NSDates and TimeIntervals - ios

Question

I'm struggling to understand how to use NSDates properly.
I have an event that instantiates a timingDate = [NSDate date];
I then later on what to record the time intervals betweeen user's touches.
So I want to find the interval between the timingDate and the user touch in milliseconds.
Then I want to reset the timingDate to be equal to the touchTime so that the next time the screen is touched I can find the differnce between the previous touch and the present touch. I hope that makes sense. But I am going around in circles because I don't understand how to use NSDates or NSIntervals. The properties interval touchTime and timingDate are all currently NSDate types - Is this right?
So I've tried a lot of different things like
-(void) touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
touchTime = timingDate;
interval = [[NSDate date] timeIntervalSinceDate:timingDate]; // should be the time difference from when the timingDate was first set and when the user touched the screen.
touchTime = [[[NSDate date]timeIntervalSinceDate:timingDate]doubleValue];
timingDate = [NSDate dateWithTimeIntervalSinceReferenceDate:touchTime];
NSLog(#"Time taken Later: %f", [[NSDate date]timeIntervalSinceDate:timingDate]);
}

Answer 1

Your code is a bit complex! You just need to calculate the difference between timingDate and the time that the touch occurred, and then set timingDate to the current time so that you can perform this calculation on every touch event.
To find the difference between timingDate and the first touch, you can use NSDate's timeIntervalSinceDate with the current time. This will return an NSTimeInterval value, which represents a time value in seconds with sub-millisecond precision. Here's an example:
NSDate *currentDate = [NSDate date];
NSTimeInterval timeInterval = [currentDate timeIntervalSinceDate:timingDate];
NSLog(#"Time taken: %f seconds / %f milliseconds",timeInterval,timeInterval*1000);
Then, in order to set your timingDate to the current time, simply use timingDate = currentDate;. This will allow you to continuously measure the time difference between touches.

Answer 2

NSTimeInterval is a double which represents seconds.
NSDate is an object that holds the date/time.
Here is an example:
#property(nonatomic, strong) NSDate * lastTouchDate;
-(void) touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
NSTimeInterval secondsSinceLastTouch = 0;
if(self.lastTouchDate){
secondsSinceLastTouch = [[NSDate date] timeIntervalSinceDate:self.lastTouchDate];
NSLog(#"It's been %.1f seconds since the user touched the screen", secondsSinceLastTouch);
}else{
NSLog(#"This is the first time the user touched the screen");
}
self.lastTouchDate = [NSDate date];
}
If you don't want the interval between the last time you touched it, do not update the self.lastTouchDate after initialization and it will be seconds since the date was initialized.

Answer 3

So, the first thing that you need to understand is that -[NSDate timeIntervalSinceDate:] returns an NSTimeInterval, which is really just a double value.
In your example above, you haven't declared your variables with types, but if you look at the value your variable interval it should be a decimal value of seconds since the time represented by timingDate.
Assuming that timingDate is an NSDate object, and it is set before this code is run, this code should print the time (seconds) to the debug console.
NSTimeInterval interval = [[NSDate date] timeIntervalSinceDate:timingDate];
NSLog(#"Time between touches %f", interval);
Here is the NSDate class documentation, in case you were having trouble finding it.
( https://developer.apple.com/library/ios/documentation/Cocoa/Reference/Foundation/Classes/NSDate_Class/index.html#//apple_ref/doc/c_ref/NSDate )