Android Paging瞭解一下？

摘要： 其實很多這種文章了，但是很多都是Kotlin甚至都是一些理論？還有一些是用VM模式(比如官方 )。作為老骨頭，我們來看看Java最基礎版的。 我的主張是：先給我看程式碼，我再考慮後面的。 Paging的功能無非是控制頁面的載入...

其實很多這種文章了，但是很多都是Kotlin甚至都是一些理論？還有一些是用VM模式(比如ofollow,noindex">官方 )。作為老骨頭，我們來看看Java最基礎版的。

我的主張是：先給我看程式碼，我再考慮後面的。

Paging的功能無非是控制頁面的載入過程（什麼時候載入上一頁/下一頁？），優點自行搜尋引擎。

時間倉促，先直接上程式碼了。後面再來完善文章：

0 引入paging庫

def pagingCommonVersion = '2.0.0-rc01'
def pagingNewestVersion = '2.1.0-beta01'

dependencies {
api 'androidx.recyclerview:recyclerview:1.0.0'
api "androidx.paging:paging-common:$pagingCommonVersion"
api "androidx.paging:paging-runtime:$pagingNewestVersion"
}

1 構建本地模擬資料

1.1 模擬實體：StudentModel

class StudentModel {
int id;
String name;

public int getId() {
return id;
}

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

public String getName() {
return name;
}

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

1.2 模擬資料倉庫CustomRepository

class CustomRepository {
List<StudentModel> studentModels = new ArrayList<>();

CustomRepository() {
for (int i = 0; i < 100; i++) {
StudentModel studentModel = new StudentModel();
studentModel.setId(i);
studentModel.setName("name" + i);
studentModels.add(studentModel);
}
}

List<StudentModel> getStudentsInit(@IntRange(from = 0) int initSize) {
return studentModels.subList(0, initSize);//包含~不包含
}

List<StudentModel> getStudentsByRange(@IntRange(from = 0) int from, int to) {
return studentModels.subList(from, to);
}

List<StudentModel> getStudentsByPage(@IntRange(from = 0) int page, int size) {
int totalPage = 0;
if (studentModels.size() % size == 0) {
totalPage = studentModels.size() / size;
} else {
totalPage = studentModels.size() / size + 1;
}
if (page >= totalPage || page < 0) {
return null;
}
if (page == totalPage - 1) {
return studentModels.subList(page * size, studentModels.size());
}
return studentModels.subList(page * size, (page + 1) * size);
}
}

2 繼承於PageKeyedDataSource 的資料來源類CustomPageDataSource

class CustomPageDataSource extends PageKeyedDataSource<Integer, StudentModel> {

private CustomRepository mCustomRepository;

CustomPageDataSource(CustomRepository customRepository) {
this.mCustomRepository = customRepository;
}

@Override
public void loadInitial(@NonNull LoadInitialParams<Integer> params, @NonNull LoadInitialCallback<Integer, StudentModel> callback) {
List<StudentModel> studentModels = mCustomRepository.getStudentsInit(params.requestedLoadSize);
callback.onResult(studentModels, null, 1);
}

@Override
public void loadBefore(@NonNull LoadParams<Integer> params, @NonNull LoadCallback<Integer, StudentModel> callback) {
//模擬網路呼叫耗時操作
new Thread(new Runnable() {
@Override
public void run() {
try {
Thread.sleep(3000);
} catch (InterruptedException e) {
e.printStackTrace();
}
List<StudentModel> studentModels = mCustomRepository.getStudentsByPage(params.key, params.requestedLoadSize);
callback.onResult(studentModels, params.key - 1);
}
}).start();

}

@Override
public void loadAfter(@NonNull LoadParams<Integer> params, @NonNull LoadCallback<Integer, StudentModel> callback) {
//模擬網路呼叫耗時操作
new Thread(new Runnable() {
@Override
public void run() {
try {
Thread.sleep(3000);
} catch (InterruptedException e) {
e.printStackTrace();
}
List<StudentModel> studentModels = mCustomRepository.getStudentsByPage(params.key, params.requestedLoadSize);
if (studentModels != null) {
callback.onResult(studentModels, params.key + 1);
}
}
}).start();

}
}

3 繼承於DataSource.Factory 的資料工廠類CustomPageDataSourceFactory

class CustomPageDataSourceFactory extends DataSource.Factory<Integer, StudentModel> {

private CustomRepository mCustomRepository;

CustomPageDataSourceFactory(CustomRepository customRepository) {
this.mCustomRepository = customRepository;
}

@NonNull
@Override
public DataSource<Integer, StudentModel> create() {
return new CustomPageDataSource(mCustomRepository);
}
}

4DiffUtil.ItemCallback

private static DiffUtil.ItemCallback<StudentModel> DIFF_CALLBACK = new DiffUtil.ItemCallback<StudentModel>() {
// Concert details may have changed if reloaded from the database,
// but ID is fixed.
@Override
public boolean areItemsTheSame(StudentModel oldConcert, StudentModel newConcert) {
return oldConcert.getId() == newConcert.getId();
}

@Override
public boolean areContentsTheSame(StudentModel oldConcert,
StudentModel newConcert) {
return oldConcert.equals(newConcert);
}
};

5 繼承於PagedListAdapter 的StudentAdapter

public class StudentAdapter extends PagedListAdapter<StudentModel, StudentAdapter.CustomViewHolder> {


protected StudentAdapter() {
super(DIFF_CALLBACK);
}

@NonNull
@Override
public CustomViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
return new CustomViewHolder(LayoutInflater.from(parent.getContext()).inflate(R.layout.item_test, parent, false));
}

@Override
public void onBindViewHolder(@NonNull CustomViewHolder holder, int position) {
StudentModel studentModel = getItem(position);
holder.tvID.setText(String.valueOf(studentModel.getId()));
holder.tvName.setText(studentModel.getName());
}

class CustomViewHolder extends RecyclerView.ViewHolder {
TextView tvID, tvName;

public CustomViewHolder(@NonNull View itemView) {
super(itemView);
tvID = itemView.findViewById(R.id.tv_id);
tvName = itemView.findViewById(R.id.tv_name);
}
}

}

6 使用MainActivity

@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
RecyclerView recyclerView = findViewById(R.id.recycler_view);
recyclerView.setLayoutManager(new LinearLayoutManager(this));
StudentAdapter studentAdapter = new StudentAdapter();
recyclerView.setAdapter(studentAdapter);

PagedList.Config config = new PagedList.Config.Builder()
.setInitialLoadSizeHint(10)//設定首次載入的數量；
.setPageSize(15)//設定每一頁載入的數量；
.setMaxSize(15 + 2 * 3)
.setPrefetchDistance(3)//設定距離最後還有多少個item時，即自動載入下一頁的資料；
.setEnablePlaceholders(true)//表示是否設定null佔位符；
.build();
CustomRepository repository = new CustomRepository();
CustomPageDataSourceFactory factory = new CustomPageDataSourceFactory(repository);
LiveData<PagedList<StudentModel>> liveData = new LivePagedListBuilder<>(factory, config).build();
//這裡的this是LifecycleOwner介面，而我們常用的AppCompatActivity已經實現啦
liveData.observe(this, studentAdapter::submitList);
}

以上程式碼只需要建立一個Activity 和對應的Adapter 所需的item_test.xml ，然後把上面的類全部copy到Activity 作為內部類就可以直接跑起來了 。當然，Activity 裡需要一個裝有RecyclerView 的activity_main.xml 。。。