Android up button isn't working

Question

I have a simple application. It consists of Activity A and Activity B.

When I start at Activity A (from the launcher icon) and then navigate to Activity B, and then hit the up button. I arrive at Activity A. Perfect.

Yet when I start activity B from the command line (simulating another app or a notification starting this activity) and I hit the up button, I just get taken to the home screen.

Here is my setup:

Activity A

public class ActivityA extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Intent intent = new Intent(ActivityA.this, ActivityB.class);
                startActivity(intent);
            }
        });

    }


}

Activity B

public class ActivityB extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_b);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        getSupportActionBar().setDisplayHomeAsUpEnabled(true);
    }
}

My manifest. Note that the parent activity is defined and my app min api is 16 (so the parent activity declaration is valid for 16 and up).

   <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">
        <activity
            android:name=".ActivityA"
            android:label="@string/app_name"
            android:theme="@style/AppTheme.NoActionBar">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        <activity
            android:name=".ActivityB"
            android:exported="true"
            android:label="@string/title_activity_b"
            android:parentActivityName=".ActivityA"
            android:theme="@style/AppTheme.NoActionBar" />
    </application>

To start ActivityB from cmd line I use: adb shell am start com.example.demo/com.example.demo.ActivityB

EDIT 1

According to docs here: https://developer.android.com/training/appbar/up-action.html

You do not need to catch the up action in the activity's onOptionsItemSelected() method. Instead, that method should call its superclass, as shown in Respond to Actions. The superclass method responds to the Up selection by navigating to the parent activity, as specified in the app manifest.

Edit 2:

Just for kicks, I tried the two code snippets here. One at a time to see if they would do the trick. But they don't. ActivityA doesn't open. ActivityB just closes.

https://developer.android.com/training/implementing-navigation/ancestral.html

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    switch (item.getItemId()) {
    // Respond to the action bar's Up/Home button
    case android.R.id.home:
        NavUtils.navigateUpFromSameTask(this);
        return true;
    }
    return super.onOptionsItemSelected(item);
}

and

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    switch (item.getItemId()) {
    // Respond to the action bar's Up/Home button
    case android.R.id.home:
        Intent upIntent = NavUtils.getParentActivityIntent(this);
        if (NavUtils.shouldUpRecreateTask(this, upIntent)) {
            // This activity is NOT part of this app's task, so create a new task
            // when navigating up, with a synthesized back stack.
            TaskStackBuilder.create(this)
                    // Add all of this activity's parents to the back stack
                    .addNextIntentWithParentStack(upIntent)
                    // Navigate up to the closest parent
                    .startActivities();
        } else {
            // This activity is part of this app's task, so simply
            // navigate up to the logical parent activity.
            NavUtils.navigateUpTo(this, upIntent);
        }
        return true;
    }
    return super.onOptionsItemSelected(item);
}

What's happening vs expected? When I open ActivityB via the cmd line and hit up, I expect to go to ActivityA while ActivityB closes. What actually happens is that ActivityB just closes.

Answer 1

That is the default bahavior, the up navigation button works with the Activity Stack, if the ActivityA isn't in the stack, the Android will return to the previous screen , in this case the Home.