Grand Central Dispatch 替代使用 NSTimer - 多次无效

Question

我在尝试使用 dispatch_source_t 时遇到问题。我想用它来延迟 PHChange 的处理 5 秒，因为 PHChange 可以在短时间内发生多次。如果能提供任何帮助，我将不胜感激。基本上我想取消之前的 dispatch_source_t 计时器，几乎就像 NSTimer.

@property (nonatomic, strong) dispatch_source_t libraryChangedTimer;

dispatch_source_t CreateTimerDispatchSource(uint64_t interval, uint64_t leeway, dispatch_queue_t queue, dispatch_block_t block)
{
    dispatch_source_t timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);

    if (timer)
    {
        dispatch_source_set_timer(timer, dispatch_walltime(NULL, 0), interval, leeway);

        dispatch_source_set_event_handler(timer, block);

        dispatch_resume(timer);
    }

    return timer;
}

- (void)libraryChanged:(PHChange *)changeInstance
{
    NSLog(@"Called immediately and it shouldn't");
}

- (void)photoLibraryDidChange:(PHChange *)changeInstance
{
    if (self.libraryChangedTimer)
    {
        dispatch_source_cancel(self.libraryChangedTimer);

        self.libraryChangedTimer = CreateTimerDispatchSource(5ull * NSEC_PER_SEC, 0, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0),^
        {
            [self libraryChanged:changeInstance];
            dispatch_source_cancel(self.libraryChangedTimer);
        });
    }
    else
    {
        self.libraryChangedTimer = CreateTimerDispatchSource(5ull * NSEC_PER_SEC, 0, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0),^
        {
            [self libraryChanged:changeInstance];
            dispatch_source_cancel(self.libraryChangedTimer);
        });
    }
}

Answer 1

我使用 dispatch_after 代码非常简单地解决了这个问题：

dispatch_source_t CreateTimerDispatchSource(uint64_t interval, uint64_t leeway, dispatch_queue_t queue, dispatch_block_t block)
{
    dispatch_source_t timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);

    if (timer)
    {
        dispatch_source_set_timer(timer, dispatch_walltime(NULL, 0), DISPATCH_TIME_FOREVER, leeway);

        dispatch_source_set_event_handler(timer, block);

        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, interval), queue,^
        {
            dispatch_resume(timer);
        });
    }

    return timer;
}

- (void)libraryChanged:(PHChange *)changeInstance
{
    // Do something 
}

- (void)photoLibraryDidChange:(PHChange *)changeInstance
{
    if (self.libraryChangedTimer)
    {
        dispatch_source_cancel(self.libraryChangedTimer);

        self.libraryChangedTimer = CreateTimerDispatchSource(5ull * NSEC_PER_SEC, 0, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0),^
        {
            [self libraryChanged:changeInstance];
            dispatch_source_cancel(self.libraryChangedTimer);
            self.libraryChangedTimer = nil;
        });
    }
    else
    {
        self.libraryChangedTimer = CreateTimerDispatchSource(5ull * NSEC_PER_SEC, 0, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0),^
        {
            [self libraryChanged:changeInstance];
            dispatch_source_cancel(self.libraryChangedTimer);
            self.libraryChangedTimer = nil;
        });
    }
}

Answer 2

你的问题是这一行：

    dispatch_source_set_timer(timer, dispatch_walltime(NULL, 0), interval, leeway);

您实际上是在指定 "now" 作为计时器的开始时间。这就是 dispatch_walltime(NULL, 0) 的计算结果。您将 interval 值作为计时器的间隔传递，这要求它在两次触发之间的那个时间段内重复。但是开始时间决定了第一次开火的时间。

你想要的是：

    dispatch_source_set_timer(timer, dispatch_walltime(NULL, interval), interval, leeway);

或者，如果您实际上不想让计时器重复：

    dispatch_source_set_timer(timer, dispatch_walltime(NULL, interval), DISPATCH_TIME_FOREVER, leeway);

Grand Central Dispatch 替代使用 NSTimer - 多次无效

Grand Central Dispatch alternative to using an NSTimer - invalidating multiple times

objective-c

grand-central-dispatch

ios

photosframework