak refs and/or side table data.
clearDeallocating_slow();
}
assert(!sidetable_present());
}
void objc_object::sidetable_clearDeallocating(){
SideTable& table = SideTables()[this];
// clear any weak table items
// clear extra retain count and deallocating bit
// (fixme warn or abort if extra retain count == 0 ?)
table.lock();
RefcountMap::iterator it = table.refcnts.find(this);
if (it != table.refcnts.end()) {
if (it->second & SIDE_TABLE_WEAKLY_REFERENCED) {
weak_clear_no_lock(&table.weak_table, (id)this);
}
table.refcnts.erase(it);
}
table.unlock();
}
void
weak_clear_no_lock(weak_table_t *weak_table, id referent_id)
{
objc_object *referent = (objc_object *)referent_id;
weak_entry_t *entry = weak_entry_for_referent(weak_table, referent);
if (entry == nil) {
/// XXX shouldn't happen, but does with mismatched CF/objc
//printf("XXX no entry for clear deallocating %p\n", referent);
return;
}
// zero out references
weak_referrer_t *referrers;
size_t count;
if (entry->out_of_line()) {
referrers = entry->referrers;
count = TABLE_SIZE(entry);
}
else {
referrers = entry->inline_referrers;
count = WEAK_INLINE_COUNT;
}
for (size_t i = 0; i < count; ++i) {
objc_object **referrer = referrers[i];
if (referrer) {
if (*referrer == referent) {
*referrer = nil;
}
else if (*referrer) {
_objc_inform("__weak variable at %p holds %p instead of %p. "
"This is probably incorrect use of "
"objc_storeWeak() and objc_loadWeak(). "
"Break on objc_weak_error to debug.\n",
referrer, (void*)*referrer, (void*)referent);
objc_weak_error();
}
}
}
weak_entry_remove(weak_table, entry);
}
自动释放池底层原理探索
上 Demo
int main(int argc, const char * argv[]) {
@autoreleasepool {
Person *p = [[[Person alloc] init] autorelease];
}
return 0;
}
clang 转为 c++ xcrun -sdk iphonesimulator clang -rewrite-objc main.m
int main(int argc, const char * argv[]) {
/* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool;
Person *p = ((Person *(*)(id, SEL))(void *)objc_msgSend)((id)((Person *(*)(id, SEL))(void *)objc_msgSend)((id)((Person *(*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("Person"), sel_registerName("alloc")), sel_registerName("init")), sel_registerName("autorelease"));
}
return 0;
}
下面的代码其实就是 objc_msgSend，有效代码是 __AtAutoreleasePool __autoreleasepool;
继续查找
struct __AtAutoreleasePool {
__AtAutoreleasePool() {
atautoreleasepoolobj = objc_autoreleasePoolPush();
}
~__AtAutoreleasePool() {
objc_autoreleasePoolPop(atautoreleasepoolobj);
}
void * atautoreleasepoolobj;
};
OC 对象本质就是结构体
-
__AtAutoreleasePool结构体中__AtAutoreleasePool是构造方法，在创建结构体的时候调用 -
~__AtAutoreleasePool是析构函数，在结构体销毁的时候调用
main 内的代码作用域，离开代表销毁。所以上面代码等价于
atautoreleasepoolobj = objc_autoreleasePoolPush();
Person *p = [[[Person alloc] init] autorelease];
objc_autoreleasePoolPop(atautoreleasepoolobj);
利用关键函数继续查看 objc4 源码
void *objc_autoreleasePoolPush(void) {
return AutoreleasePoolPage::push();
}
void objc_autoreleasePoolPop(void *ctxt) {
AutoreleasePoolPage::pop(ctxt);
}
自动释放池的主要实现依靠2个对象：__AtAutoreleasePool、AutoreleasePoolPage
objc_autoreleasePoolPush、objc_autoreleasePoolPop 底层都是调用了 AutoreleasePoolPage 对象来管理的。
查看源码
class AutoreleasePoolPage {
magic_t const magic;
id *next;
