added frame count to FrameTimeDiagnosticsPlugin (bevyengine#678)

added frame count to FrameTimeDiagnosticsPlugin

crates/bevy_diagnostic/src/frame_time_diagnostics_plugin.rs

@@ -3,28 +3,43 @@ use bevy_app::prelude::*;
 use bevy_core::Time;
 use bevy_ecs::{IntoQuerySystem, Res, ResMut};
 
-/// Adds "frame time" diagnostic to an App, specifically "frame time" and "fps"
+/// Adds "frame time" diagnostic to an App, specifically "frame time", "fps" and "frame count"
 #[derive(Default)]
 pub struct FrameTimeDiagnosticsPlugin;
 
+pub struct FrameTimeDiagnosticsState {
+    frame_count: f64,
+}
+
 impl Plugin for FrameTimeDiagnosticsPlugin {
     fn build(&self, app: &mut bevy_app::AppBuilder) {
         app.add_startup_system(Self::setup_system.system())
+            .add_resource(FrameTimeDiagnosticsState { frame_count: 0.0 })
             .add_system(Self::diagnostic_system.system());
     }
 }
 
 impl FrameTimeDiagnosticsPlugin {
     pub const FPS: DiagnosticId = DiagnosticId::from_u128(288146834822086093791974408528866909483);
+    pub const FRAME_COUNT: DiagnosticId =
+        DiagnosticId::from_u128(54021991829115352065418785002088010277);
     pub const FRAME_TIME: DiagnosticId =
-        DiagnosticId::from_u128(54021991829115352065418785002088010276);
+        DiagnosticId::from_u128(73441630925388532774622109383099159699);
 
     pub fn setup_system(mut diagnostics: ResMut<Diagnostics>) {
         diagnostics.add(Diagnostic::new(Self::FRAME_TIME, "frame_time", 20));
         diagnostics.add(Diagnostic::new(Self::FPS, "fps", 20));
+        diagnostics.add(Diagnostic::new(Self::FRAME_COUNT, "frame_count", 1));
     }
 
-    pub fn diagnostic_system(mut diagnostics: ResMut<Diagnostics>, time: Res<Time>) {
+    pub fn diagnostic_system(
+        mut diagnostics: ResMut<Diagnostics>,
+        time: Res<Time>,
+        mut state: ResMut<FrameTimeDiagnosticsState>,
+    ) {
+        state.frame_count += 1.0;
+        diagnostics.add_measurement(Self::FRAME_COUNT, state.frame_count);
+
         if time.delta_seconds_f64 == 0.0 {
             return;
         }