如何约束特定宽高比的触摸？

Question

我需要绘制一个特定宽高比（正方形，4x3,5x7,8x10等）的选择矩形，以便我可以裁剪图像。我有一些非常正常的代码可以处理在touchesBegan / Moved中自由拖动矩形的4个角，但是我有一段时间让它限制特定的宽高比而不会爬行，或者不准确。

这是我目前拥有的一些伪代码：

// this assumes that the four hitPoints are already arranged in the desired aspect ratio 

CGPoint hitPoints[4]; // arranged botLeft=0, botRight=1, topRight=2, topLeft=3
CGPoint lastPoint;
CGRect cropRect;
int activeHitPoint;

#define kMinCropSize 20

- (void) touchesBegan
{
    lastPoint = [touches locationInView:self];
    activeHitPoint = [self getWhichPointTouched];
}

- (void) touchesMoved
{
    CGPoint thisPoint = [touches locationInView:self];
    float xDiff = thisPoint.x - lastPoint.x;
    float yDiff = thisPoint.y - lastPoint.y;

    float constrain = [self getAspectRatio];  // e.g.. 8x10  = 8/10 = 0.8 

    if(ABS(xDiff) > ABS(yDiff)){
        yDiff = xDiff * constrain;
        if(activeHitPoint == 1 || activeHitPoint == 3)
            yDiff *= -1;
    }else {
        xDiff = yDiff * constrain;
        if(activeHitPoint == 1 || activeHitPoint == 3)
            xDiff *= -1;
    }

    switch(activeHitPoint){
        case 0:
        if((hitPoints[0].x + xDiff > imageDisplayRect.origin.x) &&
           (hitPoints[0].x + xDiff + kMinCropSize < hitPoints[1].x) &&
           (hitPoints[0].y + yDiff > imageDisplayRect.origin.y) &&
           (hitPoints[0].y + yDiff + kMinCropSize < hitPoints[2].y)){
                hitPoints[0].x += xDiff;
                hitPoints[0].y += yDiff;
                hitPoints[3].x = hitPoints[0].x;
                hitPoints[1].y = hitPoints[0].y;
        }
        break;
        case 1:
        if((hitPoints[1].x + xDiff < imageDisplayRect.origin.x + imageDisplayRect.size.width) &&
           (hitPoints[1].x + xDiff - kMinCropSize > hitPoints[0].x) &&
           (hitPoints[1].y + yDiff > imageDisplayRect.origin.y) &&
           (hitPoints[1].y + yDiff + kMinCropSize < hitPoints[2].y)){
                hitPoints[1].x += xDiff;
                hitPoints[1].y += yDiff;
                hitPoints[2].x = hitPoints[1].x;
                hitPoints[0].y = hitPoints[1].y;
        }
        break;
        case 2:
        if((hitPoints[2].x + xDiff < imageDisplayRect.origin.x + imageDisplayRect.size.width) &&
           (hitPoints[2].x + xDiff - kMinCropSize > hitPoints[0].x) &&
           (hitPoints[2].y + yDiff < imageDisplayRect.origin.y + imageDisplayRect.size.height) &&
           (hitPoints[2].y + yDiff - kMinCropSize > hitPoints[1].y)){
                hitPoints[2].x += xDiff;
                hitPoints[2].y += yDiff;
                hitPoints[1].x = hitPoints[2].x;
                hitPoints[3].y = hitPoints[2].y;
        }
        break;
        case 3:
        if((hitPoints[3].x + xDiff > imageDisplayRect.origin.x) &&
           (hitPoints[3].x + xDiff + kMinCropSize < hitPoints[2].x) &&
           (hitPoints[3].y + yDiff < imageDisplayRect.origin.y + imageDisplayRect.size.height) &&
           (hitPoints[3].y + yDiff - kMinCropSize > hitPoints[0].y)){
                hitPoints[3].x += xDiff;
                hitPoints[3].y += yDiff;
                hitPoints[0].x = hitPoints[3].x;
                hitPoints[2].y = hitPoints[3].y;
        }
        break;
    }

    cropRect = CGRectMake(hitPoints[0].x, hitPoints[0].y, hitPoints[1].x - hitPoints[0].x, hitPoints[2].y - hitPoints[1].y);

    lastTouch = thisTouch;
    [self setNeedsDisplay];
}

此代码几乎有效，但它不是很准确......好像有四舍五入的错误。随着时间的推移，它会失去适当的宽高比，这是令人惊讶的......以及为什么我使用花车......但它似乎并不重要。