Question

I am working on an iPhone application of drawing, user can draw a line and can remove as well so can any body help me how to remove the line or drawing?

code for draw

- (void)drawRect:(CGRect)rect {

    float newHeight;
    float newWidth;

    if (!myDrawing) {
        myDrawing = [[NSMutableArray alloc] initWithCapacity:0];
    }
    CGContextRef ctx = UIGraphicsGetCurrentContext();

    if (myPic != NULL) {
        float ratio = myPic.size.height/460;
        if (myPic.size.width/320 > ratio) {
            ratio = myPic.size.width/320;
        }

        newHeight = myPic.size.height/ratio;
        newWidth = myPic.size.width/ratio;

        [myPic drawInRect:CGRectMake(0,0,newWidth,newHeight)];
    }
    if ([myDrawing count] > 0) {
        CGContextSetLineWidth(ctx, 5);

        for (int i = 0 ; i < [myDrawing count] ; i++) {
            NSArray *thisArray = [myDrawing objectAtIndex:i];

            if ([thisArray count] > 2) {
                float thisX = [[thisArray objectAtIndex:0] floatValue];
                float thisY = [[thisArray objectAtIndex:1] floatValue];

                CGContextBeginPath(ctx);
                CGContextMoveToPoint(ctx, thisX, thisY);

                for (int j = 2; j < [thisArray count] ; j+=2) {
                    thisX = [[thisArray objectAtIndex:j] floatValue];
                    thisY = [[thisArray objectAtIndex:j+1] floatValue];

                    CGContextAddLineToPoint(ctx, thisX,thisY);
                }
                CGContextStrokePath(ctx);
            }
        }
    }
}

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

    [myDrawing addObject:[[NSMutableArray alloc] initWithCapacity:4]];

    CGPoint curPoint = [[touches anyObject] locationInView:self];
    [[myDrawing lastObject] addObject:[NSNumber numberWithFloat:curPoint.x]];
    [[myDrawing lastObject] addObject:[NSNumber numberWithFloat:curPoint.y]];
}

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

    CGPoint curPoint = [[touches anyObject] locationInView:self];
    [[myDrawing lastObject] addObject:[NSNumber numberWithFloat:curPoint.x]];
    [[myDrawing lastObject] addObject:[NSNumber numberWithFloat:curPoint.y]];

    [self setNeedsDisplay];
}

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

    CGPoint curPoint = [[touches anyObject] locationInView:self];
    [[myDrawing lastObject] addObject:[NSNumber numberWithFloat:curPoint.x]];
    [[myDrawing lastObject] addObject:[NSNumber numberWithFloat:curPoint.y]];

    [self setNeedsDisplay];

}

-(void)cancelDrawing {

    [myDrawing removeAllObjects];
    [self setNeedsDisplay];

}

Answer 1

You can use CGContextSetStrokeColorWithColor and set color of background of your view while drawing line so it will look like you are erasing line.

And if you want to provide user undo facility while drawing than you can create following function for that,

-(void)undoDrawing {

    [myDrawing removeLastObject];
    [self setNeedsDisplay];

}

Hope this helps…