More Precise CGPoint for UILongPressGestureRecognizer

问题

I am using a UILongPressGestureRecognizer which works perfectly but the data I get is not precise enough for my use case. CGPoints that I get are rounded off I think.

Example points that I get: 100.5, 103.0 etc. The decimal part is either .5 or .0 . Is there a way to get more precise points? I was hoping for something like .xxxx as in '100.8745' but .xx would do to.

The reason I need this is because I have a circular UIBezierPath, I want to restrict a drag gesture to only that circular path. The item should only be draggable along the circumference of this circle. To do this I calculated 720 points on the circle's boundary using it's radius. Now these points are .xxxx numbers. If I round them off, the drag is not as smooth around the middle section of the circle.This is because in the middle section, the equator, the points on the x-coordinate are very close together. So when I rounded of the y-coordinate, I lost a lot of points and hence the "not so smooth" drag action.

Here is how I calculate the points

for (CGFloat i = -154;i<154;i++) {

    CGPoint point = [self pointAroundCircumferenceFromCenter:center forX:i];
    [bezierPoints addObject:[NSValue valueWithCGPoint:point]];
    i = i - .5;
}

- (CGPoint)pointAroundCircumferenceFromCenter:(CGPoint)center forX:(CGFloat)x
{
CGFloat radius = 154;
CGPoint upperPoint = CGPointZero;
CGPoint lowerPoint = CGPointZero;

//theta used to be the x variable. was first calculating points using the angle
/* point.x = center.x + radius * cosf(theta);
point.y = center.y + radius * sinf(theta);*/

CGFloat y = (radius*radius) - (theta*theta);
upperPoint.x = x+156;
upperPoint.y = 230-sqrtf(y);
lowerPoint.x = x+156;
lowerPoint.y = sqrtf(y)+230;

NSLog(@"x = %f, y = %f",upperPoint.x, upperPoint.y);
[lowerPoints addObject:[NSValue valueWithCGPoint:lowerPoint]];
[upperPoints addObject:[NSValue valueWithCGPoint:upperPoint]];
return upperPoint;
}

I know the code is weird I mean why would I add the points into arrays and return one point back.

Here is how I restrict the movement

-(void)handleLongPress:(UILongPressGestureRecognizer *)recognizer{
    CGPoint finalpoint;
    CGPoint initialpoint;
    CGFloat y;
    CGFloat x;
    CGPoint tempPoint;
    if(recognizer.state == UIGestureRecognizerStateBegan){
        initialpoint = [recognizer locationInView:self.view];
        CGRect rect = CGRectMake(initialpoint.x, initialpoint.y, 40, 40);
        self.hourHand.frame = rect;
        self.hourHand.center = initialpoint;
        NSLog(@"Long Press Activated at %f,%f",initialpoint.x, initialpoint.y );
    }
    else if (recognizer.state == UIGestureRecognizerStateChanged){
        CGPoint currentPoint = [recognizer locationInView:self.view];
        x = currentPoint.x-initialpoint.x;
        y = currentPoint.y-initialpoint.y;
        tempPoint = CGPointMake( currentPoint.x,  currentPoint.y);
        NSLog(@"temp point ::%f, %f", tempPoint.x, tempPoint.y);
        tempPoint = [self givePointOnCircleForPoint:tempPoint];
        self.hourHand.center = tempPoint;
     }
    else if (recognizer.state == UIGestureRecognizerStateEnded){
    //   finalpoint = [recognizer locationInView:self.view];
        CGRect rect = CGRectMake(tempPoint.x, tempPoint.y, 20, 20);
        self.hourHand.frame = rect;
        self.hourHand.center = tempPoint;
        NSLog(@"Long Press DeActivated at %f,%f",tempPoint.x, tempPoint.y );
    }
}

-(CGPoint)givePointOnCircleForPoint:(CGPoint) point{
CGPoint resultingPoint;
for (NSValue *pointValue in allPoints){
    CGPoint pointFromArray = [pointValue CGPointValue];
    if (point.x == pointFromArray.x) {
       // if(point.y > 230.0){
            resultingPoint = pointFromArray;
            break;
       // }
    }
}

Basically, I taking the x-coordinate of the "touched point" and returning the y by comparing it to the array of points I calculated earlier.

Currently this code works for half a circle only because, each x has 2 y values because it's a circle, Ignore this because I think this can be easily dealt with.

In the picture, the white circle is the original circle, the black circle is the circle of the points I have from the code+formatting it to remove precision to fit the input I get. If you look around the equator(red highlighted part) you will see a gap between the next points. This gap is my problem.

回答1:

To answer your original question: On a device with a Retina display, one pixel is 0.5 points, so 0.5 is the best resolution you can get on this hardware. (On non-Retina devices, 1 pixel == 1 point.)

But it seems to me that you don't need that points array at all. If understand the problem correctly, you can use the following code to "restrict" (or "project") an arbitrary point to the circumference of the circle:

CGPoint center = ...; // Center of the circle
CGFloat radius = ...; // Radius of the circle
CGPoint point = ...;  // The touched point
CGPoint resultingPoint; // Resulting point on the circumference

// Distance from center to point:
CGFloat dist = hypot(point.x - center.x, point.y - center.y);
if (dist == 0) {
    // The touched point is the circle center.
    // Choose any point on the circumference:
    resultingPoint = CGPointMake(center.x + radius, center.y);
} else {
    // Project point to circle circumference:
    resultingPoint = CGPointMake(center.x + (point.x - center.x)*radius/dist,
                     center.y + (point.y - center.y)*radius/dist);
}

来源：https://stackoverflow.com/questions/20562372/more-precise-cgpoint-for-uilongpressgesturerecognizer

标签

objective-c

cocoa-touch

uigesturerecognizer

cgpoint