How to reference the current or main activity from another class

Question

I often find myself needing to access methods that require referencing some activity. For example, to use getWindowManager, I need to access some Activity. But ofte

Answer 1

I solved this by making a singleton class has an instance of the class below as a member.

public class InterActivityReferrer  {
    HashMap map;
    ArrayList reserve;

    public InterActivityReferrer() {
        map = new HashMap<>();
        reserve = new ArrayList<>();
    }

    public synchronized int attach(T obj) {
        int id;
        if (reserve.isEmpty()) {
            id = reserve.size();
        }
        else {
            id = reserve.remove(reserve.size() - 1);
        }

        map.put(id, obj);
        return id;
    }

    public synchronized T get(int id) {
        return map.get(id);
    }

    public synchronized T detach(int id) {
        T obj = map.remove(id);
        if (obj != null) reserve.add(id);
        return obj;
    }
}

This class can get a T object and return a unique integer assigned to the object by attach(). Assigned integers will not collide with each other unless HashMap fails. Each assigned integer will be freed when its corresponding object is detached by detach(). Freed integers will be reused when a new object is attached.

And from a singleton class:

public class SomeSingleton {
    ...
    private InterActivityReferrer referrer = new InterActivityReferrer<>();
    ...
    public InterActivityReferrer getReferrer() {return referrer;}
}

And from an activity that needs to be referred:

    ...
    int activityID = SomeSingleton.getInstance().getReferrer().attach(this);
    ...

Now with this, a unique integer corresponding to this activity instance is returned. And an integer can be delivered into another starting activity by using Intent and putExtra().

    ...
    Intent i = new Intent(this, AnotherActivity.class);
    i.putExtra("thisActivityID", activityID);
    startActivityForResult(i, SOME_INTEGER);
    ...

And from the another activity:

    ...
    id refereeID = getIntent().getIntExtra("thisActivityID", -1);
    Activity referredActivity = SomeSingleton.getInstance().getReferrer().get(refereeID);
    ...

And finally the activity can be referred. And InterActivityReferrer can be used for any other class.

I hope this helps.