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

activity-stackandroidback-stackgoogle-play

I've come across a bug in my application when it is launched using the "Open" button on the Google Play Store app (previously called Android Market). It seems that launching it from the Play Store uses a different Intent than launching it from the phone's application menu of icons. This is leading to multiple copies of the same Activity being launched, which are conflicting with each other.

For example, if my app consists of the Activities A-B-C, then this issue can lead to a stack of A-B-C-A.

I tried using android:launchMode="singleTask" on all the Activities to fix this problem, but it has the unwanted side-effect of clearing the Activity stack to root, whenever I hit the HOME button.

The expected behavior is: A-B-C -> HOME -> And when the app is restored, I need: A-B-C -> HOME -> A-B-C

Is there a good way to prevent launching multiple Activities of the same type, without resetting to the root activity when using the HOME button?

Best Answer

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;       
    }
}

Related Solutions

Android – Removing an activity from the history stack

You can achieve this by setting the android:noHistory attribute to "true" in the relevant <activity> entries in your AndroidManifest.xml file. For example:

<activity
    android:name=".AnyActivity"
    android:noHistory="true" />

Java – Override back button to act like home button

Most of the time you need to create a Service to perform something in the background, and your visible Activity simply controls this Service. (I'm sure the Music player works in the same way, so the example in the docs seems a bit misleading.) If that's the case, then your Activity can finish as usual and the Service will still be running.

A simpler approach is to capture the Back button press and call moveTaskToBack(true) as follows:

// 2.0 and above
@Override
public void onBackPressed() {
    moveTaskToBack(true);
}

// Before 2.0
@Override
public boolean onKeyDown(int keyCode, KeyEvent event) {
    if (keyCode == KeyEvent.KEYCODE_BACK) {
        moveTaskToBack(true);
        return true;
    }
    return super.onKeyDown(keyCode, event);
}

I think the preferred option should be for an Activity to finish normally and be able to recreate itself e.g. reading the current state from a Service if needed. But moveTaskToBack can be used as a quick alternative on occasion.

NOTE: as pointed out by Dave below Android 2.0 introduced a new onBackPressed method, and these recommendations on how to handle the Back button.

Best Answer

Related Solutions

Android – Removing an activity from the history stack

Java – Override back button to act like home button

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