Android: bug in launchMode="singleTask"? -> activity stack not preserved

Question

My main activity A has as set android:launchMode="singleTask" in the manifest. Now, whenever I start another activity from there, e.g. B and press the HOME BUTTON on the phone to return to the home screen and then again go back to my app, either via pressing the app's button or pressing the HOME BUTTONlong to show my most recent apps it doesn't preserve my activity stack and returns straight to A instead of the expected activity B.

Here the two behaviors:

Expected: A > B > HOME > B
Actual: A > B > HOME > A (bad!)

Is there a setting I'm missing or is this a bug? If the latter, is there a workaround for this until the bug is fixed?

FYI: This question has already been discussed here. However, it doesn't seem that there is any real solution to this, yet.

Answer 1

This is not a bug. When an existing singleTask activity is launched, all other activities above it in the stack will be destroyed.

When you press HOME and launch the activity again, ActivityManger calls an intent

{act=android.intent.action.MAIN cat=[android.intent.category.LAUNCHER]flag=FLAG_ACTIVITY_NEW_TASK|FLAG_ACTIVITY_RESET_IF_NEEDED cmp=A}

So the result is A > B > HOME > A.

It's different when A's launchMode is "Standard". The task which contains A will come to the foreground and keep the state the same as before.

You can create a "Standard" activity eg. C as the launcher and startActivity(A) in the onCreate method of C

OR

Just remove the launchMode="singleTask" and set FLAG_ACTIVITY_CLEAR_TOP|FLAG_ACTIVITY_SINGLE_TOP flag whenever call an intent to A

Answer 2

From http://developer.android.com/guide/topics/manifest/activity-element.html on singleTask

The system creates the activity at the root of a new task and routes the intent to it. However, if an instance of the activity already exists, the system routes the intent to existing instance through a call to its onNewIntent() method, rather than creating a new one.

This means when the action.MAIN and category.LAUNCHER flags targets your application from the Launcher, the system would rather route the intent to the existing ActivityA as opposed to creating a new task and setting a new ActivityA as the root. It would rather tear down all activities above existing task ActivityA lives in, and invoke it's onNewIntent().

If you want to capture both the behavior of singleTop and singleTask, create a separate "delegate" activity named SingleTaskActivity with the singleTask launchMode which simply invokes the singleTop activity in its onCreate() and then finishes itself. The singleTop activity would still have the MAIN/LAUNCHER intent-filters to continue acting as the application's main Launcher activity, but when other activities desire calling this singleTop activity it must instead invoke the SingleTaskActivity as to preserve the singleTask behavior. The intent being passed to the singleTask activity should also be carried over to the singleTop Activity, so something like the following has worked for me since I wanted to have both singleTask and singleTop launch modes.

<activity android:name=".activities.SingleTaskActivity"
              android:launchMode="singleTask"
              android:noHistory="true"/>

public class SingleTaskActivity extends Activity{
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        Intent intent = getIntent();
        intent.setClass(this, SingleTop.class);
        startActivity(intent);
    }
}

And your singleTop activity would continue having its singleTop launch mode.

    <activity
        android:name=".activities.SingleTopActivity"
        android:launchMode="singleTop"
        android:noHistory="true"/>

Good luck.

Answer 3

Stefan, you ever find an answer to this? I put together a testcase for this and am seeing the same (perplexing) behavior...I'll paste the code below in case anyone comes along and sees something obvious:

AndroidManifest.xml:

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
          package="com.example" >

  <uses-sdk android:minSdkVersion="3"/>

  <application android:icon="@drawable/icon" android:label="testSingleTask">

    <activity android:name=".ActivityA"
              android:launchMode="singleTask">
      <intent-filter>
        <action android:name="android.intent.action.MAIN"/>
        <category android:name="android.intent.category.LAUNCHER"/>
      </intent-filter>
    </activity>

    <activity android:name=".ActivityB"/>

  </application>
</manifest>

ActivityA.java:

public class ActivityA extends Activity implements View.OnClickListener
{
  @Override
  public void onCreate( Bundle savedInstanceState )
  {
    super.onCreate( savedInstanceState );
    setContentView( R.layout.main );
    View button = findViewById( R.id.tacos );
    button.setOnClickListener( this );
  }

  public void onClick( View view )
  {
    //Intent i = new Intent( this, ActivityB.class );
    Intent i = new Intent();
    i.setComponent( new ComponentName( this, ActivityB.class ) );
    startActivity( i );
  }
}

ActivityB.java:

public class ActivityB extends Activity
{
  @Override
  public void onCreate( Bundle savedInstanceState )
  {
    super.onCreate( savedInstanceState );
    setContentView( R.layout.layout_b );
  }
}

I tried changing minSdkVersion to no avail. This just seems to be a bug, at least according to the documentation, which states the following:

As noted above, there's never more than one instance of a "singleTask" or "singleInstance" activity, so that instance is expected to handle all new intents. A "singleInstance" activity is always at the top of the stack (since it is the only activity in the task), so it is always in position to handle the intent. However, a "singleTask" activity may or may not have other activities above it in the stack. If it does, it is not in position to handle the intent, and the intent is dropped. (Even though the intent is dropped, its arrival would have caused the task to come to the foreground, where it would remain.)