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      1 /*
      2  * Copyright 2018 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 androidx.paging;
     18 
     19 import static java.lang.annotation.RetentionPolicy.SOURCE;
     20 
     21 import androidx.annotation.IntDef;
     22 import androidx.annotation.MainThread;
     23 import androidx.annotation.NonNull;
     24 
     25 import java.lang.annotation.Retention;
     26 import java.util.Collections;
     27 import java.util.List;
     28 
     29 class PageResult<T> {
     30     @SuppressWarnings("unchecked")
     31     private static final PageResult INVALID_RESULT =
     32             new PageResult(Collections.EMPTY_LIST, 0);
     33 
     34     @SuppressWarnings("unchecked")
     35     static <T> PageResult<T> getInvalidResult() {
     36         return INVALID_RESULT;
     37     }
     38 
     39 
     40     @Retention(SOURCE)
     41     @IntDef({INIT, APPEND, PREPEND, TILE})
     42     @interface ResultType {}
     43 
     44     static final int INIT = 0;
     45 
     46     // contiguous results
     47     static final int APPEND = 1;
     48     static final int PREPEND = 2;
     49 
     50     // non-contiguous, tile result
     51     static final int TILE = 3;
     52 
     53     @NonNull
     54     public final List<T> page;
     55     @SuppressWarnings("WeakerAccess")
     56     public final int leadingNulls;
     57     @SuppressWarnings("WeakerAccess")
     58     public final int trailingNulls;
     59     @SuppressWarnings("WeakerAccess")
     60     public final int positionOffset;
     61 
     62     PageResult(@NonNull List<T> list, int leadingNulls, int trailingNulls, int positionOffset) {
     63         this.page = list;
     64         this.leadingNulls = leadingNulls;
     65         this.trailingNulls = trailingNulls;
     66         this.positionOffset = positionOffset;
     67     }
     68 
     69     PageResult(@NonNull List<T> list, int positionOffset) {
     70         this.page = list;
     71         this.leadingNulls = 0;
     72         this.trailingNulls = 0;
     73         this.positionOffset = positionOffset;
     74     }
     75 
     76     @Override
     77     public String toString() {
     78         return "Result " + leadingNulls
     79                 + ", " + page
     80                 + ", " + trailingNulls
     81                 + ", offset " + positionOffset;
     82     }
     83 
     84     public boolean isInvalid() {
     85         return this == INVALID_RESULT;
     86     }
     87 
     88     abstract static class Receiver<T> {
     89         @MainThread
     90         public abstract void onPageResult(@ResultType int type, @NonNull PageResult<T> pageResult);
     91     }
     92 }
     93