第一行代碼筆記-RecyclerView

RecyclerView是Android一個更強大的控件,其不僅可以實現和ListView同樣的效果,還有優化了ListView中的各種不足。其可以實現數據縱向滾動,也可以實現橫向滾動(ListView做不到橫向滾動)。接下來講解RecyclerView的用法。

RecyclerView 基本用法

因為RecyclerView屬於新增的控件,Android將RecyclerView定義在support庫裏。若要使用RecyclerView,第一步是要在build.gradle中添加對應的依賴庫。

添加RecyclerView 依賴庫

在app/build.gradle中的dependencies閉包添加以下內容：

implementation 'com.android.support:recyclerview-v7:27.1.1'

然後點擊頂部的Sync Now進行同步

修改 activity_main.xml

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

由於RecyclerView不是內置在系統SDK中,需要把其完整的包名路徑寫出來

新建 Fruit.java

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;
    }
}

新建 fruit_item.xml

創建ImageView來顯示水果圖片,TextView來顯示水果名字。

<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"

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

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/fruitname"
        android:layout_gravity="center_vertical"
        android:layout_marginLeft="10dp"/>

</LinearLayout>

新增適配器 FruitAdapter

為RecyclerView新增適配器FruitAdapter,並讓其繼承於RecyclerView.Adapter,把泛型指定為FruitAdapter.ViewHolder。

public class FruitAdapter extends RecyclerView.Adapter<FruitAdapter.ViewHolder> {

    private  List<Fruit> mFruitList;
    static class ViewHolder extends RecyclerView.ViewHolder{
        ImageView fruitImage;
        TextView fruitName;

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

    }

    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);
        ViewHolder holder = new ViewHolder(view);
        return holder;
    }

    @Override
    public void onBindViewHolder(ViewHolder holder, int position){

        Fruit fruit = mFruitList.get(position);
        holder.fruitImage.setImageResource(fruit.getImageId());
        holder.fruitName.setText(fruit.getName());
    }

    @Override
    public int getItemCount(){
        return mFruitList.size();
    }

• 定義內部類ViewHolder,並繼承RecyclerView.ViewHolder。傳入的View參數通常是RecyclerView子項的最外層佈局。

• FruitAdapter構造函數,用於把要展示的數據源傳入,並賦予值給全局變量mFruitList。

• FruitAdapter繼承RecyclerView.Adapter。因為必須重寫onCreateViewHolder(),onBindViewHolder()和getItemCount()三個方法

  • onCreateViewHolder()用於創建ViewHolder實例,並把加載的佈局傳入到構造函數去,再把ViewHolder實例返回。
  • onBindViewHolder()則是用於對子項的數據進行賦值,會在每個子項被滾動到屏幕內時執行。position得到當前項的Fruit實例。
  • getItemCount()返回RecyclerView的子項數目。

修改 MainActivity.java

public class MainActivity extends AppCompatActivity {

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

    @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);
        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);

        }
    }
}

LayoutManager用於指定RecyclerView的佈局方式。LinearLayoutManager指的是線性佈局。

運行效果：

修改RecyclerView 顯示效果

橫向滾動

修改 fruit_item.xml

<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="100dp"
    android:layout_height="wrap_content"
    android:orientation="vertical"

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

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/fruitname"
        android:layout_gravity="center_horizontal"
        android:layout_marginTop="10dp"/>
</LinearLayout>

把LinearLayout改成垂直排列,因為水果名字長度不一樣,把寬度改為100dp。
ImageView和TextView都改為水平居中

修改MainActivity.java

@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);
   }

通過調用setOrientation()把佈局的排列方向改為水平排列。

得益於RecyclerView的設計,我們可以通過LayoutManager實現各種不同的排列方式的佈局。

運行結果:

除了LinearLayoutManager,RecyclerView還提供了GridLayoutManager(網格佈局)和StaggeredGridLayoutManager(瀑布流佈局)

GridLayoutManager

GridLayoutManager(網格佈局)

修改MainActivity.java

修改 MainActivity.java,把

LinearLayoutManager layoutManager = new LinearLayoutManager(this);
layoutManager.setOrientation(LinearLayoutManager.HORIZONTAL);

換成

GridLayoutManager layoutManager = new GridLayoutManager(this,5);

GridLayoutManager (Context context, int spanCount)

• Context: Current context, will be used to access resources.
• spanCount int: The number of columns in the grid(網格的列數)

運行結果:

StaggeredGridLayoutManager

StaggeredGridLayoutManager(瀑布流佈局)

修改fruit_item.xml

<?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:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/fruit_image"
        android:layout_gravity="center_horizontal"/>

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

</LinearLayout>

把LinearLayout的寬度設為match_parent是因為瀑布流的寬度是 根據佈局的列數來自動適配的,而不是固定值 。(GridLayoutManager也是 根據佈局的列數來自動適配的）

修改 MainActivity.java

public class MainActivity extends AppCompatActivity {

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

    @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.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;  // 產生1-20的隨機數
        StringBuilder builder = new StringBuilder();
        for (int i =0;i<length;i++){
            builder.append(name);
        }
        return builder.toString();
    }
}

StaggeredGridLayoutManager layoutManager = new StaggeredGridLayoutManager(3,StaggeredGridLayoutManager.VERTICAL);
StaggeredGridLayoutManager傳入2個參數,第一個是佈局的列數,第二個是佈局的排列方向。

random.nextInt(20)+1 產生1-20的隨機數
運行效果:

GridLayoutManager和StaggeredGridLayout的區別

左圖是GridLayoutManager,右圖是StaggeredGridLayout。
當從顯示效果來看,已經一目瞭然。
GridLayoutManager是會固定高度的,所以會留下很多空白區域。
相反,StaggeredGridLayout並不會固定高度,以至於就算子項的高度不一致,下一行的會自動靠攏上一行。

RecyclerView 的點擊事件

修改 FruitAdapter.java

public class FruitAdapter extends RecyclerView.Adapter<FruitAdapter.ViewHolder> {

    private  List<Fruit> mFruitList;
    static class ViewHolder extends RecyclerView.ViewHolder{
        View fruitView;
        ImageView fruitImage;
        TextView fruitName;

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

    }

    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 view) {
                int position = holder.getAdapterPosition();
                Fruit fruit = mFruitList.get(position);
                Toast.makeText(view.getContext(), "you clicked view" + fruit.getName(), Toast.LENGTH_SHORT).show();
            }
        });

        holder.fruitImage.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                int position = holder.getAdapterPosition();
                Fruit fruit = mFruitList.get(position);
                Toast.makeText(view.getContext(), "you clicked image" + fruit.getName(), Toast.LENGTH_SHORT).show();
            }
        });
        return holder;
    }

  ...
}

修改ViewHolder,添加fruitView變量來保存子項最外層佈局的實例。

運行效果：