谈谈 OC 属性修饰符的本质是什么!
2018-09-27 本文已影响24人
丿行随心动
属性修饰符的本质
assign 修饰符copy 修饰符atomic 修饰符strong 修饰符weak 修饰符weakTable 实现原理
示例代码
注: 结合 runtime 源码,利用汇编反推出每一个修饰符的本质
1.使用 lldb 为每一个属性的 set 方法下断点
2.分析调试汇编代码,找到真正的操作函数
3.去 runtime 源码中找到对应的源码
@interface ViewController ()
@property (assign) NSInteger assignProperty;
@property (copy) NSString *copytext;
@property (strong) NSObject *strongProperty;
@property (weak) NSObject *weakProperty;
@end
@implementation ViewController
- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event {
[self test];
}
- (void)test {
NSObject *obj = [NSObject new];
self.assignProperty = 100;
self.copytext = @"text";
self.strongProperty = obj;
self.weakProperty = obj;
}
@end
assign 修饰符
Demo`-[ViewController setAssignProperty:]:
01. 0x102f4def8 <+0>: sub sp, sp, #0x20 ; =0x20
// 取出指向 _assignProperty 成员相对偏移量的指针
02. 0x102f4defc <+4>: adrp x8, 4
03. 0x102f4df00 <+8>: add x8, x8, #0x328 ; =0x328
04. 0x102f4df04 <+12>: str x0, [sp, #0x18]
05. 0x102f4df08 <+16>: str x1, [sp, #0x10]
06. 0x102f4df0c <+20>: str x2, [sp, #0x8]
07. 0x102f4df10 <+24>: ldr x0, [sp, #0x8]
08. 0x102f4df14 <+28>: ldr x1, [sp, #0x18]
// 取出 _assignProperty 成员相对偏移量(ldrsw 读取一个字(2个byte)的内存数据)
09. 0x102f4df18 <+32>: ldrsw x8, [x8]
// 计算出 _assignProperty 的内存地址
10. 0x102f4df1c <+36>: add x8, x1, x8
// 赋值
11. 0x102f4df20 <+40>: str x0, [x8]
12. 0x102f4df24 <+44>: add sp, sp, #0x20 ; =0x20
13. 0x102f4df28 <+48>: ret
结论: assign 修饰符没有做任何操作,本质就是得到内存空间,直接赋值
copy 修饰符
Demo`-[ViewController setCopytext:]:
0x100f79f68 <+0>: sub sp, sp, #0x30 ; =0x30
0x100f79f6c <+4>: stp x29, x30, [sp, #0x20]
0x100f79f70 <+8>: add x29, sp, #0x20 ; =0x20
0x100f79f74 <+12>: adrp x8, 4
0x100f79f78 <+16>: add x8, x8, #0x32c ; =0x32c
0x100f79f7c <+20>: stur x0, [x29, #-0x8]
0x100f79f80 <+24>: str x1, [sp, #0x10]
0x100f79f84 <+28>: str x2, [sp, #0x8]
0x100f79f88 <+32>: ldr x1, [sp, #0x10]
0x100f79f8c <+36>: ldur x0, [x29, #-0x8]
0x100f79f90 <+40>: ldrsw x3, [x8]
0x100f79f94 <+44>: ldr x8, [sp, #0x8]
0x100f79f98 <+48>: mov x2, x8
0x100f79f9c <+52>: bl 0x100f7a8c4 ; symbol stub for: objc_setProperty_nonatomic_copy
0x100f79fa0 <+56>: ldp x29, x30, [sp, #0x20]
0x100f79fa4 <+60>: add sp, sp, #0x30 ; =0x30
0x100f79fa8 <+64>: ret
结论: 第14行可以明显看出,copy 修饰符下,编译器为你调用了 objc_setProperty_nonatomic_copy,
幸运的是它确实就是 runtime 中的源码
void objc_setProperty_nonatomic_copy(id self, SEL _cmd, id newValue, ptrdiff_t offset)
