iOS-Events[3]-Multitouch Events

本文对Multitouch Events的机制进行讲解。

iOS系统的Events类型分为以下几种：Multitouch events, Accelerometer events, Remote control events。

Multitouch Events

在需要时，可以实现Touch方法，来控制Event的处理和传递，流程如下：

Creating a Subclass of UIResponder

要自定义Event的处理和传递，需要先继承这几个类：

• UIView
• UIViewController
• UIControl
• UIApplication/UIWindow

注意，接收Touches的View必须设置userInteractionEnabled为YES，并且必须是可见的，不能是透明或者隐藏的。

Implementing the Touch-Event Handling Methods in Your Subclass

继承后，需要实现的UIResponder的方法为：

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event;
- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event;
- (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event;
- (void)touchesCancelled:(NSSet *)touches withEvent:(UIEvent *)event;

注意，Cancellation是由于外部事件导致的，例如来电。

Tracking the Phase and Location of a Touch Event

可以通过UITouch获取到phase, tapCount, location, previous location以及timestamp。

Retrieving and Querying Touch Objects

UIView的multipleTouchEnabled属性默认为NO，这意外着无论同时有多少个Touch发生，当前View只接收第一个，因此可以通过anyObject来获取当前Touch：

UITouch *touch = [touches anyObject];

如果需要获取不同对象上的Touch，可以通过以下方式：

[event allTouches];
[event touchesForWindow:self.window];
[event touchesForView:self];

以绘制用户的手写轨迹为例：

@property(nonatomic, assign) size_t handWriteID;
@property(nonatomic, strong) NSMutableArray *points;

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
    UITouch *touch = [touches anyObject];
    CGPoint point = [touch locationInView:self];
    [_points addObject:[[Path alloc] initWithID:_handWriteID x:(int)point.x y:(int)point.y]];
    [self setNeedsDisplay];
}

- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event
{
    UITouch *touch = [touches anyObject];
    CGPoint point = [touch locationInView:self];
    [_points addObject:[[Path alloc] initWithID:_handWriteID x:(int)point.x y:(int)point.y]];
    [self setNeedsDisplay];
}

- (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event
{
}

-(void)touchesCancelled:(NSSet *)touches withEvent:(UIEvent *)event
{
    _handWriteID = 0;
    [_points removeAllObjects];
    [self setNeedsDisplay];
}

- (void)drawRect:(CGRect)rect
{
    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextSetLineWidth(context, 5.0);
    CGContextSetLineCap(context, kCGLineCapRound);
    for(int i = 0; i < (int)(_points.count - 1); ++i){
        Path *lastPoint = [_points objectAtIndex:i];
        Path *nowPoint = [_points objectAtIndex:i + 1];
        if(lastPoint.handWriteID == nowPoint.handWriteID){
            CGContextMoveToPoint(context, lastPoint.x, lastPoint.y);
            CGContextAddLineToPoint(context, nowPoint.x, nowPoint.y);
            CGContextStrokePath(context);
        }
    }
}

Handling One Touch Gestures

(1) Tap

- (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event {
    for (UITouch *aTouch in touches) {
        if (aTouch.tapCount >= 2) {
             // The view responds to the tap
             [self respondToDoubleTapGesture:aTouch];
        }
    }
}

(2) Swipe

这里省略了中间点的判断，只简单判断开始和结束：

#define HORIZ_SWIPE_DRAG_MIN  12
#define VERT_SWIPE_DRAG_MAX    4
 
- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
    UITouch *aTouch = [touches anyObject];
    // startTouchPosition is a property
    self.startTouchPosition = [aTouch locationInView:self];
}
 
- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {
}
 
- (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event {
    UITouch *aTouch = [touches anyObject];
    CGPoint currentTouchPosition = [aTouch locationInView:self];
 
    //  Check if direction of touch is horizontal and long enough
    if (fabsf(self.startTouchPosition.x - currentTouchPosition.x) >= HORIZ_SWIPE_DRAG_MIN &&
        fabsf(self.startTouchPosition.y - currentTouchPosition.y) <= VERT_SWIPE_DRAG_MAX)
    {
        // If touch appears to be a swipe
        if (self.startTouchPosition.x < currentTouchPosition.x) {
            [self myProcessRightSwipe:touches withEvent:event];
        } else {
            [self myProcessLeftSwipe:touches withEvent:event];
    }
    self.startTouchPosition = CGPointZero;
}
 
- (void)touchesCancelled:(NSSet *)touches withEvent:(UIEvent *)event {
    self.startTouchPosition = CGPointZero;
}

(3) Drag

- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {
    UITouch *aTouch = [touches anyObject];
    CGPoint loc = [aTouch locationInView:self];
    CGPoint prevloc = [aTouch previousLocationInView:self];
 
    CGRect myFrame = self.frame;
    float deltaX = loc.x - prevloc.x;
    float deltaY = loc.y - prevloc.y;
    myFrame.origin.x += deltaX;
    myFrame.origin.y += deltaY;
    [self setFrame:myFrame];
}

Handling Multitouch Gestures

对于同时处理多个Touch事件，需要先设置UIView的multipleTouchEnabled属性默认为YES，然后通过CFDictionaryRef来跟踪每一个Touch的phases(状态)。

注意：用CFDictionaryRef，而不是NSDictionary来跟踪，原因是NSDictionary会复制它的keys，而UITouch并没有适配NSCopying协议。

