From e96a1fb1c76501b95420290fb87ec3749446121b Mon Sep 17 00:00:00 2001
From: Tom Lister <tomlister3@gmail.com>
Date: Thu, 13 Aug 2020 19:19:56 +1000
Subject: [PATCH] ebiten: Implement
 SetClearingScreenSkipped/IsClearingScreenSkipped (#1302)

Fixes #1132
---
 run.go       | 24 ++++++++++++++++++++++--
 uicontext.go |  8 ++++++--
 2 files changed, 28 insertions(+), 4 deletions(-)

diff --git a/run.go b/run.go
index 239d1d542..b8698050a 100644
--- a/run.go
+++ b/run.go
@@ -89,8 +89,9 @@ func CurrentFPS() float64 {
 }
 
 var (
-	isDrawingSkipped = int32(0)
-	currentMaxTPS    = int32(DefaultTPS)
+	isDrawingSkipped        = int32(0)
+	isClearingScreenSkipped = int32(0)
+	currentMaxTPS           = int32(DefaultTPS)
 )
 
 func setDrawingSkipped(skipped bool) {
@@ -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.
diff --git a/uicontext.go b/uicontext.go
index 8ce01c608..bc2355ee6 100644
--- a/uicontext.go
+++ b/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.
-		c.offscreen.Clear()
+		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 {
-		c.offscreen.Clear()
+		if !IsClearingScreenSkipped() {
+			c.offscreen.Clear()
+		}
 		game.Draw(c.offscreen)
 	}