{
reallySetProperty(self, _cmd, newValue, offset, false, true, false);
}
static inline void reallySetProperty(id self,
SEL _cmd,
id newValue,
ptrdiff_t offset,
bool atomic,
bool copy,
bool mutableCopy)
{
if (offset == 0) {
object_setClass(self, newValue);
return;
}
id oldValue;
id *slot = (id*) ((char*)self + offset);
if (copy) {
newValue = [newValue copyWithZone:nil];
} else if (mutableCopy) {
newValue = [newValue mutableCopyWithZone:nil];
} else {
if (*slot == newValue) return;
newValue = objc_retain(newValue);
}
if (!atomic) {
oldValue = *slot;
*slot = newValue;
} else {
spinlock_t& slotlock = PropertyLocks[slot];
slotlock.lock();
oldValue = *slot;
*slot = newValue;
slotlock.unlock();
}
objc_release(oldValue);
}
reallySetProperty 函数分析:
1. 检查成员偏移量是否合法
2. 计算出成员的地址
3. 检查成员是否需要深浅拷贝
4. 检查是否需要原子锁操作
5. 取出 oldValue, 赋值 newValue
6. release oldValue
结论: copy 的本质就是编译器,为你调用 objc_setProperty_nonatomic_copy 函数
atomic 修饰符
由 copy 的本质, 我们可以看出实质是调用了 reallySetProperty 做了一系列操作,
所以这里就不再进行汇编分析,有兴趣的可以自己去实践
strong 修饰符
Demo`-[ViewController setStrongProperty:]:
0x102271fd4 <+0>: sub sp, sp, #0x30 ; =0x30
0x102271fd8 <+4>: stp x29, x30, [sp, #0x20]
0x102271fdc <+8>: add x29, sp, #0x20 ; =0x20
0x102271fe0 <+12>: adrp x8, 4
0x102271fe4 <+16>: add x8, x8, #0x330 ; =0x330
0x102271fe8 <+20>: stur x0, [x29, #-0x8]
0x102271fec <+24>: str x1, [sp, #0x10]
0x102271ff0 <+28>: str x2, [sp, #0x8]
0x102271ff4 <+32>: ldr x0, [sp, #0x8]
0x102271ff8 <+36>: ldur x1, [x29, #-0x8]
0x102271ffc <+40>: ldrsw x8, [x8]
0x102272000 <+44>: add x8, x1, x8
0x102272004 <+48>: str x0, [sp]
0x102272008 <+52>: mov x0, x8
0x10227200c <+56>: ldr x1, [sp]
0x102272010 <+60>: bl 0x1022728cc ; symbol stub for: objc_storeStrong
0x102272014 <+64>: ldp x29, x30, [sp, #0x20]
0x102272018 <+68>: add sp, sp, #0x30 ; =0x30
0x10227201c <+72>: ret
结论: strong 修饰符下,编译器为你调用了 objc_storeStrong 函数,
它也是一个 runtime 源码中的函数
void objc_storeStrong(id *location, id obj)
{
id prev = *location;
if (obj == prev) {
return;
}
objc_retain(obj);
*location = obj;
objc_release(prev);
}
objc_storeStrong 函数分析:
1. 取出 oldValue, 对比 newValue
2. retain newValue
3. 赋值
4. release oldValue
结论: 以上可以看出 strong 修饰的成员变量, 本质是一个对应类型二级指针,
且编译器为我们调用了 objc_storeStrong 函数来操作成员变量
weak 修饰符
Demo`-[ViewController setWeakProperty:]:
0x102272054 <+0>: sub sp, sp, #0x40 ; =0x40
0x102272058 <+4>: stp x29, x30, [sp, #0x30]
