From e7b9481aa9cc49b9af16db9463d40d1ba4554cdd Mon Sep 17 00:00:00 2001
From: Scott Bell <scott@traclabs.com>
Date: Mon, 4 Dec 2023 22:28:24 +0100
Subject: [PATCH] Destroy canvas in plots if not visible (#7263)

* first draft

* add some more debugging

* add test and remove debug

* Remove debug function

* consolidate destroy

* add better canvas name and handle if gl has gone missing

* extra check for extension
---
 .../functional/plugins/tabs/tabs.e2e.spec.js  | 16 +++-
 src/plugins/plot/chart/MctChart.vue           | 87 ++++++++++++++-----
 src/plugins/plot/draw/DrawWebGL.js            | 10 +--
 src/utils/visibility/VisibilityObserver.js    |  2 -
 4 files changed, 84 insertions(+), 31 deletions(-)

diff --git a/e2e/tests/functional/plugins/tabs/tabs.e2e.spec.js b/e2e/tests/functional/plugins/tabs/tabs.e2e.spec.js
index 79ff77c3c2..c702006a84 100644
--- a/e2e/tests/functional/plugins/tabs/tabs.e2e.spec.js
+++ b/e2e/tests/functional/plugins/tabs/tabs.e2e.spec.js
@@ -54,21 +54,35 @@ test.describe('Tabs View', () => {
     // ensure table header visible
     await expect(page.getByRole('searchbox', { name: 'message filter input' })).toBeVisible();
 
+    // no canvas (i.e., sine wave generator) in the document should be visible
+    await expect(page.locator('canvas')).toBeHidden();
+
     // select second tab
     await page.getByLabel(`${notebook.name} tab`).click();
 
     // ensure notebook visible
     await expect(page.locator('.c-notebook__drag-area')).toBeVisible();
 
+    // no canvas (i.e., sine wave generator) in the document should be visible
+    await expect(page.locator('canvas')).toBeHidden();
+
     // select third tab
     await page.getByLabel(`${sineWaveGenerator.name} tab`).click();
 
     // expect sine wave generator visible
-    expect(await page.locator('.c-plot').isVisible()).toBe(true);
+    await expect(page.locator('.c-plot')).toBeVisible();
+
+    // expect two canvases (i.e., overlay & main canvas for sine wave generator) to be visible
+    await expect(page.locator('canvas')).toHaveCount(2);
+    await expect(page.locator('canvas').nth(0)).toBeVisible();
+    await expect(page.locator('canvas').nth(1)).toBeVisible();
 
     // now try to select the first tab again
     await page.getByLabel(`${table.name} tab`).click();
     // ensure table header visible
     await expect(page.getByRole('searchbox', { name: 'message filter input' })).toBeVisible();
+
+    // no canvas (i.e., sine wave generator) in the document should be visible
+    await expect(page.locator('canvas')).toBeHidden();
   });
 });
diff --git a/src/plugins/plot/chart/MctChart.vue b/src/plugins/plot/chart/MctChart.vue
index d02867404e..cb503073b7 100644
--- a/src/plugins/plot/chart/MctChart.vue
+++ b/src/plugins/plot/chart/MctChart.vue
@@ -22,8 +22,8 @@
 
 <template>
   <div ref="chart" class="gl-plot-chart-area">
-    <canvas :style="canvasStyle"></canvas>
-    <canvas :style="canvasStyle"></canvas>
+    <canvas :style="canvasStyle" class="js-overlay-canvas"></canvas>
+    <canvas :style="canvasStyle" class="js-main-canvas"></canvas>
     <div ref="limitArea" class="js-limit-area">
       <limit-label
         v-for="(limitLabel, index) in visibleLimitLabels"
@@ -197,6 +197,10 @@ export default {
     }
   },
   mounted() {
+    this.chartVisible = true;
+    this.chartContainer = this.$refs.chart;
+    this.visibilityObserver = new IntersectionObserver(this.visibilityChanged);
+    this.visibilityObserver.observe(this.chartContainer);
     eventHelpers.extend(this);
     this.seriesModels = [];
     this.config = this.getConfig();
@@ -239,10 +243,8 @@ export default {
     this.seriesElements = new WeakMap();
     this.seriesLimits = new WeakMap();
 
