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copied!<p>You will have multiple <code>ViewHolder</code> objects in existence. </p> <p>A <code>ListView</code> by its nature doesn't create new <code>View</code> instances for each of its rows. This is so that if you have a <code>ListView</code> of a million things, you don't need to store layout information for a million things. So what do you need to store? Just the things that are on the screen. You can then reuse those views over and over again. This way, your <code>ListView</code> of a million objects can just have maybe 10 child views.</p> <p>In your custom array adapter, you will have a function called <code>getView()</code> that looks something like this:</p> <pre><code>public View getView(int position, View convertView, ViewGroup parent) { //Here, position is the index in the list, the convertView is the view to be //recycled (or created), and parent is the ListView itself. //Grab the convertView as our row of the ListView View row = convertView; //If the row is null, it means that we aren't recycling anything - so we have //to inflate the layout ourselves. if(row == null) { LayoutInflater inflater = (LayoutInflater)getContext().getSystemService(Context.LAYOUT_INFLATER_SERVICE); row = inflater.inflate(R.layout.list_item, parent, false); } //Now either row is the recycled view, or one that we've inflated. All that's left //to do is set the data of the row. In this case, assume that the row is just a //simple TextView TextView textView = (TextView) row.findViewById(R.id.listItemTextView); //Grab the item to be rendered. In this case, I'm just using a string, but //you will use your underlying object type. final String item = getItem(position); textView.setText(item); //and return the row return row; } </code></pre> <p>This will work, but take a moment and see if you can spot the inefficiency here. Think about which of the above code will be called redundantly.</p> <p>The problem is that we are calling <code>row.findViewById</code> over and over again, even though after the first time we look it up, it will never change. While if you only have a simple <code>TextView</code> in your list, it's probably not that bad, if you have a complex layout, or you have multiple views that you want to set data for, you could lose a bit of time finding your view over and over again.</p> <p>So how do we fix this? Well, it would make sense to store that TextView somewhere after we look it up. So we introduce a class called a <code>ViewHolder</code>, which "holds" the views. So inside of the adaptor, introduce an inner class like so:</p> <pre><code>private static class ViewHolder { TextView textView; } </code></pre> <p>This class is private, since it's just a caching mechanism for the adapter, and it is static so that we don't need a reference to the adapter to use it.</p> <p>This will store our view so that we don't have to call <code>row.findViewById</code> multiple times. Where should we set it? When we inflate the view for the first time. Where do we store it? Views have a custom "tag" field, which can be used to store meta-information about the view - exactly what we want! Then, if we've already seen this view, we just have to look up the tag instead of looking up each of the views within the row..</p> <p>So the if statement inside of <code>getView()</code> becomes:</p> <pre><code>//If the row is null, it means that we aren't recycling anything - so we have //to inflate the layout ourselves. ViewHolder holder = null; if(row == null) { LayoutInflater inflater = (LayoutInflater)getContext().getSystemService(Context.LAYOUT_INFLATER_SERVICE); row = inflater.inflate(R.layout.list_item, parent, false); //Now create the ViewHolder holder = new ViewHolder(); //and set its textView field to the proper value holder.textView = (TextView) row.findViewById(R.id.listItemTextView); //and store it as the 'tag' of our view row.setTag(holder); } else { //We've already seen this one before! holder = (ViewHolder) row.getTag(); } </code></pre> <p>Now, we just have to update the holder.textView's text value, since it's already a reference to the recycled view! So our final adapter's code becomes:</p> <pre><code>public View getView(int position, View convertView, ViewGroup parent) { //Here, position is the index in the list, the convertView is the view to be //recycled (or created), and parent is the ListView itself. //Grab the convertView as our row of the ListView View row = convertView; //If the row is null, it means that we aren't recycling anything - so we have //to inflate the layout ourselves. ViewHolder holder = null; if(row == null) { LayoutInflater inflater = (LayoutInflater)getContext().getSystemService(Context.LAYOUT_INFLATER_SERVICE); row = inflater.inflate(R.layout.list_item, parent, false); //Now create the ViewHolder holder = new ViewHolder(); //and set its textView field to the proper value holder.textView = (TextView) row.findViewById(R.id.listItemTextView); //and store it as the 'tag' of our view row.setTag(holder); } else { //We've already seen this one before! holder = (ViewHolder) row.getTag(); } //Grab the item to be rendered. In this case, I'm just using a string, but //you will use your underlying object type. final String item = getItem(position); //And update the ViewHolder for this View's text to the correct text. holder.textView.setText(item); //and return the row return row; } </code></pre> <p>And we're done!</p> <p>Some things to think about:</p> <ol> <li>How does this change if you have multiple views in a row that you want to change? As a challenge, make a ListView where each row has two <code>TextView</code> objects and an <code>ImageView</code></li> <li>When debugging your ListView, check a few things so that you can really see what's going on: <ol> <li>How many times ViewHolder's constructor is called.</li> <li>What the value of <code>holder.textView.getText()</code> is before you update it at the end of <code>getView()</code></li> </ol></li> </ol>

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