当手指在iPhone上移动时，如何在图像上绘制一条线

Question

I am trying to draw a line on top of a background image in my UIView tracing the movements of the user's finger. I can accomplish this by drawing the UIImage as my background and then doing my line drawing in drawRect but I have to load the image each time drawRect is called and it makes performance sluggish. Alternatively, if I use a UIImageView as my background image (just adding it to the view in IB or programatically) the lines disappear when it is dragged over the image. Is there a way to tell the lines to draw on top of the UIImageView. Thanks for your help.

Here is my code using the UIImage method:

- (void)drawRect:(CGRect)rect {
    UIImage *imageField = [UIImage imageNamed:@"field.jpg"];
    CGRect rectForImage=CGRectMake(0,0,200,200);
    [imageField drawInRect:rectForImage];

        CGContextRef context = UIGraphicsGetCurrentContext();
        CGContextMoveToPoint(context, x1, y1);
    CGContextAddLineToPoint(context, x2, y2);
    CGContextStrokePath(context);
}

Answer 1

Adding the image as the background view is the correct way to do it. The problem lies in the fact that you added the image directly to the view. Subviews are displayed on top of a view, not behind it. Correctly use a container view that holds the image view as a background, and then your line-drawing view on top of that.

Answer 2

您可以尝试将（不透明的）线绘制到透明的CALayer上，并将该CALayer放在UIView的顶部，即背景UIImage的上方。