0x10227205c <+8>: add x29, sp, #0x30 ; =0x30
0x102272060 <+12>: adrp x8, 3
0x102272064 <+16>: add x8, x8, #0x334 ; =0x334
0x102272068 <+20>: stur x0, [x29, #-0x8]
0x10227206c <+24>: stur x1, [x29, #-0x10]
0x102272070 <+28>: str x2, [sp, #0x18]
0x102272074 <+32>: ldr x0, [sp, #0x18]
0x102272078 <+36>: ldur x1, [x29, #-0x8]
0x10227207c <+40>: ldrsw x8, [x8]
0x102272080 <+44>: add x8, x1, x8
0x102272084 <+48>: str x0, [sp, #0x10]
0x102272088 <+52>: mov x0, x8
0x10227208c <+56>: ldr x1, [sp, #0x10]
0x102272090 <+60>: bl 0x1022728d8 ; symbol stub for: objc_storeWeak
0x102272094 <+64>: str x0, [sp, #0x8]
0x102272098 <+68>: ldp x29, x30, [sp, #0x30]
0x10227209c <+72>: add sp, sp, #0x40 ; =0x40
0x1022720a0 <+76>: ret
结论: weak 修饰符下,编译器为你调用了 objc_storeWeak 函数, 它也是一个 runtime 源码中的函数
id objc_storeWeak(id *location, id newObj)
{
return storeWeak<DoHaveOld, DoHaveNew, DoCrashIfDeallocating>
(location, (objc_object *)newObj);
}
static id storeWeak(id *location, objc_object *newObj)
{
// 检查参数
assert(haveOld || haveNew);
if (!haveNew) assert(newObj == nil);
Class previouslyInitializedClass = nil;
id oldObj;
SideTable *oldTable;
SideTable *newTable;
// 检查新值和旧值
retry:
if (haveOld) {
// 取出旧值
oldObj = *location;
// 取出旧值所在的 hashTable
oldTable = &SideTables()[oldObj];
} else {
oldTable = nil;
}
if (haveNew) {
// 分配新值所在的 hashTable
newTable = &SideTables()[newObj];
} else {
newTable = nil;
}
// 对 hashTable 加锁
SideTable::lockTwo<haveOld, haveNew>(oldTable, newTable);
// 检查旧值与 hashTable 中取出的值是否对应(hashTable 碰撞容错机制)
if (haveOld && *location != oldObj) {
SideTable::unlockTwo<haveOld, haveNew>(oldTable, newTable);
goto retry;
}
if (haveNew && newObj) {
// 取出新值的类对象
Class cls = newObj->getIsa();
// 检查类对象是否被初始化
if (cls != previouslyInitializedClass &&
!((objc_class *)cls)->isInitialized())
{
SideTable::unlockTwo<haveOld, haveNew>(oldTable, newTable);
_class_initialize(_class_getNonMetaClass(cls, (id)newObj));
previouslyInitializedClass = cls;
goto retry;
}
}
if (haveOld) {
// 从 hashTable 中移除旧值
weak_unregister_no_lock(&oldTable->weak_table, oldObj, location);
}
if (haveNew) {
// 向 hashTable 中插入新值
newObj = (objc_object *)
weak_register_no_lock(&newTable->weak_table,
(id)newObj,
location,
crashIfDeallocating);
if (newObj && !newObj->isTaggedPointer()) {
newObj->setWeaklyReferenced_nolock();
}
// 给成员变量赋新值
*location = (id)newObj;
}
else {
// No new value. The storage is not changed.
