ViewWrapper

This

GetLabel

If null

FindViewById

Return

GetIcon

If null

FindViewById

Return

not only holds onto the child widgets, but also lazy‑finds the child widgets. If you create a wrapper and never need a specific child, you never go through the operation for it and never have to pay for those CPU cycles.

The holder pattern also allows us to do the following:

• Consolidate all our per‑widget type casting in one place, rather than having to cast it everywhere we call

• Perhaps track other information about the row, such as state information we are not yet ready to “flush” to the underlying model

Using is a matter of creating an instance whenever we inflate a row and attaching said instance to the row via , as shown in this rewrite of :

Public class extends

Public onCreate

Super onCreate

SetContentView

SetListAdapter new IconicAdapter this

FindViewById

Private getModel

Return getListAdapter getItem

Public onListItemClick

SetText getModel

Class extends

IconicAdapter

Super

This

Public getView

Null

If null

GetLayoutInflater

Inflate null

New ViewWrapper

SetTag

Else

GetTag

GetLabel setText getModel

If getModel length

GetIcon setImageResource

Else

GetIcon setImageResource

Return

Just as we check to see if it is null in order to create the row as needed, we also pull out (or create) the corresponding row’s . Then accessing the child widgets is merely a matter of calling their associated methods on the wrapper.

Making a List…

Lists with pretty icons next to them are all fine and well. But can we create widgets whose rows contain interactive child widgets instead of just passive widgets like and ? For example, could we combine the with text in order to allow people to scroll a list of, say, songs and rate them right inside the list?

There is good news and bad news.

The good news is that interactive widgets in rows work just fine. The bad news is that it is a little tricky, specifically when it comes to taking action when the interactive widget’s state changes (e.g., a value is typed into a field). We need to store that state somewhere, since our widget will be recycled when the is scrolled. We need to be able to set the state based upon the actual word we are viewing as the is recycled, and we need to save the state when it changes so it can be restored when this particular row is scrolled back into view.

What makes this interesting is that, by default, the has absolutely no idea what model in the it is looking at. After all, the is just a widget, used in a row of a . We need to teach the rows which model they are currently displaying, so when their checkbox is checked they know which model’s state to modify.

So, let’s see how this is done, using the activity in the sample project at http://apress.com/. We’ll use the same basic classes as our previous demo–we’re showing a list of nonsense words, which you can then rate. In addition, words given a top rating will appear in all caps.