LiveDataUtil combineLatest.

app/src/main/java/org/thoughtcrime/securesms/util/DefaultValueLiveData.java

@@ -11,6 +11,7 @@ public class DefaultValueLiveData<T> extends MutableLiveData<T> {
   private final T defaultValue;
 
   public DefaultValueLiveData(@NonNull T defaultValue) {
+    super(defaultValue);
     this.defaultValue = defaultValue;
   }
 

app/src/main/java/org/thoughtcrime/securesms/util/livedata/LiveDataUtil.java

@@ -0,0 +1,67 @@
+package org.thoughtcrime.securesms.util.livedata;
+
+import androidx.annotation.NonNull;
+import androidx.lifecycle.LiveData;
+import androidx.lifecycle.MediatorLiveData;
+
+public final class LiveDataUtil {
+
+  private LiveDataUtil() {
+  }
+
+  /**
+   * Once there is non-null data on both input {@link LiveData}, the {@link Combine} function is run
+   * and produces a live data of the combined data.
+   * <p>
+   * As each live data changes, the combine function is re-run, and a new value is emitted always
+   * with the latest, non-null values.
+   */
+  public static <A, B, R> LiveData<R> combineLatest(@NonNull LiveData<A> a,
+                                                    @NonNull LiveData<B> b,
+                                                    @NonNull Combine<A, B, R> combine) {
+    return new CombineLiveData<>(a, b, combine);
+  }
+
+  public interface Combine<A, B, R> {
+    @NonNull R apply(@NonNull A a, @NonNull B b);
+  }
+
+  private static final class CombineLiveData<A, B, R> extends MediatorLiveData<R> {
+    private A a;
+    private B b;
+
+    CombineLiveData(LiveData<A> liveDataA, LiveData<B> liveDataB, Combine<A, B, R> combine) {
+      if (liveDataA == liveDataB) {
+
+        addSource(liveDataA, (a) -> {
+          if (a != null) {
+            this.a = a;
+            //noinspection unchecked: A is B if live datas are same instance
+            this.b = (B) a;
+            setValue(combine.apply(a, b));
+          }
+        });
+
+      } else {
+
+        addSource(liveDataA, (a) -> {
+          if (a != null) {
+            this.a = a;
+            if (b != null) {
+              setValue(combine.apply(a, b));
+            }
+          }
+        });
+
+        addSource(liveDataB, (b) -> {
+          if (b != null) {
+            this.b = b;
+            if (a != null) {
+              setValue(combine.apply(a, b));
+            }
+          }
+        });
+      }
+    }
+  }
+}