用set point.x获取UIBezierPath point.y

Question

我有一个UIBezierPath，当我设置点x时，我需要得到点y

谢谢

Answer 1

您将需要在这些点之间进行插值。要访问点，最简单的方法是将它们存储到NSMutableArray中。创建此数组并添加所有CGPoints，同时在绘图例程中将它们添加到UIBezierPath中。如果无法做到这一点，请参见here，了解如何从UIBezierPath中提取点。有关如何实现您想要的内容，请参阅以下代码：

-(float)getYValueFromArray:(NSArray*)a atXValue:(float)x
{
    NSValue *v1, *v2;
    float x1, x2, y1, y2;

    // iterate through all points
    for(int i=0; i<([a count]-1); i++)
    {
        // get current and next point
        v1 = [a objectAtIndex:i];
        v2 = [a objectAtIndex:i+1];

        // return if value matches v1.x or v2.x
        if(x==[v1 CGPointValue].x) return [v1 CGPointValue].y;
        if(x==[v2 CGPointValue].x) return [v2 CGPointValue].y;

        // if x is between v1.x and v2.x calculate interpolated value
        if((x>[v1 CGPointValue].x) && (x<[v2 CGPointValue].x))
        {
            x1 = [v1 CGPointValue].x;
            x2 = [v2 CGPointValue].x;
            y1 = [v1 CGPointValue].y;
            y2 = [v2 CGPointValue].y;
            return (x-x1)/(x2-x1)*(y2-y1) + y1;
        }
    }

    // should never reach this point
    return -1;
}

-(void)test
{
    NSMutableArray *a = [[NSMutableArray alloc] init];
    [a addObject:[NSValue valueWithCGPoint:CGPointMake( 0, 10)]];
    [a addObject:[NSValue valueWithCGPoint:CGPointMake(10,  5)]];
    [a addObject:[NSValue valueWithCGPoint:CGPointMake(15, 20)]];
    [a addObject:[NSValue valueWithCGPoint:CGPointMake(20, 30)]];
    [a addObject:[NSValue valueWithCGPoint:CGPointMake(35, 50)]];
    [a addObject:[NSValue valueWithCGPoint:CGPointMake(50,  0)]];

    float y = [self getYValueFromArray:a atXValue:22.5];
    NSLog(@"Y value at X=22.5 is %.2f", y);
}

Answer 2

@nullp01nter感谢你漂亮的回答。这正是我所需要的！:)下面是我的Swift版本，它是带PointXY的数组的扩展：

protocol PointXY {
    var x : CGFloat { get set }
    var y : CGFloat { get set }
}

extension CGPoint: PointXY { }

extension Array where Element: PointXY {
    func getYValue(forX x: CGFloat) -> CGFloat? {
        for index in 0..<(self.count - 1) {
            let p1 = self[index]
            let p2 = self[index + 1]
            // return p.y if a p.x matches x
            if x == p1.x { return p1.y }
            if x == p2.x { return p2.y }
            // if x is between p1.x and p2.x calculate interpolated value
            if x > p1.x && x < p2.x {
                let x1 = p1.x
                let x2 = p2.x
                let y1 = p1.y
                let y2 = p2.y
                return (x - x1) / (x2 - x1) * (y2 - y1) + y1
            }
        }
        return nil
    }
}