下面以对比起始点和结束点的位置为例，先记录起始点位置：

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
     [self cacheBeginPointForTouches:touches];
}
 
- (void)cacheBeginPointForTouches:(NSSet *)touches {
    if ([touches count] > 0) {
        for (UITouch *touch in touches) {
            CGPoint *point = (CGPoint *)CFDictionaryGetValue(touchBeginPoints, touch);
            if (point == NULL) {
                point = (CGPoint *)malloc(sizeof(CGPoint));
                CFDictionarySetValue(touchBeginPoints, touch, point);
            }
            *point = [touch locationInView:view.superview];
        }
    }
}

对比结束点位置：

- (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event {
     CGAffineTransform newTransform = [self incrementalTransformWithTouches:touches];
}
 
- (CGAffineTransform)incrementalTransformWithTouches:(NSSet *)touches {
     NSArray *sortedTouches = [[touches allObjects] sortedArrayUsingSelector:@selector(compareAddress:)];
 
     // Other code here
     CGAffineTransform transform = CGAffineTransformIdentity;
 
     UITouch *touch1 = [sortedTouches objectAtIndex:0];
     UITouch *touch2 = [sortedTouches objectAtIndex:1];
 
     CGPoint beginPoint1 = *(CGPoint *)CFDictionaryGetValue(touchBeginPoints, touch1);
     CGPoint currentPoint1 = [touch1 locationInView:view.superview];
     CGPoint beginPoint2 = *(CGPoint *)CFDictionaryGetValue(touchBeginPoints, touch2);
     CGPoint currentPoint2 = [touch2 locationInView:view.superview];
 
 
     // Compute the affine transform
     return transform;
}

判断最后一根手势离开屏幕：

- (void)touchesEnded:(NSSet*)touches withEvent:(UIEvent*)event {
    if ([touches count] == [[event touchesForView:self] count]) {
        // Last finger has lifted
    }
}

Specifying Custom Touch Event Behavior

自定义Touch Event行为，可以通过以下方式：

• 支持多个MultiTouch的事件分发:设置View的multipleTouchEnabled为YES;
• 限制单个View的事件分发:设置View的exclusiveTouch为NO，这意味着该View不能与其他View同时接受事件，如下图，如果先在B中触摸，再在A中触摸，则A不会收到事件，反之，则B不会收到事件，而B和C可以同时收到:

• 限制对SubViews的事件分发:重写hitTest:withEvents:方法，使得SubViews直接接收不到事件：

- (UIView *)hitTest:(CGPoint)point withEvent:(UIEvent *)event
{
    return self;
}

• 停止单个View的事件分发:设置View的userInteractionEnabled为NO;
• 停止App在一段时间内的事件分发:

[[UIApplication sharedApplication] beginIgnoringInteractionEvents];
[[UIApplication sharedApplication] endIgnoringInteractionEvents];

Forwarding Touch Events

一般情况下，如果一个Response对象想要处理一个Touch，则该Touch的View对象必须指向该Response对象。如果想要转发Touch事件给其他View，则该Response对象必须是自定义的UIView子类。

转发Touch事件，有两种方式，一种是UIView通过在hit-testing方法中转发给SubViews，一种是重写UIWindow的sendEvents方法：

- (void)sendEvent:(UIEvent *)event {
    for (TransformGesture *gesture in transformGestures) {
        // Collect all the touches you care about from the event
        NSSet *touches = [gesture observedTouchesForEvent:event];
        NSMutableSet *began = nil;
        NSMutableSet *moved = nil;
        NSMutableSet *ended = nil;
        NSMutableSet *canceled = nil;
 
        // Sort touches by phase to handle—-similar to normal event dispatch
        for (UITouch *touch in touches) {
            switch ([touch phase]) {
                case UITouchPhaseBegan:
                    if (!began) began = [NSMutableSet set];
                    [began addObject:touch];
                    break;
                case UITouchPhaseMoved:
                    if (!moved) moved = [NSMutableSet set];
                    [moved addObject:touch];
                    break;
                case UITouchPhaseEnded:
                    if (!ended) ended = [NSMutableSet set];
                    [ended addObject:touch];
                    break;
                case UITouchPhaseCancelled:
                    if (!canceled) canceled = [NSMutableSet set];
                    [canceled addObject:touch];
                    break;
                default:
                    break;
            }
        }
        // Call methods to handle the touches
        if (began)     [gesture touchesBegan:began withEvent:event];
        if (moved)     [gesture touchesMoved:moved withEvent:event];
        if (ended)     [gesture touchesEnded:ended withEvent:event];
        if (canceled) [gesture touchesCancelled:canceled withEvent:event];
    }
    [super sendEvent:event];
}

Best Practices for Handling Multitouch Events

一些注意事项：

• 实现Cancelation方法，保存状态;
• 如果继承UIView, UIViewController和UIResponder：实现所有的Touches方法，不要调用super方法;
• 如果是继承其他的UIKit类：不需要实现所有的Touches方法，一定要调用super方法;
• 不要转发事件给其他的UIKit类对象，转发给UIView或子类对象，并且确保其可以接收转发的;
• 不要显示地通过nextResponder方法发送事件到Responder Chain中;
• 不要对Touch的坐标取整，因为这会导致精度丢失，在一般设备上，坐标系为320×480，而在高分辨率设备上，为640×960，这意味着可能边界存在着0.5个点的情况。