Add EventStreams.animationTicks() and FPSDemo.

reactfx-demos/src/main/java/org/reactfx/demo/FPSDemo.java

@@ -0,0 +1,32 @@
+package org.reactfx.demo;
+import javafx.application.Application;
+import javafx.scene.Scene;
+import javafx.scene.control.Label;
+import javafx.scene.layout.StackPane;
+import javafx.stage.Stage;
+
+import org.reactfx.EventStreams;
+
+public class FPSDemo extends Application {
+
+    @Override
+    public void start(Stage primaryStage) {
+        Label label = new Label();
+
+        EventStreams.animationTicks()
+                .latestN(100)
+                .map(ticks -> {
+                    int n = ticks.size() - 1;
+                    return n * 1_000_000_000.0 / (ticks.get(n) - ticks.get(0));
+                })
+                .map(d -> String.format("FPS: %.3f", d))
+                .feedTo(label.textProperty());
+
+        primaryStage.setScene(new Scene(new StackPane(label), 250, 150));
+        primaryStage.show();
+    }
+
+    public static void main(String[] args) {
+        launch(args);
+    }
+}

reactfx/src/main/java/org/reactfx/EventStreams.java

@@ -10,6 +10,7 @@
 import java.util.function.Function;
 import java.util.function.Supplier;
 
+import javafx.animation.AnimationTimer;
 import javafx.beans.InvalidationListener;
 import javafx.beans.Observable;
 import javafx.beans.value.ChangeListener;
@@ -327,6 +328,28 @@ protected Subscription observeInputs() {
         };
     }
 
+    /**
+     * Returns an event stream that emits a timestamp of the current frame in
+     * nanoseconds on every frame. The timestamp has the same meaning as the
+     * argument of the {@link AnimationTimer#handle(long)} method.
+     */
+    public static EventStream<Long> animationTicks() {
+        return new EventStreamBase<Long>() {
+            private final AnimationTimer timer = new AnimationTimer() {
+                @Override
+                public void handle(long now) {
+                    emit(now);
+                }
+            };
+
+            @Override
+            protected Subscription observeInputs() {
+                timer.start();
+                return timer::stop;
+            }
+        };
+    }
+
     /**
      * Returns an event stream that emits all the events emitted from any of
      * the {@code inputs}. The event type of the returned stream is the nearest