Android :: Stop Timer When Activity Loses Focus

I have a Timer which executes a TimerTask every 30 seconds. I only want the timer to launch a new TimerTask if the Activity is displayed i.e. if the user receives a phone call or launches a new activity the Timer will stop. I then need the Timer to restart when the Activity is re-launched and comes into focus.

Now this should be easy, I override the "onWindowFocusChanged(boolean hasFocus)" method and either start or stop the timer depending on the value of has Focus. The way I start the timer is to create a new Timer object and TimerTask each time and the way I stop the Timer is to call the cancel() method on the Timer object and set timer to null.

My problem is this doesn't always work, if I launch the activity which has the Timer and switch orientations quickly (to start/stop the Activity) I find the Timer is not always canceled and I end up with multiple Timers launching TimerTasks at an ever increasing rate.