有没有办法在一根手指离开并随后返回时跟踪 UIPinchGesture？

Question

我有一个 iPad 应用程序，它广泛使用各种风格的 UIGestureRecognizers。一般来说，我是它的忠实粉丝。我对 UIPinchGestureRecognizer 有一个令人烦恼且非常具体的问题：

考虑用户做出捏合手势的场景。然后，完全不移动一根手指，移开另一根手指并将其放回不同的位置，然后继续捏合。 （这并不像听起来那么不可能）。

我的问题似乎特别在于 UIGestureRecognizer 一般无法注意到手指何时离开屏幕并触发任何类型的操作。我在其他地方（touchesBegan 和touchesEnded）也注意到了这一点，但我已经能够解决它。这次不是。

的操作：

1. 以下是UIGestureRecognizer 开始操作时
2. UIGestureRecognizerStateBegan。 n UIGestureRecognizerStateChangeds，同时捏合发生变化。
3. 当用户移开手指时出现尴尬的沉默。
4. 当用户放回手指时，出现尴尬的沉默，可能距离很远。
5. 当捏合恢复时，另一个 UIGestureRecognizerStateChanged。 5.

现在，我在第 5 步遇到了问题。由于第 3 步和第 4 步的处理程序中没有发生任何事情，因此它从第 2 步和第 5 步中的 UIGestureRecognizerStateChangeds 完美无缝过渡，但发生了很多事情。对于处理程序来说，看起来用户刚刚做了一个令人难以置信的快速捏合，在处理程序消息之间的时间内，叛逆的手指可能移动了数百个屏幕单位。

现在，如果用户实际上不可能那么快地捏捏，我可以在更新之间允许的手指位置增量上设置一个阈值。但这是可能的。快速的捏合手势确实可以让用户的手指移动如此长的距离。所以问题的高潮是这样的； UIPinchGestureRecognizer 无法区分上述奇怪的情况和快速捏合手势。我需要以非常不同的方式处理这些问题，但现在我无法区分它们。为什么操作系统无法告诉我手指何时离开屏幕？这是电容屏硬件的功能还是操作系统的错误？还是……有意设计……？

Answer 1

我认为您无法做任何事情来让 UIPinchGestureRecognizer 告诉您手指向下或向上触摸，但您可以创建自己的手势识别器子类。下面是一个类似于捏合的缩放手势识别器，它区分单点触摸和两点触摸手势（本例中的 handleOneTouchGesture 选择器为空，但您可以执行诸如计算移动距离之类的操作）。

//
//  RSScaleGestureRecognizer.h
//  Created by Jeff Argast
//

#import <Foundation/Foundation.h>

@interface RSScaleGestureRecognizer : UIGestureRecognizer {

}

@property (nonatomic, readonly) float   scale;

@end

以及实施：

//
//  RSScaleGestureRecognizer.m
//  Created by Jeff Argast
//

#import "RSScaleGestureRecognizer.h"
#import <UIKit/UIGestureRecognizerSubclass.h>

//
// Distance function
//

static CGFloat RSGetPointDistance (CGPoint p0, CGPoint p1);

//
// RSScaleGestureRecognizer private selectors
//

@interface RSScaleGestureRecognizer ()

- (void) handleTouchDown: (NSSet*) touches withEvent: (UIEvent*) event;
- (void) handleTouchMoved: (NSSet*) touches withEvent: (UIEvent*) event;
- (void) handleTouchUp: (NSSet*) touches withEvent: (UIEvent*) event;
- (void) handleOneTouchGesture: (NSSet*) allTouches;
- (void) handleTwoTouchGesture: (NSSet*) allTouches touchesMoved: (NSSet*) movedTouches;

@end

//
// UIView helper category
//

@interface UIView (RSScaleGestureRecognizer)

- (CGFloat) computeScaleFrom: (UITouch*) t0 to: (UITouch*) t1;

@end

//
// RSScaleGestureRecognizer Implementation
//

@implementation RSScaleGestureRecognizer

@synthesize scale;

- (id) initWithTarget:(id)target action:(SEL)action
{
    self = [super initWithTarget: target action: action];

    if ( self )
    {
        scale = 1.0f;
    }

    return self;
}

- (void)reset
{
    [super reset];

    scale = 1.0f;
}

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{   
    [super touchesBegan: touches withEvent: event];

    [self handleTouchDown: touches withEvent: event];
}

- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event
{
    [super touchesMoved: touches withEvent: event];

    [self handleTouchMoved: touches withEvent: event];
}

- (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event
{   
    [super touchesEnded: touches withEvent: event]; 

