feat(observable): Implement observable .once (#5309)

Node has a handy one-liner method on its EventEmitter called "once".
It is similar to "on", but fires a single time and automatically unsubscribes.

tns-core-modules/data/observable/observable.d.ts

@@ -95,6 +95,14 @@ export class Observable {
      */
     on(event: "propertyChange", callback: (data: EventData) => void, thisArg?: any);
 
+    /**
+     * Adds one-time listener function for the event named `event`.
+     * @param event Name of the event to attach to.
+     * @param callback A function to be called when the specified event is raised.
+     * @param thisArg An optional parameter which when set will be used as "this" in callback method call.
+     */
+    once(event: string, callback: (data: EventData) => void, thisArg?: any);
+
     /**
      * Shortcut alias to the removeEventListener method.
      */

tns-core-modules/data/observable/observable.ts

@@ -3,6 +3,7 @@
 interface ListenerEntry {
     callback: (data: EventData) => void;
     thisArg: any;
+    once?: true;
 }
 
 let _wrappedIndex = 0;
@@ -56,6 +57,11 @@ export class Observable implements ObservableDefinition {
         this.addEventListener(eventNames, callback, thisArg);
     }
 
+    public once(event: string, callback: (data: EventData) => void, thisArg?: any) {
+        const list = this._getEventList(event, true);
+        list.push({ callback, thisArg, once: true });
+    }
+
     public off(eventNames: string, callback?: any, thisArg?: any) {
         this.removeEventListener(eventNames, callback, thisArg);
     }
@@ -120,6 +126,9 @@ export class Observable implements ObservableDefinition {
 
         for (let i = observers.length - 1; i >= 0; i--) {
             let entry = observers[i];
+            if (entry.once) {
+                observers.splice(i, 1);
+            }
             if (entry.thisArg) {
                 entry.callback.apply(entry.thisArg, [data]);
             } else {