ListView如果不使用一些技巧来提升它的运行效率，那么ListView的性能就会非常差。ListView的扩展性也不够好，它只能实现数据纵向滚动的效果，如果想要实现横向滚动的话，ListView是做不到的。
为此，Android提供了一个更加强大的滚动控件--RecyclerView。RecyclerView不仅可以轻松实现和ListView同样的效果，还优化了ListView中存在的各种不足之处。
一、RecyclerView的基本用法
首先在项目的build.gradle中添加相应的依赖库，打开app/build.gradle文件，在dependencies闭包中添加如下内容：
dependencies {
compile fileTree(dir: 'libs', include: ['*.jar'])
androidTestCompile('com.android.support.test.espresso:espresso-core:2.2.2', {
exclude group: 'com.android.support', module: 'support-annotations'
})
compile 'com.android.support:appcompat-v7:26.+'
compile 'com.android.support:recyclerview-v7:26.+'
testCompile 'junit:junit:4.12'
}添加完之后点击Sync Now来进行同步，接着修改activity_main.xml中的代码，如下所示：
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="match_parent"
android:layout_height="match_parent">
<android.support.v7.widget.RecyclerView
android:id="@+id/recycler_view"
android:layout_width="match_parent"
android:layout_height="match_parent"/>
</LinearLayout> 新建Fruit类，代码如下：
public class Fruit
{
private String name;
private int imageId;
public Fruit(String name, int imageId)
{
this.name = name;
this.imageId = imageId;
}
public String getName()
{
return name;
}
public int getImageId()
{
return imageId;
}
}fruit_item.xml文件内容如下：
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_margin="5dp"
android:orientation="horizontal">
<ImageView
android:id="@+id/fruit_image"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginLeft="10dp"
android:src="@drawable/apple_pic"/>
<TextView
android:id="@+id/fruit_name"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="center_vertical"
android:layout_marginLeft="10dp"
android:text="苹果"
android:textColor="#FF0000"
android:textSize="20sp"/>
</LinearLayout>public class FruitAdapter extends RecyclerView.Adapter<FruitAdapter.ViewHolder>
{
private List<Fruit> fruitList;
public FruitAdapter(List<Fruit> fruitList)
{
this.fruitList = fruitList;
}
@Override
public ViewHolder onCreateViewHolder(ViewGroup parent, int viewType)
{
View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.fruit_item, parent, false);
ViewHolder viewHolder = new ViewHolder(view);
return viewHolder;
}
@Override
public void onBindViewHolder(ViewHolder holder, int position)
{
Fruit fruit = fruitList.get(position);
holder.fruitImage.setImageResource(fruit.getImageId());
holder.fruitName.setText(fruit.getName());
}
@Override
public int getItemCount()
{
return fruitList == null ? 0 : fruitList.size();
}
static class ViewHolder extends RecyclerView.ViewHolder
{
ImageView fruitImage;
TextView fruitName;
public ViewHolder(View itemView)
{
super(itemView);
fruitImage = itemView.findViewById(R.id.fruit_image);
fruitName = itemView.findViewById(R.id.fruit_name);
}
}
}修改MainActivity中的代码，如下所示：
public class MainActivity extends AppCompatActivity
{
private List fruitList = new ArrayList<>();
@Override
protected void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
initFruits(); // 初始化水果数据源
RecyclerView recyclerView = (RecyclerView) findViewById(R.id.recycler_view);
LinearLayoutManager layoutManager = new LinearLayoutManager(this);
recyclerView.setLayoutManager(layoutManager);
FruitAdapter adapter = new FruitAdapter(fruitList);
recyclerView.setAdapter(adapter);
}
private void initFruits()
{
for (int i = 0; i < 2; i++)
{
Fruit apple = new Fruit("Apple", R.drawable.apple_pic);
fruitList.add(apple);
Fruit banana = new Fruit("Banana", R.drawable.banana_pic);
fruitList.add(banana);
Fruit orange = new Fruit("Orange", R.drawable.orange_pic);
fruitList.add(orange);
Fruit watermelon = new Fruit("Watermelon", R.drawable.watermelon_pic);
fruitList.add(watermelon);
Fruit pear = new Fruit("Pear", R.drawable.pear_pic);
fruitList.add(pear);
Fruit grape = new Fruit("Grape", R.drawable.grape_pic);
fruitList.add(grape);
Fruit pineapple = new Fruit("Pineapple", R.drawable.pineapple_pic);
fruitList.add(pineapple);
Fruit strawberry = new Fruit("Strawberry", R.drawable.strawberry_pic);
fruitList.add(strawberry);
Fruit cherry = new Fruit("Cherry", R.drawable.cherry_pic);
fruitList.add(cherry);
Fruit mango = new Fruit("Mango", R.drawable.mango_pic);
fruitList.add(mango);
}
}
}可以看到，这里使用了initFruits()方法初始化所有的水果数据。接着获取RecyclerView实例，为其设置布局管理器LinearLayoutManager对象，LinearLayoutManager是线性布局的意思，可以实现和ListView同样的效果。接下来创建FruitAdapter实例，将水果数据传入到FruitAdapter的构造函数中，最后调用RecyclerView的setAdapter()方法来完成适配器设置，这样RecyclerView和数据之间的关联就建立完成了。
