How do I prevent Android taking a screenshot when my app goes to the background?

Question

Try FLAG_SECURE:

public class FlagSecureTestActivity extends Activity {
  @Override
  public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    getWindow().setFlags(WindowManager.LayoutParams.FLAG_SECURE,
                         WindowManager.LayoutParams.FLAG_SECURE);

    setContentView(R.layout.main);
  }
}

This definitely secures against manual screenshots and automatic screenshots from the ICS recent-tasks history. It also secures against screen recording (e.g., apps using the media projection APIs).

UPDATE: it also secures against Now On Tap or other assistants on Android 6.0; they will not get access to the details of widgets and containers in your UI if the user brings up the assistant.

UPDATE #2: however, not everything in the activity will be protected. Any pop-up windows — Dialog, Spinner, AutoCompleteTextView, action bar overflow, etc. — will be insecure. You can fix the Dialog problem by calling getWindow() on it and setting FLAG_SECURE. The rest... gets tricky. See this blog post for more.

Be careful about using WindowManager.LayoutParams.FLAG_SECURE, on some devices (verified on Samsung Galaxy ACE, e.g. GT-S5830) this will make the view scrambled. Looks like a Samsung specific bug. I recommend the following:

if(android.os.Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.HONEYCOMB) {
    getWindow().setFlags(WindowManager.LayoutParams.FLAG_SECURE, WindowManager.LayoutParams.FLAG_SECURE);
}

This is what a scrambled screen looks like:

This is working properly on ICS Samsung phones though, so I'm assuming problem is isolated to Gingerbread devices (or older).

The solution provided by CommonsWare continues to be valid also in Lollipop.

Just a note, if you want to continue to not see snapshots in recent list for the entire app, ALL the implemented activities should specify in the onCreate() method the flag getWindow().addFlags(WindowManager.LayoutParams.FLAG_SECURE); before setContentView();

Otherwise a snapshot in the recent list will show the first activity without the flag if the user navigated through it.

Here is a solution for hiding content of an app by covering it with a splash screen when the app is put into the background. This is not using the FLAG_SECURE technique, I simply override the onPause and onResume methods of the screens and modify the view to show one that covers everything in the back.

https://stackoverflow.com/a/52976001/6686912

In case if someone is looking for a solution in which the app must secure (screen overlay) when the app is background or stick of all running app and the in-app app should allow screenshot. Try Below:-

@Override
    protected void onResume() {
        super.onResume();
        getWindow().clearFlags(WindowManager.LayoutParams.FLAG_SECURE);
    }

    @Override
    protected void onPause() {
        super.onResume();
        getWindow().setFlags(WindowManager.LayoutParams.FLAG_SECURE, WindowManager.LayoutParams.FLAG_SECURE);
    }