问清除CGContext行

EN

- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {

    if(isDrawing == YES) {

        UITouch *touch = [touches anyObject];
        CGPoint currentPoint = [touch locationInView:mainImageView.superview];

        UIGraphicsBeginImageContext(mainImageView.frame.size);
        [mainImageView.image drawInRect:CGRectMake(0, 0, mainImageView.frame.size.width, mainImageView.frame.size.height)];

        CGContextSetLineCap(UIGraphicsGetCurrentContext(), kCGLineCapRound);
        CGContextSetLineWidth(UIGraphicsGetCurrentContext(), dimension);

        const CGFloat *components = CGColorGetComponents([color CGColor]);
        CGContextSetRGBStrokeColor(UIGraphicsGetCurrentContext(), components[0], components[1], components[2], components[3]);

        CGContextBeginPath(UIGraphicsGetCurrentContext());
        CGContextMoveToPoint(UIGraphicsGetCurrentContext(), lastPoint.x, lastPoint.y);
        CGContextAddLineToPoint(UIGraphicsGetCurrentContext(), currentPoint.x, currentPoint.y);
        CGContextStrokePath(UIGraphicsGetCurrentContext());

        mainImageView.image = UIGraphicsGetImageFromCurrentImageContext();
        UIGraphicsEndImageContext();

        lastPoint = currentPoint;

    } else {
        //Clear current line (like a rubber would do)
    }
}