paging组件的用法和意义

[{"rating":["9.7","50"],"rank":1,"cover_url":"https://img2.doubanio.com\/view\/photo\/s_ratio_poster\/public\/p480747492.webp","is_playable":true,"id":"1292052","types":["犯罪","剧情"],"regions":["美国"],"title":"肖申克的救赎","url":"https:\/\/movie.douban.com\/subject\/1292052\/","release_date":"1994-09-10","actor_count":25,"vote_count":2912430,"score":"9.7","actors":["蒂姆·罗宾斯","摩根·弗里曼","鲍勃·冈顿","威廉姆·赛德勒","克兰西·布朗","吉尔·贝罗斯","马克·罗斯顿","詹姆斯·惠特摩","杰弗里·德曼","拉里·布兰登伯格","尼尔·吉恩托利","布赖恩·利比","大卫·普罗瓦尔","约瑟夫·劳格诺","祖德·塞克利拉","保罗·麦克兰尼","芮妮·布莱恩","阿方索·弗里曼","V·J·福斯特","弗兰克·梅德拉诺","马克·迈尔斯","尼尔·萨默斯","耐德·巴拉米","布赖恩·戴拉特","唐·麦克马纳斯"],"is_watched":false},{"rating":["9.6","50"],"rank":2,"cover_url":"https://img1.doubanio.com\/view\/photo\/s_ratio_poster\/public\/p2561716440.webp","is_playable":true,"id":"1291546","types":["剧情","爱情","同性"],"regions":["中国大陆","中国香港"],"title":"霸王别姬","url":"https:\/\/movie.douban.com\/subject\/1291546\/","release_date":"1993-07-26","actor_count":28,"vote_count":2150114,"score":"9.6","actors":["张国荣","张丰毅","巩俐","葛优","英达","蒋雯丽","吴大维","吕齐","雷汉","尹治","马明威","费振翔","智一桐","李春","赵海龙","李丹","童弟","沈慧芬","黄斐","徐杰","黄磊","冯远征","杨立新","方征","周璞","隋永清","宋小川","杜广沛"],"is_watched":false},{"rating":["9.6","50"],"rank":3,"cover_url":"https://img2.doubanio.com\/view\/photo\/s_ratio_poster\/public\/p2578474613.webp","is_playable":true,"id":"1292063","types":["剧情","喜剧","爱情","战争"],"regions":["意大利"],"title":"美丽人生","url":"https:\/\/movie.douban.com\/subject\/1292063\/","release_date":"2020-01-03","actor_count":29,"vote_count":1332240,"score":"9.6","actors":["罗伯托·贝尼尼","尼可莱塔·布拉斯基","乔治·坎塔里尼","朱斯蒂诺·杜拉诺","赛尔乔·比尼·布斯特里克","玛丽萨·帕雷德斯","霍斯特·布赫霍尔茨","利迪娅·阿方西","朱利亚娜·洛约迪切","亚美利哥·丰塔尼","彼得·德·席尔瓦","弗朗西斯·古佐","拉法埃拉·莱博罗尼","克劳迪奥·阿方西","吉尔·巴罗尼","马西莫·比安奇","恩尼奥·孔萨尔维","吉安卡尔洛·科森蒂诺","阿伦·克雷格","汉尼斯·赫尔曼","弗兰科·梅斯科利尼","安东尼奥·普雷斯特","吉娜·诺维勒","理查德·塞梅尔","安德烈提多娜","迪尔克·范登贝格","奥梅罗·安东努蒂","沈晓谦","张欣"],"is_watched":false},{"rating":["9.6","50"],"rank":4,"cover_url":"https://img2.doubanio.com\/view\/photo\/s_ratio_poster\/public\/p492406163.webp","is_playable":true,"id":"1295124","types":["剧情","历史","战争"],"regions":["美国"],"title":"辛德勒的名单","url":"https:\/\/movie.douban.com\/subject\/1295124\/","release_date":"1993-11-30","actor_count":49,"vote_count":1110744,"score":"9.6","actors":["连姆·尼森","本·金斯利","拉尔夫·费因斯","卡罗琳·古多尔","乔纳森·萨加尔","艾伯丝·戴维兹","马尔戈萨·格贝尔","马克·伊瓦涅","碧翠斯·马科拉","安德烈·瑟韦林","弗里德里希·冯·图恩","克齐斯茨托夫·拉夫特","诺伯特·魏塞尔","维斯瓦夫·科马萨","布拉德·雅各布维茨","Maciej Orlos","皮奥特·赛尔沃斯","Tadeusz Huk","马丁·塞梅洛格","托马斯·德德克","奥拉夫·卢巴申科","马瑞安·格林卡","约亨·尼克尔","阿格涅兹卡·克鲁科沃娜","阿格尼兹卡·旺格","托马斯·莫里斯","佐久间玲","吴俊全","约阿希姆·保罗·阿斯波克","彭河","戈兹·奥托","玛雅·奥丝塔泽斯卡","Maciej Kozlowski","艾尔文·莱德","Eugeniusz Priwieziencew","Marta Bizon","埃兹拉·达甘","吉恩·莱赫纳","Razia Israeli","拉米·希尔伯格","布兰科·拉斯蒂格","路德格·皮斯特","埃琳娜·勒文松","胡契克·卡勒塔","塔德乌什·布拉德茨基","亨里克·比斯塔","帕维·德朗柯","耶日·诺瓦克","安娜·穆查"],"is_watched":false},{"rating":["9.6","50"],"rank":5,"cover_url":"https://img1.doubanio.com\/view\/photo\/s_ratio_poster\/public\/p1505392928.webp","is_playable":true,"id":"1296141","types":["剧情","犯罪","悬疑"],"regions":["美国"],"title":"控方证人","url":"https:\/\/movie.douban.com\/subject\/1296141\/","release_date":"1957-12-17","actor_count":51,"vote_count":564999,"score":"9.6","actors":["泰隆·鲍华","玛琳·黛德丽","查尔斯·劳顿","埃尔莎·兰彻斯特","约翰·威廉姆斯","亨利·丹尼尔","伊安·沃尔夫","托林·撒切尔","诺玛·威登","尤娜·奥康纳","茹塔·李","贝丝·弗劳尔斯","比尔·厄尔文","J·帕特·奥马利","本·怀特","Paul Kruger","Jack Raine","Paul Power","乔治·佩林","威廉·H·奥布莱恩","奥托拉内史密斯","Frank McClure","Colin Kenny","Jeanne Lafayette","Wilbur Mack","Fred Rapport","利奥达·理查德斯","Glen Walters","Arthur Tovey","伯特史蒂文斯","Cap Somers","Lucille Sewall","斯考特西顿","Norbert Schiller","杰弗里·塞尔","John Roy","Al Roberts","Art Howard","Stuart Hall","Francis Compton","Philip Tonge","帕特里克·艾亨","富兰克林·法纳姆","玛乔丽·伊顿","史蒂夫·卡鲁瑟斯","George Calliga","乔治布鲁格曼","丹尼·鲍沙其","Brandon Beach","埃迪·贝克","沃尔特·培根"],"is_watched":false}]

