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Question

I've a question about ListView and listeners. Suppose i have a listview in my app. Each item of it contains a checkbox. And i've the following code:

public View getView(final int position, View convertView, ViewGroup parent) {

    CheckBox checkbox = (CheckBox)v.findViewById(R.id.checkbox);

    checkbox.setOnCheckedChangeListener(new OnCheckedChangeListener(){

        public void onCheckedChanged(CompoundButton buttonView, boolean isChecked) {
            // some code
        }               
    });

}

As you can see I set setOnCheckedChangeListener every time when getView method is called. So is it right way to set listener? Or should I set it once? Is it bad in terms of performance? Or it doesn't matter? And if I set the listener multiple times whether it will overwrite previous listener or will i have multiple listeners for this event?

Answer 1

Is it bad in terms of performance? Or it doesn't matter?

It does not matter. The setter just assign the object.

And if I set the listener multiple times whether it will overwrite previous listener or will i have multiple listeners for this event?

you will have just one for each instance of the checkbox upon you called setOnCheckedChangeListener . The ListView is particular case due of its recycling mechanism

Answer 2

Each listener that you set will overwrite the previous one (if any).
Setting a listener, or anything else, will consume a super small amount of time, however here you're also creating a new anonymous class which will take longer.

For the maximum performance, I would make a few adjustments:

1. Use convertView s to cache the views
2. Use ViewHolder pattern to avoid multiple findViewById calls
3. Use a single listener for all checkboxes and set it only on the cached views (if convertView == null )
4. Save the position of the item that the CheckBox is located in as the CheckBox's tag

Here's an example:

private static class ViewHolder {
    CheckBox checkBox;
}

private CompoundButton.OnCheckedChangeListener mCheckListener = new CompoundButton
        .OnCheckedChangeListener() {
    @Override
    public void onCheckedChanged(CompoundButton buttonView, boolean isChecked) {
        Log.e("TAG", "CheckBox position: " + buttonView.getTag());
    }
};

@Override
public View getView(int position, View convertView, ViewGroup parent) {
    ViewHolder holder;
    if (convertView == null) {
        /* Inflate the layout here */

        holder = new ViewHolder();
        holder.checkBox = (CheckBox) convertView.findViewById(R.id.checkbox);
        /* Find other views in you layout here */

        holder.checkBox.setOnCheckedChangeListener(mCheckListener);
        // Set the holder as tag, so you can fetch it later for re-use
        convertView.setTag(holder);
    } else {
        // Fetch the holder
        holder = (ViewHolder) convertView.getTag();
    }

    // Set the checkbox position
    holder.checkBox.setTag(position);

    return convertView;
}

If you're not familiar with using convertViews and the ViewHolder pattern you should watch The World of ListView that explains it all.