pthread_t const thread;
AutoreleasePoolPage * const parent;
AutoreleasePoolPage *child;
uint32_t const depth;
uint32_t hiwat;
}
- 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。child 指向下一个对象，parent 指向上一个对象
id * begin() {
return (id *) ((uint8_t *)this+sizeof(*this));
}
id * end() {
return (id *) ((uint8_t *)this+SIZE);
}
其中 begin 方法返回 autoreleasePoolPage 对象中开始存储 autorelease 对象的开始地址
end 方法返回 autoreleasePoolPage 对象中结束存储 autorelease 对象的开始地址
调用 AutoreleasePoolPage::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 AutoreleasePoolPage::pop 方法时传入一个 POOL_BOUNDARY 的内存地址，系统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
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;
}
来个骚一些的例子
int main(int argc, const char * argv[]) {
@autoreleasepool {
Person *p1 = [[[Person alloc] init] autorelease];
Person *p2 = [[[Person alloc] init] autorelease];
@autoreleasepool {
Person *p3 = [[[Person alloc] init] autorelease];
@autoreleasepool {
Person *p4 = [[[Person alloc] init] autorelease];
}
}
}
return 0;
}
main 方法内部3个 autoreleasepool 底层怎么样工作的？
3个@auto releasepool， 系统遇到第一个的时候底层就是初始化一个结构体 __AtAutoreleasePool，结构体构造方法内部调用 AutoreleasePoolPage::push 方法，系统给 AutoreleasePoolPage 真正保存 autorelease 对象的地方存储进一个 POOL_BOUNDARY 对象，然后储存 P1、P2 对象地址，遇到第二个则继续初始化结构体，调用 push 方法，存储一个 POOL_BOUNDARY 对象，继续保存 P3，遇到第三个则继续初始化结构体，调用 push 方法，存储一个 POOL_BOUNDARY 对象，继续保存 P4。
当结束第三个大括号的时候，第三个结构体对象，调用析构函数，内部调用 AutoreleasePoolPage::pop 方法，会从最后一个入栈的对象开始发送 release 消息，直到遇到 POOL_BOUNDARY 对象。
紧接着第二个大括号结束，第二个结构体对象析构函数执行，内部调用 AutoreleasePoolPage::pop 方法，会从最后一个入栈的对象开始发送 release 消息，直到遇到 POOL_BOUNDARY 对象。
第一个同理。
小窍门，对于上述原理的分析可以用源码中看到的 AutoreleasePoolPage 对象的 printAll 方法。
static void printAll() {
_objc_inform("##############");
_objc_inform("AUTORELEASE POOLS for thread %p", pthread_self());
AutoreleasePoolPage *page;
ptrdiff_t objects = 0;
for (page = coldPage(); page; page = page->child) {
objects += page->next - page->begin();
}
_objc_inform("%llu releases pending.", (unsigned long long)objects);
if (haveEmptyPoolPlaceholder()) {
_objc_inform("[%p] ................ PAGE (placeholder)",
EMPTY_POOL_PLACEHOLDER);
_objc_inform("[%p] ################ POOL (placeholder)",
EMPTY_POOL_PLACEHOLDER);
}
else {
for (page = coldPage(); page; page = page->child) {
page->print();
}
}
_objc_inform("##############");
}
void _objc_autoreleasePoolPrint(void) {
AutoreleasePoolPage::printAll();
}
查了下 printAll 函数的使用方，就只有 _objc_autoreleasePoolPrint 函数。且可以看到在 objc4 objc-internal.h 头文件中有将该函数 export 出去，也就是可以在外部链接该符号。
OBJC_EXPORT void _objc_autoreleasePoolPrint(void) OBJC_AVAILABLE(10.7, 5.0, 9.0, 1.0, 2.0);
所以我们在测试 Demo 中将 _objc_autoreleasePoolPrint 函数声明下。在打印下
extern void _objc_autoreleasePoolPrint(void);
int main(int argc, const char * argv[]) {
@autoreleasepool {
Person *p1 = [[[Person alloc] init] autorelease];
Person *p2 = [[[Person alloc] init] autorelease];
@autoreleasepool {
Person *p3 = [[[Person alloc] init] autorelease];
@autoreleasepool {
Person *p4 = [[[Person alloc] init] autorelease];
_objc_autoreleasePoolPrint();
}
}
}
return 0;
}
objc[23132]: ##############
objc[23132]: AUTORELEASE POOLS for thread 0x100094600
objc[23132]: 7 releases pending.
