Android L中的RecyclerView 、CardView 、Palette的使用

Android L版本中新增了RecyclerView、CardView 、Palette。RecyclerView、CardView为用于显示复杂视图的新增Widget。Palette作为调色板类，可以让你从图像中提取突出的颜色。

RecyclerView

RecyclerView作为替代ListView使用，RecyclerView标准化了ViewHolder，ListView中convertView是复用的，在RecyclerView中，是把ViewHolder作为缓存的单位了，然后convertView作为ViewHolder的成员变量保持在ViewHolder中，也就是说，假设没有屏幕显示10个条目，则会创建10个ViewHolder缓存起来，每次复用的是ViewHolder，所以他把getView这个方法变为了onCreateViewHolder。 ViewHolder更适合多种子布局的列表，尤其IM的对话列表。RecyclerView不提供setOnItemClickListener方法，你可以在ViewHolder中添加事件。

RecyclerView可以实现横向、纵向滑动视图：

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，可以设置成水平。

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;
    }
}

设置横向：

@Override
   protected void onCreate(Bundle savedInstanceState) {
       super.onCreate(savedInstanceState);
       setContentView(R.layout.activity_recycler_view_horizontal);
       // specify an adapter (see also next example)
       List<MyAdapter.Item> itemList = new ArrayList<MyAdapter.Item>();
       for (int i = 0; i < 100; i++)
           itemList.add(new MyAdapter.Item("Item " + i, "world"));
       mAdapter = new MyAdapter(itemList);
       mRecyclerViewHorizontal = (RecyclerView) findViewById(R.id.my_recycler_view_horizontal);
       mRecyclerViewHorizontal.setHasFixedSize(true);
       // use a linear layout manager
       LinearLayoutManager mLayoutManager = new LinearLayoutManager(this);
       mLayoutManager.setOrientation(LinearLayoutManager.HORIZONTAL);
       mRecyclerViewHorizontal.setLayoutManager(mLayoutManager);
       mRecyclerViewHorizontal.setAdapter(mAdapter);
   }

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>

Palette

Palette从图像中提取突出的颜色，这样可以把色值赋给ActionBar、或者其他，可以让界面整个色调统一，效果见上图（Palette）。

Palette这个类中提取以下突出的颜色：

Vibrant （有活力）

Vibrant dark（有活力暗色）

Vibrant light（有活力亮色）

Muted （柔和）

Muted dark（柔和暗色）

Muted light（柔和亮色）

提取色值代码如下：

Bitmap bm = BitmapFactory.decodeResource(getResources(), item.image);
    Palette palette = Palette.generate(bm);
    if (palette.getLightVibrantColor() != null) {
          name.setBackgroundColor(palette.getLightVibrantColor().getRgb());
          getSupportActionBar().setBackgroundDrawable(new ColorDrawable(palette.getLightVibrantColor().getRgb()));
    }
}