All examples of the new paging library have been with Room library and Room creates a Data Source for us. In my own case, I need to create my custom data source.
Here is a method in my view model class that ought to return the live data. My livedata always returns null.
LiveData<PagedList<ApiResult>> getData(){
LivePagedListProvider<Integer,ApiResult> p = new LivePagedListProvider<Integer, ApiResult>() {
@Override
protected DataSource<Integer, ApiResult> createDataSource() {
return new DataClass();
}
};
listLiveData = p.create(0,new PagedList.Config.Builder()
.setPageSize(5) //number of items loaded at once
.setPrefetchDistance(0)// the distance to the end of already loaded list before new data is loaded
.build());
return listLiveData;
}
And here is the Data class
public class DataClass extends TiledDataSource<ApiResult> {
private List<ApiResult> result = new ArrayList<>();
@Override
public int countItems() {
return result.size();
}
@Override
public List<ApiResult> loadRange(int startPosition, int count) {
Call<String> call = NetworkModule.providesWebService().makeRequest();
call.enqueue(new Callback<String>() {
@Override
public void onResponse(@NonNull Call<String> call, @NonNull Response<String> response) {
Log.i(DataClass.this.getClass().getSimpleName() + " - onResponse", String.valueOf(response));
result = parseJson(response.body());
}
@Override
public void onFailure(@NonNull Call<String> call, @NonNull Throwable t) {
Log.i(DataClass.this.getClass().getSimpleName() + " - onFailure", t.getMessage());
}
});
return result;
}
}
I think this can help:
1. countItems() should return DataSource.COUNT_UNDEFINED
2. loadRange(int startPosition, int count): You can directly execute your query.
3. So now you can delete result global variable
Also, turnoff placeholders:
listLiveData = p.create(0,new PagedList.Config.Builder()
.setPageSize(5) //number of items loaded at once
.setPrefetchDistance(10) //Must be >0 since placeholders are off
.setEnablePlaceholders(false)
.build());
Here is the updated code for Data Class:
public class DataClass extends TiledDataSource<ApiResult> {
@Override
public int countItems() {
return DataSource.COUNT_UNDEFINED;
}
@Override
public List<ApiResult> loadRange(int startPosition, int count) {
Call<String> call = NetworkModule.providesWebService().makeRequest();
Response<String> response = call.execute();
return parseJson(response.body());
}
You can check an example project here: https://github.com/brainail/.samples/tree/master/ArchPagingLibraryWithNetwork
You can create a Custom Data Source, usually we built backend API to fetch data that takes pagenumber as a parameter to return the specified page.
For this situation you can use PageKeyedDataSource. Here is a sample code of PageKeyedDataSource using the StackOverflow API. The below code is using Retrofit to get the data from the StackOverflow API.
public class ItemDataSource extends PageKeyedDataSource<Integer, Item> {
//the size of a page that we want
public static final int PAGE_SIZE = 50;
//we will start from the first page which is 1
private static final int FIRST_PAGE = 1;
//we need to fetch from stackoverflow
private static final String SITE_NAME = "stackoverflow";
//this will be called once to load the initial data
@Override
public void loadInitial(@NonNull LoadInitialParams<Integer> params, @NonNull final LoadInitialCallback<Integer, Item> callback) {
RetrofitClient.getInstance()
.getApi().getAnswers(FIRST_PAGE, PAGE_SIZE, SITE_NAME)
.enqueue(new Callback<StackApiResponse>() {
@Override
public void onResponse(Call<StackApiResponse> call, Response<StackApiResponse> response) {
if (response.body() != null) {
callback.onResult(response.body().items, null, FIRST_PAGE + 1);
}
}
@Override
public void onFailure(Call<StackApiResponse> call, Throwable t) {
}
});
}
//this will load the previous page
@Override
public void loadBefore(@NonNull final LoadParams<Integer> params, @NonNull final LoadCallback<Integer, Item> callback) {
RetrofitClient.getInstance()
.getApi().getAnswers(params.key, PAGE_SIZE, SITE_NAME)
.enqueue(new Callback<StackApiResponse>() {
@Override
public void onResponse(Call<StackApiResponse> call, Response<StackApiResponse> response) {
//if the current page is greater than one
//we are decrementing the page number
//else there is no previous page
Integer adjacentKey = (params.key > 1) ? params.key - 1 : null;
if (response.body() != null) {
//passing the loaded data
//and the previous page key
callback.onResult(response.body().items, adjacentKey);
}
}
@Override
public void onFailure(Call<StackApiResponse> call, Throwable t) {
}
});
}
//this will load the next page
@Override
public void loadAfter(@NonNull final LoadParams<Integer> params, @NonNull final LoadCallback<Integer, Item> callback) {
RetrofitClient.getInstance()
.getApi()
.getAnswers(params.key, PAGE_SIZE, SITE_NAME)
.enqueue(new Callback<StackApiResponse>() {
@Override
public void onResponse(Call<StackApiResponse> call, Response<StackApiResponse> response) {
if (response.body() != null) {
//if the response has next page
//incrementing the next page number
Integer key = response.body().has_more ? params.key + 1 : null;
//passing the loaded data and next page value
callback.onResult(response.body().items, key);
}
}
@Override
public void onFailure(Call<StackApiResponse> call, Throwable t) {
}
});
}
}
Here you can see we are getting the result by making a Retrofit call and then we are pushing the result to callback.
For a detailed, step by step explanation go through this Android Paging Library Tutorial.
来源：https://stackoverflow.com/questions/46625740/paging-library-without-room