: Beginning Android

Using the Holder Pattern

Using the Holder Pattern

Another somewhat expensive operation we do a lot with fancy views is call findViewById(). This dives into our inflated row and pulls out widgets by their assigned identifiers so we can customize the widget contents (e.g., change the text of a TextView, change the icon in an ImageView). Since findViewById() can find widgets anywhere in the tree of children of the rows root View, this could take a fair number of instructions to execute, particularly if we keep having to re-find widgets we had found once before.

In some GUI toolkits, this problem is avoided by having the composite Views, like our rows, be declared totally in program code (in this case, Java). Then accessing individual widgets is merely a matter of calling a getter or accessing a field. And you can certainly do that with Android, but the code gets rather verbose. We need a way that lets us use the layout XML yet cache our rows key child widgets so we have to find them only once. Thats where the holder pattern comes into play, in a class well call ViewWrapper.

All View objects have getTag() and setTag() methods. These allow you to associate an arbitrary object with the widget. That holder pattern uses that tag to hold an object that, in turn, holds each of the child widgets of interest. By attaching that holder to the row View, every time we use the row, we already have access to the child widgets we care about, without having to call findViewById() again.

So, lets take a look at one of these holder classes (taken from the FancyLists/ViewWrapper sample projectat http://apress.com/):

class ViewWrapper {
View base;
TextView label =null;
ImageView icon =null;
ViewWrapper(View base) {
this.base = base;
}
TextViewgetLabel() {
if (label==null) {
label = (TextView)base.findViewById(R.id.label);
}
return(label);
}
ImageViewgetIcon() {
if (icon==null) {
icon = (ImageView)base.findViewById(R.id.icon);
}
return(icon);
}
}
ViewWrapper
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 findViewById() 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 findViewById()

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

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

public class ViewWrapperDemo extends ListActivity {
TextView selection;
String[] items={"lorem", "ipsum", "dolor", "sit", "amet",
"consectetuer", "adipiscing", "elit", "morbi", "vel",
"ligula", "vitae", "arcu", "aliquet", "mollis",
"etiam", "vel", "erat", "placerat", "ante",
"porttitor", "sodales", "pellentesque", "augue",
"purus"};
@Override
public void onCreate(Bundle icicle) {
super.onCreate(icicle);
setContentView(R.layout.main);
setListAdapter(new IconicAdapter(this));
selection = (TextView)findViewById(R.id.selection);
}
private String getModel(int position) {
return(((IconicAdapter)getListAdapter()).getItem(position));
}
public void onListItemClick(ListView parent, View v,
int position, long id) {
selection.setText(getModel(position));
}
class IconicAdapter extends ArrayAdapter<String> {
Activity context;
IconicAdapter(Activity context) {
super(context, R.layout.row, items);
this.context = context;
}
public View getView(int position, View convertView,
ViewGroup parent) {
View row = convertView;
ViewWrapper wrapper = null;
if (row==null) {
LayoutInflater inflater = context.getLayoutInflater();
row = inflater.inflate(R.layout.row, null);
wrapper = new ViewWrapper(row);
row.setTag(wrapper);
} else {
wrapper = (ViewWrapper)row.getTag();
}
wrapper.getLabel().setText(getModel(position));
if (getModel(position).length() > 4) {
wrapper.getIcon().setImageResource(R.drawable.delete);
} else {
wrapper.getIcon().setImageResource(R.drawable.ok);
}
return(row);
}
}
}

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


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