0

I have some problems on BackPressed when using the ActivityGroup under TabHost. To illustrate, first of all, codes are drilled down in layers as follows:

TabGroupActivity class

The following TabGroupActivity.java is referenced from an online tutorial.

public class TabGroupActivity extends ActivityGroup 
{    
    private ArrayList<String> mIdList;

    @Override
    public void onCreate(Bundle savedInstanceState) 
    {
        super.onCreate(savedInstanceState);       
        if (mIdList == null) mIdList = new ArrayList<String>();
    }

    /**
     * This is called when a child activity of this one calls its finish method. 
     * This implementation calls {@link LocalActivityManager#destroyActivity} on the child activity
     * and starts the previous activity.
     * If the last child activity just called finish(),this activity (the parent),
     * calls finish to finish the entire group.
     */
  @Override
  public void finishFromChild(Activity child) 
  {
      LocalActivityManager manager = getLocalActivityManager();
      int index = mIdList.size()-1;

      if (index < 1) 
      {
          finish();
          return;
      }

      manager.destroyActivity(mIdList.get(index), true);
      mIdList.remove(index); index--;
      String lastId = mIdList.get(index);
      Activity a = manager.getActivity(lastId); 
      Intent lastIntent = a.getIntent();      // LINE 49
      Window newWindow = manager.startActivity(lastId, lastIntent);
      setContentView(newWindow.getDecorView());
  }

  /**
   * Starts an Activity as a child Activity to this.
   * @param Id Unique identifier of the activity to be started.
   * @param intent The Intent describing the activity to be started.
   * @throws android.content.ActivityNotFoundException.
   */
  public void startChildActivity(String Id, Intent intent) 
  {     
      Window window = getLocalActivityManager().startActivity(Id,intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP));
      if (window != null) 
      {
          mIdList.add(Id);
          setContentView(window.getDecorView()); 
      }    
  }

  /**
   * The primary purpose is to prevent systems before android.os.Build.VERSION_CODES.ECLAIR
   * from calling their default KeyEvent.KEYCODE_BACK during onKeyDown.
   */
  @Override
  public boolean onKeyDown(int keyCode, KeyEvent event) 
  {
      if (keyCode == KeyEvent.KEYCODE_BACK) 
      {
          //preventing default implementation previous to android.os.Build.VERSION_CODES.ECLAIR
          return true;
      }
      return super.onKeyDown(keyCode, event);
  }

  /**
   * Overrides the default implementation for KeyEvent.KEYCODE_BACK 
   * so that all systems call onBackPressed().
   */
  @Override
  public boolean onKeyUp(int keyCode, KeyEvent event) 
  {
      if (keyCode == KeyEvent.KEYCODE_BACK) 
      {
          onBackPressed(); //LINE 88
          return true;
      }
      return super.onKeyUp(keyCode, event);
  }

  /**
   * If a Child Activity handles KeyEvent.KEYCODE_BACK.
   * Simply override and add this method.
   */
  @Override
  public void  onBackPressed  () 
  {
      int length = mIdList.size();
      if ( length > 1) 
      {
          Activity current = getLocalActivityManager().getActivity(mIdList.get(length-1));
          current.finish(); //LINE 103
      }  
  }
}

Tools_GroupActivity class

public class Tools_GroupActivity extends TabGroupActivity
{
    @Override
    public void onCreate(Bundle savedInstanceState) 
    {
        super.onCreate(savedInstanceState);
        startChildActivity("Tools_index", new Intent(this, Tools_index.class));
    }
}

Tools_index.class

It is a page with 9 buttons, for example when pressing button 1, it will invoke the Tools_1_diamond claass:

public void tools_button1_click (View view) 
{
    Intent edit = new Intent(getParent(), Tools_1_Diamond.class);
    TabGroupActivity parentActivity = (TabGroupActivity)getParent();
    parentActivity.startChildActivity("Tools_1_Diamond", edit);   
}

