画一条虚线 ————————————————

 UIImageView *imageView1 = [[UIImageView alloc]initWithFrame:CGRectMake(0, 200, 320, 20)];
    [self.view addSubview:imageView1];
    
    
    UIGraphicsBeginImageContext(imageView1.frame.size);   //开始画线
    [imageView1.image drawInRect:CGRectMake(0, 0, imageView1.frame.size.width, imageView1.frame.size.height)];
    CGContextSetLineCap(UIGraphicsGetCurrentContext(), kCGLineCapRound);  //设置线条终点形状
    
    
    const CGFloat lengths[] = {5,3};
    CGContextRef line = UIGraphicsGetCurrentContext();
    CGContextSetStrokeColorWithColor(line, [UIColor redColor].CGColor);
    
    CGContextSetLineDash(line, 0, lengths, 2);  //画虚线
    CGContextMoveToPoint(line, 0.0, 20.0);    //开始画线
    CGContextAddLineToPoint(line, 310.0, 20.0);
    CGContextStrokePath(line);
    
    imageView1.image = UIGraphicsGetImageFromCurrentImageContext();