Are convertViews being reused in a ListView?

Question

I am using this code to layout my ListView, using a different layout based on some data:

@Override
public View getView(int i, View convertView, ViewGroup viewGroup) {
    ViewHolder viewHolder;

    MyInfo myInfo = getItem(i);
    String label = myInfo.getLabel();

    if (convertView == null) {
        if (!"".equals(label)) {
            convertView = LayoutInflater.from(getContext()).inflate(R.layout.info_grey, null);
            Log.d(SapphireApplication.TAG, "GREY, label=" + label);
        } else {
            convertView = LayoutInflater.from(getContext()).inflate(R.layout.info_plain, null);
            Log.d(SapphireApplication.TAG, "PLAIN, label=" + label);
        }

        viewHolder = new ViewHolder();
        viewHolder.tvLabel = convertView.findViewById(R.id.tvLabel);

        convertView.setTag(viewHolder);
    } else {
        viewHolder = (ViewHolder)convertView.getTag();
    }

    viewHolder.tvLabel.setText(label);

    return convertView;
}

However, the Log.d is never done for some items in the list. Does that mean Android re-uses an existing convertView, causing it (in this case) to use the wrong layout?

Answer 1

Yes. They are being re-used. And that is the reason you are seeing that message log for few items only.

@Override
public View getView(int i, View convertView, ViewGroup viewGroup) {

    if (convertView == null) {
        // convertView is null. It means the ListView does not have a view to give to you
        // This way, you need to create a new one.
        // You will enter here until the ListView has enough Views to fill
        // the screen.
        // So, just inflate the view and set the View holder here.
        // Don't customize your view here (set text, contenet etc)
        // So, any log message here will be printed only when the ListView becomes visible (and when you scroll to next item)
        // After that, views will be re-used so convertView will no longer be null
    } else {
        // ListView gave a convertView to you. It means that you are receiving a View
        // that was created in the past and it is be re-used now.
        // At this moment, convertView still has the content of the old item it was 
        // representing.
        // This view was created in the statement above and after user scrolled the ListView
        // it becomes hidden and ready to be re-used.
        // Don't customize the view here.. just get the ViewHolder from the View
    }

    // Here you customize the View. Set content, text, color, background etc
    // The ViewHolder is just a helpful class to help you to access
    // all View inside the convertView without needing to perform the
    // findViewById again.

    return convertView;
}

In this basic example however, all convertViews are similar. They were inflated from the same layout. This works fine for when you have a single view type per line. all items are similar but with different content. This still works if you have small differences. For example, you can inflate same layout and control the visibility of some of its Views according to the position (position 1 has an image and position 2 don't).

However, there are some cases where you really need to inflate different layouts per row. For example, the "grey" layout is very different from the "plain" layout. On this case, you need to update you code as follows:

private static final int GREY = 1;
private static final int PLAIN = 2;
private static final int TOTAL_VIEW_TYPES = 2; // Grey and Plain

@Override
public int getViewTypeCount() {
    // Tell the list view that you have two types of Views (Grey and plain)
    return TOTAL_VIEW_TYPES;
}

@Override
public int getItemViewType(int position) {
    // You must inform view type for given position
    String label = myInfo.getLabel();
    if (!"".equals(label)) {
        return GREY; 
    } else {
        return PLAIN;
    }
}

    @Override
public View getView(int position, View convertView, ViewGroup viewGroup) {

    if (convertView == null) {
        // If view is null, you must create the view. But you need to create the
        // correct view for given position
        if(getItemViewType(position) == GREY) {
            // Inflate grey;
        } else { 
            // inflate plain
        }
    } else {
        // convertView is not null. It is being reused.
        // Android will give you the proper view here. If you are expecting
        // a plain type, that's what you will get. Android won't re-use
        // plain layout where you are expecting to have the grey layout.
        // It will re-use the proper view for each position (following to the getItemViewType()).
        // ListView is very robust.
    }

    // Update the view here.. Just remember that here you may have two different
    // types of view.. grey or plain.
    return convertView;
}

The most cool about theses concepts is that they are valid for any view that uses this View<->Adapter relation.. RecyclerView, ListView, Spinner, PagerView etc.