Jetpack学习-7-Paging

Paging是什么

分页库可一次加载和显示一块数据，按需载入部分数据减少网络带宽和系统资源的使用量。

简单使用

引入Paging

dependencies {
    implementation 'androidx.paging:paging-runtime:2.1.2'
}

定义Bean

public class Student {
    private String id;
    private String name;
    private String gender;

    public String getId() {
        return id;
    }

    public String getName() {
        return name;
    }

    public String getGender() {
        return gender;
    }

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

    public void setName(String name) {
        this.name = name;
    }

    public void setGender(String gender) {
        this.gender = gender;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) {
            return true;
        }
        if (o == null || getClass() != o.getClass()) {
            return false;
        }
        Student student = (Student) o;
        return Objects.equals(id, student.id) &&
                Objects.equals(name, student.name) &&
                Objects.equals(gender, student.gender);
    }

    @Override
    public int hashCode() {
        return Objects.hash(id, name, gender);
    }
}

需要重写equals和hasCode方法，后面比较数据会用到。

定义DataSource

/**
 * @author: George.ren
 * date: 2020/8/11
 * PageKeyedDataSource：按页加载
 * ItemKeyedDataSource：按条目加载
 * PositionalDataSource：按位置加载(从n到n+x)
 */
public class StudentDataSource extends PositionalDataSource<Student> {
    private static final String TAG = "-paging-StudentData";

    @Override
    public void loadInitial(@NonNull LoadInitialParams params, @NonNull LoadInitialCallback<Student> callback) {
        Log.d(TAG, "loadInitial-初始化数据-数据量:" + 10);
        // 模拟数据源；开始位置；总共数据数；
        // 注意：模拟数据源的开始位置最好和 第二个参数开始位置一致。
        // 如果模拟数据源个数小于 第三个参数，则触发分页(loadRange去加载下一页)
        //
        // 如果请求数据大小不符合要求直接crash--》PagedStorage-->insertPage--> page introduces incorrect tiling
        // 例如第三个参数 小于 每页个数
        callback.onResult(getStudents(0, Config.PER_PAGE_SIZE), 0, Config.INIT_SIZE);
        // 可以不写三个参数
//        callback.onResult(getStudents(0, Config.PER_PAGE_SIZE), 0);
    }

    @Override
    public void loadRange(@NonNull LoadRangeParams params, @NonNull LoadRangeCallback<Student> callback) {
        Log.d(TAG, "-下一页数据-loadRange-start:" + params.startPosition + " size:" + params.loadSize);
        callback.onResult(getStudents(params.startPosition, params.loadSize));
    }

    /**
     * 模拟数据源
     *
     * @param startPosition 开始加载位置
     * @param pageSize      每页个数
     * @return
     */
    private List<Student> getStudents(int startPosition, int pageSize) {
        Log.d(TAG, "模拟数据源-getStudents-startPosition:" + startPosition + " pageSize:"
                + pageSize + " endPo:" + (startPosition + pageSize - 1));
        List<Student> list = new ArrayList<>();
        for (int i = startPosition; i < startPosition + pageSize; i++) {
            Student student = new Student();
            student.setId("ID:" + i);
            student.setName("名称:" + i);
            student.setGender("性别:" + i);
            list.add(student);
        }
        return list;
    }
}

定义继承自PositionalDataSource，这是一个固定大小的数据源。
获取数据方法getStudents，重写loadInitial首次加载数据被调用，loadRange方法(分页加载)

定义DataSourceFactory

public class StudentDataSourceFactory extends DataSource.Factory<Integer,Student>{
    @NonNull
    @Override
    public DataSource<Integer, Student> create() {
        StudentDataSource dataSource = new StudentDataSource();
        return dataSource;
    }
}

工厂模式，控制数据源StudentDataSource

定义ViewModel

public class StudentViewModel extends ViewModel {
    private final LiveData<PagedList<Student>> listLiveData;

