draw a vertical line where the screen was tapped - ios

Question

I want to draw a vertical line on the place where the screen was tapped. Because the average finger is wider than 1 pixel wide I want to do this "in steps". So basically, the line can only be drawn every 25px. And I want to figure out the nearest location where I could draw a line.
For example if the finger tapps 30 pixels from the left side of my upper view, I want to draw a vertical line 25 pixels from the left side of the view. If the screen is tapped 40 pixels from the left, i want the line drawn 50 pixels from the left side. (So there can only be one line every 25 pixels and and I want to draw the nearest one.
Any idea how I could do this?
Drawing the line is easy:
UIView *lineView = [[UIView alloc] initWithFrame:CGRectMake(100.0, 0.0, 1, 320.0)];
lineView.backgroundColor = [UIColor whiteColor];
[parentView addSubview:lineView];
But I have no idea how to find where the user tapped the screen.

Answer 1

To choose the nearest vertical line aligned to 25-point boundaries use this to calculate the correct x-value:
CGFloat spacing = 25.0f;
NSInteger lineNumber = (NSInteger)((touchPoint.x + (spacing / 2.0f)) / spacing);
CGFloat snapX = spacing * lineNumber;
Here's what happens in the code above:
Add half of the spacing value onto the touch point - this is because the 'snap' process in the next step will always find the previous line, so by adding half of the spacing value we ensure it will 'snap' to the nearest line.
Calculate the line number by dividing by the spacing and casting the value into an integer. This truncates the fractional part of the result so we now have the integer line number (0, 1, 2, 3, etc.).
Multiply by the original spacing to get the actual x-value of the line you want to draw (0, 25, 50, 75, etc.).

Answer 2

Create a transparent custom UIView that covers the whole screen (except the status bar) and override its (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event.
You can get the touch point with:
UITouch *touch = [touches anyObject];
CGPoint touchPoint = [touch locationInView:self];

Answer 3

You can use ssteinberg code:
UITouch *touch = [touches anyObject];
CGPoint touchPoint = [touch locationInView:self];
or adding a UITapGestureRecognizer to your view, which it is probably easier:
UITapGestureRecognizer* tapRecognizer = [[UITapGestureRecognizer alloc] initWithTarget:self action:#selector(handleTap:)];
[yourParentView addGestureRecognizer:tapRecognizer];
[tapRecognizer release];
...
- (IBAction)handleTap:(UITapGestureRecognizer *)recognizer {
CGPoint touchPoint = [recognizer locationInView:self.view];
...
And in addition, draw your line at every 25px by adding something like this:
CGFloat padding = 25.0;
NSInteger xPosition = round(touchPoint.x / padding) * padding;
[self drawLineAt:xPosition];

Answer 4

Refer This if you find it suitable :
- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
if (lineView) {
[lineView removeFromSuperview];
NSSet *allTouchesSet = [event allTouches];
NSArray *allTouches = [NSArray arrayWithArray:[allTouchesSet allObjects]];
int count = [allTouches count];
if (count == 1) {
UITouch *touch = [[event allTouches] anyObject];
tapPoint = [touch locationInView:self];
NSLog(#"tapPoint: %#",NSStringFromCGPoint(tapPoint));
UIView *lineView = [[UIView alloc] initWithFrame:CGRectMake(tapPoint.x, 0.0, 1, 320.0)];
lineView.backgroundColor = [UIColor whiteColor];
[parentView addSubview:lineView];
}
}
}