public class RetrofitClient {
    private static final String BASE_URL="https://movie.douban.com";
    private static RetrofitClient retrofitClient;
    private final Retrofit retrofit;

    private RetrofitClient(){
        retrofit=new Retrofit.Builder()
                .baseUrl(BASE_URL)
                .addConverterFactory(GsonConverterFactory.create())  //添加转换器
                .client(getClient())  //添加OkHttp的拦截器
                .build();
    }
    public static synchronized RetrofitClient getInstance(){
        if(retrofitClient==null){
            retrofitClient=new RetrofitClient();
        }
        return retrofitClient;
    }
    public Api getApi(){
        return retrofit.create(Api.class);
    }
    private OkHttpClient getClient(){
        OkHttpClient client=new OkHttpClient.Builder()
                .addInterceptor(new Interceptor() {
                    @Override
                    public Response intercept(Chain chain) throws IOException { //拦截器用于添加请求参数，会在源url上进行修改
                        Request original = chain.request();
                        HttpUrl url = original.url();
                        HttpUrl currentUrl = url.newBuilder()
                                .addQueryParameter("interval_id", "100:90")
                                .addQueryParameter("type", "11")
                                .addQueryParameter("action", "")
                                .build();
                        Request newRequest = original.newBuilder()
                                .url(currentUrl)
                                .build();
                        return chain.proceed(newRequest);
                    }
                })
                .build();
        return client;
    }
}

