如何确定平移手势的真实结束速度？

Question

当使用UIPanGestureRecognizer并检测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。如何在手势结束时获取真实速度？

我这样创建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];

我的动作方法的开始在这里：

- (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相同（翻译相同，但我只关心速度），尽管我按住手指大约 30 秒没有移动它，然后抬起手指手指。您可以从条目的时间戳中判断时间。在我的手指不动 30 秒后肯定不会报告速度。

我已经用 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，才会发生平移手势。如果您保持手指不动，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;
    }   
 }

Answer 2

2019 年操作方法...

这是真正知道手指抬起时速度的唯一方法：

有一些变量...

var cat: CADisplayLink? = nil
var prevTime = CFAbsoluteTimeGetCurrent()
var lastKnownPosition: CGFloat = 0
var lastKnownActualVelocity: Double = 0

然后……

@objc func _checkVelocityEveryTrueFrame() {
    let newTime = CFAbsoluteTimeGetCurrent()
    let frameTime = newTime - prevTime
    prevTime = newTime

    let newPos = yourConstraint.constant
    lastKnownActualVelocity = Double(newPos - lastKnownPosition) / frameTime
    lastKnownPosition = newPos
    print("- \(frameTime) \(lastKnownPosition) \(lastKnownActualVelocity)")
}

@objc func dragOrFlick(_ p: UIPanGestureRecognizer) {
    if p.state == .began {
        cat?.invalidate()
        cat = nil
        cat = CADisplayLink(target: self,
             selector: #selector(_checkVelocityEveryTrueFrame))
        cat?.add(to: .main, forMode: .common)
    }

    if p.state == .changed {
        ... p.translation(in: ...
        yourConstraint.constant = new position...
    }

    if p.state == .ended {
        cat?.invalidate()
        cat = nil
        let trueFinalVelocity = lastKnownActualVelocity
        print("trueFinalVelocity is truly \(trueFinalVelocity)")
    }
}

就是这样。据我所知，没有更简单的方法。

⁺脚注。正如任何游戏程序员都会告诉你的那样，即使这也有点粗制滥造；它在一帧上给出了柏拉图式的速度：纯粹主义者会在可讨论的帧数上稍微平滑它：/这是一个棘手的问题。