我以为我已经很了解弱引用和 block ，但是在尝试下面的代码片段时，有一些我不明白的地方。

方法测试 1 : 一切正常，对象不保留

方法测试 2 : 我不明白为什么对象似乎一直保留到方法结束 测试3 !甚至明确设置 object = nil在方法结束时 测试 2 不会改变任何东西。

方法测试3 : 对象没有保留。为什么是方法测试 2 不是这样吗？

作为一个附带问题，我实际上想知道弱变量是否是线程安全的？即，如果我在尝试从不同线程访问弱变量时永远不会得到任何 BAD_ACCESS 异常。

@interface Object : NSObject
@property (nonatomic) NSInteger index;
@end

@implementation Object

- (id)initWithIndex:(NSInteger) index {
    if (self = [super init]) {
        _index = index;
    }
    return self;
}

- (void)dealloc {
    NSLog(@"Deallocating object %d", _index);
}

@end

- (void) test1 {
    NSLog(@"test1");
    Object* object = [[Object alloc] initWithIndex:1];
    NSLog(@"Object: %@", object);
    __weak Object* weakObject = object;
    dispatch_async(dispatch_queue_create(NULL, NULL), ^{
        //NSLog(@"Weak object: %@", weakObject);
        [NSThread sleepForTimeInterval:2];
        NSLog(@"Exiting dispatch");
    });
    [NSThread sleepForTimeInterval:1];
    NSLog(@"Exiting method");
}

- (void) test2 {
    NSLog(@"test2");
    Object* object = [[Object alloc] initWithIndex:2];
    NSLog(@"Object: %@", object);
    __weak Object* weakObject = object;
    dispatch_async(dispatch_queue_create(NULL, NULL), ^{
        NSLog(@"Weak object: %@", weakObject);
        [NSThread sleepForTimeInterval:2];
        NSLog(@"Exiting dispatch");
    });
    [NSThread sleepForTimeInterval:1];
    NSLog(@"Exiting method");
}

- (void) test3 {
    NSLog(@"test3");
    Object* object = [[Object alloc] initWithIndex:3];
    NSLog(@"Object: %@", object);
    NSValue *weakObject = [NSValue valueWithNonretainedObject:object];
    dispatch_async(dispatch_queue_create(NULL, NULL), ^{
        NSLog(@"Weak object: %@", [weakObject nonretainedObjectValue]);
        [NSThread sleepForTimeInterval:2];
        NSLog(@"Exiting dispatch");
    });
    [NSThread sleepForTimeInterval:1];
    NSLog(@"Exiting method");
}

- (void) test {
    [self test1];
    [NSThread sleepForTimeInterval:3];
    [self test2];
    [NSThread sleepForTimeInterval:3];
    [self test3];
}

2013-05-11 19:09:56.753 test[1628:c07] test1
2013-05-11 19:09:56.754 test[1628:c07] Object: <Object: 0x7565940>
2013-05-11 19:09:57.755 test[1628:c07] Exiting method
2013-05-11 19:09:57.756 test[1628:c07] Deallocating object 1
2013-05-11 19:09:58.759 test[1628:1503] Exiting dispatch
2013-05-11 19:10:00.758 test[1628:c07] test2
2013-05-11 19:10:00.758 test[1628:c07] Object: <Object: 0x71c8260>
2013-05-11 19:10:00.759 test[1628:1503] Weak object: <Object: 0x71c8260>
2013-05-11 19:10:01.760 test[1628:c07] Exiting method
2013-05-11 19:10:02.760 test[1628:1503] Exiting dispatch
2013-05-11 19:10:04.761 test[1628:c07] test3
2013-05-11 19:10:04.762 test[1628:c07] Object: <Object: 0x71825f0>
2013-05-11 19:10:04.763 test[1628:1503] Weak object: <Object: 0x71825f0>
2013-05-11 19:10:05.764 test[1628:c07] Exiting method
2013-05-11 19:10:05.764 test[1628:c07] Deallocating object 3
2013-05-11 19:10:05.767 test[1628:c07] Deallocating object 2
2013-05-11 19:10:06.764 test[1628:1503] Exiting dispatch

最佳答案

在我触及你的一些问题之前，我对你的三个测试有两个观察:

Method test2: I don't understand why the object seems to get retained until the end of method test3! Even explicitly setting object = nil at the end of method test2 does not change anything.

- (void) test2 {
    NSLog(@"test2");
    Object* object = [[Object alloc] initWithIndex:2];
    NSLog(@"Object: %@", object);
    __weak Object* weakObject = object;
    dispatch_async(dispatch_queue_create(NULL, NULL), ^{
        [NSThread sleepForTimeInterval:2];      // new sleep
        NSLog(@"Weak object: %@", weakObject);
        [NSThread sleepForTimeInterval:2];
        NSLog(@"Exiting dispatch");
    });
    // [NSThread sleepForTimeInterval:1];       // not really necessary
    NSLog(@"Exiting method");
}

Method test3: the object is not retained. Why is method test2 not behaving like this?

- (void) test3 {
    NSLog(@"test3");
    Object* object = [[Object alloc] initWithIndex:3];
    NSLog(@"Object: %@", object);
    NSValue *weakObject = [NSValue valueWithNonretainedObject:object];
    dispatch_async(dispatch_queue_create(NULL, NULL), ^{
        NSLog(@"Weak object: %@", [weakObject nonretainedObjectValue]);
        [NSThread sleepForTimeInterval:2];
        NSLog(@"Exiting dispatch");
    });
//    [NSThread sleepForTimeInterval:1];
    NSLog(@"Exiting method");
}

As a side question, I was actually wondering if weak variables are thread safe? ie if I will never get any BAD_ACCESS exception when trying to access a weak variable from different threads.

- (void)doSomethingLater
{
    __weak Object *weakSelf = self;

    double delayInSeconds = 10.0;
    dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, (int64_t)(delayInSeconds * NSEC_PER_SEC));
    dispatch_after(popTime, dispatch_get_main_queue(), ^(void){
        Object *strongSelf = weakSelf;
        NSLog(@"%d", strongSelf->_index); // **BAD** - this can crash of `self` has been released by this point
    });
}

- (void)doSomethingLater
{
    __weak Object *weakSelf = self;

    double delayInSeconds = 10.0;
    dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, (int64_t)(delayInSeconds * NSEC_PER_SEC));
    dispatch_after(popTime, dispatch_get_main_queue(), ^(void){
        Object *strongSelf = weakSelf;
        if (strongSelf) {
            NSLog(@"%d", strongSelf->_index);
        }
    });
}

关于ios - ARC block ，弱和保留计数，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/16499739/