    /**
     * 构造方法赋值
     * 加载第一页数据
     */
    public StudentViewModel() {
        StudentDataSourceFactory factory = new StudentDataSourceFactory();
        // 工厂：创建DataSource对象；每页大小
        this.listLiveData = new LivePagedListBuilder<>(factory, getPageConfig()).build();
    }

    private PagedList.Config getPageConfig(){
        return new PagedList.Config.Builder()
                .setPageSize(Config.PER_PAGE_SIZE) // 分页数据量。DataSource的loadRange中的值
                .setPrefetchDistance(Config.PRE_SIZE)// 提前多少数据继续加载。上滑动加载更多，距离最后item还有2个会触发。下滑刷新同理
                .setInitialLoadSizeHint(Config.INIT_SIZE)// 初始化加载数量
                .setEnablePlaceholders(false)// 配置是否启动PlaceHolders
                .build();
    }
    public LiveData<PagedList<Student>> getListLiveData() {
        return listLiveData;
    }
}

常量类：

public class Config {
    public static final int PER_PAGE_SIZE = 8;
    public static final int PRE_SIZE = 2;
    public static final int INIT_SIZE = 8;
}

创建工厂数据源，配置分页信息(每页数据量，初始化数据量等信息)

定义Adapter

public class RecyclerPagingAdapter extends PagedListAdapter<Student, RecyclerPagingAdapter.RecyclerViewHolder> {

    private static DiffUtil.ItemCallback<Student> DIFF_STUDENT = new DiffUtil.ItemCallback<Student>() {
        @Override
        public boolean areItemsTheSame(@NonNull Student oldItem, @NonNull Student newItem) {
            return oldItem.getId() == newItem.getId();
        }

        @Override
        public boolean areContentsTheSame(@NonNull Student oldItem, @NonNull Student newItem) {
            return oldItem.equals(newItem);
        }
    };

    public RecyclerPagingAdapter() {
        super(DIFF_STUDENT);
    }

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

    @Override
    public void onBindViewHolder(@NonNull RecyclerViewHolder holder, int position) {
        // 从数据源中获取对应Data数据，可能为空
        Student student = getItem(position);
        if (student == null) {
            holder.tvId.setText("加载中...");
            holder.tvName.setText("加载中...");
            holder.tvGender.setText("加载中...");
        } else {
            holder.tvId.setText(student.getId());
            holder.tvName.setText(student.getName());
            holder.tvGender.setText(student.getGender());
        }
    }

    public static class RecyclerViewHolder extends RecyclerView.ViewHolder {
        TextView tvId;
        TextView tvName;
        TextView tvGender;

        public RecyclerViewHolder(@NonNull View itemView) {
            super(itemView);
            tvId = itemView.findViewById(R.id.tvId);
            tvName = itemView.findViewById(R.id.tvName);
            tvGender = itemView.findViewById(R.id.tvGender);
        }
    }
}

列表页item配置，diff处理用到equals

显示数据

public class PagingActivity extends AppCompatActivity {
    private static final String TAG = "-paging-PagingActivity";
    private RecyclerView recyclerView;
    private RecyclerPagingAdapter adapter;
    private StudentViewModel viewModel;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_paging);
        recyclerView = findViewById(R.id.recyclerView);
        adapter = new RecyclerPagingAdapter();
        // 第一次初始化
        refresh();
        recyclerView.setAdapter(adapter);
        recyclerView.setLayoutManager(new LinearLayoutManager(this));
    }

    private void refresh(){
        adapter.submitList(null);
        viewModel = new ViewModelProvider(this,new ViewModelProvider.NewInstanceFactory()).get(StudentViewModel.class);
        // 获取数据，监听数据改变
        viewModel.getListLiveData().observe(this, new Observer<PagedList<Student>>() {
            @Override
            public void onChanged(PagedList<Student> students) {
                Log.d(TAG, "数据源改变-onChanged-size:"+students.size());
                // 设定新的数据源
                adapter.submitList(students);
            }
        });
    }

    /**
     * 刷新数据源：可以模仿下拉刷新触发点
     * @param view
     */
    public void onRefresh(View view) {
        refresh();
    }
}

原理

通过Paging可以做分页处理。
清除adapter里的数据：submitList()传null值，可以模仿刷新

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