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      1 /*
      2  * Copyright (C) 2017 The Android Open Source Project
      3  *
      4  * Licensed under the Apache License, Version 2.0 (the "License");
      5  * you may not use this file except in compliance with the License.
      6  * You may obtain a copy of the License at
      7  *
      8  *      http://www.apache.org/licenses/LICENSE-2.0
      9  *
     10  * Unless required by applicable law or agreed to in writing, software
     11  * distributed under the License is distributed on an "AS IS" BASIS,
     12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
     13  * See the License for the specific language governing permissions and
     14  * limitations under the License.
     15  */
     16 
     17 package android.arch.paging;
     18 
     19 import android.support.annotation.Nullable;
     20 import android.support.annotation.RestrictTo;
     21 import android.support.annotation.WorkerThread;
     22 
     23 import java.util.ArrayList;
     24 import java.util.Collections;
     25 import java.util.List;
     26 
     27 /**
     28  * Simplest data source form that provides all of its data through a single loadRange() method.
     29  * <p>
     30  * Requires that your data resides in positions <code>0</code> through <code>N</code>, where
     31  * <code>N</code> is the value returned from {@link #countItems()}. You must return the exact number
     32  * requested, so that the data as returned can be safely prepended/appended to what has already
     33  * been loaded.
     34  * <p>
     35  * For more flexibility in how many items to load, or to avoid counting your data source, override
     36  * {@link PositionalDataSource} directly.
     37  *
     38  * @param  Value type returned by the data source.
     39  *
     40  * @hide
     41  */
     42 @RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
     43 public abstract class BoundedDataSource<Value> extends PositionalDataSource<Value> {
     44     /**
     45      * Called to load items at from the specified position range.
     46      *
     47      * @param startPosition Index of first item to load.
     48      * @param loadCount     Exact number of items to load. Returning a different number will cause
     49      *                      an exception to be thrown.
     50      * @return List of loaded items. Null if the BoundedDataSource is no longer valid, and should
     51      *         not be queried again.
     52      */
     53     @WorkerThread
     54     @Nullable
     55     public abstract List<Value> loadRange(int startPosition, int loadCount);
     56 
     57     @WorkerThread
     58     @Nullable
     59     @Override
     60     public List<Value> loadAfter(int startIndex, int pageSize) {
     61         return loadRange(startIndex, pageSize);
     62     }
     63 
     64     @WorkerThread
     65     @Nullable
     66     @Override
     67     public List<Value> loadBefore(int startIndex, int pageSize) {
     68         if (startIndex < 0) {
     69             return new ArrayList<>();
     70         }
     71         int loadSize = Math.min(pageSize, startIndex + 1);
     72         startIndex = startIndex - loadSize + 1;
     73         List<Value> result = loadRange(startIndex, loadSize);
     74         if (result != null) {
     75             if (result.size() != loadSize) {
     76                 throw new IllegalStateException("invalid number of items returned.");
     77             }
     78             Collections.reverse(result);
     79         }
     80         return result;
     81     }
     82 }
     83