public class Movie{

    private List<String> rating;
    private Integer    rank;
    private String cover_url;
    private String    is_playable;
    private String    id;
    private List<String> types;
    private List<String> regions;
    private String    title;
    private String    url;
    private String    release_date;
    private Integer    actor_count;
    private Integer    vote_count;
    private String    score;
    private List<String> actors;
    private String    is_watched;

    public List<String> getRating() {
        return this.rating;
    }

    public void setRating(List<String> rating) {
        this.rating = rating;
    }

    public Integer getRank() {
        return this.rank;
    }

    public void setRank(Integer rank) {
        this.rank = rank;
    }

    public String getCover_url() {
        return this.cover_url;
    }

    public void setCover_url(String cover_url) {
        this.cover_url = cover_url;
    }

    public String getIs_playable() {
        return this.is_playable;
    }

    public void setIs_playable(String is_playable) {
        this.is_playable = is_playable;
    }

    public String getId() {
        return this.id;
    }

    public void setId(String id) {
        this.id = id;
    }

    public List<String> getTypes() {
        return this.types;
    }

    public void setTypes(List<String> types) {
        this.types = types;
    }

    public List<String> getRegions() {
        return this.regions;
    }

    public void setRegions(List<String> regions) {
        this.regions = regions;
    }

    public String getTitle() {
        return this.title;
    }

    public void setTitle(String title) {
        this.title = title;
    }

    public String getUrl() {
        return this.url;
    }

    public void setUrl(String url) {
        this.url = url;
    }

    public String getRelease_date() {
        return this.release_date;
    }

    public void setRelease_date(String release_date) {
        this.release_date = release_date;
    }

    public Integer getActor_count() {
        return this.actor_count;
    }

    public void setActor_count(Integer actor_count) {
        this.actor_count = actor_count;
    }

    public Integer getVote_count() {
        return this.vote_count;
    }

    public void setVote_count(Integer vote_count) {
        this.vote_count = vote_count;
    }

    public String getScore() {
        return this.score;
    }

    public void setScore(String score) {
        this.score = score;
    }

    public List<String> getActors() {
        return this.actors;
    }

    public void setActors(List<String> actors) {
        this.actors = actors;
    }

    public String getIs_watched() {
        return this.is_watched;
    }

    public void setIs_watched(String is_watched) {
        this.is_watched = is_watched;
    }

    @Override
    public String toString() {
        return "Movie{" +
                "rating=" + rating +
                ", rank=" + rank +
                ", cover_url='" + cover_url + '\'' +
                ", is_playable='" + is_playable + '\'' +
                ", id='" + id + '\'' +
                ", types=" + types +
                ", regions=" + regions +
                ", title='" + title + '\'' +
                ", url='" + url + '\'' +
                ", release_date='" + release_date + '\'' +
                ", actor_count=" + actor_count +
                ", vote_count=" + vote_count +
                ", score='" + score + '\'' +
                ", actors=" + actors +
                ", is_watched='" + is_watched + '\'' +
                '}';
    }
}

public class MovieDataSource extends PositionalDataSource<Movie> {
    public static final int pageSize=5;
    @Override
    public void loadInitial(@NonNull LoadInitialParams loadInitialParams, @NonNull LoadInitialCallback<Movie> loadInitialCallback) {//负责第一页数据的加载
        int startPosition=0;
        RetrofitClient.getInstance()
                .getApi()
                .getMovies(startPosition,pageSize)
                .enqueue(new Callback<List<Movie>>() {
                    @Override
                    public void onResponse(Call<List<Movie>> call, Response<List<Movie>> response) {
                        if(response.body()!=null){
                            loadInitialCallback.onResult(response.body(),0,250);//将数据返回给PagedList，250是指数据源中的数据总数，当调用了setEnablePlaceHolder(true)方法时，必须传入此参数，以便预留位置
                        }
                    }

                    @Override
                    public void onFailure(Call<List<Movie>> call, Throwable t) {

                    }
                });
    }

    @Override
    public void loadRange(@NonNull LoadRangeParams loadRangeParams, @NonNull LoadRangeCallback<Movie> loadRangeCallback) {//负责第一页之后数据的加载
        RetrofitClient.getInstance()
                .getApi()
                .getMovies(loadRangeParams.startPosition,pageSize)  //在你滑动手机屏幕到底部请求下一页的数据时，loadRangeParams.startPosition会自动维护，不需要你手动修改
                .enqueue(new Callback<List<Movie>>() {
                    @Override
                    public void onResponse(Call<List<Movie>> call, Response<List<Movie>> response) {
                        if(response.body()!=null){
                            loadRangeCallback.onResult(response.body());
                        }
                    }