}
// 为 hashTable 开锁
SideTable::unlockTwo<haveOld, haveNew>(oldTable, newTable);
return (id)newObj;
}
结论: 综上所述,我们可以得出,weak 修饰的成员变量实际也是一个对应类型的二级指针,
且编译器为我们调用了 objc_storeWeak 函数,来操作成员变量和对应的hashTable,
接下来将继续深入 weakTable 实现原理
weakTable 实现原理
alignas(StripedMap<SideTable>) static uint8_t
SideTableBuf[sizeof(StripedMap<SideTable>)];
static void SideTableInit() {
new (SideTableBuf) StripedMap<SideTable>();
}
static StripedMap<SideTable>& SideTables() {
return *reinterpret_cast<StripedMap<SideTable>*>(SideTableBuf);
}
注: weakTable 是由一个静态的 SideTableBuf 对象所维护,其类型为 <StripedMap<SideTable> *>
template<typename T>
class StripedMap {
enum { CacheLineSize = 64 };
#if TARGET_OS_EMBEDDED
enum { StripeCount = 8 };
#else
enum { StripeCount = 64 };
#endif
struct PaddedT {
T value alignas(CacheLineSize);
};
PaddedT array[StripeCount];
static unsigned int indexForPointer(const void *p) {
uintptr_t addr = reinterpret_cast<uintptr_t>(p);
return ((addr >> 4) ^ (addr >> 9)) % StripeCount;
}
public:
T& operator[] (const void *p) {
return array[indexForPointer(p)].value;
}
const T& operator[] (const void *p) const {
return const_cast<StripedMap<T>>(this)[p];
}
.
.
.
}
注: StripeMap 是一个散列表,其成员 PaddedT array[StripeCount](这里分为64个桶),
PaddedT 内部维护着 SideTable 类型的对象,
函数 static unsigned int indexForPointer(const void *p) 将 weak 对象指针的 hash % 64 分发入桶.
enum HaveOld { DontHaveOld = false, DoHaveOld = true };
enum HaveNew { DontHaveNew = false, DoHaveNew = true };
struct SideTable {
spinlock_t slock;
RefcountMap refcnts;
weak_table_t weak_table;
SideTable() {
memset(&weak_table, 0, sizeof(weak_table));
}
~SideTable() {
_objc_fatal("Do not delete SideTable.");
}
void lock() { slock.lock(); }
void unlock() { slock.unlock(); }
void forceReset() { slock.forceReset(); }
// Address-ordered lock discipline for a pair of side tables.
template<HaveOld, HaveNew>
static void lockTwo(SideTable *lock1, SideTable *lock2);
template<HaveOld, HaveNew>
static void unlockTwo(SideTable *lock1, SideTable *lock2);
};
注: SideTable 中 slock 负责资源的线程安全, 并维护着真正的 weakTable
struct weak_table_t {
weak_entry_t *weak_entries;
size_t num_entries;
uintptr_t mask;
uintptr_t max_hash_displacement;
};
/// Adds an (object, weak pointer) pair to the weak table.
id weak_register_no_lock(weak_table_t *weak_table, id referent,
id *referrer, bool crashIfDeallocating);
/// Removes an (object, weak pointer) pair from the weak table.
void weak_unregister_no_lock(weak_table_t *weak_table, id referent, id *referrer);
#if DEBUG
/// Returns true if an object is weakly referenced somewhere.
bool weak_is_registered_no_lock(weak_table_t *weak_table, id referent);
#endif
/// Called on object destruction. Sets all remaining weak pointers to nil.
void weak_clear_no_lock(weak_table_t *weak_table, id referent);
struct weak_entry_t {
DisguisedPtr<objc_object> referent;
.
.
.
}
注: 这里可以看出 weak_table_t 管理着一个数组, 每个元素为 <weak_entry_t *>,
weak_entry_t 才真正存储着我们需要的 weak 对象容器
回到函数 static id storeWeak(id *location, objc_object *newObj)
我们可以看到:
1.元素移除函数为 void weak_unregister_no_lock(weak_table_t *weak_table, id referent, id *referrer);
2.元素插入函数为 id weak_register_no_lock(weak_table_t *weak_table, id referent, id *referrer, bool crashIfDeallocating);
void weak_unregister_no_lock(weak_table_t *weak_table, id referent_id,
id *referrer_id)
{
objc_object *referent = (objc_object *)referent_id;
objc_object **referrer = (objc_object **)referrer_id;
weak_entry_t *entry;
if (!referent) return;
if ((entry = weak_entry_for_referent(weak_table, referent))) {
remove_referrer(entry, referrer);
bool empty = true;
if (entry->out_of_line() && entry->num_refs != 0) {
empty = false;
}
else {
for (size_t i = 0; i < WEAK_INLINE_COUNT; i++) {
if (entry->inline_referrers[i]) {
empty = false;
break;
}
}
}
if (empty) {
weak_entry_remove(weak_table, entry);
}
}
// Do not set *referrer = nil. objc_storeWeak() requires that the
// value not change.
