How to communicate Application state across threads & other general threading questions...

I'm trying to better understand how threading & backgrounding work in Xamarin and iOS. A couple of questions:

1. What is the best / simplest way for code within a thread to determine the current Application State (Running, Inactive, Backgrounded, Terminated)?

2. In which thread do events occur? For example, I have a High Precision Timer that is running continuously in its own thread and which regularly triggers an OnPulse event. I have several custom UIControls & a ViewController that attach event handlers to this OnPulse event. I assume that these controls exist and operate within the Main UI thread because they were created within the main AppDelegate class and updating their values updates the GUI. When the OnPulse event handler executes, which context will it be under?