前几天在 iOS 圈内流传着 "一个关于历年来 weak 的面试题答案" 的段子, 感觉有点搞怪 O(∩_∩)O~~. 是的, 做技术开发门槛越来越高了...
结合查看 objc 源码, 我写了个简单测试 demo, 关于对象的三个修饰词__strong,__weak,__unsafe_unretained, 测试结果用三张图表示.
- - (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
- {
- #pragma clang diagnostic push
- #pragma clang diagnostic ignored "-Wunused-variable"
- __strong ZYClass *strongZYClass;
- __weak ZYClass *weakZYClass;
- __unsafe_unretained ZYClass *unsafeZYClass;
- #pragma clang diagnostic pop
- NSLog(@"test begin");
- {
- ZYClass *zyClass = [[ZYClass alloc] init];
- strongZYClass = zyClass;
- // weakZYClass = zyClass;
- // unsafeZYClass = zyClass;
- }
- NSLog(@"test over%@",strongZYClass);
- }
01__strong 指针引用对象. PNG
02__weak 指针引用对象. PNG
03__unsafe_unretained 指针引用对象. PNG
"zyClass" 定义的作用域如下:
- {
- ZYClass *zyClass = [[ZYClass alloc] init];
- strongZYClass = zyClass;
- // weakZYClass = zyClass;
- // unsafeZYClass = zyClass;
- }
鉴于__strong 指针对对象有强引用关系, 所以 "zyClass" 在出作用域后并没有立即销毁;
__weak 指针对对象是弱引用关系, 不持有引用对象. 所以 "zyClass" 在出作用域后就销毁了;
__unsafe_unretained 指针对对象是弱引用关系, 不持有引用对象. 所以 "zyClass" 在出作用域后就销毁了.(与__weak 不同的是,__weak 引用的对象销毁后, 系统会将对象置为 nil, 而__unsafe_unretained 不这么做, 导致 EXC_BAD_ACCESS 错误.)
weak 指针帮我们干了啥?
当一个对象释放的时候, 会执行 "- (void)dealloc {}" 方法, 在 objc 源码的 "NSObject.mm" 中找到了该函数以及相关调用流程:
- // Replaced by NSZombies
- - (void)dealloc {
- _objc_rootDealloc(self);
- }
- void _objc_rootDealloc(id obj)
- {
- assert(obj);
- obj->rootDealloc();
- }
- inline void objc_object::rootDealloc()
- {
- if (isTaggedPointer()) return; // fixme necessary?
- if (fastpath(isa.nonpointer &&
- !isa.weakly_referenced &&
- !isa.has_assoc &&
- !isa.has_cxx_dtor &&
- !isa.has_sidetable_rc))
- {
- 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;
- }
- /***********************************************************************
- * objc_destructInstance
- * Destroys an instance without freeing memory.
- * Calls C++ destructors.
- * Calls ARC ivar cleanup.
- * Removes associative references.
- * Returns `obj`. Does nothing if `obj` is 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.
- // 清除对象的成员变量
- if (cxx) object_cxxDestruct(obj);
- // 清除对象的关联对象
- if (assoc) _object_remove_assocations(obj);
- obj->clearDeallocating();
- }
- 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());
- }
- // Slow path of clearDeallocating()
- // for objects with nonpointer isa
- // that were ever weakly referenced
- // or whose retain count ever overflowed to the side table.
- NEVER_INLINE void objc_object::clearDeallocating_slow()
- {
- assert(isa.nonpointer && (isa.weakly_referenced || isa.has_sidetable_rc));
- SideTable& table = SideTables()[this];
- table.lock();
- if (isa.weakly_referenced) {
- weak_clear_no_lock(&table.weak_table, (id)this);
- }
- if (isa.has_sidetable_rc) {
- table.refcnts.erase(this);
- }
- table.unlock();
- }
- /**
- * Called by dealloc; nils out all weak pointers that point to the
- * provided object so that they can no longer be used.
- *
- * @param weak_table
- * @param referent The object being deallocated.
- */
- 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);
- }
- /**
- * Return the weak reference table entry for the given referent.
- * If there is no entry for referent, return NULL.
- * Performs a lookup.
- *
- * @param weak_table
- * @param referent The object. Must not be nil.
- *
- * @return The table of weak referrers to this object.
- */
- static weak_entry_t * weak_entry_for_referent(weak_table_t *weak_table, objc_object *referent)
- {
- assert(referent);
- weak_entry_t *weak_entries = weak_table->weak_entries;
- if (!weak_entries) return nil;
- size_t begin = hash_pointer(referent) & weak_table->mask;
- size_t index = begin;
- size_t hash_displacement = 0;
- while (weak_table->weak_entries[index].referent != referent) {
- index = (index+1) & weak_table->mask;
- if (index == begin) bad_weak_table(weak_table->weak_entries);
- hash_displacement++;
- if (hash_displacement> weak_table->max_hash_displacement) {
- return nil;
- }
- }
- return &weak_table->weak_entries[index];
- }
inline void objc_object::rootDealloc() 中有一句判断 if (isTaggedPointer()) return; // fixme necessary?, 这个地方条件成立就会 return , 而不会释放对象, 为什么?
实际上苹果在 64 位系统之后推出了 "Tagged Pointer" 技术来优化 NSNumber,NSString,NSDate 等小对象的存储, 在没有引入 "Tagged Pointer" 技术之前, NSNumber 等对象需要动态分配内存, 维护引用技术, NSNumber 指针存储的是 NSNumber 对象的地址值. iOS 引入 "Tagged Pointer" 技术之后, NSNumber 指针里面存储的数据变成了 "Tag+Data", 也就是直接将数据存储在指针中. 仅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.
"Tagged Pointer" 的好处是: 一方面节约计算机内存, 另一方面因为可以直接从指针中读取数据, 可以节约之前 objc_msgSend 流程消耗的时间.
回到正题:
当一个对象被回收的时候调用流程:
- 1 -(void)dealloc ->
- 2 _objc_rootDealloc(id obj) ->
- 3 objc_object::rootDealloc() ->
- 4 object_dispose(id obj) ->
- 5 objc_destructInstance(id obj) ->
- 6 objc_object::clearDeallocating() ->
- 7 objc_object::clearDeallocating_slow() ->
- 8 weak_clear_no_lock(weak_table_t weak_table, id referent_id) ->
- **9 weak_entry_for_referent(weak_table_t *weak_table, objc_object *referent)
前面 1~6 都很好理解. 7 开始到关键点: side
来源: https://juejin.im/entry/5beb11a351882516fa6375cd