1 files changed, 51 insertions, 0 deletions
diff --git a/taler-kotlin-common/src/main/java/net/taler/common/Event.kt b/taler-kotlin-common/src/main/java/net/taler/common/Event.kt
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+++ b/taler-kotlin-common/src/main/java/net/taler/common/Event.kt
@@ -0,0 +1,51 @@
+/*
+ * This file is part of GNU Taler
+ * (C) 2020 Taler Systems S.A.
+ *
+ * GNU Taler is free software; you can redistribute it and/or modify it under the
+ * terms of the GNU General Public License as published by the Free Software
+ * Foundation; either version 3, or (at your option) any later version.
+ *
+ * GNU Taler is distributed in the hope that it will be useful, but WITHOUT ANY
+ * WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
+ * A PARTICULAR PURPOSE.  See the GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along with
+ * GNU Taler; see the file COPYING.  If not, see <http://www.gnu.org/licenses/>
+ */
+
+package net.taler.common
+
+import androidx.lifecycle.LiveData
+import androidx.lifecycle.Observer
+import java.util.concurrent.atomic.AtomicBoolean
+
+/**
+ * Used as a wrapper for data that is exposed via a [LiveData] that represents an one-time event.
+ */
+open class Event<out T>(private val content: T) {
+
+    private val isConsumed = AtomicBoolean(false)
+
+    /**
+     * Returns the content and prevents its use again.
+     */
+    fun getIfNotConsumed(): T? {
+        return if (isConsumed.compareAndSet(false, true)) content else null
+    }
+
+}
+
+fun <T> T.toEvent() = Event(this)
+
+/**
+ * An [Observer] for [Event]s, simplifying the pattern of checking if the [Event]'s content has
+ * already been consumed.
+ *
+ * [onEvent] is *only* called if the [Event]'s contents has not been consumed.
+ */
+class EventObserver<T>(private val onEvent: (T) -> Unit) : Observer<Event<T>> {
+    override fun onChanged(event: Event<T>?) {
+        event?.getIfNotConsumed()?.let { onEvent(it) }
+    }
+}