With the preview version of android L, Google has introduced two new widgets:
- Cardview
- RecyclerView
The RecyclerView comes into play when the purpose is to show many Views repeatedly, lists, grids, etc ..., when the views don't fit into the screen.
RecyclerView implements a system to accomplish this task easily and efficiently.
Unlike the ListView, GridView, etc ... the RecyclerView is dedicated solely to what its name suggests, recycle, reuse resources and avoid the repeated use of the costly findViewById(), do not worry about the visual appearance, that purpose if for the LayoutManager, One class, one task, that is the philosophy that follows the RecyclerView API, a bundle of inner classes, each with a responsibility:
- Adapter
- ViewHolder
- LayoutManager
- ItemDecoration
- ItemAnimator
- Adapter
RecyclerView.Adapter<VH>
This class is responsible for creating Views needed for each element in a row or cell, RecyclerView also is very close to ViewHolder, having to be indicated in the declaration of the class.
Many could think that this is not new, that Google and advise that pattern time ago , this time Google force us the implementation in the Adapter, step forward, no doubt.
The method OnCreateViewHolder(parentViewGroup ViewGroup, int i) initializes the ViewHolder on the adapter:
Override
public ViewHolder onCreateViewHolder (parentViewGroup ViewGroup, int i) {
View rowView = LayoutInflater.from (parentViewGroup.getContext ())
.inflate (R.layout.list_basic_item, parentViewGroup, false);
return new ViewHolder (rowView);
} The ```onBindViewHolder(ViewHolder viewholder, int position)```method is used to set the content of the Views
Override
public void onBindViewHolder (ViewHolder viewHolder, int position) {
SampleModel RowData = sampleData.get end (position);
viewHolder.textViewSample.setText (rowData.getSampleText ());
viewHolder.itemView.setTag (RowData);
} As I was saying, ViewHolder pattern is nothing new, Google actually recommend using it in a while, you can think the ViewHolder as a cache of views can reuse rather than create them again.
Override
public static class extends ViewHolder RecyclerView.ViewHolder {
private final TextView textViewSample;
public ViewHolder (View itemView) {
super (itemView);
textViewSample = (TextView) itemView.findViewById (
R.id.textViewSample);
}
} ##RecyclerView.LayoutManager
The LayoutManager is responsible to layout all views in the RecyclerView, specifying the LinearLayoutManager allows access among other items displayed on the screen as the first element, last, or, for example, the last fully visible, this horizontally or vertical, in the example used in a vertical arrangement.
LinearLayoutManager LinearLayoutManager mLayoutManager = new (this);
recyclerView.setLayoutManager (mLayoutManager); ##RecyclerView.ItemDecorator
Another important class is the ItemDecorator, this class can alter the views of the RecycleView, this also provides also provides an element called insets (margins) that can be applied to views without modifying the layout parameters.
In the example shown as ItemDecorators used to draw a small divider between the elements of RecyclerView:
saulmm.com.recyclerviewproject package;
android.content.Context import;
android.content.res.TypedArray import;
android.graphics.Canvas import;
android.graphics.drawable.Drawable import;
android.support.v7.widget.RecyclerView import;
android.view.View import;
public class extends SampleDivider RecyclerView.ItemDecoration {
private static final int [] = {ATTRS android.R.attr.listDivider};
private Drawable mDivider;
public SampleDivider (Context context) {
TypedArray a = context.obtainStyledAttributes (ATTRS);
a.getDrawable mDivider = (0);
a.recycle ();
}
Override
public void onDrawOver (Canvas c, RecyclerView parent) {
parent.getPaddingLeft int left = ();
int right = parent.getWidth () - parent.getPaddingRight ();
parent.getChildCount childCount = int ();
for (int i = 0; i <childCount; ++ i) {
Child = parent.getChildAt view (i);
RecyclerView.LayoutParams params = (RecyclerView.LayoutParams) child
.getLayoutParams ();
child.getBottom int top = () + params.bottomMargin;
int bottom = top + mDivider.getIntrinsicHeight ();
mDivider.setBounds (left, top, right, bottom);
mDivider.draw (c);
}
}
} The ItemAnimator class as the name suggests, animates the RecyclerView when added or remove and an element, RecyclerView implements a DefaultItemAnimator by default.
The RecyclerView must know when an item is inserted or deleted, with elements like ListViews, GridViews this was accomplished by calling notifyDataSetChanged () method, in terms of performance, it is quite costly as it redraws all items in the layout, the same with the RecyclerView is using notifyItemInserted() method to add and notifyItemRemoved () to remove, updating only the appropriate part.
##References: Wolfram RITTMEYER - A Glance at Android's first RecyclerView Wires are obsolete - Building a RecyclerView Android L preview Reference