}
注: 遍历数组找到 weak 对象,根据 num_refs(weak 对象引用计数)进行移除
id weak_register_no_lock(weak_table_t *weak_table, id referent_id,
id *referrer_id, bool crashIfDeallocating)
{
objc_object *referent = (objc_object *)referent_id;
objc_object **referrer = (objc_object **)referrer_id;
if (!referent || referent->isTaggedPointer()) return referent_id;
// ensure that the referenced object is viable
bool deallocating;
if (!referent->ISA()->hasCustomRR()) {
deallocating = referent->rootIsDeallocating();
}
else {
BOOL (*allowsWeakReference)(objc_object *, SEL) =
(BOOL(*)(objc_object *, SEL))
object_getMethodImplementation((id)referent,
SEL_allowsWeakReference);
if ((IMP)allowsWeakReference == _objc_msgForward) {
return nil;
}
deallocating =
! (*allowsWeakReference)(referent, SEL_allowsWeakReference);
}
if (deallocating) {
if (crashIfDeallocating) {
_objc_fatal("Cannot form weak reference to instance (%p) of "
"class %s. It is possible that this object was "
"over-released, or is in the process of deallocation.",
(void*)referent, object_getClassName((id)referent));
} else {
return nil;
}
}
// now remember it and where it is being stored
weak_entry_t *entry;
if ((entry = weak_entry_for_referent(weak_table, referent))) {
append_referrer(entry, referrer);
}
else {
weak_entry_t new_entry(referent, referrer);
weak_grow_maybe(weak_table);
weak_entry_insert(weak_table, &new_entry);
}
// Do not set *referrer. objc_storeWeak() requires that the
// value not change.
return referent_id;
}
static void weak_entry_insert(weak_table_t *weak_table, weak_entry_t *new_entry)
{
weak_entry_t *weak_entries = weak_table->weak_entries;
assert(weak_entries != nil);
size_t begin = hash_pointer(new_entry->referent) & (weak_table->mask);
size_t index = begin;
size_t hash_displacement = 0;
while (weak_entries[index].referent != nil) {
index = (index+1) & weak_table->mask;
if (index == begin) bad_weak_table(weak_entries);
hash_displacement++;
}
weak_entries[index] = *new_entry;
weak_table->num_entries++;
if (hash_displacement > weak_table->max_hash_displacement) {
weak_table->max_hash_displacement = hash_displacement;
}
}
static void weak_resize(weak_table_t *weak_table, size_t new_size)
{
size_t old_size = TABLE_SIZE(weak_table);
weak_entry_t *old_entries = weak_table->weak_entries;
weak_entry_t *new_entries = (weak_entry_t *)
calloc(new_size, sizeof(weak_entry_t));
weak_table->mask = new_size - 1;
weak_table->weak_entries = new_entries;
weak_table->max_hash_displacement = 0;
weak_table->num_entries = 0; // restored by weak_entry_insert below
if (old_entries) {
weak_entry_t *entry;
weak_entry_t *end = old_entries + old_size;
for (entry = old_entries; entry < end; entry++) {
if (entry->referent) {
weak_entry_insert(weak_table, entry);
}
}
free(old_entries);
}
}
注: 该函数负责 weak_table_t 的插入(weak_entry_insert)和扩容(weak_grow_maybe)
其中 weak_entry_insert 也是采用的散列分布的方式插入元素,使用了一次线性探测法来解决 hash 碰撞问题
github: https://github.com/huxiaoluder
掘金: https://juejin.im/user/5b76e3a3f265da435a484d37
邮箱: huxiaoluder@163.com