                    @Override
                    public void onFailure(Call<List<Movie>> call, Throwable t) {

                    }
                });
    }
}

public class MovieDataSourceFactory extends DataSource.Factory<Integer, Movie>{
    @NonNull
    @Override
    public DataSource<Integer, Movie> create() {
        return new MovieDataSource();
    }
}

public class MovieViewModel extends ViewModel {
    public LiveData<PagedList<Movie>> moviePagedList;
    public MovieViewModel(){
        PagedList.Config config=new PagedList.Config.Builder()
                .setEnablePlaceholders(true)   //设置是否为那些数量已知，但尚未加载出来的数据预留位置
                .setPageSize(5)
                .setPrefetchDistance(3)  //设置当距离底部还有多少数据时加载下一页数据
                .setInitialLoadSizeHint(20)  //设置首次加载数据的数量
                .setMaxSize(65536*5)　
                .build();
        moviePagedList=new LivePagedListBuilder<>(new MovieDataSourceFactory(),config).build(); //MovieDataSource中的onResult方法会把服务器返回的数据传递到PagedList当中
    }
}

public class MoviePagedListAdapter extends PagedListAdapter<Movie, MoviePagedListAdapter.ViewHolder> {
    private final Context context;
    public MoviePagedListAdapter(Context context){
        super(diffCallback);
        this.context=context;
    }
    private static final DiffUtil.ItemCallback<Movie> diffCallback=new DiffUtil.ItemCallback<Movie>() {
        @Override
        public boolean areItemsTheSame(@NonNull Movie oldItem, @NonNull Movie newItem) {//根据id来判断两条数据是否是同一条数据
            return oldItem.getId().equals(newItem.getId());
        }

        @SuppressLint("DiffUtilEquals")
        @Override
        public boolean areContentsTheSame(@NonNull Movie oldItem, @NonNull Movie newItem) {//比较两条数据的内容是否一样
            return oldItem.equals(newItem);
        }
    };
    @NonNull
    @Override
    public ViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
        LayoutInflater inflater=LayoutInflater.from(context);
        MovieItemBinding movieItemBinding= DataBindingUtil.inflate(inflater,R.layout.movie_item,parent,false);
        return new ViewHolder(movieItemBinding);
    }

    @Override
    public void onBindViewHolder(@NonNull ViewHolder holder, int position) {
        Movie movie = getItem(position);//从PagedList中获取数据，如果没有的话，PagedList会通知DataSource获取下一页的数据
        if(movie!=null){
            holder.movieItemBinding.setMovie(movie);
        }
    }

    public static class ViewHolder extends RecyclerView.ViewHolder{
        public MovieItemBinding movieItemBinding;
        public ViewHolder(MovieItemBinding movieItemBinding) {
            super(movieItemBinding.getRoot());
            this.movieItemBinding=movieItemBinding;
        }
    }
}

public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        ActivityMainBinding activityMainBinding= DataBindingUtil.setContentView(this,R.layout.activity_main);
        activityMainBinding.recyclerView.setLayoutManager(new LinearLayoutManager(this));
        MoviePagedListAdapter moviePagedListAdapter=new MoviePagedListAdapter(this);
        activityMainBinding.recyclerView.setAdapter(moviePagedListAdapter);
        MovieViewModel movieViewModel = new ViewModelProvider(this).get(MovieViewModel.class);
        movieViewModel.moviePagedList.observe(this, new Observer<PagedList<Movie>>() {
            @Override
            public void onChanged(PagedList<Movie> movies) {
                moviePagedListAdapter.submitList(movies);//当PagedList数据发生变化时，通知适配器更新数据，然后用getItem()方法获取数据
            }
        });
    }
}

<?xml version="1.0" encoding="utf-8"?>
<layout xmlns:android="http://schemas.android.com/apk/res/android">
    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical">
        <androidx.recyclerview.widget.RecyclerView
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:id="@+id/recyclerView"/>
    </LinearLayout>
</layout>

<layout xmlns:android="http://schemas.android.com/apk/res/android">
    <data>
        <variable
            name="movie"
            type="com.example.paging.model.Movie" />
    </data>
    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="vertical">
        <TextView
            android:id="@+id/movieName"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@{movie.title}"/>
    </LinearLayout>
</layout>