[移动开发]Android控件之RecyclerView

1.RecycleView的基本用法

RecyclerView是比ListView更强大的滚动控件，不仅可以实现ListView的效果，还优化了ListView中存在的不足。可以实现横屏滚动和竖屏滚动。
首先新建一个RecyclerViewTest项目，并让AndroidStudio自动创建好活动。由于RecyclerView属于新增的控件，Android团队为了让RecyclerView在所有的Android版本上都能使用，将其定义在support库当中。要使用RecyclerView控件就必须在项目的build.gradle中添加相应的依赖库。

1.1.添加依赖库

(1)在app/build.gradle文件中dependencies闭包中添加语句，compile 'com.android.support:recyclerview-v7:24.2.1’已经被弃用了，需要添加语句implementation ‘androidx.recyclerview:recyclerview:1.1.0’ 。添加完之后点击右上角Sync Now进行同步。
(2) 还有一种简便方法就是在->Project Structure->Dependencies->点击 ‘+’，选择1.Library Dependency。

搜索recyclerview，选择1.1.0，点击OK。

1.2.在布局中加入RecyclerView控件

定义id，宽度和高度。需要注意的是，需要写出完整的包路径，因为RecyclerView不是内置在系统SDK中的。

 <androidx.recyclerview.widget.RecyclerView
        android:id="@+id/recycler_view"
        android:layout_width="match_parent"
        android:layout_height="match_parent"/>

1.3.准备适配器

（1）新建Fruit类和fruit_item.xml文件。

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;
    }
}

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_margin="5dp">

    <ImageView
        android:id="@+id/fruit_image"
        android:layout_width="150dp"
        android:layout_height="150dp" />

    <TextView
        android:id="@+id/fruit_name"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center"
        android:layout_marginTop="10dp"/>

</LinearLayout>

（2）新建FruitAdapter类。继承RecyclerView.Adapter,泛型指定为FruitAdapter.ViewHelder，ViewHelder是在FruitAdapter中定义的内部类。

public class FruitAdapter extends RecyclerView.Adapter<FruitAdapter.ViewHolder> {
    private List<Fruit> mFruitList;
    static class ViewHolder extends RecyclerView.ViewHolder {
        ImageView fruitImage;
        TextView fruitName;
//view参数通常指RecyclerView子项的最外层布局
        public ViewHolder(View view) {
            super(view);
            fruitImage = (ImageView) view.findViewById(R.id.fruit_image);
            fruitName = (TextView) view.findViewById(R.id.fruit_name);
        }
    }
//构造函数，把要展示的数据源传递进来，并赋值给全局变量mFruitList
    public FruitAdapter(List<Fruit> fruitList) {
        mFruitList = fruitList;
    }
//重写 onCreateViewHolder()方法，创建ViewHolder实例
    @Override
    public ViewHolder onCreateViewHolder(ViewGroup parent,int viewType) {
        View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.fruit_item,parent,false);
        ViewHolder holder = new ViewHolder(view);
        return holder;
    }
//重写onBindViewHolder()方法
    @Override
    public void onBindViewHolder(ViewHolder holder,int position) {
        Fruit fruit = mFruitList.get(position);
        holder.fruitImage.setImageResource(fruit.getImageId());
        holder.fruitName.setText(fruit.getName());
    }
//重写getItemCount()方法，返回总共有多少个子项
    @Override
    public int getItemCount() {
        return mFruitList.size();
    }
}

1.4.使用RecyclerView

public class MainActivity extends AppCompatActivity {

    private List<Fruit> fruitList = new ArrayList<>();
    private Object GridLayoutManager;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        initFruits();//初始化水果数据
        RecyclerView recyclerView = (RecyclerView) findViewById(R.id.recycler_view);
//LayoutManager用于指定RecyclerView的布局方法，LinearLayoutManager为线性布局
        LinearLayoutManager layoutManager = new LinearLayoutManager(this);
        recyclerView.setLayoutManager(layoutManager);
        //创建FruitAdapter实例
        FruitAdapter adapter = new FruitAdapter(fruitList);
        //设置适配器
        recyclerView.setAdapter(adapter);
    }
    private void initFruits() {
            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.waterme_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);
    }
}