-    let canvasEls = this.$parent.$refs.chartContainer.querySelectorAll('canvas');
-    const mainCanvas = canvasEls[1];
-    const overlayCanvas = canvasEls[0];
-    if (this.initializeCanvas(mainCanvas, overlayCanvas)) {
+    const canvasReadyForDrawing = this.readyCanvasForDrawing();
+    if (canvasReadyForDrawing) {
       this.draw();
     }
 
@@ -256,6 +258,7 @@ export default {
   },
   beforeUnmount() {
     this.destroy();
+    this.visibilityObserver.unobserve(this.chartContainer);
   },
   methods: {
     getConfig() {
@@ -272,6 +275,26 @@ export default {
 
       return config;
     },
+    visibilityChanged([entry]) {
+      if (entry.target === this.chartContainer) {
+        const wasVisible = this.chartVisible;
+        this.chartVisible = entry.isIntersecting;
+        if (!this.chartVisible) {
+          // destroy the chart
+          this.destroyCanvas();
+        } else if (!wasVisible && this.chartVisible) {
+          // rebuild the chart
+          this.buildCanvasElements();
+          const canvasInitialized = this.readyCanvasForDrawing();
+          if (canvasInitialized) {
+            this.draw();
+          }
+          this.$emit('plot-reinitialize-canvas');
+        } else if (wasVisible && this.chartVisible) {
+          // ignore, moving on
+        }
+      }
+    },
     reDraw(newXKey, oldXKey, series) {
       this.changeInterpolate(newXKey, oldXKey, series);
       this.changeMarkers(newXKey, oldXKey, series);
@@ -417,13 +440,12 @@ export default {
       this.scheduleDraw();
     },
     destroy() {
+      this.destroyCanvas();
       this.isDestroyed = true;
-      this.stopListening();
       this.lines.forEach((line) => line.destroy());
       this.limitLines.forEach((line) => line.destroy());
       this.pointSets.forEach((pointSet) => pointSet.destroy());
       this.alarmSets.forEach((alarmSet) => alarmSet.destroy());
-      DrawLoader.releaseDrawAPI(this.drawAPI);
     },
     resetYOffsetAndSeriesDataForYAxis(yAxisId) {
       delete this.offset[yAxisId].y;
@@ -477,33 +499,49 @@ export default {
         return this.offset[yAxisId].y(pSeries.getYVal(point));
       }.bind(this);
     },
-
-    initializeCanvas(canvas, overlay) {
-      this.canvas = canvas;
-      this.overlay = overlay;
-      this.drawAPI = DrawLoader.getDrawAPI(canvas, overlay);
+    destroyCanvas() {
+      if (this.isDestroyed) {
+        return;
+      }
+      this.stopListening(this.drawAPI);
+      DrawLoader.releaseDrawAPI(this.drawAPI);
+      if (this.chartContainer) {
+        const canvasElements = this.chartContainer.querySelectorAll('canvas');
+        canvasElements.forEach((canvas) => {
+          canvas.parentNode.removeChild(canvas);
+        });
+      }
+    },
+    readyCanvasForDrawing() {
+      const canvasEls = this.chartContainer.querySelectorAll('canvas');
+      const mainCanvas = canvasEls[1];
+      const overlayCanvas = canvasEls[0];
+      this.canvas = mainCanvas;
+      this.overlay = overlayCanvas;
+      this.drawAPI = DrawLoader.getDrawAPI(mainCanvas, overlayCanvas);
       if (this.drawAPI) {
         this.listenTo(this.drawAPI, 'error', this.fallbackToCanvas, this);
       }
 
