I want to nslog the device motion timestamp property .
The device motion is in the class CMMotionManager.devicemotion.timestamp
Any ideas.
2 Answers
Here the solution I put in place because the date is according to Apple documentation :
The time stamp is the amount of time in seconds since the phone booted.
I first save the originDate at the first mesure (if my NSDate is nil).
[self.motionManager startGyroUpdatesToQueue:self.queue withHandler:^(CMGyroData *gyroData, NSError *error) {
if (self.originDate == nil) {
self.originDate = [NSDate dateWithTimeIntervalSinceNow:-gyroData.timestamp];
}
}];
Then I can display when I want the real date like this :
[NSDate dateWithTimeInterval:mygyroData.timestamp sinceDate:self.originDate]
Don't forget to reset the originDate to nil if you need to restart some mesure.
1 Comment
William henderson
After trying many variations of @ben-mosher's approach, I think this approach is best. It's a bit more of a pain to implement, but it doesn't rely on internal implementation details of -[NSProcessInfo systemUptime] and is thus a more defensive solution.
Edit: Please see Nicolas Lauquin's answer. Per the comments, the following solution is not correct but is retained here for history (and because I can't delete it since it is currently marked accepted).
The timestamp property is an NSTimeInterval, so you should be able to do:
NSLog(@"Motion at time: %@",
[NSDate dateWithTimeIntervalSinceReferenceDate:devicemotion.timestamp]);
NSTimeInterval is just a typedef'd double type, so you could use %f instead of %@ and log it directly.
Also, the docs don't indicate whether this timestamp is set against Apple's reference date or the standard *nix date, so you may need to use [NSDate dateWithTimeIntervalSince1970:] if the aforementioned method returns dates far in the future.
As @davidbitton has suggested the CMDeviceMotion's timestamp is relative to the last device boot, the correct NSDate could be derived by
NSDate *startupTime = [NSDate dateWithTimeIntervalSinceNow:
-1 * [[NSProcessInfo processInfo] systemUptime]];
NSDate *deviceMotionDate = [NSDate dateWithTimeInterval:devicemotion.timestamp
sinceDate:startupTime];
This should yield a roughly accurate NSDate object, assuming @davidbitton is correct. (reference: NSProcessInfo -systemUptime)
However, given how complicated this is, I would now suggest for simplicity that, given the nature of the timestamp property, that you log it in a format string as something like
"... event logged at %0.2f seconds since startup...", devicemotion.timestamp
4 Comments
davidbitton
This doesn't work because the timestamp for a device motion sample is an NSTimeInterval from when the device was last booted. It is NOT from either of the common epochs of 1/1/2001 or 1970.
Ben Mosher
@davidbitton + flaggers + downvoter: updated with a (convoluted?) way to derive an
NSDate given that the CMDeviceMotion.timestamp property is based on uptime.William henderson
This does not work either. -[NSProcessInfo systemUptime] will not give you the correct value while the CPU is asleep.
Ben Mosher
Wild. I don't think the CPU could be asleep when I wrote this. That's pretty cool.
timestampfrom CMLogItem.