WASM: Avoid frame copy, pause when not in focus

cmd/dendy-wasm/main.go

@@ -21,7 +21,7 @@ const (
 //go:embed nestest.nes
 var bootROM []byte
 
-func createSystem(joy *input.Joystick, romData []byte) (*system.System, error) {
+func create(joy *input.Joystick, romData []byte) (*system.System, error) {
 	rom, err := ines.NewFromBuffer(romData)
 	if err != nil {
 		return nil, fmt.Errorf("failed to load ROM: %v", err)
@@ -38,10 +38,10 @@ func createSystem(joy *input.Joystick, romData []byte) (*system.System, error) {
 }
 
 func main() {
-	log.SetFlags(0)
+	log.SetFlags(0) // disable timestamps
 	joystick := input.NewJoystick()
 
-	nes, err := createSystem(joystick, bootROM)
+	nes, err := create(joystick, bootROM)
 	if err != nil {
 		log.Fatalf("[ERROR] failed to initialize: %v", err)
 	}
@@ -53,43 +53,43 @@ func main() {
 	)
 
 	js.Global().Set("runFrame", js.FuncOf(func(this js.Value, args []js.Value) any {
+		buttons := args[0].Int()
 		start := time.Now()
-		frameBuf := args[0]
-		buttons := args[1].Int()
+		var framePtr uintptr
 
 		for {
 			nes.Tick()
 			if nes.FrameReady() {
 				frame := nes.Frame()
-				frameBytes := unsafe.Slice((*byte)(unsafe.Pointer(&frame[0])), len(frame)*4)
-				js.CopyBytesToJS(frameBuf, frameBytes) // TODO: can we avoid copying here?
 				joystick.SetButtons(uint8(buttons))
+				framePtr = uintptr(unsafe.Pointer(&frame[0]))
 				break
 			}
 		}
 
 		if debugFrameTime {
-			frameTimeSum += time.Since(start)
+			frameTime := time.Since(start)
+			frameTimeSum += frameTime
 			frameCount++
 
 			if frameCount%120 == 0 {
 				runtime.ReadMemStats(&mem)
-				elapsed := frameTimeSum / time.Duration(frameCount)
-				log.Printf("[DEBUG] frame time: %s, memory: %d", elapsed, mem.Alloc)
+				avgFrameTime := frameTimeSum / time.Duration(frameCount)
+				log.Printf("[INFO] frame time: %v, memory: %d", avgFrameTime, mem.HeapAlloc)
 				frameTimeSum = 0
 				frameCount = 0
 			}
 		}
 
-		return nil
+		return framePtr
 	}))
 
 	js.Global().Set("uploadROM", js.FuncOf(func(this js.Value, args []js.Value) any {
 		data := js.Global().Get("Uint8Array").New(args[0])
 		romData := make([]byte, data.Length())
 		js.CopyBytesToGo(romData, data)
 
-		nes2, err := createSystem(joystick, romData)
+		nes2, err := create(joystick, romData)
 		if err != nil {
 			log.Printf("[ERROR] failed to initialize: %v", err)
 			return false

web/main.js

@@ -6,11 +6,9 @@ const documentReady = new Promise((resolve) => {
   }
 });
 
-let wasm = null;
 const go = new Go();
 const wasmReady = WebAssembly.instantiateStreaming(fetch("dendy.wasm"), go.importObject).then((result) => {
-  wasm = result.instance;
-  go.run(wasm);
+  go.run(result.instance);
 });
 
 Promise.all([wasmReady, documentReady]).then(() => {
@@ -28,8 +26,6 @@ Promise.all([wasmReady, documentReady]).then(() => {
 
   let ctx = canvas.getContext("2d");
   ctx.imageSmoothingEnabled = false;
-
-  let imageData = ctx.createImageData(width, height);
   let buttonsPressed = 0;
 
   const BUTTON_A = 1 << 0;
@@ -90,8 +86,28 @@ Promise.all([wasmReady, documentReady]).then(() => {
     });
   }
 
-  setInterval(() => {
-    runFrame(imageData.data, buttonsPressed);
-    ctx.putImageData(imageData, 0, 0);
-  }, 1000 / targetFPS);
-});
+  function isInFocus() {
+    return document.hasFocus() && document.visibilityState === "visible";
+  }
+
+  function gameLoop() {
+    let nextFrame = () => {
+      let start = performance.now();
+
+      if (isInFocus()) {
+        let framePtr = runFrame(buttonsPressed);
+        let memPtr = go._inst.exports.mem?.buffer || go._inst.exports.memory.buffer; // latter is for TinyGo
+        let image = new ImageData(new Uint8ClampedArray(memPtr, framePtr, width * height * 4), width, height);
+        ctx.putImageData(image, 0, 0);
+      }
+
+      let elapsed = performance.now() - start;
+      let nextTimeout = Math.max(0, (1000 / targetFPS) - elapsed);
+      setTimeout(nextFrame, nextTimeout);
+    };
+
+    nextFrame();
+  }
+
+  gameLoop();
+});