ebiten: Implement SetClearingScreenSkipped/IsClearingScreenSkipped (#1302)

Fixes #1132

run.go

@@ -90,6 +90,7 @@ func CurrentFPS() float64 {
 
 var (
 	isDrawingSkipped        = int32(0)
+	isClearingScreenSkipped = int32(0)
 	currentMaxTPS           = int32(DefaultTPS)
 )
 
@@ -101,6 +102,25 @@ func setDrawingSkipped(skipped bool) {
 	atomic.StoreInt32(&isDrawingSkipped, v)
 }
 
+// SetClearingScreenSkipped enables or disables the clearing of the screen at the beginning of each frame.
+// The default value is false and the screen is cleared each frame by default.
+//
+// SetClearingScreenSkipped is concurrent-safe.
+func SetClearingScreenSkipped(skipped bool) {
+	v := int32(0)
+	if skipped {
+		v = 1
+	}
+	atomic.StoreInt32(&isClearingScreenSkipped, v)
+}
+
+// IsClearingScreenSkipped returns true if the frame isn't cleared at the beginning.
+//
+// IsClearingScreenSkipped is concurrent-safe.
+func IsClearingScreenSkipped() bool {
+	return atomic.LoadInt32(&isClearingScreenSkipped) != 0
+}
+
 // IsDrawingSkipped returns true if rendering result is not adopted.
 // It is recommended to skip drawing images or screen
 // when IsDrawingSkipped is true.

uicontext.go

@@ -290,7 +290,9 @@ func (c *uiContext) update() error {
 
 		// Multiple successive Clear call should be integrated into one graphics command, then
 		// calling Clear on every Update should not affect the performance.
+		if !IsClearingScreenSkipped() {
 			c.offscreen.Clear()
+		}
 		if err := c.game.Update(c.offscreen); err != nil {
 			return err
 		}
@@ -306,7 +308,9 @@ func (c *uiContext) draw() {
 	}
 
 	if game, ok := c.game.(interface{ Draw(*Image) }); ok {
+		if !IsClearingScreenSkipped() {
 			c.offscreen.Clear()
+		}
 		game.Draw(c.offscreen)
 	}