objc[23132]: [0x10080a000] ................ PAGE (hot) (cold)
objc[23132]: [0x10080a038] ################ POOL 0x10080a038
objc[23132]: [0x10080a040] 0x10075f060 Person
objc[23132]: [0x10080a048] 0x10075f0c0 Person
objc[23132]: [0x10080a050] ################ POOL 0x10080a050
objc[23132]: [0x10080a058] 0x10075f0e0 Person
objc[23132]: [0x10080a060] ################ POOL 0x10080a060
objc[23132]: [0x10080a068] 0x10075f100 Person
objc[23132]: ##############
可以看到打印结果和上面的分析是一致的（和上面的图片对比看看）
再来个 Demo，验证下 AutoreleasePoolPage 一页满情况
extern void _objc_autoreleasePoolPrint(void);
int main(int argc, const char * argv[]) {
@autoreleasepool {
Person *p1 = [[[Person alloc] init] autorelease];
Person *p2 = [[[Person alloc] init] autorelease];
@autoreleasepool {
for (NSInteger index = 0; index<600; index++) {
Person *p3 = [[[Person alloc] init] autorelease];
}
@autoreleasepool {
Person *p4 = [[[Person alloc] init] autorelease];
_objc_autoreleasePoolPrint();
}
}
}
return 0;
}
objc[23504]: ##############
objc[23504]: AUTORELEASE POOLS for thread 0x100094600
objc[23504]: 606 releases pending.
objc[23504]: [0x10080d000] ................ PAGE (full) (cold)
objc[23504]: [0x10080d038] ################ POOL 0x10080d038
objc[23504]: [0x10080d040] 0x1007092f0 Person
objc[23504]: [0x10080d048] 0x100709350 Person
objc[23504]: [0x10080d050] ################ POOL 0x10080d050
objc[23504]: [0x10080d058] 0x100753250 Person
objc[23504]: [0x10080d060] 0x100753270 Person
objc[23504]: [0x10080d068] 0x100753290 Person
objc[23504]: [0x10080d070] 0x1007532b0 Person
objc[23504]: [0x10080d078] 0x1007532d0 Person
objc[23504]: [0x10080d080] 0x1007532f0 Person
objc[23504]: [0x10080d088] 0x100753310 Person
objc[23504]: [0x10080d090] 0x100753330 Person
objc[23504]: [0x10080d098] 0x100753680 Person
objc[23504]: [0x10080d0a0] 0x1007536a0 Person
objc[23504]: [0x10080d0a8] 0x1007536c0 Person
objc[23504]: [0x10080d0b0] 0x1007536e0 Person
objc[23504]: [0x10080d0b8] 0x100753700 Person
objc[23504]: [0x10080d0c0] 0x100753720 Person
objc[23504]: [0x10080d0c8] 0x100753740 Person
objc[23504]: [0x10080d0d0] 0x100753760 Person
objc[23504]: [0x10080d0d8] 0x100753780 Person
objc[23504]: [0x10080d0e0] 0x1007537a0 Person
objc[23504]: [0x10080d0e8] 0x1007537c0 Person
objc[23504]: [0x10080d0f0] 0x1007537e0 Person
objc[23504]: [0x10080d0f8] 0x100753800 Person
objc[23504]: [0x10080d100] 0x100753820 Person
objc[23504]: [0x10080d108] 0x100753840 Person
objc[23504]: [0x10080d110] 0x100753860 Person
objc[23504]: [0x10080d118] 0x100753880 Person
objc[23504]: [0x10080d120] 0x1007538a0 Person
objc[23504]: [0x10080d128] 0x1007538c0 Person
objc[23504]: [0x10080d130] 0x1007538e0 Person
objc[23504]: [0x10080d138] 0x100753900 Person
objc[23504]: [0x10080d140] 0x100753920 Person
objc[23504]: [0x10080d148] 0x100753940 Person
objc[23504]: [0x10080d150] 0x100753960 Person
objc[23504]: [0x10080d158] 0x100753980 Person
