Question

I have an app with two screens, and buttons that switch between them. One screen is mainly a listView, but the other has a game engine in it (AndEngine) and it can take a while for the game engine to init the first time the activity is created.
So switching between them can cause big delays when going to a new game Engine. However, if I just use the back button, the previous game engine loads much faster.
Is there a way I can specify to crete only one game engine and always bring up that instance of the activity? Right now I am using startIntent() to swap screens. Is there some other way?
A way that allows me to just have a single instance of each activity and swap between them?

Answer 1

Android provides a simple way to switch between activities using a TabHost. You can also use it to switch between Fragments as explained under TabActivity. Alternatively, you can add a FrameLayout to your activity, programmatically instantiate the fragments and attach/show/hide them when needed.

Your res/layout/main.xml would look like this:

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
             android:layout_height="fill_parent"
             android:layout_width="match_parent"
             android:id="@+id/mainframe">
</FrameLayout>

And assuming the v4 support library is used, your activity would look like this:

public MyActivity extends FragmentActivity {

    private Fragment mListFrag;
    private Fragment mGameFrag;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        mListFrag = new MyListFragment();
        mGameFrag = new MyGameFragment();

        FragmentManager fm = getSupportFragmentManager();
        fm.beginTransaction()
            .add(
                R.id.mainframe,
                mListFrag,
                MyListFragment.class.getName())
            .add(
                R.id.mainframe,
                mGameFrag,
                MyGameFragment.class.getName())
            .detach(mGameFrag)
            .commit();

        fm.executePendingTransactions();
    }

    public void showList() {
        getSupportFragmentManager().beginTransaction()
            .hide(mGameFrag)
            .show(mListFrag)
            .commit();
    }

    public void showGame() {
        FragmentTransaction ft = getSupportFragmentManager().beginTransaction();
        if (mGameFrag.isDetached()) {
            ft.attach(mGameFrag);
        }
        ft.hide(mListFrag).show(mGameFrag).commit();
    }
}

Observe that MyGameFragment.onCreateView isn’t called until it is first attached. After that, hiding and showing the fragments allows the user to switch without delay.

Edit: I realise now you wanted 2 activities with their own button. I’ve updated the code to reflect this. From the OnClickListeners simply call the relevant activity functions like so:

((MyActivity) MyListFragment.this.getActivity()).showGame();