[HTMLCanvasElement.toBlob] Default callback version without scheduler

Source/core/html/HTMLCanvasElement.cpp

Index: Source/core/html/HTMLCanvasElement.cpp
diff --git a/Source/core/html/HTMLCanvasElement.cpp b/Source/core/html/HTMLCanvasElement.cpp
index 50e6afbb428e0bea70f3aace7a0a07cf64e98161..21f45a9b8164e68aefc0c88282349410d1129b11 100644
--- a/Source/core/html/HTMLCanvasElement.cpp
+++ b/Source/core/html/HTMLCanvasElement.cpp
@@ -34,6 +34,7 @@
 #include "core/HTMLNames.h"
 #include "core/dom/Document.h"
 #include "core/dom/ExceptionCode.h"
+#include "core/fileapi/File.h"
 #include "core/frame/LocalFrame.h"
 #include "core/frame/Settings.h"
 #include "core/html/ImageData.h"
@@ -54,6 +55,8 @@
 #include "platform/graphics/gpu/AcceleratedImageBufferSurface.h"
 #include "platform/transforms/AffineTransform.h"
 #include "public/platform/Platform.h"
+#include "public/platform/WebTraceLocation.h"
+#include "wtf/Functional.h"
 #include <math.h>
 #include <v8.h>
 
@@ -538,6 +541,40 @@ String HTMLCanvasElement::toDataURL(const String& mimeType, const ScriptValue& q
     return toDataURLInternal(mimeType, qualityPtr, BackBuffer);
 }
 
+void HTMLCanvasElement::toBlob(FileCallback* callback, const String& mimeType, const ScriptValue& qualityArgument, ExceptionState& exceptionState) const
+{
+    if (!originClean()) {
+        exceptionState.throwSecurityError("Tainted canvases may not be exported.");
+        return;
+    }
+
+    File* resultBlob = nullptr;
+    if (!isPaintable()) {
+        // If the canvas element's bitmap has no pixels
+        return;

[Noel Gordon, 2015/08/28 08:53:47]

I don't think the spec allows us to just return here.  What does the spec say to
do in this case?

[xlai (Olivia), 2015/08/28 19:54:19]

Acknowledged. The spec says "If the canvas element's bitmap has no pixels (i.e.
either its horizontal dimension or its vertical dimension is zero) then let
result be null.". I think I need to fix this in the next round of code review
issue by passing the null resultBlob to the call back function.

+    }
+
+    double quality;
+    if (!qualityArgument.isEmpty()) {
+        v8::Local<v8::Value> v8Value = qualityArgument.v8Value();
+        if (v8Value->IsNumber()) {
+            quality = v8Value.As<v8::Number>()->Value();
+        }
+    }
+
+    String encodingMimeType = toEncodingMimeType(mimeType);
+
+    ImageData* imageData = toImageData(BackBuffer);
+    ScopedDisposal<ImageData> disposer(imageData);
+
+    // Perform image encoding
+    Vector<char> encodedImage;
+    ImageDataBuffer(imageData->size(), imageData->data()->data()).encodeImage(encodingMimeType, &quality, &encodedImage);

[Noel Gordon, 2015/08/28 08:53:47]

encodeImage is bool returning (success/failure).  toDataURL must return "data:,"
on failure, and the <canvas> spec requires that  toBlob similarly handle the
failure case.  What does the spec say to do on failure?

Secondly, the image is encoded on the main thread here, which is not really in
the spirit of the toBlob spec (wants an aysnc encode). Is there a plan to ensure
that this image encode does not jank the main thread?  No point doing toBlob
otherwise.

[xlai (Olivia), 2015/08/28 19:54:19]

Acknowledged. I will explain in the overall message I sent to you.

+    resultBlob = File::create(encodedImage.data(), encodedImage.size(), encodingMimeType);
+
+    Platform::current()->mainThread()->postTask(FROM_HERE, bind(&FileCallback::handleEvent, callback, resultBlob));
+}
+
 SecurityOrigin* HTMLCanvasElement::securityOrigin() const
 {
     return document().securityOrigin();