Material Design UI Widgets

 Android L 开发者预览支持库提供两个新的Widgets，RecyclerView和CardView。使用这两个Widgets可以显示复杂的Listview和卡片布局，这两个Widgets默认使用Material design。

RecyclerView

    RecyclerView是一个更高级柔性版本的Listview，RecyclerView是一个能包含很多视图的容器，它能完美的处理循环和滚动。在item动态变化的Listview使用RecyclerView。

RecyclerView使用很简单，因为它提供了：

1、定位item的布局管理器

2、常见的item操作默认动画

你能够灵活的为RecyclerView自定义布局管理器和动画。

使用RecyclerView，必须使用指定一个adapter、定义一个布局管理器。创建adapter必须继承自RecyclerView.Adapter。实施的细节需要看数据类型和需要的视图。

   RecyclerView widget

    RecyclerView 提供了 LayoutManager，RecylerView 不负责子 View 的布局，我们可以自定义 LayoutManager 来实现不同的布局效果，目前只提供了LinearLayoutManager。 LinearLayoutManager 可以指定方向，默认是垂直， 可以指定水平， 这样就轻松实现了水平的 ListView。

RecyclerView Demo：

1、布局文件

 <!-- A RecyclerView with some commonly used attributes -->
 <android.support.v7.widget.RecyclerView
     android:id="@+id/my_recycler_view"
     android:scrollbars="vertical"
     android:layout_width="match_parent"
     android:layout_height="match_parent"/>

2、Activity文件

 public class MyActivity extends Activity {
     private RecyclerView mRecyclerView;
     private RecyclerView.Adapter mAdapter;
     private RecyclerView.LayoutManager mLayoutManager;

     @Override
     protected void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
         setContentView(R.layout.my_activity);
         mRecyclerView = (RecyclerView) findViewById(R.id.my_recycler_view);

         // improve performance if you know that changes in content
         // do not change the size of the RecyclerView
         mRecyclerView.setHasFixedSize(true);

         // use a linear layout manager
         mLayoutManager = new LinearLayoutManager(this);
         mRecyclerView.setLayoutManager(mLayoutManager);

         // specify an adapter (see also next example)
         mAdapter = new MyAdapter(myDataset);
         mRecyclerView.setAdapter(mAdapter);
     }
     ...
 }
 To create a simple adapter:

 public class MyAdapter extends RecyclerView.Adapter<MyAdapter.ViewHolder> {
     private String[] mDataset;

     // Provide a reference to the type of views that you are using
     // (custom viewholder)
     public static class ViewHolder extends RecyclerView.ViewHolder {
         public TextView mTextView;
         public ViewHolder(TextView v) {
             super(v);
             mTextView = v;
         }
     }

     // Provide a suitable constructor (depends on the kind of dataset)
     public MyAdapter(String[] myDataset) {
         mDataset = myDataset;
     }

     // Create new views (invoked by the layout manager)
     @Override
     public MyAdapter.ViewHolder onCreateViewHolder(ViewGroup parent,
                                                    int viewType) {
         // create a new view
         View v = LayoutInflater.from(parent.getContext())
                                .inflate(R.layout.my_text_view, null);
         // set the view's size, margins, paddings and layout parameters
         ...
         ViewHolder vh = new ViewHolder(v);
         return vh;
     }

     // Replace the contents of a view (invoked by the layout manager)
     @Override
     public void onBindViewHolder(ViewHolder holder, int position) {
         // - get element from your dataset at this position
         // - replace the contents of the view with that element
         holder.mTextView.setText(mDataset[position]);

     }

     // Return the size of your dataset (invoked by the layout manager)
     @Override
     public int getItemCount() {
         return mDataset.length;
     }
 }

3、Recycler adapter

 public class MyAdapter extends RecyclerView.Adapter<MyAdapter.ViewHolder> {
     private String[] mDataset;

     // Provide a reference to the type of views that you are using
     // (custom viewholder)
     public static class ViewHolder extends RecyclerView.ViewHolder {
         public TextView mTextView;
         public ViewHolder(TextView v) {
             super(v);
             mTextView = v;
         }
     }

     // Provide a suitable constructor (depends on the kind of dataset)
     public MyAdapter(String[] myDataset) {
         mDataset = myDataset;
     }

     // Create new views (invoked by the layout manager)
     @Override
     public MyAdapter.ViewHolder onCreateViewHolder(ViewGroup parent,
                                                    int viewType) {
         // create a new view
         View v = LayoutInflater.from(parent.getContext())
                                .inflate(R.layout.my_text_view, null);
         // set the view's size, margins, paddings and layout parameters
         ...
         ViewHolder vh = new ViewHolder(v);
         return vh;
     }

     // Replace the contents of a view (invoked by the layout manager)
     @Override
     public void onBindViewHolder(ViewHolder holder, int position) {
         // - get element from your dataset at this position
         // - replace the contents of the view with that element
         holder.mTextView.setText(mDataset[position]);

     }

     // Return the size of your dataset (invoked by the layout manager)
     @Override
     public int getItemCount() {
         return mDataset.length;
     }
 }

RecyclerView 的标准化了 ViewHolder， 编写 Adapter 面向的是 ViewHoder 而不在是View 了， 复用的逻辑被封装了， 写起来更加简单。

CardView

CardView继承自FrameLayout类，可以在一个卡片布局中一致性的显示内容，卡片可以包含圆角和阴影。

可以使用android:elevation属性，创建一个阴影的卡片。

怎样指定CardView的属性：

1、使用android:cardCornerRadius属性指定圆角半径

2、使用CardView.setRadius 设置圆角半径。

3、使用 android:cardBackgroundColor属性设置卡片颜色

在创建布局文件中创建CardView：

 <!-- A CardView that contains a TextView -->
 <android.support.v7.widget.CardView
     xmlns:card_view="http://schemas.android.com/apk/res-auto"
     android:id="@+id/card_view"
     android:layout_gravity="center"
     android:layout_width="200dp"
     android:layout_height="200dp"
     card_view:cardCornerRadius="4dp">

     <TextView
         android:id="@+id/info_text"
         android:layout_width="match_parent"
         android:layout_height="match_parent" />
 </android.support.v7.widget.CardView>