iPad Air上的Bezier Curve绘图问题

Question

我有以下代码，允许绘制曲线。用户绘制直线并且一旦触摸结束，控制手柄就会出现，用户可以将线修改为所需的曲线。它适用于所有iPad上的IOS 6和IOS 7。但它在IOS 8上运行的iPad Air和iPad mini 2上运行起来很奇怪。当用户向一个方向拉动手柄时，起始点移动到相反的位置。所以它就像反向坐标计算。

-(void)drawStraightLine
{
    CGContextRef context = UIGraphicsGetCurrentContext();

    // set the line properties
    CGContextSetStrokeColorWithColor(context, self.lineColor.CGColor);
    CGContextSetLineCap(context, kCGLineCapRound);
    CGContextSetLineWidth(context, 2.5);
    CGContextSetAlpha(context, 1.0);

    // draw the line
    CGContextMoveToPoint(context, d.x, d.y);
    CGContextAddLineToPoint(context, c.x, c.y);
    CGContextStrokePath(context);
}

-(void)showHandles
{
    CGContextRef ctx = UIGraphicsGetCurrentContext();
    CGContextSetStrokeColorWithColor(ctx, [UIColor grayColor].CGColor);

    CGContextMoveToPoint(ctx, a.x, a.y);
    CGContextAddLineToPoint(ctx, b.x, b.y);
    CGContextMoveToPoint(ctx, c.x, c.y);
    CGContextAddLineToPoint(ctx, d.x, d.y);
    CGContextStrokePath(ctx);

    CGContextMoveToPoint(ctx, a.x, a.y);
    for (double t = 0; t < 1; t += EPSILON) {
        CGPoint p = [self bezier:t];
        CGContextAddLineToPoint(ctx, p.x, p.y);
    }
    CGContextSetStrokeColorWithColor(ctx, self.lineColor.CGColor);
    CGContextSetLineWidth(ctx, 2.5);
    CGContextSetLineCap(ctx, kCGLineCapRound);
    CGContextSetLineJoin(ctx, kCGLineJoinRound);
    CGContextStrokePath(ctx);

    //[self showLines:ctx];
    CGContextSetFillColorWithColor(ctx, [UIColor blackColor].CGColor);
    CGContextFillEllipseInRect(ctx, CGRectMake(a.x-5, a.y-5, 10, 10));
    CGContextFillEllipseInRect(ctx, CGRectMake(b.x-5, b.y-5, 10, 10));
    CGContextFillEllipseInRect(ctx, CGRectMake(c.x-5, c.y-5, 10, 10));
    CGContextFillEllipseInRect(ctx, CGRectMake(d.x-5, d.y-5, 10, 10));

    CGContextSetFillColorWithColor(ctx, [UIColor clearColor].CGColor);
    CGContextSelectFont(ctx, "Helvetica", 18.0, kCGEncodingMacRoman);
    CGContextSetTextDrawingMode(ctx, kCGTextFill);
    CGContextSetTextMatrix(ctx, CGAffineTransformMakeScale(1.0, -1.0));

    NSString *label = [NSString stringWithFormat:@"(%.0f,%.0f)", a.x, a.y];

    label = [NSString stringWithFormat:@"(%.0f,%.0f)", c.x, c.y];

    CGContextShowTextAtPoint(ctx, fmax(30, fmin(self.bounds.size.width - 80, c.x - 10)), fmax(30, fmin(self.bounds.size.height - 44, c.y + 10)), [label UTF8String], [label length]);

    //[NSString drawAtPoint:aPoint withAttributes:dictOfAttributes];

    label = [NSString stringWithFormat:@"(%.0f,%.0f)", d.x, d.y];
    CGContextShowTextAtPoint(ctx, fmax(30, fmin(self.bounds.size.width - 80, d.x - 10)), fmax(30, fmin(self.bounds.size.height - 44, d.y + 10)), [label UTF8String], [label length]);


    label = [NSString stringWithFormat:@"(%.0f,%.0f)", b.x, b.y];

    CGContextShowTextAtPoint(ctx, fmax(30, fmin(self.bounds.size.width - 80, b.x - 10)), fmax(30, fmin(self.bounds.size.height - 44, b.y + 10)), [label UTF8String], [label length]);

    CGContextShowTextAtPoint(ctx, fmax(30, fmin(self.bounds.size.width - 80, a.x - 10)), fmax(30, fmin(self.bounds.size.height - 44, a.y + 10)), [label UTF8String], [label length]);
}