在res中新建文件夹drawable-xhdpi文件存放图片。

2.实现横向滚动，网格布局和瀑布流布局

2.1.实现横向滚动

对fruit_item.xml布局进行修改，将fruit_item里的元素改成垂直排列（默认是水平排列）。layout_marginTop让图片和文字保持距离

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="100dp"
    android:layout_height="wrap_content"
    android:layout_margin="5dp">

    <ImageView
        android:id="@+id/fruit_image"
        android:layout_width="150dp"
        android:layout_height="150dp"
        android:layout_gravity="center_horizontal"/>

    <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"/>

</LinearLayout>

在MainActivity中调用LinearLayoutManager的setOrientation()方法来设置布局的排列方向，默认是纵向排列，传入的LinearLayoutManager.HORIZONTAL参数表示让布局横向排列

@Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        initFruits();
        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);
    }

2.2.实现网格布局

在onCreate()修改LayoutManager的布局方式为GridLayoutManager，传入的第一个参数表示上下文，第二个参数用于指定布局的列数，传入3表示把布局分为3列。

  @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        initFruits();
        RecyclerView recyclerView = (RecyclerView) findViewById(R.id.recycler_view);
        GridLayoutManager layoutManager = new GridLayoutManager(MainActivity.this,3);
        recyclerView.setLayoutManager(layoutManager);
        FruitAdapter adapter = new FruitAdapter(fruitList);
        recyclerView.setAdapter(adapter);
    }

2.3.实现瀑布流布局

对fruit_item.xml布局进行修改，将LinearLayout的宽度改为match_parent，因为瀑布流的宽度是根据布局的列数自动适配的，不是一个固定值。使用layout_margin属性让子项之间互留一点间距。将TextView的对齐属性改成了左对齐。

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_margin="5dp">

    <ImageView
        android:id="@+id/fruit_image"
        android:layout_width="150dp"
        android:layout_height="150dp"
        android:layout_gravity="center_horizontal"/>

    <TextView
        android:id="@+id/fruit_name"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="left"
        android:layout_marginTop="10dp"/>

</LinearLayout>

在onCreate()中修改布局，StaggeredGridLayoutManager的构造函数接收两个参数，第一个参数用于指定布局的列数，第二个参数用于指定布局的排列方向，传入的StaggeredGridLayoutManager.VERTICAL表示让布局纵向排列。

public class MainActivity extends AppCompatActivity {

    private List<Fruit> fruitList = new ArrayList<>();
    private Object GridLayoutManager;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        initFruits();
        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.waterme_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);
        }
    }
//getRandomLengthName()方法使用random对象创造1到20之间的随机数，将传入的字符串参数（也就是name)重复随机遍，使得每个子项高度不一致。
    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();
    }
}

3.RecyclerView的点击事件

RecyclerView需要我们自己给子项具体的View去注册事件。这样的好处是当想要点击子项里面具体的某个按钮时就很方便。

public class FruitAdapter extends RecyclerView.Adapter<FruitAdapter.ViewHolder> {
    private List<Fruit> mFruitList;
    static class ViewHolder extends RecyclerView.ViewHolder {
        ImageView fruitImage;
        TextView fruitName;
        View fruitView;//保存子项最外层布局的实例

        public ViewHolder(View view) {
            super(view);
            fruitView = view;
            fruitImage = (ImageView) view.findViewById(R.id.fruit_image);
            fruitName = (TextView) view.findViewById(R.id.fruit_name);
        }
    }

    public FruitAdapter(List<Fruit> fruitList) {
        mFruitList = fruitList;
    }

    @Override
    public ViewHolder onCreateViewHolder(ViewGroup parent,int viewType) {
        View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.fruit_item,parent,false);
        final ViewHolder holder = new ViewHolder(view);
        holder.fruitView.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                int position = holder.getAdapterPosition();
                Fruit fruit = mFruitList.get(position);
                Toast.makeText(v.getContext(),"you clicked view" + fruit.getName(),
                        Toast.LENGTH_SHORT).show();
            }
        });
        holder.fruitImage.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                int position = holder.getAdapterPosition();
                Fruit fruit = mFruitList.get(position);
                Toast.makeText(v.getContext(),"you clicked image " +
                        fruit.getName(),Toast.LENGTH_SHORT).show();;
            }
        });
        return holder;
    }
    ...
 }