来自A线程的Android调用活动B的方法

I have a main Activity A, in which I create a background thread to load data from db. After the loading is done, I want to update the list which might have been already shown in a child activity B (If user has navigated to B in the meanwhile). If user hasnt yet navigated to B, its not an issue. I have a main Activity A, in which I create a b