Devicemotion is not updating values - ios

Question

I am unable to figure out why the roll, pitch and yaw values are giving 0.0000 when logged.. I am sure it is a something that i miss but i cant figure it out..
This is the code:
//ViewController.m
#import "ViewController.h"
#interface ViewController (){
}
#property (nonatomic) CMMotionManager *motionManager;
#end
#implementation ViewController
- (void)viewDidLoad
{
[super viewDidLoad];
// Do any additional setup after loading the view, typically from a nib.
self.motionManager = [[CMMotionManager alloc] init];
CMDeviceMotion *devMotion = [[CMDeviceMotion alloc]init];
if ([self.motionManager isDeviceMotionAvailable]) {
NSLog(#"Device Motion Available");
[self.motionManager setDeviceMotionUpdateInterval:1.0/30.0];
// Pull mechanism is used
[self.motionManager startDeviceMotionUpdates];
}
devMotion = self.motionManager.deviceMotion;
NSLog(#"Roll Pitch and Yaw are %f, %f, %f",devMotion.attitude.roll, devMotion.attitude.pitch, devMotion.attitude.yaw);
}
I have gone thru this similar question: SO Question
Please help me understand this..
Thanks..
Updated Code:
#implementation ViewController
- (void)viewDidLoad
{
[super viewDidLoad];
// Do any additional setup after loading the view, typically from a nib.
self.motionManager = [[CMMotionManager alloc] init];
if ([self.motionManager isDeviceMotionAvailable]) {
NSLog(#"Device Motion Available");
[self.motionManager setDeviceMotionUpdateInterval:1.0/30.0];
// Pull mechanism is used
[self.motionManager startDeviceMotionUpdates];
}
CMDeviceMotion *devMotion = self.motionManager.deviceMotion;
NSLog(#"*Roll,Pitch and Yaw are %f, %f, %f",devMotion.attitude.roll, devMotion.attitude.pitch, devMotion.attitude.yaw);
self.timer = [NSTimer scheduledTimerWithTimeInterval:1.0/30.0 target:self selector:#selector(updateValues:) userInfo:nil repeats:YES];
}
-(void) updateValues:(NSTimer *)timer{
CMDeviceMotion *currDeviceMotion = self.motionManager.deviceMotion;
NSLog(#"Roll Pitch and Yaw are %f, %f, %f",currDeviceMotion.attitude.roll, currDeviceMotion.attitude.pitch, currDeviceMotion.attitude.yaw);
}
This code also has a large part of its initial values as 0.000000 . After that it starts to get values... So i guess there is some delay for startDeviceMotionUpdates for providing values to deviceMotion. So looks like i need to figure out how to save the first non zero values.

Answer 1

Notice that your devMotion variable won’t hold usable values (or any at all) right away as CMMotionManager takes a while to update the deviceMotion property after startDeviceMotionUpdates. So you should have some sort of timer that fires periodically and reads that property. Your linked article does something similar.
As long as the gyroscope is starting up, the deviceMotion property will be null (you’re only seeing 0.0 because messages to nil return zero).
As a sidenote: your call to [[CMDeviceMotion alloc] init] is useless as you then override the variable the result was assigned to with self.motionManager.deviceMotion.