我的类Canvas继承自 UIImageView （在背景是set image background.png），并且在方法touchesMoved中我绘制了一些颜色的手势：
UITouch *touch = [touches anyObject];
CGPoint currentLocation = [touch locationInView:self];
UIGraphicsBeginImageContext(self.frame.size);
CGContextRef ctx = UIGraphicsGetCurrentContext();
[self.image drawInRect:CGRectMake(0, 0, self.frame.size.width, self.frame.size.height)];
CGContextSetLineCap(ctx, kCGLineCapRound);
CGContextSetLineWidth(ctx, width);
CGContextSetRGBStrokeColor(ctx, red, green, blue, alpha);
CGContextBeginPath(ctx);
CGContextMoveToPoint(ctx, location.x, location.y);
CGContextAddLineToPoint(ctx, currentLocation.x, currentLocation.y);
CGContextStrokePath(ctx);
self.image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
location = currentLocation;
我有ERASE按钮，如何清除画布的上下文但是将background.png保留为背景，只删除用户使用触摸绘制的内容（我有属性画布）？