ios - 如何忽略多点触控序列中的某些 UITouch 点

Question

我正在为 ipad 开发一个绘图应用程序，我提供全屏绘图。所以就像我们现在一样，用户可能会用手腕支撑或将手放在屏幕上进行书写。所以我的目标是让用户在手腕/手部支撑下自由书写。

但应用程序应该只检测手指画画，或者忽略/拒绝手腕和手部触摸并删除它们

我开始研究它，我创建了一个启用了多点触控的示例项目。

下面是我的代码

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

    mouseSwiped = NO;

    for (UITouch *touch in touches)
    {
        NSString *key = [NSString stringWithFormat:@"%d", (int) touch];
        lastPoint = [touch locationInView:self.view];

        [touchPaths setObject:[NSValue valueWithCGPoint:lastPoint] forKey:key];
    }

}


- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event
{    
    mouseSwiped = YES;
    CGPoint lastPoint3;

    for (UITouch *touch in touches)
    {
        NSString *key = [NSString stringWithFormat:@"%d", (int) touch];

        lastPoint = [[touchPaths objectForKey:key] CGPointValue];


        currentPoint1 = [touch locationInView:self.view];

        NSLog(@"Y:%f",currentPoint1.y);


        UIGraphicsBeginImageContext(self.view.frame.size);
        [self.tempDrawImage.image drawInRect:CGRectMake(0, 0, self.view.frame.size.width, self.view.frame.size.height)];
        CGContextSetLineCap(UIGraphicsGetCurrentContext(), kCGLineCapRound);
        CGContextSetLineWidth(UIGraphicsGetCurrentContext(), brush );
        CGContextSetRGBStrokeColor(UIGraphicsGetCurrentContext(), red, green, blue, 1.0);
        CGContextSetBlendMode(UIGraphicsGetCurrentContext(),kCGBlendModeNormal);
        CGContextBeginPath(UIGraphicsGetCurrentContext());
        CGContextMoveToPoint(UIGraphicsGetCurrentContext(), lastPoint.x, lastPoint.y);
        CGContextAddLineToPoint(UIGraphicsGetCurrentContext(), currentPoint1.x, currentPoint1.y);

        CGContextStrokePath(UIGraphicsGetCurrentContext());
        self.tempDrawImage.image = UIGraphicsGetImageFromCurrentImageContext();
        [self.tempDrawImage setAlpha:opacity];
        UIGraphicsEndImageContext();

       [touchPaths setObject:[NSValue valueWithCGPoint:currentPoint1] forKey:key];
    }
}

所以这适用于任意数量的触摸，但我不明白如何在绘图时拒绝那些手掌/手触摸，只绘制用户用他的手指/手写笔绘制的内容。

目前如果我画画，我得到这个东西，下面是图像

这里是我用手撑画的，你可以在“你好”下面看到他们画了一些奇怪的画。我怎么能拒绝那些触摸并删除它们只画你好

谢谢

兰 git

Answer 1

一种解决方案是将最上面的点击存储在 touchesBegan 中，并且只绘制这个。

正如您所指出的，您不应该保留 UITouch 实例，因此我建议改用弱引用。

这只会绘制一个触摸。如果你想绘制多个手指的触摸，你需要另一种过滤掉手的方法(例如，许多绘图应用程序有用户设置来告诉应用程序手的姿势，但这当然更复杂)。

这里有一个关于如何做到这一点的想法:

#import <QuartzCore/QuartzCore.h>

@interface TViewController () {
    // We store a weak reference to the current touch that is tracked
    // for drawing.
    __weak UITouch* drawingTouch;
    // This is the previous point we drawed to, or the first point the user tapped.
    CGPoint touchStartPoint;
}
@end
@interface _TDrawView : UIView {
@public
    CGLayerRef persistentLayer, tempLayer;
}
-(void)commitDrawing;
-(void)discardDrawing;
@end