objc[23504]: [0x10080d160] 0x1007539a0 Person
objc[23504]: [0x10080d168] 0x1007539c0 Person
objc[23504]: [0x10080d170] 0x1007539e0 Person
objc[23504]: [0x10080d178] 0x100753a00 Person
objc[23504]: [0x10080d180] 0x100753a20 Person
objc[23504]: [0x10080d188] 0x100753a40 Person
objc[23504]: [0x10080d190] 0x100753a60 Person
objc[23504]: [0x10080d198] 0x100753a80 Person
objc[23504]: [0x10080d1a0] 0x100753aa0 Person
objc[23504]: [0x10080d1a8] 0x100753ac0 Person
objc[23504]: [0x10080d1b0] 0x100753ae0 Person
objc[23504]: [0x10080d1b8] 0x100753b00 Person
objc[23504]: [0x10080d1c0] 0x100753b20 Person
objc[23504]: [0x10080d1c8] 0x100753b40 Person
objc[23504]: [0x10080d1d0] 0x100753b60 Person
objc[23504]: [0x10080d1d8] 0x100753b80 Person
objc[23504]: [0x10080d1e0] 0x100753ba0 Person
objc[23504]: [0x10080d1e8] 0x100753bc0 Person
objc[23504]: [0x10080d1f0] 0x100753be0 Person
objc[23504]: [0x10080d1f8] 0x100753c00 Person
objc[23504]: [0x10080d200] 0x100753c20 Person
objc[23504]: [0x10080d208] 0x100753c40 Person
objc[23504]: [0x10080d210] 0x100753c60 Person
objc[23504]: [0x10080d218] 0x100753c80 Person
objc[23504]: [0x10080d220] 0x100753ca0 Person
objc[23504]: [0x10080d228] 0x100753cc0 Person
objc[23504]: [0x10080d230] 0x100753ce0 Person
objc[23504]: [0x10080d238] 0x100753d00 Person
objc[23504]: [0x10080d240] 0x100753d20 Person
objc[23504]: [0x10080d248] 0x100753d40 Person
objc[23504]: [0x10080d250] 0x100753d60 Person
objc[23504]: [0x10080d258] 0x100753d80 Person
objc[23504]: [0x10080d260] 0x100753da0 Person
objc[23504]: [0x10080d268] 0x100753dc0 Person
objc[23504]: [0x10080d270] 0x100753de0 Person
objc[23504]: [0x10080d278] 0x100753e00 Person
objc[23504]: [0x10080d280] 0x100753e20 Person
objc[23504]: [0x10080d288] 0x100753e40 Person
objc[23504]: [0x10080d290] 0x100753e60 Person
objc[23504]: [0x10080d298] 0x100753e80 Person
objc[23504]: [0x10080d2a0] 0x100753ea0 Person
objc[23504]: [0x10080d2a8] 0x100753ec0 Person
objc[23504]: [0x10080d2b0] 0x100753ee0 Person
objc[23504]: [0x10080d2b8] 0x100753f00 Person
objc[23504]: [0x10080d2c0] 0x100753f20 Person
objc[23504]: [0x10080d2c8] 0x100753f40 Person
objc[23504]: [0x10080d2d0] 0x100753f60 Person
objc[23504]: [0x10080d2d8] 0x100753f80 Person
objc[23504]: [0x10080d2e0] 0x100753fa0 Person
objc[23504]: [0x10080d2e8] 0x100753fc0 Person
objc[23504]: [0x10080d2f0] 0x100753fe0 Person
objc[23504]: [0x10080d2f8] 0x100754000 Person
objc[23504]: [0x10080d300] 0x100754020 Person
objc[23504]: [0x10080d308] 0x100754040 Person
objc[23504]: [0x10080d310] 0x100754060 Person
objc[23504]: [0x10080d318] 0x100754080 Person
objc[23504]: [0x10080d320] 0x1007540a0 Person
objc[23504]: [0x10080d328] 0x1007540c0 Person
objc[23504]: [0x10080d330] 0x1007540e0 Person
objc[23504]: [0x10080d338] 0x100754100 Person
objc[23504]: [0x10080d340] 0x100754120 Person
objc[23504]: [0x10080d348] 0x100754140 Person
objc[23504]: [0x10080d350] 0x100754160 Person
objc[23504]: [0x10080d358] 0x100754180 Person
