Creating Lists and Cards 创建列表和卡片

To create complex lists and cards with material design styles in your apps, you can use the RecyclerView and CardView widgets.

//想要创建复杂的带有material design风格的列表，使用RecyclerView 和 CardView widget

Create Lists

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The RecyclerView widget is a more advanced and flexible version of ListView. This widget is a container for displaying large data sets that can be scrolled very efficiently by maintaining a limited number of views. Use the RecyclerView widget when you have data collections whose elements change at runtime based on user action or network events.

The RecyclerView class simplifies the display and handling of large data sets by providing:

• Layout managers for positioning items
• Default animations for common item operations, such as removal or addition of items //对移除或者添加等常用条目的行为都有默认的动画

You also have the flexibility to define custom layout managers and animations for RecyclerView widgets.

Figure 1. The RecyclerView widget.

To use the RecyclerView widget, you have to specify an adapter and a layout manager. To create an adapter, extend the RecyclerView.Adapter class. The details of the implementation depend on the specifics of your dataset and the type of views. For more information, see the examples below.

Figure 2 - Lists with RecyclerView.

A layout manager positions item views inside a RecyclerView and determines when to reuse item views that are no longer visible to the user. To reuse (or recycle) a view, a layout manager may ask the adapter to replace the contents of the view with a different element from the dataset. Recycling views in this manner improves performance by avoiding the creation of unnecessary views or performing expensive findViewById() lookups.

//layout manager负责排布recyclerview中的视图条目，并且决定当一些条目不在可见时什么时候重用条目。为了重用view，layout manager可能会向adapter请求用不同的数据库的数据带替换view中的内容。

RecyclerView provides these built-in layout managers:

• LinearLayoutManager shows items in a vertical or horizontal scrolling list. //以水平或者垂直的滚动列表形式显示条目
• GridLayoutManager shows items in a grid. //以网格形式显示
• StaggeredGridLayoutManager shows items in a staggered grid.//以交错网格形式展示

To create a custom layout manager, extend the RecyclerView.LayoutManager class.

Animations

Animations for adding and removing items are enabled by default in RecyclerView. To customize these animations, extend the RecyclerView.ItemAnimator class and use the RecyclerView.setItemAnimator()method.

//为了定制动画，继承RecyclerView.ItemAnimator 类，并且使用RecyclerView.setItemAnimator()方法来设置

Examples

The following code example demonstrates how to add the RecyclerView to a layout:

<!-- 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"/>

Once you have added a RecyclerView widget to your layout, obtain a handle to the object, connect it to a layout manager, and attach an adapter for the data to be displayed:

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);

        // use this setting to improve performance if you know that changes
        // in content do not change the layout size of the RecyclerView
        //使用这个设置来提高性能，如果你已经知道了内容的修改不会改变recyclerview的布局大小
        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);
    }
    ...
}

The adapter provides access to the items in your data set, creates views for items, and replaces the content of some of the views with new data items when the original item is no longer visible. The following code example shows a simple implementation for a data set that consists of an array of strings displayed using TextView widgets:

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

    // Provide a reference to the views for each data item
    // Complex data items may need more than one view per item, and
    // you provide access to all the views for a data item in a view holder
    public static class ViewHolder extends RecyclerView.ViewHolder {
        // each data item is just a string in this case
        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, parent, false);
        // 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;
    }
}

Figure 3. Card examples.

Create Cards

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CardView extends the FrameLayout class and lets you show information inside cards that have a consistent look across the platform. CardView widgets can have shadows and rounded corners.

To create a card with a shadow, use the card_view:cardElevation attribute. CardView uses real elevation and dynamic shadows on Android 5.0 (API level 21) and above and falls back to a programmatic shadow implementation on earlier versions. For more information, see Maintaining Compatibility.

//为了创建带有阴影的卡片，使用card_view:cardElevation属性，cardview使用真是的正视图和动态的阴影效果在Android 5.0及以上的平台上，在较早版本上使用代码来实现。

Use these properties to customize the appearance of the CardView widget:

• To set the corner radius in your layouts, use the card_view:cardCornerRadius attribute. //为了设置布局的圆角，使用card_view:cardCornerRadius属性
• To set the corner radius in your code, use the CardView.setRadius method. //在代码中设置圆角，使用CardView.setRadius方法
• To set the background color of a card, use the card_view:cardBackgroundColor attribute. //给卡片设置背景的属性

The following code example shows you how to include a CardView widget in your layout:

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    xmlns:card_view="http://schemas.android.com/apk/res-auto"
    ... >
    <!-- 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>
</LinearLayout>

For more information, see the API reference for CardView.

Add Dependencies

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The RecyclerView and CardView widgets are part of the v7 Support Libraries. To use these widgets in your project, add these Gradle dependencies to your app's module:

dependencies {
    ...
    compile 'com.android.support:cardview-v7:21.0.+'
    compile 'com.android.support:recyclerview-v7:21.0.+'
}