运行程序，可以看到以下结果：
二、实现横向滚动和瀑布流布局
修改fruit_item.xml，如下所示：
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="100dp"
android:layout_height="wrap_content"
android:layout_margin="5dp"
android:orientation="vertical">
<ImageView
android:id="@+id/fruit_image"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="center_horizontal"
android:layout_marginLeft="10dp"
android:src="@drawable/apple_pic"/>
<TextView
android:id="@+id/fruit_name"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="center_horizontal"
android:layout_marginTop="10dp"
android:textColor="#FF0000"
android:textSize="20sp"/>
</LinearLayout>修改MainActivity的代码，如下所示：
@Override
protected void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
initFruits(); // 初始化水果数据源
RecyclerView recyclerView = (RecyclerView) findViewById(R.id.recycler_view);
LinearLayoutManager layoutManager = new LinearLayoutManager(this);
layoutManager.setOrientation(LinearLayoutManager.HORIZONTAL);
recyclerView.setLayoutManager(layoutManager);
FruitAdapter adapter = new FruitAdapter(fruitList);
recyclerView.setAdapter(adapter);
}在MainActivity中只增加了一行代码，调用LinearLayoutManager的setOrientation()方法来设置布局的排列方向，默认是纵向排列的，传入LinearLayoutManager.HORIZONTAL表示让布局横向排列，这样RecyclerView就可以横向滚动了。
重新运行程序，结果如下：
之所以ListView很难实现的效果但在RecyclerView上可以轻松实现，是因为ListView的布局排列是由自身去管理的，而RecyclerView则将这个工作交给了LayoutManager，LayoutManager中定制了一套可扩展的布局排列接口，子类只要按照接口的规范来实现，就能定制出各种不同排列方式的布局了。
除了LinearLayoutManager之外，RecyclerView还提供了GlidLayoutManager和StaggeredGridLayoutManager这两种内置的布局排列方式。GlidLayoutManager可以用于实现网格布局，StaggeredGridLayoutManager可以用于实现瀑布流布局。接下来实现一下炫酷的瀑布流布局。
首先修改fruit_item.xml，代码如下：
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:orientation="vertical"
android:layout_margin="5dp">
<ImageView
android:id="@+id/fruit_image"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="center_horizontal"
android:src="@drawable/apple_pic"/>
<TextView
android:id="@+id/fruit_name"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="left"
android:layout_marginTop="10dp"
android:textColor="#FF0000"
android:textSize="20sp"/>
</LinearLayout>接着修改MainActivity，代码如下：
public class MainActivity extends AppCompatActivity
{
private List fruitList = new ArrayList<>();
@Override
protected void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
initFruits(); // 初始化水果数据源
RecyclerView recyclerView = (RecyclerView) findViewById(R.id.recycler_view);
StaggeredGridLayoutManager layoutManager = new StaggeredGridLayoutManager(3, StaggeredGridLayoutManager.VERTICAL);
recyclerView.setLayoutManager(layoutManager);
FruitAdapter adapter = new FruitAdapter(fruitList);
recyclerView.setAdapter(adapter);
}
private void initFruits()
{
for (int i = 0; i < 2; i++)
{
Fruit apple = new Fruit(getRandomLengthName("Apple"), R.drawable.apple_pic);
fruitList.add(apple);
Fruit banana = new Fruit(getRandomLengthName("Banana"), R.drawable.banana_pic);
fruitList.add(banana);
Fruit orange = new Fruit(getRandomLengthName("Orange"), R.drawable.orange_pic);
fruitList.add(orange);
Fruit watermelon = new Fruit(getRandomLengthName("Watermelon"), R.drawable.watermelon_pic);
fruitList.add(watermelon);
Fruit pear = new Fruit(getRandomLengthName("Pear"), R.drawable.pear_pic);
fruitList.add(pear);
Fruit grape = new Fruit(getRandomLengthName("Grape"), R.drawable.grape_pic);
fruitList.add(grape);
Fruit pineapple = new Fruit(getRandomLengthName("Pineapple"), R.drawable.pineapple_pic);
fruitList.add(pineapple);
Fruit strawberry = new Fruit(getRandomLengthName("Strawberry"), R.drawable.strawberry_pic);
fruitList.add(strawberry);
Fruit cherry = new Fruit(getRandomLengthName("Cherry"), R.drawable.cherry_pic);
fruitList.add(cherry);
Fruit mango = new Fruit(getRandomLengthName("Mango"), R.drawable.mango_pic);
fruitList.add(mango);
}
}
private String getRandomLengthName(String name)
{
Random random = new Random();
int length = random.nextInt(20) + 1;
StringBuilder builder = new StringBuilder();
for (int i = 0; i < length; i++)
{
builder.append(name);
}
return builder.toString();
}
}在onCreate()方法中，创建了一个StaggeredGridLayoutManager的实例。StaggeredGridLayoutManager接收两个参数的构造函数，第一个是指定布局的列数，传入3代表把布局分为3列；第二个是指定布局的排列方向，传入StaggeredGridLayoutManager.VERTICAL表示会让布局纵向排列，最后再把创建好的实例设置到RecyclerView当中就可以了。
重新运行程序，结果如下：
