Use observer pattern for label change

app/src/main/java/ch/dissem/apps/abit/util/Observable.kt

@@ -0,0 +1,37 @@
+package ch.dissem.apps.abit.util
+
+import kotlin.properties.Delegates
+
+/**
+ * A simple observable implementation that should be mostly
+ */
+class Observable<T>(value: T) {
+    private val observers = mutableMapOf<Any, (T) -> Unit>()
+
+    var value: T by Delegates.observable(value, { _, old, new ->
+        if (old != new) {
+            observers.values.forEach { it.invoke(new) }
+        }
+    })
+
+    /**
+     * The key will make sure the observer can easily be removed. Usually the key should be either
+     * the object that created the observer, or the observer itself, if it's easily available.
+     *
+     * Note that a map is used for observers, so if you define more than one observer with the same
+     * key, all previous ones will be removed. Also, the observers will be notified in no specific
+     * order.
+     *
+     * To prevent memory leaks, the observer must be removed if it isn't used anymore.
+     */
+    fun addObserver(key: Any, observer: (T) -> Unit) {
+        observers.put(key, observer)
+    }
+
+    /**
+     * Remove the observer that was registered with the given key.
+     */
+    fun removeObserver(key: Any) {
+        observers.remove(key)
+    }
+}