objc[23504]: [0x10080d360] 0x1007541a0 Person
objc[23504]: [0x10080d368] 0x1007541c0 Person
objc[23504]: [0x10080d370] 0x1007541e0 Person
objc[23504]: [0x10080d378] 0x100754200 Person
objc[23504]: [0x10080d380] 0x100754220 Person
objc[23504]: [0x10080d388] 0x100754240 Person
objc[23504]: [0x10080d390] 0x100754260 Person
objc[23504]: [0x10080d398] 0x100754280 Person
objc[23504]: [0x10080d3a0] 0x1007542a0 Person
objc[23504]: [0x10080d3a8] 0x1007542c0 Person
objc[23504]: [0x10080d3b0] 0x1007542e0 Person
objc[23504]: [0x10080d3b8] 0x100754300 Person
objc[23504]: [0x10080d3c0] 0x100754320 Person
objc[23504]: [0x10080d3c8] 0x100754340 Person
objc[23504]: [0x10080d3d0] 0x100754360 Person
objc[23504]: [0x10080d3d8] 0x100754380 Person
objc[23504]: [0x10080d3e0] 0x1007543a0 Person
objc[23504]: [0x10080d3e8] 0x1007543c0 Person
objc[23504]: [0x10080d3f0] 0x1007543e0 Person
objc[23504]: [0x10080d3f8] 0x100754400 Person
objc[23504]: [0x10080d400] 0x100754420 Person
objc[23504]: [0x10080d408] 0x100754440 Person
objc[23504]: [0x10080d410] 0x100754460 Person
objc[23504]: [0x10080d418] 0x100754480 Person
objc[23504]: [0x10080d420] 0x1007544a0 Person
objc[23504]: [0x10080d428] 0x1007544c0 Person
objc[23504]: [0x10080d430] 0x1007544e0 Person
objc[23504]: [0x10080d438] 0x100754500 Person
objc[23504]: [0x10080d440] 0x100754520 Person
objc[23504]: [0x10080d448] 0x100754540 Person
objc[23504]: [0x10080d450] 0x100754560 Person
objc[23504]: [0x10080d458] 0x100754580 Person
objc[23504]: [0x10080d460] 0x1007545a0 Person
objc[23504]: [0x10080d468] 0x1007545c0 Person
objc[23504]: [0x10080d470] 0x1007545e0 Person
objc[23504]: [0x10080d478] 0x100754600 Person
objc[23504]: [0x10080d480] 0x100754620 Person
objc[23504]: [0x10080d488] 0x100754640 Person
objc[23504]: [0x10080d490] 0x100754660 Person
objc[23504]: [0x10080d498] 0x100754680 Person
objc[23504]: [0x10080d4a0] 0x1007546a0 Person
objc[23504]: [0x10080d4a8] 0x1007546c0 Person
objc[23504]: [0x10080d4b0] 0x1007546e0 Person
objc[23504]: [0x10080d4b8] 0x100754700 Person
objc[23504]: [0x10080d4c0] 0x100754720 Person
objc[23504]: [0x10080d4c8] 0x100754740 Person
objc[23504]: [0x10080d4d0] 0x100754760 Person
objc[23504]: [0x10080d4d8] 0x100754780 Person
objc[23504]: [0x10080d4e0] 0x1007547a0 Person
objc[23504]: [0x10080d4e8] 0x1007547c0 Person
objc[23504]: [0x10080d4f0] 0x1007547e0 Person
objc[23504]: [0x10080d4f8] 0x100754800 Person
objc[23504]: [0x10080d500] 0x100754820 Person
objc[23504]: [0x10080d508] 0x100754840 Person
objc[23504]: [0x10080d510] 0x100754860 Person
objc[23504]: [0x10080d518] 0x100754880 Person
objc[23504]: [0x10080d520] 0x1007548a0 Person
objc[23504]: [0x10080d528] 0x1007548c0 Person
objc[23504]: [0x10080d530] 0x1007548e0 Person
objc[23504]: [0x10080d538] 0x100754900 Person
objc[23504]: [0x10080d540] 0x100754920 Person
objc[23504]: [0x10080d548] 0x100754940 Person
objc[23504]: [0x10080d550] 0x100754960 Person
