iOS detect movement of user

Question

I want to create a simple app that draws a simple line on screen when I move my phone on the Y-axis from a start point to end point, for example from point a(0,0) to point b

Answer 1

You need to initialize the motion manager and then check motion.userAcceleration.y value for an appropriate acceleration value (measured in meters / second / second).

In the example below I check for 0.05 which I've found is a fairly decent forward move of the phone. I also wait until the user slows down significantly (-Y value) before drawing. Adjusting the device MotionUpdateInterval will will determine the responsiveness of your app to changes in speed. Right now it is sampling at 1/60 seconds.

motionManager = [[CMMotionManager alloc] init];
motionManager.deviceMotionUpdateInterval = 1.0/60.0;
[motionManager startDeviceMotionUpdatesToQueue:[NSOperationQueue currentQueue] withHandler:^(CMDeviceMotion *motion, NSError *error) {
    NSLog(@"Y value is: %f", motion.userAcceleration.y);
    if (motion.userAcceleration.y > 0.05) { 
        //a solid move forward starts 
        lineLength++; //increment a line length value
    } 
    if (motion.userAcceleration.y < -0.02 && lineLength > 10) {
        /*user has abruptly slowed indicating end of the move forward.
         * we also make sure we have more than 10 events 
         */
        [self drawLine]; /* writing drawLine method
                          * and quartz2d path code is left to the 
                          * op or others  */
        [motionManager stopDeviceMotionUpdates];
    }
}];

Note this code assumes that the phone is lying flat or slightly tilted and that the user is pushing forward (away from themselves, or moving with phone) in portrait mode.