Touch Events and UIResponder - iOS Programming: The Big Nerd Ranch Guide - page 550

Now let's return to the methods for handling touch events. First, in TouchDrawView.m ,
override touchesBegan:withEvent: to create a new Line instance and store it in
an NSMutableDictionary .
- (void)touchesBegan:(NSSet *)touches
withEvent:(UIEvent *)event
{
for (UITouch *t in touches) {
// Is this a double tap?
if ([t tapCount] > 1) {
[self clearAll];
return;
}
// Use the touch object (packed in an NSValue) as the key
NSValue *key = [NSValue valueWithNonretainedObject:t];
// Create a line for the value
CGPoint loc = [t locationInView:self];
Line *newLine = [[Line alloc] init];
[newLine setBegin:loc];
[newLine setEnd:loc];
// Put pair in dictionary
[linesInProcess setObject:newLine forKey:key];
}
}