objc[23504]: [0x10080d558] 0x100754980 Person
objc[23504]: [0x10080d560] 0x1007549a0 Person
objc[23504]: [0x10080d568] 0x1007549c0 Person
objc[23504]: [0x10080d570] 0x1007549e0 Person
objc[23504]: [0x10080d578] 0x100754a00 Person
objc[23504]: [0x10080d580] 0x100754a20 Person
objc[23504]: [0x10080d588] 0x100754a40 Person
objc[23504]: [0x10080d590] 0x100754a60 Person
objc[23504]: [0x10080d598] 0x100754a80 Person
objc[23504]: [0x10080d5a0] 0x100754aa0 Person
objc[23504]: [0x10080d5a8] 0x100754ac0 Person
objc[23504]: [0x10080d5b0] 0x100754ae0 Person
objc[23504]: [0x10080d5b8] 0x100754b00 Person
objc[23504]: [0x10080d5c0] 0x100754b20 Person
objc[23504]: [0x10080d5c8] 0x100754b40 Person
objc[23504]: [0x10080d5d0] 0x100754b60 Person
objc[23504]: [0x10080d5d8] 0x100754b80 Person
objc[23504]: [0x10080d5e0] 0x100754ba0 Person
objc[23504]: [0x10080d5e8] 0x100754bc0 Person
objc[23504]: [0x10080d5f0] 0x100754be0 Person
objc[23504]: [0x10080d5f8] 0x100754c00 Person
objc[23504]: [0x10080d600] 0x100754c20 Person
objc[23504]: [0x10080d608] 0x100754c40 Person
objc[23504]: [0x10080d610] 0x100754c60 Person
objc[23504]: [0x10080d618] 0x100754c80 Person
objc[23504]: [0x10080d620] 0x100754ca0 Person
objc[23504]: [0x10080d628] 0x100754cc0 Person
objc[23504]: [0x10080d630] 0x100754ce0 Person
objc[23504]: [0x10080d638] 0x100754d00 Person
objc[23504]: [0x10080d640] 0x100754d20 Person
objc[23504]: [0x10080d648] 0x100754d40 Person
objc[23504]: [0x10080d650] 0x100754d60 Person
objc[23504]: [0x10080d658] 0x100754d80 Person
objc[23504]: [0x10080d660] 0x100754da0 Person
objc[23504]: [0x10080d668] 0x100754dc0 Person
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objc[23504]: [0x100817170] 0x100752550 Person
objc[23504]: [0x100817178] 0x1007556d0 Person
objc[23504]: [0x100817180] 0x1007556f0 Person
objc[23504]: [0x100817188] 0x100755710 Person
objc[23504]: [0x100817190] 0x100755730 Person
objc[23504]: [0x100817198] 0x100755750 Person
objc[23504]: [0x1008171a0] 0x100755770 Person
objc[23504]: [0x1008171a8] 0x100755790 Person
objc[23504]: [0x1008171b0] 0x1007557b0 Person
objc[23504]: [0x1008171b8] 0x1007557d0 Person
objc[23504]: [0x1008171c0] 0x1007557f0 Person
objc[23504]: [0x1008171c8] 0x100755810 Person
objc[23504]: [0x1008171d0] 0x100755830 Person
objc[23504]: [0x1008171d8] 0x100755850 Person
objc[23504]: [0x1008171e0] 0x100755870 Person
objc[23504]: [0x1008171e8] 0x100755890 Person
objc[23504]: [0x1008171f0] 0x1007558b0 Person
objc[23504]: [0x1008171f8] 0x1007558d0 Person
objc[23504]: [0x100817200] 0x1007558f0 Person
objc[23504]: [0x100817208] 0x100755910 Person
objc[23504]: [0x100817210] 0x100755930 Person
objc[23504]: [0x100817218] 0x100755950 Person
objc[23504]: [0x100817220] 0x100755970 Person
objc[23504]: [0x100817228] 0x100755990 Person
objc[23504]: [0x100817230] 0x1007559b0 Person
objc[23504]: [0x100817238] 0x1007559d0 Person
objc[23504]: [0x100817240] 0x1007559f0 Person
