Resolve #57 Add support of SingleLiveEvent<Void>

lifecycle/src/main/java/ru/touchin/lifecycle/livedata/EmptySingleLiveEvent.kt

@@ -0,0 +1,28 @@
+package ru.touchin.lifecycle.livedata
+
+import androidx.annotation.MainThread
+
+/**
+ * A lifecycle-aware observable that sends only new updates after subscription, used for events like
+ * navigation and Snackbar messages.
+ *
+ *
+ * This avoids a common problem with events: on configuration change (like rotation) an update
+ * can be emitted if the observer is active. This LiveData only calls the observable if there's an
+ * explicit call to setValue() or call().
+ *
+ *
+ * Note that only one observer is going to be notified of changes.
+ *
+ * This version of SingleLiveEvent supports empty events
+ */
+class EmptySingleLiveEvent : SingleLiveEvent<Void?>() {
+    @MainThread
+    fun call() {
+        value = null
+    }
+
+    fun postCall() {
+        super.postValue(null)
+    }
+}

lifecycle/src/main/java/ru/touchin/lifecycle/livedata/SingleLiveEvent.kt

@@ -5,7 +5,7 @@ import androidx.lifecycle.MutableLiveData
 import androidx.lifecycle.Observer
 import java.util.concurrent.atomic.AtomicBoolean
 
-class SingleLiveEvent<T> : MutableLiveData<T>() {
+open class SingleLiveEvent<T> : MutableLiveData<T>() {
 
     private val pending = AtomicBoolean(false)