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:
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
}
}
}
public class Tools_GroupActivity extends TabGroupActivity
{
@Override
public void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
startChildActivity("Tools_index", new Intent(this, 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);
}
Tools_1_diamond classThe 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);
}
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.
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,
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.
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.
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)
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.