Android Jetpack-Paging使用

参考：

https://developer.android.google.cn/topic/libraries/architecture/paging/

Concert

public class Concert {

    private String title;
    private String content;
    private String author;

    public String getTitle() {
        return title;
    }

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

    public String getContent() {
        return content;
    }

    public void setContent(String content) {
        this.content = content;
    }

    public String getAuthor() {
        return author;
    }

    public void setAuthor(String author) {
        this.author = author;
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj) {
            return true;
        }
        if (obj == null) {
            return false;
        }
        if (this.getClass() == obj.getClass()) {
            return true;
        }
        Concert concert = (Concert) obj;
        return Objects.equals(title, concert.title) && Objects.equals(content, concert.content) && Objects.equals(author, concert.author);
    }

    @Override
    public int hashCode() {
        return Objects.hash(title, content, author);
    }

}

PagedListAdapter

参考：
https://developer.android.google.cn/topic/libraries/architecture/paging/ui

这个Adapter就是一个RecyclerView的Adapter。

不过我们在使用paging实现RecyclerView的分页加载效果， 不能直接继承RecyclerView的Adapter，而是需要继承PagedListAdapter。

public class RecyclerAdapter extends PagedListAdapter<Concert, RecyclerAdapter.RecyclerViewHolder> {

    protected RecyclerAdapter() {
        super(DIFF_CALLBACK);
    }

    @NonNull
    @Override
    public RecyclerViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
        View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.item_recyclerview, parent, false);
        return new RecyclerViewHolder(view);
    }

    @Override
    public void onBindViewHolder(@NonNull RecyclerViewHolder holder, int position) {
        Concert concert = getItem(position);
        if (concert != null) {
            holder.mTitleTextView.setText(concert.getTitle());
            holder.mAuthorTextView.setText(concert.getAuthor());
            holder.mContentTextView.setText(concert.getContent());
        }
    }

    public static class RecyclerViewHolder extends RecyclerView.ViewHolder {

        TextView mTitleTextView;
        TextView mAuthorTextView;
        TextView mContentTextView;

        public RecyclerViewHolder(View itemView) {
            super(itemView);
            mTitleTextView = itemView.findViewById(R.id.title);
            mAuthorTextView = itemView.findViewById(R.id.author);
            mContentTextView = itemView.findViewById(R.id.content);
        }
    }

    //需要oldConcert与新 newConcert 比较才能得出变化的数据
    private static DiffUtil.ItemCallback<Concert> DIFF_CALLBACK =
            new DiffUtil.ItemCallback<Concert>() {
                // 判断Item是否已经存在
                @Override
                public boolean areItemsTheSame(Concert oldConcert, Concert newConcert) {
                    return oldConcert.getTitle().equals(newConcert.getTitle());
                }

                // 如果Item已经存在则会调用此方法，判断Item的内容是否一致
                @Override
                public boolean areContentsTheSame(Concert oldConcert, Concert newConcert) {
                    return oldConcert.equals(newConcert);
                }
            };

}

ConcertDataSource

/**
 * DataSource:顾名思义，数据源，获取数据是通过它实现的。
 * 官方文档上，实现的是ItemKeyedDataSource,而我这里实现的是PositionalDataSource
 */
public class ConcertDataSource extends PositionalDataSource<Concert> {

    /**
     * 加载初始化数据，可以这么来理解，加载的是第一页的数据。
     * 形象的说，当我们第一次打开页面，需要回调此方法来获取数据。
     */
    @Override
    public void loadInitial(@NonNull LoadInitialParams params, @NonNull final LoadInitialCallback<Concert> callback) {
        callback.onResult(fetchItems(0, 20), 0, 10000);
    }

    /**
     * 当有了初始化数据之后，滑动的时候如果需要加载数据的话，会调用此方法。
     */
    @Override
    public void loadRange(@NonNull LoadRangeParams params, @NonNull final LoadRangeCallback<Concert> callback) {
        callback.onResult(fetchItems(params.startPosition, params.loadSize));
    }

    private List<Concert> fetchItems(int startPosition, int pageSize) {
        List<Concert> list = new ArrayList<>();
        for (int i = startPosition; i < startPosition + pageSize; i++) {
            Concert concert = new Concert();
            concert.setAuthor("author = " + i);
            concert.setContent("content = " + i);
            concert.setTitle("title = " + i);
            list.add(concert);
        }
        return list;
    }

}

ConcertFactory

public class ConcertFactory extends DataSource.Factory<Integer, Concert> {

    private MutableLiveData<ConcertDataSource> mSourceLiveData =
            new MutableLiveData<>();

    @Override
    public DataSource<Integer, Concert> create() {
        ConcertDataSource concertDataSource = new ConcertDataSource();
        mSourceLiveData.postValue(concertDataSource);
        return concertDataSource;
    }
}

ConcertViewModel

/**
 * PagedList: 数据源获取的数据最终靠PagedList来承载。
 * 对于PagedList,我们可以这样来理解，它就是一页数据的集合。
 * 每请求一页，就是新的一个PagedList对象。
 */
public class ConcertViewModel extends ViewModel {

    private final LiveData<PagedList<Concert>> convertList;
    private DataSource<Integer, Concert> concertDataSource;

    public ConcertViewModel() {
        ConcertFactory concertFactory = new ConcertFactory();
        concertDataSource = concertFactory.create();
        convertList = new LivePagedListBuilder<>(concertFactory, 20).build();
    }

    public void invalidateDataSource() {
        concertDataSource.invalidate();
    }

    public LiveData<PagedList<Concert>> getConvertList() {
        return convertList;
    }
}

MainActivity

public class MainActivity extends AppCompatActivity {

    private RecyclerView mRecyclerView;
    private RecyclerAdapter adapter;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mRecyclerView = findViewById(R.id.recyclerView);
        adapter = new RecyclerAdapter();
        ConcertViewModel viewModel =
                ViewModelProviders.of(this).get(ConcertViewModel.class);
        
//        viewModel.getConvertList().observe(this, concerts -> adapter.submitList(concerts));
        viewModel.getConvertList().observe(this, new Observer<PagedList<Concert>>() {
            @Override
            public void onChanged(@Nullable PagedList<Concert> concerts) {
                adapter.submitList(concerts);
                Log.i("hongx","数据更新了");
            }
        });

        mRecyclerView.setAdapter(adapter);
        mRecyclerView.setLayoutManager(new LinearLayoutManager(this, LinearLayoutManager.VERTICAL, false));

    }
}

Github：
https://github.com/345166018/HxJetpack/tree/master/HxPaging