objective-c - 弱引用何时在Objective-C中更新为nil？

This question already has answers here:

Why isn’t my weak reference cleared right after the strong ones are gone?

(4个答案)


7年前关闭。




请考虑以下两种情况:

// case 1
NSObject *strongOne = [[NSObject alloc] init];
NSObject * __weak weakOne = strongOne;

if (weakOne) {
    NSLog(@"weakOne is not nil.");
} else {
    NSLog(@"weakOne is nil.");
}

strongOne = nil;

if (weakOne) {
    NSLog(@"weakOne is not nil.");
} else {
    NSLog(@"weakOne is nil.");
}

输出此:

weakOne is not nil.
weakOne is not nil.

和

// case 2
NSObject *strongOne = [[NSObject alloc] init];
NSObject * __weak weakOne = strongOne;

strongOne = nil;

if (weakOne) {
    NSLog(@"weakOne is not nil.");
} else {
    NSLog(@"weakOne is nil.");
}

输出此:

weakOne is nil.

据我所知，当释放strongOne时，对同一对象的弱引用应更新为nil。

我的问题:为什么只在case 2中发生？

最佳答案

我认为这是因为当您使用带有weakOne的if语句时，将增加自动释放池中的保留计数。因此，直到自动释放池耗尽后，弱指针才会为零。

 // Try this
NSObject *strongOne = [[NSObject alloc] init];
NSObject * __weak weakOne = strongOne; //count 1
@autoreleasepool {

    if (weakOne) {
        NSLog(@"weakOne is not nil."); //count 2
    } else {
        NSLog(@"weakOne is nil.");
    }

    strongOne = nil; // count 1

    if (weakOne) {
        NSLog(@"weakOne is not nil.");
    } else {
        NSLog(@"weakOne is nil.");
    }

} // count 0, therefore the weakOne become nil

if (weakOne) {
    NSLog(@"weakOne is not nil.");
} else {
    NSLog(@"weakOne is nil.");
}