1.为什么要使用weak关键字修饰对象
因为使用weak修饰的对象在对象释放的时候,引用它的指针会自动被置为nil;
2. weak和__unsafe_unretained差别,是不是__unsafe_unretained因为weak出现而不再使用了
__unsafe_unretained和weak区别在于,所引用的对象在释放的时候,引用它的指针会不会自动被置为nil,可能会产生野指针,但这并不代表__unsafe_unretained就没用了,在一些情况下合理使用__unsafe_unretained能够带来一定性能上的提升.
3.weak的底层的实现
weak存储.png
参照代码:
SideTable
struct SideTable {
weak_table_t weak_table;
....
};
weak_table
struct weak_table_t {
weak_entry_t *weak_entries;
};
weak_entry_t
struct weak_entry_t {
DisguisedPtr<objc_object> referent;
union {
struct {
weak_referrer_t *referrers;
};
};
...
};
可以得到一个结论,系统为我们创建了一个全局的weak_table,这个表里面有一个weak_entries这样的一个一维数组,重点关注这个weak_entries这个数组中的每个结构体weak_entry_t,其中referent为被弱引用的对象, 而referrers则是指向这个弱引用的的地址,举个例子
@interface Dog : NSObject
@property (nonatomic,weak) NSObject *referent;
@end
// 暂时可认为这个referent和weak_entry_t结构体中的referent是一致的,而referrers[0] = & referent;(ps.假设目前就一个弱指针指向referent)
4.weak修饰对象的存储
4.1 property中使用weak修饰
@property (nonatomic,weak) NSObject *referent;
// 底层实现函数入口
id objc_storeWeak(id *location, id newObj)
{
return storeWeak<DoHaveOld, DoHaveNew, DoCrashIfDeallocating>
(location, (objc_object *)newObj);
}
4.2 使用__weak修饰对象
__weak NSObject *referent
// 底层实现函数入口
id objc_initWeak(id *location, id newObj)
{
if (!newObj) {
*location = nil;
return nil;
}
return storeWeak<DontHaveOld, DoHaveNew, DoCrashIfDeallocating>
(location, (objc_object*)newObj);
}
其实不论是使用weak还是__weak底层都是调用storeWeak这个函数,区别在于模板的第一个参数HaveOld,官方解释如下
If HaveOld is true, the variable has an existing value
that needs to be cleaned up. This value might be nil.
进入到storeWeak函数中
static id storeWeak(id *location, objc_object *newObj)
{
...
if (haveNew) {
newObj = (objc_object *)
weak_register_no_lock(&newTable->weak_table, (id)newObj, location,
crashIfDeallocating);
// Do not set *location anywhere else. That would introduce a race.
*location = (id)newObj;
}
...
}
关注weak_register_no_lock函数
id weak_register_no_lock(weak_table_t *weak_table, id referent_id,
id *referrer_id, bool crashIfDeallocating)
{
// 被弱引用的对象
objc_object *referent = (objc_object *)referent_id;
// 指向弱引用对象的指针
objc_object **referrer = (objc_object **)referrer_id;
// 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;
}
优先检查weak_table中是否存在referent作为key的的weak_entry_t,如果存在,则插入一个新的指向这个弱引用对象的referrer地址,对应的关系如下图:
unit_weak_entry.png
先看看在没有
weak_entry_t存储了referent的时候如何处理,代码如下
weak_entry_t new_entry(referent, referrer);
weak_grow_maybe(weak_table);
weak_entry_insert(weak_table, &new_entry);
先为这个referent创建了一个weak_entry_t,目前也就一个referrer弱引用referent,接下来判定weak_table的存储空间是否足够
static void weak_grow_maybe(weak_table_t *weak_table)
{
size_t old_size = TABLE_SIZE(weak_table);
// Grow if at least 3/4 full.
if (weak_table->num_entries >= old_size * 3 / 4) {
weak_resize(weak_table, old_size ? old_size*2 : 64);
}
}
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));
// 因为mask为2^n,所以-1,是的mask等于全1的二进制
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
// 重新将老的数据插入到插入到新分配的空间中
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);
}
}
当weak_table的num_entries大于总量的3/4,其中这个总量存储在weak_table的mask字段中,初始使用64,以后每次扩容为上次大小的2倍.
接下来插入这个新的weak_entry_t
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++;
}
// 这个index即为弱引用对象的地址,hash偏移后产生的
weak_entries[index] = *new_entry;
weak_table->num_entries++;
if (hash_displacement > weak_table->max_hash_displacement) {
weak_table->max_hash_displacement = hash_displacement;
}
}
weak_entry_insert的算法算是__weak实现的精华所在,如果直接使用弱引用对象的地址作为index,那么weak_entries的大小就要alloc对应系统位数的内存大小,显然不可能,这样内存空间将会全部被占用.因此出现了上面这个方法,根据存储对象的数量,动态申请内存,再根据引用对象的地址mask后,一定是小于TABLE_SIZE,但是可能有两个不同的对象,结尾的地址是相同的,这个时候就需要特殊处理,每次index++,直到这个index对应的位置没有被使用.
5.weak修饰对象的释放
当使用weak修饰的对象被释放,调用流程如下图所示
被弱引用对象的释放.png
从图可以看出
referent对象在释放以后,会去判定是否在weak表中有数据,如果存在会利用weak_clear_no_lock函数,将referrers全部置为nil,代码如下
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);
// 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) {
// 这是为什么使用weak修饰属性,在释放的时候会被置为nil的原因
*referrer = nil;
}
}
}
// 从weak_table中移除这个entry
weak_entry_remove(weak_table, entry);
}
当某个对象拥有weak属性,在这个对象被释放的时候,会调用下面这个方法,将引用地址置为nil
void objc_destroyWeak(id *location)
{
(void)storeWeak<DoHaveOld, DontHaveNew, DontCrashIfDeallocating>
(location, nil);
}
static id
storeWeak(id *location, objc_object *newObj)
{
// Clean up old value, if any.
if (haveOld) {
weak_unregister_no_lock(&oldTable->weak_table, oldObj, location);
}
}
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))) {
// 移除弱引用
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;
}
}
}
// 没有弱引用指向这个对象,移除这个entry
if (empty) {
weak_entry_remove(weak_table, entry);
}
}
}
static void remove_referrer(weak_entry_t *entry, objc_object **old_referrer)
{
if (! entry->out_of_line()) {
for (size_t i = 0; i < WEAK_INLINE_COUNT; i++) {
if (entry->inline_referrers[i] == old_referrer) {
entry->inline_referrers[i] = nil;
return;
}
}
}
size_t begin = w_hash_pointer(old_referrer) & (entry->mask);
size_t index = begin;
size_t hash_displacement = 0;
while (entry->referrers[index] != old_referrer) {
index = (index+1) & entry->mask;
if (index == begin) bad_weak_table(entry);
hash_displacement++;
}
entry->referrers[index] = nil;
entry->num_refs--;
}
