From ca443d6e6c46aefeef600a84bece426908dbd951 Mon Sep 17 00:00:00 2001
From: pacien
Date: Fri, 20 Jul 2018 22:18:36 +0200
Subject: Fix canvas flicker

---
 pointless/viewer/screen.js | 96 ++++++++++++++++++++++++----------------------
 1 file changed, 50 insertions(+), 46 deletions(-)

(limited to 'pointless/viewer/screen.js')

diff --git a/pointless/viewer/screen.js b/pointless/viewer/screen.js
index 42ea781..9ba4574 100644
--- a/pointless/viewer/screen.js
+++ b/pointless/viewer/screen.js
@@ -11,79 +11,83 @@ class Screen {
   constructor(window, secondary=false, withTimer=false) {
     this.window = window;
     this.secondary = secondary;
-    this.canvas = window.document.getElementById("screen");
-    this.context = this.canvas.getContext("2d");
-    this.page = null;
 
-    var self = this;
-    this.window.addEventListener("resize", function() {
-      self._refreshPage();
-    });
+    this.canvasId = "screen";
+    this.page = null;
 
     this.timer = withTimer ? new Timer(window) : null;
     this.pageTurnCount = 0;
+
+    this._registerListeners();
   }
 
   setPage(page) {
-    if (this.pageTurnCount === 1 && this.timer != null)
+    if (this.pageTurnCount++ === 1 && this.timer != null)
       this.timer.start();
 
     this.page = page;
-    this._changePage();
-    this.pageTurnCount++;
+    this._repaint();
+  }
+
+  _registerListeners() {
+    const self = this;
+    this.window.addEventListener("resize", function() {
+      self._repaint();
+    });
   }
 
-  _resizeScreen(ratio) {
-    var windowRatio = this.window.innerWidth / this.window.innerHeight;
-    var scaleFactor = ratio / windowRatio;
-    this.canvas.width = this.window.innerWidth * Math.min(scaleFactor, 1);
-    this.canvas.height = this.window.innerHeight / Math.max(scaleFactor, 1);
+  _getScreenSize(ratio) {
+    const windowRatio = this.window.innerWidth / this.window.innerHeight;
+    const horizontalScaleFactor = ratio / windowRatio;
+    return {
+      width: this.window.innerWidth * Math.min(horizontalScaleFactor, 1),
+      height: this.window.innerHeight / Math.max(horizontalScaleFactor, 1)
+    };
   }
 
-  _setOffset() {
-    var xOffset = this.secondary ? -this.canvas.width : 0;
-    this.context.transform(1, 0, 0, 1, xOffset, 0);
+  _getSlideSizeRatio() {
+    const viewport = this.page.getViewport(1);
+    return (viewport.width / 2) / viewport.height;
   }
 
-  _makeWorkCanvas(width, height) {
-    var canvas = document.createElement("canvas");
+  _newCanvas(width, height, xOffset, yOffset) {
+    const canvas = document.createElement("canvas");
     canvas.width = width;
     canvas.height = height;
-    return canvas;
+
+    const context = canvas.getContext("2d");
+    context.transform(1, 0, 0, 1, xOffset, yOffset);
+
+    return { canvas: canvas, context: context };
   }
 
-  _transferCanvas(source) {
-    this.context.drawImage(source, 0, 0);
+  _showCanvas(canvas) {
+    const oldCanvas = this.window.document.getElementById(this.canvasId);
+    canvas.id = oldCanvas.id;
+    canvas.classList = oldCanvas.classList;
+    oldCanvas.replaceWith(canvas);
   }
 
-  _paintPage() {
-    var renderRatio = this.canvas.height / this.page.getViewport(1).height;
-    var renderViewport = this.page.getViewport(renderRatio);
-    var workCanvas = this._makeWorkCanvas(renderViewport.width, renderViewport.height);
-    var workContext = workCanvas.getContext("2d");
-    var renderContext = { canvasContext: workContext, viewport: renderViewport };
+  _render(canvas, context, scaleFactor) {
+    const renderContext = {
+      canvasContext: context,
+      viewport: this.page.getViewport(scaleFactor)
+    };
 
-    var self = this;
+    const self = this;
     this.page.render(renderContext).then(function() {
-      self._transferCanvas(workCanvas);
+      self._showCanvas(canvas);
     });
   }
 
-  _changePage() {
-    this.canvas.style.opacity = 0;
-    var self = this;
-    setTimeout(function() {
-      self._refreshPage();
-      self.canvas.style.opacity = 1;
-    }, 300);
-  }
-
-  _refreshPage() {
+  _repaint() {
     if (this.page == null) return;
-    var viewport = this.page.getViewport(1);
-    var ratio = (viewport.width / 2) / viewport.height;
-    this._resizeScreen(ratio);
-    this._setOffset();
-    this._paintPage();
+
+    const screenRatio = this._getSlideSizeRatio();
+    const { width, height } = this._getScreenSize(screenRatio);
+    const scaleFactor = height / this.page.getViewport(1).height;
+    const xOffset = this.secondary ? -width : 0;
+    const { canvas, context } = this._newCanvas(width, height, xOffset, 0);
+    this._render(canvas, context, scaleFactor);
   }
 }
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