Intent - if activity is running, bring it to front, else start a new one (from notification)

Question

You can use this:

<activity 
   android:name=".YourActivity"
   android:launchMode="singleTask"/>

which will work similar to "singleInstance" but it won't have that weird animation.

I think the best way to do it and in a simple manner is to start the activity normally, but set that activity in the manifest with the singleInstance property. With this you practically approach both issues you are having right now, by bringing the activity to the front all the time, and letting the OS automatically create a new one if no activity exists or bring to the front the currently existing activity (thanks to the singleInstance property).

This is the way an activity is declared as a single instance:

<activity 
   android:name=".YourActivity"
   android:launchMode="singleInstance"/>

Also to avoid a choppy animation when launching through singleInstance, you could use instead "singleTask", both are very similar but the difference is explained here as per Google's documentation:

<activity 
   android:name=".YourActivity"
   android:launchMode="singleTask"/>

singleInstance is the same as "singleTask", except that the system doesn't launch any other activities into the task holding the instance. The activity is always the single and only member of its task.

Hope this helps.

Regards!

I think what you need is in singleTop Activity, rather than a singleTask or singleInstance.

<activity android:name=".MyActivity"
          android:launchMode="singleTop"
          ...the rest... >

What the documentation says does perfectly suit your needs:

[...] 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.

On top of that (no pun intended), I had exactly the same need as you. I tested all the launchMode flags to figure out how they actually behave in practice and as a result singleTop is actually the best for this: no weird animation, app displayed once in the recent applications list (unlike singleInstance that displays it twice due to the fact it doesn't allow any other Activity to be part of its task), and proper behavious regardless the target Activity already exists or not.

This is old thread, but for all those who is still seeking for answer, here is my solution. It is purely in code, without manifest settings:

private static PendingIntent prepareIntent(Context context) {
  Intent intent = new Intent(context, MainActivity.class);
  intent.setFlags(Intent.FLAG_ACTIVITY_SINGLE_TOP | Intent.FLAG_ACTIVITY_CLEAR_TOP);      
  return PendingIntent.getActivity(context, NON_ZERO_REQUEST_CODE, intent, 
    PendingIntent.FLAG_UPDATE_CURRENT);
}

Here FLAG_ACTIVITY_SINGLE_TOP opens existing activity if it is at the top of task stack. If it is not at the top, but inside stack, FLAG_ACTIVITY_CLEAR_TOP will remove all activities on top of target activity and show it. If activity is not in the task stack, new one will be created. A crucially important point to mention - second parameter of PendingIntent.getActivity(), i.e. requestCode should have non-zero value (I even called it NON_ZERO_REQUEST_CODE in my snippet), otherwise those intent flags will not work. I have no idea why it is as it is. Flag FLAG_ACTIVITY_SINGLE_TOP is interchangeable with android:launchMode="singleTop" in activity tag in manifest.

I know it is old, but nothing from the above were fitting to my application.

Without changing manifests and other configuration, here is the code to bring your app back to front - or opening it when it is closed

Intent notificationIntent = context.getPackageManager().getLaunchIntentForPackage(context.getPackageName());
notificationIntent.setPackage(null); // The golden row !!!
notificationIntent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_RESET_TASK_IF_NEEDED);

I tried this, and it worked even though the IDE was complaining about the code

Intent notificationIntent = new Intent(THIS_CONTEXT, MainActivity.class);
    notificationIntent.setFlags(Intent.FLAG_ACTIVITY_REORDER_TO_FRONT);
    PendingIntent intent = PendingIntent.getActivity(THIS_CONTEXT, 0, notificationIntent, Intent.FLAG_ACTIVITY_REORDER_TO_FRONT);

    NotificationCompat.Builder mBuilder = new NotificationCompat.Builder(THIS_CONTEXT)
            .setSmallIcon(R.drawable.cast_ic_notification_0)
            .setContentTitle("Title")
            .setContentText("Content")
            .setContentIntent(intent)
            .setPriority(PRIORITY_HIGH) //private static final PRIORITY_HIGH = 5;
            .setAutoCancel(true)
            /*.setDefaults(Notification.DEFAULT_SOUND | Notification.DEFAULT_VIBRATE | Notification.DEFAULT_LIGHTS)*/;
    NotificationManager mNotificationManager = (NotificationManager) THIS_CONTEXT.getSystemService(Context.NOTIFICATION_SERVICE);
    mNotificationManager.notify(0, mBuilder.build());

Since you say you want to start your activity if it's not started already, maybe you wouldn't mind restarting it. I tested a ton of suggestions and flag combinations for the intent as well, this will always bring the activity you need to the front, though it won't keep any state previously associated with it.

intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TASK|Intent.FLAG_ACTIVITY_NEW_TASK);

API11+ only.