Inside Tools_1_diamond class

The layout has a button that can back to the Tools_index page, as follows:

public void button_tools_subpage_back_click (View view)   
{  
    Intent edit = new Intent(getParent(), Tools_index.class);
    TabGroupActivity parentActivity = (TabGroupActivity)getParent();
    parentActivity.startChildActivity("Tools_index", edit); 
}   

@Override  
public void onBackPressed()   
{  
    Intent edit = new Intent(getParent(), Tools_index.class);
    TabGroupActivity parentActivity = (TabGroupActivity)getParent();
    parentActivity.startChildActivity("Tools_index", edit); 
}

Problem reproduced:

Case A: 1. When pressing self-designed button in Tools_1_Diamond class, (instead of the backpressed) the app can return properly back to the Tools_index page.

  1. Then, inside the Tools_index page, if the user at this time press the BackPressed Button, the app will go back to Tools_1_Diamond page,

  2. and then further press the BackPress Button, error FATAL EXCEPTION: main java.lang.NullPointerException will occur.

Case B: When all the way just using BackPress without touching the self-designed back button, everything runs smoothly.

Question / Intention:

I think the problem should be the self-designed button does not reduce the ArrayList similar to BackPress method...but...

How could this be solved? I would like to offer user that when pressing either the self-designed back button or the BackPress method will return to the Tools_index page, and further pressing the BackPress in the Tools_index page will offer no reaction, or even better, to pop up an alertDialog asking user whether want to quit the app.

Logcat:

06-19 22:40:03.570: D/webviewglue(23395): nativeDestroy view: 0x51909960
06-19 22:40:06.585: D/AndroidRuntime(23395): Shutting down VM
06-19 22:40:06.585: W/dalvikvm(23395): threadid=1: thread exiting with uncaught exception (group=0x40fe72a0)
06-19 22:40:06.670: E/AndroidRuntime(23395): FATAL EXCEPTION: main
06-19 22:40:06.670: E/AndroidRuntime(23395): java.lang.NullPointerException
06-19 22:40:06.670: E/AndroidRuntime(23395):    at com.abc.abc_v1.TabGroupActivity.finishFromChild(TabGroupActivity.java:49)
06-19 22:40:06.670: E/AndroidRuntime(23395):    at android.app.Activity.finish(Activity.java:4215)
06-19 22:40:06.670: E/AndroidRuntime(23395):    at com.abc.abc_v1.TabGroupActivity.onBackPressed(TabGroupActivity.java:103)
06-19 22:40:06.670: E/AndroidRuntime(23395):    at com.abc.abc_v1.TabGroupActivity.onKeyUp(TabGroupActivity.java:88)
06-19 22:40:06.670: E/AndroidRuntime(23395):    at android.view.KeyEvent.dispatch(KeyEvent.java:2729)
06-19 22:40:06.670: E/AndroidRuntime(23395):    at android.app.Activity.dispatchKeyEvent(Activity.java:2431)
06-19 22:40:06.670: E/AndroidRuntime(23395):    at com.android.internal.policy.impl.PhoneWindow$DecorView.dispatchKeyEvent(PhoneWindow.java:2033)
06-19 22:40:06.670: E/AndroidRuntime(23395):    at android.view.ViewGroup.dispatchKeyEvent(ViewGroup.java:1363)
06-19 22:40:06.670: E/AndroidRuntime(23395):    at android.view.ViewGroup.dispatchKeyEvent(ViewGroup.java:1363)
06-19 22:40:06.670: E/AndroidRuntime(23395):    at android.view.ViewGroup.dispatchKeyEvent(ViewGroup.java:1363)
06-19 22:40:06.670: E/AndroidRuntime(23395):    at android.widget.TabHost.dispatchKeyEvent(TabHost.java:309)
06-19 22:40:06.670: E/AndroidRuntime(23395):    at android.view.ViewGroup.dispatchKeyEvent(ViewGroup.java:1363)
06-19 22:40:06.670: E/AndroidRuntime(23395):    at android.view.ViewGroup.dispatchKeyEvent(ViewGroup.java:1363)
06-19 22:40:06.670: E/AndroidRuntime(23395):    at android.view.ViewGroup.dispatchKeyEvent(ViewGroup.java:1363)
06-19 22:40:06.670: E/AndroidRuntime(23395):    at com.android.internal.policy.impl.PhoneWindow$DecorView.superDispatchKeyEvent(PhoneWindow.java:2106)
06-19 22:40:06.670: E/AndroidRuntime(23395):    at com.android.internal.policy.impl.PhoneWindow.superDispatchKeyEvent(PhoneWindow.java:1466)
06-19 22:40:06.670: E/AndroidRuntime(23395):    at android.app.Activity.dispatchKeyEvent(Activity.java:2426)
06-19 22:40:06.670: E/AndroidRuntime(23395):    at com.android.internal.policy.impl.PhoneWindow$DecorView.dispatchKeyEvent(PhoneWindow.java:2033)
06-19 22:40:06.670: E/AndroidRuntime(23395):    at android.view.ViewRootImpl.deliverKeyEventPostIme(ViewRootImpl.java:3852)
06-19 22:40:06.670: E/AndroidRuntime(23395):    at android.view.ViewRootImpl.handleImeFinishedEvent(ViewRootImpl.java:3800)
06-19 22:40:06.670: E/AndroidRuntime(23395):    at android.view.ViewRootImpl$ViewRootHandler.handleMessage(ViewRootImpl.java:2935)
06-19 22:40:06.670: E/AndroidRuntime(23395):    at android.os.Handler.dispatchMessage(Handler.java:99)
06-19 22:40:06.670: E/AndroidRuntime(23395):    at android.os.Looper.loop(Looper.java:137)
06-19 22:40:06.670: E/AndroidRuntime(23395):    at android.app.ActivityThread.main(ActivityThread.java:4921)
06-19 22:40:06.670: E/AndroidRuntime(23395):    at java.lang.reflect.Method.invokeNative(Native Method)
06-19 22:40:06.670: E/AndroidRuntime(23395):    at java.lang.reflect.Method.invoke(Method.java:511)
06-19 22:40:06.670: E/AndroidRuntime(23395):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1027)
06-19 22:40:06.670: E/AndroidRuntime(23395):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:794)
06-19 22:40:06.670: E/AndroidRuntime(23395):    at dalvik.system.NativeStart.main(Native Method)
pearmak
  • 4,979
  • 15
  • 64
  • 122

