NSDate, NSTimeInterval, Calculating time passed and Core Data

Question

I'm creating a timesheet/stopwatch application of sorts for the iPhone.
I am using an NSDate in CoreData and following this pattern:
1) Start:
Session* session = (Session*)\[NSEntityDescription insertNewObjectForEntityForName:@"Session" inManagedObjectContext:managedObjectContext_\];
session.startTime = \[NSDate date\];
2) Calculate time passed and update label:
NSTimeInterval timeDiff = -1*\[session.startTime timeIntervalSinceNow\];
cell.textLabel.text = \[NSString stringWithFormat:@"%f", timeDiff\];
I also see that I could use:
NSTimeInterval timeDiff = \[\[NSDate date\] timeIntervalSinceDate:session.startTime\];
cell.textLabel.text = \[NSString stringWithFormat:@"%f", timeDiff\];
I don't like this since it is needlessly creating a lot of dates - and there could be 100s of these timers running at the same time ... or, does timeIntervalSinceNow possibly also create a new date evertime under the covers and I'm pre-optimizing or something too early. Can anyone suggest a better way to do this? For instance, initially, I was simply using NSTimeInterval:
NSTimeInterval startTime = \[NSDate timeIntervalSinceReferenceDate\];
NSTimeInterval endTime = \[NSDate timeIntervalSinceReferenceDate\] - startTime;
But I end up struggling with the best way to manage this as part of a CoreData entity. Should I uses a double? or a Date? I understand that a Date is implemented as an NSTimeInterval but I can't assign or subtract an NSTimeInterval directly to my entity.date
session.startTime = \[NSDate timeIntervalSinceReferenceDate\];
NSTimeInterval timeDiff = \[NSDate timeIntervalSinceReferenceDate\] - session.startTime;
Obviously, this won't compile when startTime is a Date object but I think it illustrates what I'm after. Trying to 'lightly' calculate time passed by using the start time assigned to an Entity. Maybe the original way is just fine. I'm just looking for suggestions. I guess I could try running this inside a profiler. As a side note, is there a better way to print the stopwatch output to the screen? It seems like overkill to constantly update a Label's text property ... hundreds, maybe thousands of times especially when there may be 10 or so timers running on the same screen. Does anyone have any advice for stopwatch type 'update the time' label behavior?
I'm doing this in an NSTimer potentially set to fire every .01s ... IE: this method will get hit quite a bit ...
Thanks.
Message was edited by: LutherBaker

Answer 1

LutherBaker wrote:
I don't like this since it is needlessly creating a lot of dates - and there could be 100s of these timers running at the same time ... Can anyone suggest a better way to do this?
That doesn't sound good. Why would you have so many?
I'm just looking for suggestions. I guess I could try running this inside a profiler. As a side note, is there a better way to print the stopwatch output to the screen? It seems like overkill to constantly update a Label's text property ... hundreds, maybe thousands of times especially when there may be 10 or so timers running on the same screen. Does anyone have any advice for stopwatch type 'update the time' label behavior?
I'm doing this in an NSTimer potentially set to fire every .01s ... IE: this method will get hit quite a bit ...
That doesn't seem like a side note. That seems like the main point.
As "good" video is typically around 30 frames per second, it doesn't make any sense to update the user interface any more frequently than that. Also, you really only need one timer. Let that timer update all the values. You may not need any timers. You could do everything via key-value coding and let the OS handle all the work.