objc[23504]: [0x100817248] 0x100755a10 Person
objc[23504]: [0x100817250] 0x100755a30 Person
objc[23504]: [0x100817258] 0x100755a50 Person
objc[23504]: [0x100817260] 0x100755a70 Person
objc[23504]: [0x100817268] 0x100755a90 Person
objc[23504]: [0x100817270] 0x100755ab0 Person
objc[23504]: [0x100817278] 0x100755ad0 Person
objc[23504]: [0x100817280] 0x100755af0 Person
objc[23504]: [0x100817288] 0x100755b10 Person
objc[23504]: [0x100817290] 0x100755b30 Person
objc[23504]: [0x100817298] 0x100755b50 Person
objc[23504]: [0x1008172a0] 0x100755b70 Person
objc[23504]: [0x1008172a8] 0x100755b90 Person
objc[23504]: [0x1008172b0] 0x100755bb0 Person
objc[23504]: [0x1008172b8] 0x100755bd0 Person
objc[23504]: [0x1008172c0] 0x100755bf0 Person
objc[23504]: [0x1008172c8] 0x100755c10 Person
objc[23504]: [0x1008172d0] 0x100755c30 Person
objc[23504]: [0x1008172d8] 0x100755c50 Person
objc[23504]: [0x1008172e0] 0x100755c70 Person
objc[23504]: [0x1008172e8] 0x100755c90 Person
objc[23504]: [0x1008172f0] 0x100755cb0 Person
objc[23504]: [0x1008172f8] 0x100755cd0 Person
objc[23504]: [0x100817300] 0x100755cf0 Person
objc[23504]: [0x100817308] 0x100755d10 Person
objc[23504]: [0x100817310] 0x100755d30 Person
objc[23504]: [0x100817318] 0x100755d50 Person
objc[23504]: [0x100817320] 0x100755d70 Person
objc[23504]: [0x100817328] 0x100755d90 Person
objc[23504]: [0x100817330] 0x100755db0 Person
objc[23504]: [0x100817338] 0x100755dd0 Person
objc[23504]: [0x100817340] 0x100755df0 Person
objc[23504]: [0x100817348] 0x100755e10 Person
objc[23504]: [0x100817350] ################ POOL 0x100817350
objc[23504]: [0x100817358] 0x100755e30 Person
objc[23504]: ##############
可以看到当600*8=4800字节，所以一页肯定存不下，可以看到
................ PAGE (full) (cold) page 右边有个 cold、hot。cold 代表不是当前页，hot 代表当前页。
继续看看对象调用 autorelease 方法做了什么事情？
- (id)autorelease {
return ((id)self)->rootAutorelease();
}
inline id objc_object::rootAutorelease() {
if (isTaggedPointer()) return (id)this;
if (prepareOptimizedReturn(ReturnAtPlus1)) return (id)this;
return rootAutorelease2();
}
_attribute__((noinline,used)) id objc_object::rootAutorelease2() {
assert(!isTaggedPointer());
return AutoreleasePoolPage::autorelease((id)this);
}
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 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);
}
}
查看 NSObject autorelease 方法调用链路可以看到最后还是调用 AutoreleasePoolPage 的 add 方法（会判断有没有页、有没有满）
autorelease 对象什么时候调用 release 方法
其实也就是 autorelease 和 RunLoop 的关系。
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()。还监听了kCFRunLoopBeforeExit事件，会调用objc_autoreleasePoolPop()
结合 RunLoop 运行图
-
01 通知 Observer 进入 Loop 会调用
objc_autoreleasePoolPush -
做一堆其他事情
-
07 在将要休眠的时候先调用
objc_autoreleasePoolPop，再调用objc_autoreleasePoolPush -
等待唤醒做一堆其他事情，回到第二步
-
07 又开始休眠，先调用
objc_autoreleasePoolPop，再调用objc_autoreleasePoolPush -
11 没任务将要休眠，调用
objc_autoreleasePoolPop
可以看到 objc_autoreleasePoolPush、objc_autoreleasePoolPop 成对调用，贯穿 RunLoop