Como posso apagar o conteúdo na frente UIImage passando o dedo sobre ele e exibir UIImage que é o lado de trás do UIView.

Eu usei o código a seguir- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event para apagar a imagem frontal:

-(void)eraseDrawingContents
{
    UIGraphicsBeginImageContext(frontImage.frame.size);
    [frontImage.image drawInRect:CGRectMake(0, 0, frontImage.frame.size.width, frontImage.frame.size.height)];

    CGContextSaveGState(UIGraphicsGetCurrentContext());
    CGContextSetShouldAntialias(UIGraphicsGetCurrentContext(), YES);
    CGContextSetLineCap(UIGraphicsGetCurrentContext(), kCGLineCapRound);
    CGContextSetLineWidth(UIGraphicsGetCurrentContext(), 25.0);
    CGContextSetShadowWithColor(UIGraphicsGetCurrentContext(), CGSizeMake(0, 0), 50, [[UIColor whiteColor]CGColor]);

    CGMutablePathRef path = CGPathCreateMutable();
    CGPathMoveToPoint(path, nil, lastPoint.x, lastPoint.y);
    CGPathAddLineToPoint(path, nil, currentPoint.x, currentPoint.y);
    CGContextSetBlendMode(UIGraphicsGetCurrentContext(), kCGBlendModeClear);
    CGContextAddPath(UIGraphicsGetCurrentContext(), path);
    CGContextStrokePath(UIGraphicsGetCurrentContext());

    frontImage.image = UIGraphicsGetImageFromCurrentImageContext();
    CGContextRestoreGState(UIGraphicsGetCurrentContext());
    UIGraphicsEndImageContext();

    lastPoint = currentPoint;
}

Eu consegui seguir a saída.

Mas eu preciso de saída assim.

Como podemos alcançar essa funcionalidade?

Por favor me ajude.