Difference between 2 dates in seconds ios - ios

Question

I have an app where content is displayed to the user. I now want to find out how many seconds a user actually views that content for. So in my header file, I've declared an
NSDate *startTime;
NSDate *endTime;
Then in my viewWillAppear
startTime = [NSDate date];
Then in my viewWillDisappear
endTime = [NSDate date];
NSTimeInterval secs = [endTime timeIntervalSinceDate:startTime];
NSLog(#"Seconds --------> %f", secs);
However, the app crashes, with different errors sometimes. Sometimes it's a memory leak, sometimes it's a problem with the NSTimeInterval, and sometimes it crashes after going back to the content for a second time.
Any ideas on to fix this?

Answer 1

since you are not using ARC, when you write
startTime = [NSDate date];
you do not retain startTime, so it is deallocated before -viewWillDisappear is called. Try
startTime = [[NSDate date] retain];
Also, I recommend to use ARC. There should be much less errors with memory management with it, than without it

Answer 2

You should declare a property with retain for the start date. Your date is getting released before you can calculate the time difference.
So declare
#property (nonatomic, retain) NSDate *startDate
- (void)viewWillAppear:(BOOL)animated
{
[super viewWillAppear:animated];
[self setStartDate: [NSDate date]];
}
- (void)viewWillDisappear:(BOOL)animated
{
[super viewWillDisappear:animated];
NSLog(#"Seconds --------> %f",[[NSDate date] timeIntervalSinceDate: self.startDate]);
}
Don't forget to cleanup.
- (void)dealloc
{
[self.startDate release];
[super dealloc];
}