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      1 /*
      2  * Copyright 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 androidx.work.impl.utils;
     18 
     19 import android.arch.core.util.Function;
     20 import android.arch.lifecycle.LiveData;
     21 import android.arch.lifecycle.MediatorLiveData;
     22 import android.arch.lifecycle.Observer;
     23 import android.support.annotation.NonNull;
     24 import android.support.annotation.Nullable;
     25 import android.support.annotation.RestrictTo;
     26 
     27 import androidx.work.impl.utils.taskexecutor.WorkManagerTaskExecutor;
     28 
     29 /**
     30  * Utility methods for {@link LiveData}.
     31  *
     32  * @hide
     33  */
     34 @RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
     35 public class LiveDataUtils {
     36 
     37     /**
     38      * Creates a new {@link LiveData} object that maps the values of {@code inputLiveData} using
     39      * {@code mappingMethod} on a background thread, but only triggers its observers when the mapped
     40      * values actually change.
     41      *
     42      * @param inputLiveData An input {@link LiveData}
     43      * @param mappingMethod A {@link Function} that maps input of type {@code In} to output of type
     44      *                      {@code Out}
     45      * @param  The type of data for {@code inputLiveData}
     46      * @param  The type of data to output
     47      * @return A new {@link LiveData} of type {@code Out}
     48      */
     49     public static <In, Out> LiveData<Out> dedupedMappedLiveDataFor(
     50             @NonNull LiveData<In> inputLiveData,
     51             @NonNull final Function<In, Out> mappingMethod) {
     52         final MediatorLiveData<Out> outputLiveData = new MediatorLiveData<>();
     53         outputLiveData.addSource(inputLiveData, new Observer<In>() {
     54             @Override
     55             public void onChanged(@Nullable final In input) {
     56                 WorkManagerTaskExecutor.getInstance().executeOnBackgroundThread(new Runnable() {
     57                     @Override
     58                     public void run() {
     59                         synchronized (outputLiveData) {
     60                             Out newOutput = mappingMethod.apply(input);
     61                             Out previousOutput = outputLiveData.getValue();
     62                             if (previousOutput == null && newOutput != null) {
     63                                 outputLiveData.postValue(newOutput);
     64                             } else if (
     65                                     previousOutput != null && !previousOutput.equals(newOutput)) {
     66                                 outputLiveData.postValue(newOutput);
     67                             }
     68                         }
     69                     }
     70                 });
     71             }
     72         });
     73         return outputLiveData;
     74     }
     75 
     76     private LiveDataUtils() {
     77     }
     78 }
     79