Question

I have an application in which I’d like one row at a time to have a certain color. This seems to work about 95% of the time, but sometimes instead of having just one row with this color, it will allow multiple rows to have the color. Specifically, a row is set to have the “special” color when it is tapped. In rare instances, the last row tapped will retain the color despite a call to setBackgroundColor attempting to make it otherwise.

private OnItemClickListener mDirectoryListener = new OnItemClickListener(){
    public void onItemClick(AdapterView parent, View view, int pos, long id){
        if (stdir.getStationCount() == pos) {
            stdir.moreStations();  
            return;
        }

        if (playingView != null)
            playingView.setBackgroundColor(Color.DKGRAY);

        view.setBackgroundColor(Color.MAGENTA);
        playingView = view;
        playStation(pos);
    }
};

I have confirmed with print statements that the code setting the row to gray is always called.

Can anyone imagine a reason why this code might intermittently fail? If there is a pattern or condition that causes it, I can’t tell.

I thought it might have something to do with the activity lifecycle setting the “playingView” variable back to null, but I can’t reliably reproduce the problem by switching activities or locking the phone.

private class DirectoryAdapter extends ArrayAdapter {
    private ArrayList<Station> items;

    public DirectoryAdapter(Context c, int resLayoutId, ArrayList<Station> stations){
        super(c, resLayoutId, stations);            
        this.items = stations;
    }

    public int getCount(){
        return items.size() + 1;
    }

    public View getView(int position, View convertView, ViewGroup parent){
        View v = convertView;
        LayoutInflater vi = (LayoutInflater)getContext().getSystemService(Context.LAYOUT_INFLATER_SERVICE);

        if (position == this.items.size()) {
            v = vi.inflate(R.layout.morerow, null);
            return v;
        }

        Station station = this.items.get(position);
        v = vi.inflate(R.layout.songrow, null);

        if (station.playing)
            v.setBackgroundColor(Color.MAGENTA);
        else if (station.visited)
            v.setBackgroundColor(Color.DKGRAY);
        else
            v.setBackgroundColor(Color.BLACK);

        TextView title  = (TextView)v.findViewById(R.id.title);
        title.setText(station.name);
        return v;
    }    
};

Answer 1

ListViews don’t create instances of contained views for every item in the list, but only for ones that are actually visible on the screen. For performance reasons, they try and maintain as few views as possible, and recycle them. That’s what the convertView parameter is.

When a view scrolls off the screen, it may be recycled or destroyed. You can’t hold a reference to an old view and assume that it will refer to the item you expect it to in the future. You should save the ID of the list item you need and look that up instead.

Moreover, there are a couple of other issues with your implementation (from a best practices standpoint). You seem to be ignoring the convertView parameter and creating a new view from scratch each time. That can cause your application to bog down a bit while scrolling if you have a long list. Secondly, instead of adding the “more” element the way you do, you’re better of setting it with setFooterView().

There’s an excellent talk on the ListView from Google I/O 2010 that covers these and other issues. It’s an hour long, but definitely worth watching in its entirety.