ebiten: Ensure that Update is called at least once before Draw in the first frame

Fixes #1155

run.go

@@ -35,6 +35,9 @@ type Game interface {
 	// With Draw (the recommended way), Update updates only the game logic and Draw draws the screen.
 	// In this case, the argument screen's updated content by Update is not adopted for the actual game screen,
 	// and the screen's updated content by Draw is adopted instead.
+	// In the first frame, it is ensured that Update is called at least once before Draw. You can use Update
+	// to initialize the game state. After the first frame, Update might not be called or might be called
+	// multiple times for one frame. The frequency is determined by the current TPS (tick-per-second).
 	//
 	// Without Draw (the legacy way), Update updates the game logic and also draws the screen.
 	// In this case, the argument screen's updated content by Update is adopted for the actual game screen.

uicontext.go

@@ -50,6 +50,8 @@ type uiContext struct {
 	offscreen *Image
 	screen    *Image
 
+	updateCalled bool
+
 	// scaleForWindow is the scale of a window. This doesn't represent the scale on fullscreen. This value works
 	// only on desktops.
 	//
@@ -263,6 +265,13 @@ func (c *uiContext) update() error {
 	_, hasDraw := c.game.(interface{ Draw(*Image) })
 
 	updateCount := clock.Update(MaxTPS())
+
+	// Ensure that Update is called once before Draw so that Update can be used for initialization.
+	if !c.updateCalled && updateCount == 0 {
+		updateCount = 1
+		c.updateCalled = true
+	}
+
 	for i := 0; i < updateCount; i++ {
 		c.updateOffscreen()