fix(TextMetrics): rtl direction + start/end textAlign (#2510)

* fix(TextMetrics): rtl direction + start/end textAlign
* changelog
* update tests
* expose resolveTextAlignment
* jest test

Author: ShaMan123

CHANGELOG.md

@@ -15,6 +15,7 @@ project adheres to [Semantic Versioning](http://semver.org/).
 * Fix fetching prebuilds during installation on certain newer versions of Node (#2497)
 * Fixed issue with fillText that was breaking subsequent fillText calls (#2171)
 * Fix svg rendering when the image is resized (#2498)
+* Fix measureText with direction rtl textAlign start/end
 
 3.1.0
 ==================

src/CanvasRenderingContext2d.cc

@@ -2557,6 +2557,20 @@ inline double getBaselineAdjustment(PangoLayout* layout, short baseline) {
   }
 }
 
+text_align_t
+Context2d::resolveTextAlignment() {
+  text_align_t alignment = state->textAlignment;
+
+  // Convert start/end to left/right based on direction
+  if (alignment == TEXT_ALIGNMENT_START) {
+    return (state->direction == "rtl") ? TEXT_ALIGNMENT_RIGHT : TEXT_ALIGNMENT_LEFT;
+  } else if (alignment == TEXT_ALIGNMENT_END) {
+    return (state->direction == "rtl") ? TEXT_ALIGNMENT_LEFT : TEXT_ALIGNMENT_RIGHT;
+  }
+
+  return alignment;
+}
+
 /*
  * Set text path for the string in the layout at (x, y).
  * This function is called by paintText and won't behave correctly
@@ -2567,14 +2581,7 @@ inline double getBaselineAdjustment(PangoLayout* layout, short baseline) {
 void
 Context2d::setTextPath(double x, double y) {
   PangoRectangle logical_rect;
-  text_align_t alignment = state->textAlignment;
-
-  // Convert start/end to left/right based on direction
-  if (alignment == TEXT_ALIGNMENT_START) {
-    alignment = (state->direction == "rtl") ? TEXT_ALIGNMENT_RIGHT : TEXT_ALIGNMENT_LEFT;
-  } else if (alignment == TEXT_ALIGNMENT_END) {
-    alignment = (state->direction == "rtl") ? TEXT_ALIGNMENT_LEFT : TEXT_ALIGNMENT_RIGHT;
-  }
+  text_align_t alignment = resolveTextAlignment();
 
   switch (alignment) {
     case TEXT_ALIGNMENT_CENTER:
@@ -2816,16 +2823,16 @@ Context2d::MeasureText(const Napi::CallbackInfo& info) {
 
   metrics = PANGO_LAYOUT_GET_METRICS(layout);
 
+  text_align_t alignment = resolveTextAlignment();
+
   double x_offset;
-  switch (state->textAlignment) {
+  switch (alignment) {
     case TEXT_ALIGNMENT_CENTER:
       x_offset = logical_rect.width / 2.;
       break;
-    case TEXT_ALIGNMENT_END:
     case TEXT_ALIGNMENT_RIGHT:
       x_offset = logical_rect.width;
       break;
-    case TEXT_ALIGNMENT_START:
     case TEXT_ALIGNMENT_LEFT:
     default:
       x_offset = 0.0;

src/CanvasRenderingContext2d.h

@@ -222,6 +222,7 @@ class Context2d : public Napi::ObjectWrap<Context2d> {
     void _setStrokePattern(Napi::Value arg);
     void checkFonts();
     void paintText(const Napi::CallbackInfo&, bool);
+    text_align_t resolveTextAlignment();
     Napi::Reference<Napi::Value> _fillStyle;
     Napi::Reference<Napi::Value> _strokeStyle;
     Canvas *_canvas;

test/canvas.test.js

@@ -1026,6 +1026,39 @@ describe('Canvas', function () {
       assertApprox(rm.actualBoundingBoxLeft, 19, 6)
       assertApprox(rm.actualBoundingBoxRight, 1, 6)
     })
+
+    it('resolves text alignment wrt Context2d#direction #2508', function () {
+      const canvas = createCanvas(0, 0)
+      const ctx = canvas.getContext('2d')
+
+      ctx.textAlign = "left";
+      const leftMetrics = ctx.measureText('hello');
+      assert(leftMetrics.actualBoundingBoxLeft < leftMetrics.actualBoundingBoxRight, "leftMetrics.actualBoundingBoxLeft < leftMetrics.actualBoundingBoxRight");
+      
+      ctx.textAlign = "right";
+      const rightMetrics = ctx.measureText('hello');
+      assert(rightMetrics.actualBoundingBoxLeft > rightMetrics.actualBoundingBoxRight, "metrics.actualBoundingBoxLeft > metrics.actualBoundingBoxRight");
+      
+      ctx.textAlign = "start";
+      
+      ctx.direction = "ltr";
+      const ltrStartMetrics = ctx.measureText('hello');
+      assert.deepStrictEqual(ltrStartMetrics, leftMetrics, "ltr start metrics should equal left metrics");
+        
+      ctx.direction = "rtl";
+      const rtlStartMetrics = ctx.measureText('hello');
+      assert.deepStrictEqual(rtlStartMetrics, rightMetrics, "rtl start metrics should equal right metrics");
+      
+      ctx.textAlign = "end";
+      
+      ctx.direction = "ltr";
+      const ltrEndMetrics = ctx.measureText('hello');
+      assert.deepStrictEqual(ltrEndMetrics, rightMetrics, "ltr end metrics should equal right metrics");
+        
+      ctx.direction = "rtl";
+      const rtlEndMetrics = ctx.measureText('hello');
+      assert.deepStrictEqual(rtlEndMetrics, leftMetrics, "rtl end metrics should equal left metrics");
+    })
   })
 
   it('Context2d#fillText()', function () {