1 Answers1

0

First

The problem could be that in TabGroupActivity, after you have done:

mIdList.remove(index);

you don't check that index is still valid, ie bigger than or equal to one. The following code should probably be:

if (index > 0) {
     index--;
     String lastId = mIdList.get(index);
     Intent lastIntent = manager.getActivity(lastId).getIntent(); //LINE49
     Window newWindow = manager.startActivity(lastId, lastIntent);
     setContentView(newWindow.getDecorView());
}
else {
    // Handle case where there are no children left.
}

Second

It is worth noting that LocalActivityManager was deprecated in API level 13, and so, unless you are only targeting pre-13 devices, it shouldn't be used.

Even if you do decide to proceed with using it, there are some conditions under which it won't actually start a new Activity. See here.

Neil Townsend
  • 6,024
  • 5
  • 35
  • 52
  • thanks for prompt reply! I have tried using your suggestion shown above but fails, making the same error. Judging from the fact that if pressing solely the BackPressed button the whole code can be implemented without any error. Therefore I dont know whether it is because that pressing the own-designed back button is different from pressing the BackPress, and if yes, could there be any way to let the system know if user presses the own-designed back button, to execute the onKeyUp method that would be executed by the OnBackPressed? – pearmak Jun 19 '13 at 16:37
  • Have you tried splitting line 49 into two lines to see which one fails: `Activity a = manager.getActivity(lastId);` then `Intent lastIntent = a.getIntent();` – Neil Townsend Jun 19 '13 at 17:15
  • So, `a` is null, so `lastId` is not returning an actiivty. I'll have a think. – Neil Townsend Jun 19 '13 at 18:18