@implementation TViewController

- (void) loadView
{
    self.view = [[_TDrawView alloc] initWithFrame:[UIScreen mainScreen].bounds];
    self.view.opaque = YES;
    self.view.multipleTouchEnabled = YES;
    self.view.backgroundColor = [UIColor whiteColor];
}

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
    // Start with what we currently have
    UITouch* topmostTouch = self->drawingTouch;
    // Find the top-most touch
    for (UITouch *touch in touches) {
        CGPoint lastPoint = [touch locationInView:self.view];
        if(!topmostTouch || [topmostTouch locationInView:self.view].y > lastPoint.y) {
            topmostTouch = touch;
            touchStartPoint = lastPoint;
        }
    }
    // A new finger became the drawing finger, discard any previous 
    // strokes since last touchesEnded
    if(self->drawingTouch != nil && self->drawingTouch != topmostTouch) {
        [(_TDrawView*)self.view discardDrawing];
    }
    self->drawingTouch = topmostTouch;
}

- (void) touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event {
    // Always commit the current stroke to the persistent layer if the user
    // releases a finger. This could need some tweaking for optimal user experience.
    self->drawingTouch = nil;
    [(_TDrawView*)self.view commitDrawing];
    [self.view setNeedsDisplay];
}

- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event
{
    const CGFloat red=0, green=0, blue=0, brush=1;
    for (UITouch *touch in touches) {
        // Find the touch that we track for drawing
        if(touch == self->drawingTouch) {
            CGPoint currentPoint = [touch locationInView:self.view];

            // Draw stroke first in temporary layer
            CGContextRef ctx = CGLayerGetContext(((_TDrawView*)self.view)->tempLayer);
            CGContextSetLineCap(ctx, kCGLineCapRound);
            CGContextSetLineWidth(ctx, brush );
            CGContextSetRGBStrokeColor(ctx, red, green, blue, 1.0);
            CGContextSetBlendMode(ctx,kCGBlendModeNormal);
            CGContextBeginPath(ctx);
            CGContextMoveToPoint(ctx, touchStartPoint.x, touchStartPoint.y);
            CGContextAddLineToPoint(ctx, currentPoint.x, currentPoint.y);
            CGContextStrokePath(ctx);
            // Update the points so that the next line segment is drawn from where
            // we left off
            touchStartPoint = currentPoint;
            // repaint the layer
            [self.view setNeedsDisplay];
        }
    }
}

@end

@implementation _TDrawView

- (void) finalize {
    if(persistentLayer) CGLayerRelease(persistentLayer);
    if(tempLayer) CGLayerRelease(tempLayer);
}

- (void) drawRect:(CGRect)rect {
    CGContextRef ctx = UIGraphicsGetCurrentContext();
    if(!persistentLayer) persistentLayer = CGLayerCreateWithContext(ctx, self.bounds.size, nil);
    if(!tempLayer) tempLayer = CGLayerCreateWithContext(ctx, self.bounds.size, nil);

    // Draw the persistant drawing
    CGContextDrawLayerAtPoint(ctx, CGPointMake(0, 0), persistentLayer);
    // Overlay with the temporary drawing
    CGContextDrawLayerAtPoint(ctx, CGPointMake(0, 0), tempLayer);
}

- (void)commitDrawing {
    // Persist the temporary drawing
    CGContextRef ctx = CGLayerGetContext(persistentLayer);
    CGContextDrawLayerAtPoint(ctx, CGPointMake(0, 0), tempLayer);
    [self discardDrawing];
}
- (void)discardDrawing {
    // Clears the temporary layer
    CGContextRef ctx = CGLayerGetContext(tempLayer);
    CGContextClearRect(ctx, self.bounds);
    CGContextFlush(ctx);
}
@end

编辑:我添加了这样的逻辑，即如果检测到新触摸，如果当前正在绘制任何具有更高 y 值的笔画，它将被删除，正如我们在评论中所讨论的那样。

覆盖是通过绘制两个 CGLayer 完成的。此代码可以针对性能进行大量优化，应将其更多地视为示例，而不是生产就绪代码。