Android ListView Selection Problem

Question

Hi All,

I apologize for the following long question...

I have a LinearLayout which contains a ListView and some other items. As for the ListView, each on of its rows is a LinearLayout that contains 3 views - Checkbox, ImageView and TextView (from left to right - horizontal). Since I wanted the whole row to be selected when using the trackball (to be highlighted with a background color), I set the all three views inside the LinearLayout row as not focusable, and it worked.

Now I'm having 2 problems regarding this ListView. First, I want that whenever I touch a row in the ListView (with my finger), to get the same behavior as when using the trackball - means that I want the row to be selected (highlighted). What's happening right now is that when I touch the row it really becomes selected, but when I release my finger the selection is gone (much like happens in device's contact list).

Second - from a Menu, I can display a new LinearLayout instead the one that contains the ListView (different application's screen). When this happens, I still stores the object of the LinearLayout that contains the ListView, because I want to be able to re-display it later without creating it from scratch. The problem is that when I re-disaply the LinearLayout with the ListView, none of the ListView's rows are selected, even if a ceratin row was selected when the the LinearLayout with the ListView "left" the screen.

Sorry again for the long post.

Thanks!

Answer 1

Yeah, From an iOS developer's perspective, I find that it is extremely hard to apply features like "set default selection when starting" and "remember selection status after user clicked row" to ListView.

So let's start with "remember selection" first.The problem is that even if you know that you can use selector xml to define highlight/pressed/focus style.But that style will not be kept after user clicked that row. For instance, I have a highlighting selector xml (list_selector.xml under res/drawable folder) like this (but you may have other fields need to highlight like text color of textview in row):

<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android">
    <item android:drawable="@drawable/list_selector_pressed" android:state_pressed="true" />
    <item android:drawable="@drawable/list_selector_pressed" android:state_selected="true" />
</selector>    

and list_selector_pressed.xml which defined the highlighting style--set the background color to a gray color :

<?xml version="1.0" encoding="utf-8"?>
<layer-list xmlns:android="http://schemas.android.com/apk/res/android">
    <item>
        <shape xmlns:android="http://schemas.android.com/apk/res/android">
            <solid android:color="@color/dark_gray" />
        </shape>
    </item>
</layer-list>

So as @David Hedlund suggested:

Rather, assign an OnItemClickListener, and have it store away the id of the selected item into some variable.

you need to create a instance variable on top of your class:

    private View currentSelectedView;

then go to

@Override
public void onListItemClick(ListView l, View v, int position, long id) {
    if (currentSelectedView != null && currentSelectedView != v) {
        unhighlightCurrentRow(currentSelectedView);
    }

    currentSelectedView = v;
    highlightCurrentRow(currentSelectedView);
    //other codes 
    }

Pretty simple: we check if currentSelectedView is null or current clicked view or not. we first to unhighlight any style by calling method unhighlightCurrentRow(currentSelectedView)---you may wonder why we pass instant variable currentSelectedView as parameter, I will explain it later. Then we assign view to currentSelectedView and highlight current row; so that the style will persist after user's clicking is done.

private void unhighlightCurrentRow(View rowView) {
    rowView.setBackgroundColor(Color.TRANSPARENT);
    TextView textView = (TextView) rowView.findViewById(R.id.menuTitle);
    textView.setTextColor(getResources().getColor(R.color.white));
}

private void highlightCurrentRow(View rowView) {
    rowView.setBackgroundColor(getResources().getColor(
            R.color.dark_gray));
    TextView textView = (TextView) rowView.findViewById(R.id.menuTitle);
    textView.setTextColor(getResources().getColor(R.color.yellow));

} 

Aha, that's it. That is how we implement "remember selection" for list view. As you see, we have to duplicate the codes for styling both in xml and java code--pretty stupid :(

Next about "set default selection". You may think that you can do this

listView.setAdapter(adatper)
listView.setSelection(0);
currentSelectedView = listView.getChildAt(0);
highlightCurrentRow(currentSelectedView);

in onCreate() in activity or onActivityCreated() in fragment.
But if you run it , you will get NullPointer exception and why ? because at this time, the listview is not rendered yet and Android doesn't like iOS which have viewWillAppear. SO you have to create an instant variable to remember whether it is first time to render listview cell and in onListItemClick to unset that variable:

So under currentSelectedView declaration:

private Boolean firstTimeStartup = true;

then add methods : suppose we want to highlight the first row in list view:

public class HomeAdapter extends ArrayAdapter<String> {
    int layoutResourceId;

    public HomeAdapter(Context context, int textViewResourceId,
            ArrayList<String> objects) {
        super(context, textViewResourceId, objects);
        layoutResourceId = textViewResourceId;
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        if (convertView == null) {
            convertView = LayoutInflater.from(getContext()).inflate(
                    layoutResourceId, null);
        }

        if (firstTimeStartup && postion == 0) {
            highlightCurrentRow(convertView);
        } else {
            unhighlightCurrentRow(convertView);
        }

        TextView title = (TextView) convertView
                .findViewById(R.id.menuTitle);
        title.setText(getItem(position));
        return convertView;
    }
}

Pretty simple. But you need to make some changes in onListItemClick method:

@Override
public void onListItemClick(ListView l, View v, int position, long id) {

    if (firstTimeStartup) {// first time  highlight first row
        currentSelectedView = l.getChildAt(0);
    }
    firstTimeStartup = false; 
    if (currentSelectedView != null && currentSelectedView != v) {
        unhighlightCurrentRow(currentSelectedView);
    }

    currentSelectedView = v;
    highlightCurrentRow(currentSelectedView);

     //other codes
}

There you go! Enjoy Android :)

Answer 2

private void setListviewSelection(final ListView list, final int pos) {
list.post(new Runnable() 
   {
    @Override
    public void run() 
      {
        list.setSelection(pos);
        View v = list.getChildAt(pos);
        if (v != null) 
        {
            v.requestFocus();
        }
    }
});
}

Above helps me setting a row focussed in the list.