    [self handleTouchUp: touches withEvent: event];
}

- (void)touchesCancelled:(NSSet *)touches withEvent:(UIEvent *)event
{
    [super touchesCancelled: touches withEvent: event];

    [self handleTouchUp: touches withEvent: event];
}

- (void) handleTouchDown: (NSSet*) touches withEvent: (UIEvent*) event
{
    switch ( self. state )
    {
        case UIGestureRecognizerStateBegan:
        case UIGestureRecognizerStateChanged:
            break;

        case UIGestureRecognizerStatePossible:
            {
                NSSet* allTouches = [event touchesForGestureRecognizer: self];

                if ( allTouches.count > 2 )
                {
                    self.state = UIGestureRecognizerStateFailed;
                    return;
                }
            }
            break;

        default:
            self.state = UIGestureRecognizerStateFailed;
    }
}

- (void) handleTouchMoved: (NSSet*) movedTouches withEvent: (UIEvent*) event
{
    NSSet* allTouches = [event touchesForGestureRecognizer: self];

    switch ( allTouches.count )
    {
        case 1:
        {
            [self handleOneTouchGesture: allTouches];
        }
        break;

        case 2:
        {
            [self handleTwoTouchGesture: allTouches touchesMoved: movedTouches];
        }
        break;
    }
}

- (void) handleTouchUp: (NSSet*) touches withEvent: (UIEvent*) event
{   
    NSSet* allTouches = [event touchesForGestureRecognizer: self];

    int touchesRemaining = allTouches.count - touches.count;

    if ( touchesRemaining > 0 )
        return;

    switch ( self.state )
    {
        case UIGestureRecognizerStateBegan:
        case UIGestureRecognizerStateChanged:
            self.state = UIGestureRecognizerStateEnded;
            break;

        default:
            self.state = UIGestureRecognizerStateFailed;
    }

}

- (void) handleOneTouchGesture: (NSSet*) allTouches
{
    // Do something special here if desired when only one finger is touching
    return;
}

- (void) handleTwoTouchGesture: (NSSet*) allTouches touchesMoved: (NSSet*) movedTouches
{
    UIGestureRecognizerState currentState = self.state;

    switch ( currentState )
    {
        case UIGestureRecognizerStatePossible:  
        case UIGestureRecognizerStateBegan:
        case UIGestureRecognizerStateChanged:
            {
                UIView* selfView        = self.view;
                NSEnumerator* touchEnum = [allTouches objectEnumerator];
                UITouch* firstTouch     = [touchEnum nextObject];
                UITouch* secondTouch    = [touchEnum nextObject];

                scale = scale * [selfView computeScaleFrom: firstTouch to: secondTouch];        

                if ( currentState == UIGestureRecognizerStatePossible )
                {
                    self.state = UIGestureRecognizerStateBegan;
                }
                else 
                {
                    self.state = UIGestureRecognizerStateChanged;
                }
            }
            break;


        default:
            self.state = UIGestureRecognizerStateFailed;
    }
}

@end

//
// UIVIew category implementation
//

@implementation UIView (RSScaleGestureRecognizer)

- (CGFloat) computeScaleFrom: (UITouch*) t0 to: (UITouch*) t1
{
    UITouchPhase t0Phase = t0.phase;

    if ( (t0Phase == UITouchPhaseEnded) || (t0Phase == UITouchPhaseCancelled) || (t0Phase == UITouchPhaseBegan) )
        return 1.0;

    UITouchPhase t1Phase = t1.phase;

    if ( (t1Phase == UITouchPhaseEnded) || (t1Phase == UITouchPhaseCancelled) || (t1Phase == UITouchPhaseBegan) )
        return 1.0;

    CGPoint oldFirstPoint = [t0 previousLocationInView:self];
    CGPoint oldSecondPoint = [t1 previousLocationInView:self];
    CGFloat oldLength = RSGetPointDistance (oldFirstPoint, oldSecondPoint);

    CGPoint currentFirstPoint = [t0 locationInView:self];
    CGPoint currentSecondPoint = [t1 locationInView:self ];
    CGFloat currentLength = RSGetPointDistance (currentFirstPoint, currentSecondPoint);

    // Avoid divide by zero
    if ( oldLength < 0.01f )
        return 1.0f;

    return currentLength / oldLength;
}

@end

//
// Distance function implementation
//

CGFloat RSGetPointDistance (CGPoint p0, CGPoint p1)
{
    CGFloat xDiff = p0.x - p1.x;
    CGFloat yDiff = p0.y - p1.y;

    return sqrtf ((xDiff * xDiff) + (yDiff * yDiff));
}