       return Boolean(this.drawAPI);
     },
-    fallbackToCanvas() {
-      this.stopListening(this.drawAPI);
-      DrawLoader.releaseDrawAPI(this.drawAPI);
-      // Have to throw away the old canvas elements and replace with new
-      // canvas elements in order to get new drawing contexts.
+    buildCanvasElements() {
       const div = document.createElement('div');
       div.innerHTML = `
-      <canvas style="position: absolute; background: none; width: 100%; height: 100%;"></canvas>
-      <canvas style="position: absolute; background: none; width: 100%; height: 100%;"></canvas>
+      <canvas style="position: absolute; background: none; width: 100%; height: 100%;" class="js-overlay-canvas"></canvas>
+      <canvas style="position: absolute; background: none; width: 100%; height: 100%;" class="js-main-canvas"></canvas>
       `;
       const mainCanvas = div.querySelectorAll('canvas')[1];
       const overlayCanvas = div.querySelectorAll('canvas')[0];
-      this.canvas.parentNode.replaceChild(mainCanvas, this.canvas);
+      this.chartContainer.appendChild(mainCanvas, this.canvas);
       this.canvas = mainCanvas;
-      this.overlay.parentNode.replaceChild(overlayCanvas, this.overlay);
+      this.chartContainer.appendChild(overlayCanvas, this.overlay);
       this.overlay = overlayCanvas;
+    },
+    fallbackToCanvas() {
+      console.warn(`📈 fallback to 2D canvas`);
+      this.destroyCanvas();
+      this.buildCanvasElements();
       this.drawAPI = DrawLoader.getFallbackDrawAPI(this.canvas, this.overlay);
       this.$emit('plot-reinitialize-canvas');
     },
@@ -653,7 +691,7 @@ export default {
     },
     draw() {
       this.drawScheduled = false;
-      if (this.isDestroyed) {
+      if (this.isDestroyed || !this.chartVisible) {
         return;
       }
 
@@ -681,6 +719,9 @@ export default {
       });
     },
     updateViewport(yAxisId) {
+      if (!this.chartVisible) {
+        return;
+      }
       const mainYAxisId = this.config.yAxis.get('id');
       const xRange = this.config.xAxis.get('displayRange');
       let yRange;
diff --git a/src/plugins/plot/draw/DrawWebGL.js b/src/plugins/plot/draw/DrawWebGL.js
index f9c4b16ef1..f9b2fc0ad7 100644
--- a/src/plugins/plot/draw/DrawWebGL.js
+++ b/src/plugins/plot/draw/DrawWebGL.js
@@ -154,14 +154,14 @@ DrawWebGL.prototype.initContext = function () {
 DrawWebGL.prototype.destroy = function () {
   // Lose the context and delete all associated resources
   // https://developer.mozilla.org/en-US/docs/Web/API/WebGL_API/WebGL_best_practices#lose_contexts_eagerly
-  this.gl.getExtension('WEBGL_lose_context').loseContext();
-  this.gl.deleteBuffer(this.buffer);
+  this.gl?.getExtension('WEBGL_lose_context')?.loseContext();
+  this.gl?.deleteBuffer(this.buffer);
   this.buffer = undefined;
-  this.gl.deleteProgram(this.program);
+  this.gl?.deleteProgram(this.program);
   this.program = undefined;
-  this.gl.deleteShader(this.vertexShader);
+  this.gl?.deleteShader(this.vertexShader);
   this.vertexShader = undefined;
-  this.gl.deleteShader(this.fragmentShader);
+  this.gl?.deleteShader(this.fragmentShader);
   this.fragmentShader = undefined;
   this.gl = undefined;
 
diff --git a/src/utils/visibility/VisibilityObserver.js b/src/utils/visibility/VisibilityObserver.js
index c78086d754..2088c6aed5 100644
--- a/src/utils/visibility/VisibilityObserver.js
+++ b/src/utils/visibility/VisibilityObserver.js
@@ -38,8 +38,6 @@ export default class VisibilityObserver {
     if (!element) {
       throw new Error(`VisibilityObserver must be created with an element`);
     }
-    // set the id to some random 4 letters
-    this.id = Math.random().toString(36).substring(2, 6);
     this.#element = element;
     this.isIntersecting = true;