对象的Weak属性在什么时候被设置成nil?

先说结论:
对象被释放的时候, 其dealloc方法执行之前, 它的所有weak属性都已经被设置为nil. 因此, 如果期望在dealloc里访问weak属性, 那是不行的.

项目最近遇到一个坑, 原因是对weak属性何时被设置为nil理解不够. 问题抽象如下:

@interface CheckBase : NSObject

- (void)printMessage;

@end

@class WeakInternalPlayerHolder;

@interface InternalPlayer : CheckBase

@property (strong, nonatomic) WeakInternalPlayerHolder *weakSelfHolder;

@end

@interface WeakInternalPlayerHolder : CheckBase

@property (weak, nonatomic) InternalPlayer *weakInterPlayer;
- (void) doSomeWork;
@end

@interface Player : CheckBase

@property (strong, nonatomic) InternalPlayer *interPlayer;

+ (void)doTest;

@end

@implementation CheckBase

-(void)dealloc {
    NSLog(@"%@ dealloc", NSStringFromClass([self class]));
}

- (void)printMessage {
    NSLog(@"message from %@", NSStringFromClass([self class]));
}

@end


@implementation InternalPlayer

- (instancetype)init
{
    self = [super init];
    if (self) {
        _weakSelfHolder = [[WeakInternalPlayerHolder alloc] init];
        _weakSelfHolder.weakInterPlayer = self;
    }
    return self;
}

-(void)dealloc {
    NSLog(@"self:%@ weakself:%@", self, self.weakSelfHolder.weakInterPlayer);
    [self.weakSelfHolder doSomeWork];
}

@end

@implementation WeakInternalPlayerHolder

- (void)doSomeWork {
    [self.weakInterPlayer printMessage];
}

@end

@implementation Player

- (instancetype)init
{
    self = [super init];
    if (self) {
        _interPlayer = [[InternalPlayer alloc] init];
    }
    return self;
}

- (void)dealloc
{
    NSLog(@"internal obj:%@", self.interPlayer);
    [self.interPlayer printMessage];
}

+ (void)doTest {
    Player *p = [[Player alloc] init];
    NSLog(@"weak internal player:%@", p.interPlayer.weakSelfHolder.weakInterPlayer);
    p = nil;
    NSLog(@"all release");
}

@end

简而言之, 上述问题对象关系是这样的:
Player---(强引用)--->InternalPlayer---(强引用)---->WeakInternalPlayerHolder---(弱引用)--->InternalPlayer.
我们期望在InternalPlayer的dealloc方法通过弱引用的指针, 让InternalPlayer打印一个消息.
然而, 运行[Player doTest]得到如下日志输出:

weak internal player:<InternalPlayer: 0x61800000eb20>
internal obj:<InternalPlayer: 0x61800000eb20>
message from InternalPlayer
Player dealloc
self:<InternalPlayer: 0x61800000eb20> weakself:(null)
InternalPlayer dealloc
WeakInternalPlayerHolder dealloc
all release

其中的语句[self.weakSelfHolder doSomeWork];并没有得到预期的结果.
从其中的日志:self:<InternalPlayer: 0x61800000eb20> weakself:(null)
看出来弱引用对象在dealloc里已经是nil了.
这个问题从直觉上比较难接受: InternalPlayer对象分明还没有释放(日志打印self是正确的地址), 然而它的一个弱引用却已经是nil了.