How to prevent multiple instances of an Activity when it is launched with different Intents

Question

Add this to onCreate and you should be good to go:

// Possible work around for market launches. See https://issuetracker.google.com/issues/36907463
// for more details. Essentially, the market launches the main activity on top of other activities.
// we never want this to happen. Instead, we check if we are the root and if not, we finish.
if (!isTaskRoot()) {
    final Intent intent = getIntent();
    if (intent.hasCategory(Intent.CATEGORY_LAUNCHER) && Intent.ACTION_MAIN.equals(intent.getAction())) {
        Log.w(LOG_TAG, "Main Activity is not the root.  Finishing Main Activity instead of launching.");
        finish();
        return;       
    }
}

I'm just going to explain why it fails, and how to reproduce this bug programmatically so you can incorporate this in your test suite:

1. When you launch an app through Eclipse or Market App, it launches with intent flags: FLAG_ACTIVITY_NEW_TASK.

2. When launching through the launcher (home), it uses flags: FLAG_ACTIVITY_NEW_TASK | FLAG_ACTIVITY_BROUGHT_TO_FRONT | FLAG_ACTIVITY_RESET_TASK_IF_NEEDED, and uses action "MAIN" and category "LAUNCHER".

If you would like to reproduce this in a test case, use these steps:

adb shell am start -f 0x10000000 -n com.testfairy.tests.regression.taskroot/.MainActivity 

Then do whatever is needed to get to the other activity. For my purposes, I just placed a button that starts another activity. Then, go back to the launcher (home) with:

adb shell am start -W -c android.intent.category.HOME -a android.intent.action.MAIN

And simulate launching it via the launcher with this:

adb shell am start -a "android.intent.action.MAIN" -c "android.intent.category.LAUNCHER" -f 0x10600000 -n com.testfairy.tests.regression.taskroot/.MainActivity

If you haven't incorporated the isTaskRoot() workaround, this will reproduce the problem. We use this in our automatic testing to make sure this bug never occurs again.

Hope this helps!

Have you tried the singleTop launch mode?

Here is some of the description from http://developer.android.com/guide/topics/manifest/activity-element.html:

... a new instance of a "singleTop" activity may also be created to handle a new intent. However, if the target task already has an existing instance of the activity at the top of its stack, that instance will receive the new intent (in an onNewIntent() call); a new instance is not created. In other circumstances — for example, if an existing instance of the "singleTop" activity is in the target task, but not at the top of the stack, or if it's at the top of a stack, but not in the target task — a new instance would be created and pushed on the stack.

Perhaps it is this issue? Or some other form of the same bug?

I realize that the question does not have anything to do with Xamarin Android but I wanted to post something since I did not see it anywhere else.

To fix this in Xamarin Android I used the code from @DuaneHomick and added into MainActivity.OnCreate(). The difference with Xamarin is that is must go after Xamarin.Forms.Forms.Init(this, bundle); and LoadApplication(new App());. So my OnCreate() would look like:

protected override void OnCreate(Bundle bundle) {
    base.OnCreate(bundle);

    Xamarin.Forms.Forms.Init(this, bundle);
    LoadApplication(new App());

    if(!IsTaskRoot) {
        Intent intent = Intent;
        string action = intent.Action;
        if(intent.HasCategory(Intent.CategoryLauncher) && action != null && action.Equals(Intent.ActionMain, System.StringComparison.OrdinalIgnoreCase)) {
            System.Console.WriteLine("\nIn APP.Droid.MainActivity.OnCreate() - Finishing Activity and returning since a second MainActivity has been created.\n");
            Finish();
            return; //Not necessary if there is no code below
        }
    }
}

*Edit: Since Android 6.0, the above solution is not enough for certain situations. I have now also set LaunchMode to SingleTask, which seems to have made things work correctly once again. Not sure what effects this might have on other things though unfortunately.