iphone - 如何确定平移手势的真实最终速度？

Question

当使用 UIPanGestureRecognizer 并检测到 UIGestureRecognizerStateEnded 时，手势的速度不是真实的速度。相反，它是之前调用我的操作方法的旧速度。如何获取手势结束时的真实速度？

我像这样创建我的 UIPanGestureRecognizer:

    UIPanGestureRecognizer* panGestureRecognizer = [[UIPanGestureRecognizer alloc] initWithTarget:self action:@selector(panGestureRecognized:)];
    [panGestureRecognizer setMaximumNumberOfTouches:2];
    [panGestureRecognizer setMinimumNumberOfTouches:1];
    [panGestureRecognizer setDelegate:self];
    [panGestureRecognizer setDelaysTouchesBegan:NO];
    [panGestureRecognizer setDelaysTouchesEnded:NO];
    [panGestureRecognizer setCancelsTouchesInView:NO];
    [self addGestureRecognizer:panGestureRecognizer];

我的 Action 方法的开头在这里:

- (IBAction) panGestureRecognized:(UIPanGestureRecognizer *)recognizer {

    UIGestureRecognizerState state = recognizer.state;

    CGPoint gestureTranslation = [recognizer translationInView:self];
    CGPoint gestureVelocity = [recognizer velocityInView:self];

    [CBAppDelegate log:@"panGestureRecognized: state: %s\n    translation: (%f, %f)\n    velocity: (%f, %f)", [self toString:state], gestureTranslation.x, gestureTranslation.y, gestureVelocity.x, gestureVelocity.y];

日志输出示例:

2013-09-30_10:46:32.830 panGestureRecognized: state: UIGestureRecognizerStateChanged
    translation: (-283.000000, 2.000000)
    velocity: (-43.046783, 45.551472)
2013-09-30_10:47:02.942 panGestureRecognized: state: UIGestureRecognizerStateEnded
    translation: (-283.000000, 2.000000)
    velocity: (-43.046783, 45.551472)

如您所见，两个日志条目中的速度是相同的(与翻译相同的故事，但我只关心速度)，尽管我按住手指大约 30 秒而不移动它，然后抬起手指。您可以从条目的时间戳中判断时间。在我的手指不动 30 秒后，当然不应该报告速度。

我已经使用适用于 iOS 6.1 的 iPhone 的 iOS 模拟器对此进行了测试。

Answer 1

velocityInView 方法仅在平移发生时定义。也就是说，只有当您实际移动手指时，才会出现平移手势。如果您的手指保持不动，它实际上不会触发平移手势。

这意味着没有内置方法可以知道手势结束时的移动速度。您可以执行一些操作，例如检查状态值为 UIGestureRecognizerStateChanged 和 UIGestureRecognizerStateEnded 的最后一个事件之间的时间差。然后您可以调整此阈值以获得所需的行为。

例如

- (IBAction) panGestureRecognized:(UIPanGestureRecognizer *)recognizer {

    UIGestureRecognizerState state = recognizer.state;

    CGPoint gestureTranslation = [recognizer translationInView:self];
    CGPoint gestureVelocity = [recognizer velocityInView:self];

    if ( state == UIGestureRecognizerStateChanged )
         _lastChange = CFAbsoluteTimeGetCurrent();
    else if ( state == UIGestureRecognizerStateEnded ) {
         double curTime = CFAbsoluteTimeGetCurrent(); 
         double timeElapsed = curTime - _lastChange;
         if ( timeElapsed < MY_THRESHOLD )
              finalSpeed = gestureVelocity;
         else
              finalSpeed = CGPointZero;
    }   
 }