Android, how to detect that the activity is back from another activity?

Question

For Android, suppose 3 activities, a1 a2 a3, with their click hierarchy, a1->a2->a3.

My questions is in a2, how to detect it is from a3 by pressing the back key, instead of entering from a1?

Answer 1

You can use onActivityResult to check for return from another activity. Put this code within your a2 activity.

Declare the request code as a constant at the top of your activity:

public static final int OPEN_NEW_ACTIVITY = 123;

Put this where you start the new activity:

Intent intent = new Intent(this, NewActivity.class);
startActivityForResult(intent, OPEN_NEW_ACTIVITY);

Do something when the activity is finished. Documentation suggests that you use resultCode, but depending on the situation, your result can either be RESULT_OK or RESULT_CANCELED when the button is pressed. So I would leave it out.

@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
    if (requestCode == OPEN_NEW_ACTIVITY) {
        // Execute your code on back here
        // ....
    }
}

You should take care to run onActivityResult on the Activity itself, and not a Fragment.

You don't actually have to put any code in the a3 activity, but you can send data back if you like.