【Android】recyclerView-1-简单使用

首先来看一个简单的用法。

1、在build.gradle（app）中的dependencies闭包中添加远程依赖

dependencies {
    compile fileTree(dir: 'libs', include: ['*.jar'])
    testCompile 'junit:junit:4.12'
    compile 'com.android.support:appcompat-v7:24.2.1'
    compile 'com.android.support:recyclerview-v7:24.2.1'
}

2、在大布局里添加RecyclerView控件

<?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="match_parent">
    <android.support.v7.widget.RecyclerView
        android:id="@+id/recycler_view"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />
</LinearLayout>

3、新建RecyclerView的子布局fruit_item.xml

<?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="wrap_content"
        android:layout_height="wrap_content"
        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>

4、新建一个ViewHolder类，继承RecyclerView.Holder

public 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.fruit_name);
    }
}

5、新建一个Fruit类

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;
    }
}

6、新建一个FruitAdapter类，继承自RecyclerView.Adapter类。

public class FruitAdapter extends RecyclerView.Adapter<ViewHolder>{
    private List<Fruit> mFruitList;
    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;
    }

    @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();
    }

}

7、在Activity里初始化配置

public class MainActivity extends AppCompatActivity {
    private List<Fruit> fruitList = new ArrayList<Fruit>();
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        RecyclerView recyclerView = (RecyclerView) findViewById(R.id.recycler_view);
        StaggeredGridLayoutManager layoutManager = new StaggeredGridLayoutManager(1, StaggeredGridLayoutManager.HORIZONTAL);
        recyclerView.setLayoutManager(layoutManager);
        initFruits();
        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);
        }
    }
}

解析：

我之前觉得Recyclerview很难用，因为我觉得它很繁琐，很乱，我也不明白它的机制，只是觉得很神奇，只要这么配置，它就自己能生成一个可滑动的列表组件。后来我终于明白它的原理（没看过源码，自以为的原理），才发现原来是这样。

它的核心是一个adapter，adapter的主要任务，遍历一个集合，遍历过程中将集合元素的属性和子项布局绑定。

我们拿上面的代码来分析：

public class FruitAdapter extends RecyclerView.Adapter<ViewHolder>{
    private List<Fruit> mFruitList;
    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);
        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();
    }

}

先看它的构造方法：获得一个集合对象，这个对象的元素里所包含的属性，应该就是我们想在列表项里，单个模块想展示的内容。

然后看onCreateViewHolder()方法：ViewHolder这个类，其实是一个引用暂存类，它的引用变量直接指向了子项布局的组件的对象。所以这个方法就是子项布局的引用都存到holder这个引用指向的对象里。

再看onBindViewHolder()方法：这个方法应该在遍历集合元素的时候调用，遍历时会将holder指向的布局组件进行资源设置，其实应该是根据position生成了一个子项布局的拷贝，遍历时都设置一番，我们最终应该只看到一个组件，并且属性为最后一次的设置，由此可知是在设置拷贝的对象的属性。

最后看getItemCount()方法：这个应该就是遍历的长度。

以上分析，纯属猜测和脑补，有什么不对的，我也没办法啊。