I am having my first play around with Core Motion and have struck an issue and I can't work out what is going on. I am sure it is going to be a simple issue. Here is some code:
.h file
#import <UIKit/UIKit.h>
#import <CoreMotion/CoreMotion.h>
@interface TempVC : UIViewController {
CMMotionManager *motionManager;
CMAttitude *referenceAttitude;
CMAttitude *attitude;
}
- (void)updateData;
@end
.m file
#import "TempVC.h"
#define DEGREES(radians) (radians / M_PI * 180.0)
@implementation TempVC
- (void)viewDidLoad
{
[super viewDidLoad];
self.view.backgroundColor = [UIColor brownColor];
motionManager = [[CMMotionManager alloc] init];
referenceAttitude = nil;
CMDeviceMotion *dm = motionManager.deviceMotion;
attitude = dm.attitude;
referenceAttitude = [attitude retain];
motionManager.deviceMotionUpdateInterval = 0.1;
[motionManager startDeviceMotionUpdates];
[NSTimer scheduledTimerWithTimeInterval:0.1 target:self selector:@selector(updateData) userInfo:nil repeats:YES];
}
- (void)updateData
{
attitude = motionManager.deviceMotion.attitude;
[attitude multiplyByInverseOfAttitude:referenceAttitude];
NSLog(@"\n Roll: %.0f\n Pitch: %.0f\n Yaw: %.0f",DEGREES(attitude.roll), DEGREES(attitude.pitch), DEGREES(attitude.yaw));
}
This code will crash with no real indication of what goes wrong. If I comment out
[attitude multiplyByInverseOfAttitude:referenceAttitude];
Everything works fine??? Have I implemented the reference wrong?
You access CMMotionManager's deviceMotion property before starting updates. There you store your referenceAttitude which will still be nil at that time. When updateData is called a few millis later [attitude multiplyByInverseOfAttitude:referenceAttitude]; will fail.
Try instead to set the referenceAttitude on the first call of updateData and it should work.