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 often my code for using these methods is in some other class that has no reference to an activity. Up until now, I've either stored a reference to the main activity or passed the context of some activity to the class. Is there some better way to do this?

Answer 1

public static Activity getLaunchActivity()
{
    final Class<?> activityThreadClass = Class.forName("android.app.ActivityThread");

    final Method methodApp = activityThreadClass.getMethod("currentApplication");
    Application App = (Application) methodApp.invoke(null, (Object[]) null);
    Intent launcherIntent = App.getPackageManager().getLaunchIntentForPackage(App.getPackageName());
    ActivityInfo launchActivityInfo = launcherIntent.resolveActivityInfo(App.getPackageManager(), 0);
    Class<?> clazz;
    try
    {
        clazz = Class.forName(launchActivityInfo.name);
        if(clazz != null)
            return Activity.class.cast(clazz.newInstance());
    }
    catch (Exception e)
    {}

    return null;
}

Answer 2

I'm new to android so my suggestion may look guffy but what if you'll just create a reference to your activity as private property and assign that in OnCreate method? You can even create your CustomActivity with OnCreate like that and derive all your activities from your CustomActivity, not the generic Activity provided by android.

class blah extends Activity{
  private Activity activityReference;
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        activityReference = this;
    }

}

after that you could use that the way you want, i.e. in

Intent i = new Intent(activityReference, SomeOtherActivity.class)

etc

Answer 3

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

public class InterActivityReferrer <T> {
    HashMap<Integer, T> map;
    ArrayList<Integer> 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<Activity> referrer = new InterActivityReferrer<>();
    ...
    public InterActivityReferrer<Activity> 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.

Answer 4

I found a way to get the Activity to a non-activity class that I have not seen discussed in forums. This was after numerous failed attempts at using getApplicationContext() and of passing the context in as a parameter to constructors, none of which gave Activity. I saw that my adapters were casting the incoming context to Activity so I made the same cast to my non-activity class constructors:

public class HandleDropdown extends Application{
...
    public Activity activity;
...
public HandleDropdown() {
    super();
}
public HandleDropdown(Activity context) {
    this.activity = context;
    this.context = context;
}
public void DropList(View v,Activity context) {
    this.activity = context;
    this.context = context; 
...
}

After doing this cast conversion of Context to Activity I could use this.activity wherever I needed an Activity context.

Answer 5

If you already have a valid context, just use this: Activity activity = (Activity) context;

Answer 6

Just a guess since I haven't done this but it might work.

1) Get your applicationContext by making your Android Application class a Singleton.

2) Get your ActivityManager class from the context.

3) Get a list of RunningTaskInfos using getRunningTasks() on the ActivityManager.

4) Get the first RunningTaskInfo element from the list which should be the most recent task launched.

5) Call topActivity on that RunningTaskInfo which should return you the top activity on the activity stack for that task.

Now, this seems like a LOT more work than any of the other methods mentioned here, but you can probably encapsulate this in a static class and just call it whenever. It seems like it might be the only way to get the top activity on the stack without adding references to the activities.

Answer 7

We built a framework for this. We have a BaseActivity class that inherits from Activity and it overrides all the lifecycle methods and has some static (class) variables that keep track of the activity stack. If anything wants to know what the current activity is, it just calls a static method in BaseActivity that returns the activity on top of our privately-managed stack.

It is kinda hacky, but it works. I'm not sure I would recommend it though.

Answer 8

You can make you application instance a singleton, and use it when you need a Context

An example is in this question:
Android Application as Singleton

This way, when you need a Context, you can get it with
Context context = MyApplication.getInstance()

This might not be the cleanest solution, but it has worked well for me so far

Answer 9

Handle the Intent in the class you want to do these methods, and send your information to it in a Bundle like so:

    Intent i = new Intent("android.intent.action.MAIN");
    i.setComponent(new ComponentName("com.my.pkg","com.my.pkg.myActivity"));
    Bundle data = new Bundle();

    i.putExtras(data);

    startActivityForResult(i);

Then use an OnActivityResultListener to grab the new data.

Answer 10

Passing context is better way for refrence Activity.

You can pass Context to another class.

IN Activity ::

AnotherClass Obj  = new AnotherClass(this);

IN Another Class

class AnotherClass{

public AnotherClass(Context Context){

    }

}

Answer 11

You can implement the necessary methods in your activity and implement a Handler. Then, simply pass a handler instance to your classes, where you can obtain a message for handler and send it to target.

Answer 12

There are many ways for Activities communication.

you can use:

• the startActivityForResult method

• a system of broadcast message and receiver (you can broadcast an event from the actual activity, and register a receiver in the target activity. Remember that the target activity must be previously initialized and non finished)

• as you say, store a reference of the target activity wherever you need.