Iphone 为什么程序背景会减慢UIScrollView的速度？

我的问题代码是在UIView drawRect中绘制的过程背景中：然后将其作为子视图添加到UIScrollView。程序代码如下。它绘制了长方体形状，看起来有点像天际线。你知道为什么这会减慢my UIScrollView的速度吗？它似乎只是在第一次滚动时变慢了，然后它就好像被缓存了一样。背景宽度可以高达1000像素，有时甚至更宽。请看图片

- (void)drawRect:(CGRect)rect
{
UIBezierPath *vertLine = [[UIBezierPath alloc] init];

[vertLine moveToPoint:CGPointMake(0,self.frame.size.height)];

int detail = 10;
int ranNum = 0;
int count = self.bounds.size.width/detail;
CGFloat heightIncrement = 0.0;

CGFloat minHeight = self.frame.size.height;

CGFloat xPos = 0;
CGFloat yPos = self.frame.size.height-20;

for (int i =0; i<count; i++)
{
    ranNum += (arc4random() % 9)-5;

    yPos -= (arc4random() % 30);
    [vertLine addLineToPoint:CGPointMake(xPos,yPos)];

    xPos += (arc4random() % 20)+10;
    [vertLine addLineToPoint:CGPointMake(xPos,yPos)];

    yPos += (arc4random() % 30);
    [vertLine addLineToPoint:CGPointMake(xPos,yPos)];

    xPos += (arc4random() % 30);
    [vertLine addLineToPoint:CGPointMake(xPos,yPos)];

    if (yPos>self.frame.size.height-10) {
        yPos = self.frame.size.height-10;
    }

    if (yPos<self.frame.size.height-50) {
        yPos = self.frame.size.height-50;
    }

}

[vertLine addLineToPoint:CGPointMake(count*20,(self.frame.size.height))];

[[UIColor colorWithRed:0.0/255.0 green:38.0/255.0 blue:51.0/255 alpha:1] setFill];

[vertLine fill];

}

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可能是因为您的绘图超出了滚动范围。尝试：

[yourScrollView setContentOffest:CGPointMake(0,0)];