在我的WPF应用程序中捕获没有焦点的按键事件

I have developed an On Screen Keyboard in WPF. I need to capture the key press event (via Key Board) in order to keep a track of Caps Lock, Shift etc (whether they are pressed). Please note that my application loses focus when any other application (say notepad) is opened.I have developed an On Screen Keyboard in WPF.