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/*
* Copyright (C) 2017 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package android.arch.paging;
import android.arch.core.executor.ArchTaskExecutor;
import android.arch.lifecycle.ComputableLiveData;
import android.arch.lifecycle.LiveData;
import android.support.annotation.AnyThread;
import android.support.annotation.NonNull;
import android.support.annotation.Nullable;
import android.support.annotation.WorkerThread;
/**
* Provides a {@code LiveData<PagedList>}, given a means to construct a DataSource.
* <p>
* Return type for data-loading system of an application or library to produce a
* {@code LiveData<PagedList>}, while leaving the details of the paging mechanism up to the
* consumer.
* <p>
* If you're using Room, it can generate a LivePagedListProvider from a query:
* <pre>
* {@literal @}Dao
* interface UserDao {
* {@literal @}Query("SELECT * FROM user ORDER BY lastName ASC")
* public abstract LivePagedListProvider<Integer, User> usersByLastName();
* }</pre>
* In the above sample, {@code Integer} is used because it is the {@code Key} type of
* {@link TiledDataSource}. Currently, Room can only generate a {@code LIMIT}/{@code OFFSET},
* position based loader that uses TiledDataSource under the hood, and specifying {@code Integer}
* here lets you pass an initial loading position as an integer.
* <p>
* In the future, Room plans to offer other key types to support paging content with a
* {@link KeyedDataSource}.
*
* @param <Key> Type of input valued used to load data from the DataSource. Must be integer if
* you're using TiledDataSource.
* @param <Value> Data type produced by the DataSource, and held by the PagedLists.
*
* @see PagedListAdapter
* @see DataSource
* @see PagedList
*/
public abstract class LivePagedListProvider<Key, Value> {
/**
* Construct a new data source to be wrapped in a new PagedList, which will be returned
* through the LiveData.
*
* @return The data source.
*/
@WorkerThread
protected abstract DataSource<Key, Value> createDataSource();
/**
* Creates a LiveData of PagedLists, given the page size.
* <p>
* This LiveData can be passed to a {@link PagedListAdapter} to be displayed with a
* {@link android.support.v7.widget.RecyclerView}.
*
* @param initialLoadKey Initial key used to load initial data from the data source.
* @param pageSize Page size defining how many items are loaded from a data source at a time.
* Recommended to be multiple times the size of item displayed at once.
*
* @return The LiveData of PagedLists.
*/
@AnyThread
@NonNull
public LiveData<PagedList<Value>> create(@Nullable Key initialLoadKey, int pageSize) {
return create(initialLoadKey,
new PagedList.Config.Builder()
.setPageSize(pageSize)
.build());
}
/**
* Creates a LiveData of PagedLists, given the PagedList.Config.
* <p>
* This LiveData can be passed to a {@link PagedListAdapter} to be displayed with a
* {@link android.support.v7.widget.RecyclerView}.
*
* @param initialLoadKey Initial key to pass to the data source to initialize data with.
* @param config PagedList.Config to use with created PagedLists. This specifies how the
* lists will load data.
*
* @return The LiveData of PagedLists.
*/
@AnyThread
@NonNull
public LiveData<PagedList<Value>> create(@Nullable final Key initialLoadKey,
final PagedList.Config config) {
return new ComputableLiveData<PagedList<Value>>() {
@Nullable
private PagedList<Value> mList;
@Nullable
private DataSource<Key, Value> mDataSource;
private final DataSource.InvalidatedCallback mCallback =
new DataSource.InvalidatedCallback() {
@Override
public void onInvalidated() {
invalidate();
}
};
@Override
protected PagedList<Value> compute() {
@Nullable Key initializeKey = initialLoadKey;
if (mList != null) {
//noinspection unchecked
initializeKey = (Key) mList.getLastKey();
}
do {
if (mDataSource != null) {
mDataSource.removeInvalidatedCallback(mCallback);
}
mDataSource = createDataSource();
mDataSource.addInvalidatedCallback(mCallback);
mList = new PagedList.Builder<Key, Value>()
.setDataSource(mDataSource)
.setMainThreadExecutor(ArchTaskExecutor.getMainThreadExecutor())
.setBackgroundThreadExecutor(
ArchTaskExecutor.getIOThreadExecutor())
.setConfig(config)
.setInitialKey(initializeKey)
.build();
} while (mList.isDetached());
return mList;
}
}.getLiveData();
}
}
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