Bug 1517786 - Implement the getContextAttributes() method for CanvasRenderingContext2D. r=gfx-reviewers,lsalzman,emilio

This has been in the HTML spec for a while now (see https://html.spec.whatwg.org/multipage/canvas.html#2dcontext),
so we should probably go ahead and implement it.

Differential Revision: https://phabricator.services.mozilla.com/D184499

dom/canvas/CanvasRenderingContext2D.cpp

@@ -1136,6 +1136,17 @@ JSObject* CanvasRenderingContext2D::WrapObject(
   return CanvasRenderingContext2D_Binding::Wrap(aCx, this, aGivenProto);
 }
 
+void CanvasRenderingContext2D::GetContextAttributes(
+    CanvasRenderingContext2DSettings& aSettings) const {
+  aSettings = CanvasRenderingContext2DSettings();
+
+  aSettings.mAlpha = mContextAttributesHasAlpha;
+  aSettings.mWillReadFrequently = mWillReadFrequently;
+
+  // We don't support the 'desynchronized' and 'colorSpace' attributes, so
+  // those just keep their default values.
+}
+
 CanvasRenderingContext2D::ColorStyleCacheEntry
 CanvasRenderingContext2D::ParseColorSlow(const nsACString& aString) {
   ColorStyleCacheEntry result{nsCString(aString)};
@@ -1881,7 +1892,7 @@ CanvasRenderingContext2D::SetContextOptions(JSContext* aCx,
   // drawtarget yet
   MOZ_ASSERT(!mTarget);
 
-  ContextAttributes2D attributes;
+  CanvasRenderingContext2DSettings attributes;
   if (!attributes.Init(aCx, aOptions)) {
     aRvForDictionaryInit.Throw(NS_ERROR_UNEXPECTED);
     return NS_ERROR_UNEXPECTED;

dom/canvas/CanvasRenderingContext2D.h

@@ -96,6 +96,8 @@ class CanvasRenderingContext2D : public nsICanvasRenderingContextInternal,
     return mCanvasElement->GetOriginalCanvas();
   }
 
+  void GetContextAttributes(CanvasRenderingContext2DSettings& aSettings) const;
+
   void OnMemoryPressure() override;
   void OnBeforePaintTransaction() override;
   void OnDidPaintTransaction() override;

dom/webidl/CanvasRenderingContext2D.webidl

@@ -24,11 +24,16 @@ enum CanvasFontVariantCaps { "normal", "small-caps", "all-small-caps", "petite-c
 enum CanvasTextRendering { "auto", "optimizeSpeed", "optimizeLegibility", "geometricPrecision" };
 
 [GenerateInit]
-dictionary ContextAttributes2D {
-  // whether or not we're planning to do a lot of readback operations
-  boolean willReadFrequently = false;
+dictionary CanvasRenderingContext2DSettings {
   // signal if the canvas contains an alpha channel
   boolean alpha = true;
+
+  boolean desynchronized = false;
+
+  PredefinedColorSpace colorSpace = "srgb";
+
+  // whether or not we're planning to do a lot of readback operations
+  boolean willReadFrequently = false;
 };
 
 dictionary HitRegionOptions {
@@ -54,6 +59,8 @@ interface CanvasRenderingContext2D {
   // associated with a canvas.
   readonly attribute HTMLCanvasElement? canvas;
 
+  CanvasRenderingContext2DSettings getContextAttributes();
+
   // Show the caret if appropriate when drawing
   [Func="CanvasUtils::HasDrawWindowPrivilege"]
   const unsigned long DRAWWINDOW_DRAW_CARET   = 0x01;

testing/web-platform/meta/html/canvas/element/manual/context-attributes/getContextAttributes.html.ini

@@ -1,17 +0,0 @@
-[getContextAttributes.html]
-  expected:
-    if (os == "android") and fission: [OK, TIMEOUT]
-  [Test context creation attributes alpha: true]
-    expected: FAIL
-
-  [Test context creation attributes alpha: false]
-    expected: FAIL
-
-  [Test default context creation attributes]
-    expected: FAIL
-
-  [Test context creation attributes desynchronized: false]
-    expected: FAIL
-
-  [Test context creation attributes willReadFrequently: true]
-    expected: FAIL