Android RecyclerView Load More, Endless Scrolling

这篇文章参考:

https://www.journaldev.com/24041/android-recyclerview-load-more-endless-scrolling

这一篇文章讲的是RecyclerView的endless scrolling

我们都见过APP向上滑动然后又一个旋转小圈圈（ProgressBar）在那转呀转，转完之后就加载出更多的Item。是不是很神奇，那接下来我们讲解一下是如何实现的吧。

其实就是在RecyclerView的Adapter中的数据末尾加上null,通过null来作为提示显示圆圈和加载更多。

首先是旋转小圆圈，我们在Resource的layout文件夹中创建circle_wait.xml, root为linearLayout:

<?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:id="@+id/progress_linear_layout"
    android:orientation="vertical">

    <ProgressBar
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/upload_circle"
        android:layout_gravity="center_horizontal"/>

</LinearLayout>

RecyclerView其中包含的View的Layout我就掠过了

然后我们需要在RecyclerView中的Adapter中加上null的情况:

package com.journaldev.androidrecyclerviewloadmore;

import android.support.annotation.NonNull;
import android.support.v7.widget.RecyclerView;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ProgressBar;
import android.widget.TextView;

import java.util.List;

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

    //两种情况
    //第一种，正常的Item，即VIEW_TYPE_ITEM
    //第二种, null的Item,代表正在加载中即VIEW_TYPE_LOADING
    private final int VIEW_TYPE_ITEM = 0;
    private final int VIEW_TYPE_LOADING = 1;

    //这个是Adapter绑定的数据
    public List<String> mItemList;


    //构造函数传入数据
    public RecyclerViewAdapter(List<String> itemList) {

        mItemList = itemList;
    }

    
    //RecyclerView创建ViewHolder时调用的函数，用来返回Viewholder
    @NonNull
    @Override
    public RecyclerView.ViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
        if (viewType == VIEW_TYPE_ITEM) { //正常的Item
            View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.item_row, parent, false);
            return new ItemViewHolder(view); //返回正常的View holder
        } else { //否则返回ProgressBar的ViewHolder
            View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.item_loading, parent, false);
            return new LoadingViewHolder(view);
        }
    }

    //为ViewHolder中的View绑定数据
    @Override
    public void onBindViewHolder(@NonNull RecyclerView.ViewHolder viewHolder, int position) {

        if (viewHolder instanceof ItemViewHolder) { //如果是正常的Item，正常绑定数据
            populateItemRows((ItemViewHolder) viewHolder, position);
        } else if (viewHolder instanceof LoadingViewHolder) { //如果是ProgressBar的Loading界面，什么都不做
            showLoadingView((LoadingViewHolder) viewHolder, position);
        }

    }

    //RecyclerView用来控制显示View的多少
    @Override
    public int getItemCount() {
        return mItemList == null ? 0 : mItemList.size();
    }

    /**
     * The following method decides the type of ViewHolder to display in the RecyclerView
     * 下面就两种Item（正常和null）的ViewHolder定义
     * @param position
     * @return
     */
    //我们在onCreateViewHolder的时候判断的类型，其实是在这里给的
    @Override
    public int getItemViewType(int position) {
        return mItemList.get(position) == null ? VIEW_TYPE_LOADING : VIEW_TYPE_ITEM;
    }


    //普通Item的ViewHolder
    private class ItemViewHolder extends RecyclerView.ViewHolder {

        TextView tvItem;

        public ItemViewHolder(@NonNull View itemView) {
            super(itemView);

            tvItem = itemView.findViewById(R.id.tvItem);
        }
    }

    //null的item的ViewHolder
    private class LoadingViewHolder extends RecyclerView.ViewHolder {

        ProgressBar progressBar;

        public LoadingViewHolder(@NonNull View itemView) {
            super(itemView);
            progressBar = itemView.findViewById(R.id.progressBar);
        }
    }

    //如果是null的item的绑定，我们什么都不做，留空就好，因为我们只需要将ProgressBar显示在界面上就ok
    private void showLoadingView(LoadingViewHolder viewHolder, int position) {
        //ProgressBar would be displayed

    }

    //普通Item的绑定
    private void populateItemRows(ItemViewHolder viewHolder, int position) {

        String item = mItemList.get(position);
        viewHolder.tvItem.setText(item);

    }
    
    //获取绑定在Adapter中的数据，方便再Listener中使用
    public List<String> getListFromAdapter(int index) {
        return mItemList;
    }


}

接下来就是MainActivity里面，将Adapter绑定到RecyclerView上，并且为RecyclerView添加上Scroll Listner:

package com.journaldev.androidrecyclerviewloadmore;

import android.os.Handler;
import android.support.annotation.NonNull;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;

import java.util.ArrayList;

public class MainActivity extends AppCompatActivity {

    RecyclerView recyclerView;
    RecyclerViewAdapter recyclerViewAdapter;
    ArrayList<String> rowsArrayList = new ArrayList<>();

    boolean isLoading = false;

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

        //找到recyclerView
        recyclerView = findViewById(R.id.recyclerView);
        //填充数据
        populateData();
        //绑定Adapter
        initAdapter();
        //绑定Scroll Listener
        initScrollListener();


    }

    //填充数据
    private void populateData() {
        int i = 0;
        while (i < 10) {
            rowsArrayList.add("Item " + i);
            i++;
        }
    }

    //初始化Adapter，为其绑定上数据，再将其绑定到recyclerView上
    private void initAdapter() {

        recyclerViewAdapter = new RecyclerViewAdapter(rowsArrayList);
        recyclerView.setAdapter(recyclerViewAdapter);
    }

    //初始化Scroll Listener
    private void initScrollListener() {
        //为recyclerView加上Scroll Listener
        recyclerView.addOnScrollListener(new RecyclerView.OnScrollListener() {
            
            @Override
            public void onScrollStateChanged(@NonNull RecyclerView recyclerView, int newState) {
                super.onScrollStateChanged(recyclerView, newState);
            }

            //我们实现这个方法，因为我们要Listen合适滑倒最底下
            @Override
            public void onScrolled(@NonNull RecyclerView recyclerView, int dx, int dy) {
                super.onScrolled(recyclerView, dx, dy);
                
                //从recyclerView中获取LinearManager
                LinearLayoutManager linearLayoutManager = (LinearLayoutManager) recyclerView.getLayoutManager();

                //因为一直再监听，加上条件，只监听一次。只添加一个progress bar
                if (!isLoading) {
                    if (linearLayoutManager != null && linearLayoutManager.findLastCompletelyVisibleItemPosition() == rowsArrayList.size() - 1) { //当当前数据全部划完
                        //bottom of list!
                        //从Adapter中获取数据
                        rowsArrayList = recyclerView.getListFromAdapter();
                        //在数据最尾部加上null
                        rowsArrayList.add(null);
                        //提示recycler view插入了一个数据了，请更新数据吧
                        //更新完后屏幕上便显示了progress bar
                        recyclerView.getAdapter().notifyItemInserted(rowsArrayList.size() - 1);
                        isLoading = true; //设置为true后便不会一直add null,出现很多个progress bar
                        
                        //这里再扩充你的数据
                        
                        //扩充完毕后用recyclerView中的adapter notify dataset的变化
                        //例如: recyclerView.getAdapter().notifyDataSetChanged();
                        
                        //最后loading完毕后把isLoading变为false便又可以load更多data
                        isLoading = false;
                    }
                }
            }
        });


    }

}