Detect When GUI.Text Field gets focus.

i am trying to work with getting a numeric keyboard to pop up instead of the regular one for my app i am making. I am having trouble in the fact that if just keeps opening the keyboard. I understand why this is happening for the as the onGui gets called, somewhere around every frame. So i am looking for a solution to this problem and i was hoping that there was just some way of detecting when an object gains focus. Any help would be much appreciated on this matter. If it matters i am developing my app for android.