Add methods for creating V8 extras-based ReadableStreams from C++

third_party/WebKit/Source/core/streams/UnderlyingSourceBase.cpp

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "core/streams/UnderlyingSourceBase.h"
+
+#include "bindings/core/v8/ScriptPromise.h"
+#include "bindings/core/v8/ScriptState.h"
+#include "bindings/core/v8/ScriptValue.h"
+#include "core/streams/ReadableStreamController.h"
+#include <v8.h>
+
+namespace blink {
+
+ScriptPromise UnderlyingSourceBase::startWrapper(ScriptState* scriptState, ScriptValue stream)
+{
+    // Cannot call start twice (e.g., cannot use the same UnderlyingSourceBase to construct multiple streams)
+    ASSERT(!m_controller);
+
+    // In ReadableStream.js, we special-case externally-controlled streams by having them pass themselves to start
+    // as the first argument. This allows us to create a ReadableStreamController.
+
+    m_controller = new ReadableStreamController(stream);
+
+    return start(scriptState);
+}
+
+ScriptPromise UnderlyingSourceBase::start(ScriptState* scriptState)
+{
+    return ScriptPromise::cast(scriptState, v8::Undefined(scriptState->isolate()));
+}
+
+ScriptPromise UnderlyingSourceBase::pull(ScriptState* scriptState)
+{
+    return ScriptPromise::cast(scriptState, v8::Undefined(scriptState->isolate()));
+}
+
+ScriptPromise UnderlyingSourceBase::cancelWrapper(ScriptState* scriptState, ScriptValue reason)
+{
+    m_controller->noteHasBeenCanceled();
+    return cancel(scriptState, reason);
+}
+
+ScriptPromise UnderlyingSourceBase::cancel(ScriptState* scriptState, ScriptValue reason)
+{
+    return ScriptPromise::cast(scriptState, v8::Undefined(scriptState->isolate()));
+}
+
+bool UnderlyingSourceBase::hasPendingActivity() const
+{
+    if (executionContext()->activeDOMObjectsAreStopped())

[haraken, 2016/02/04 01:27:50]

It looks a bit strange that you need to have the activeDOMObjectsAreStopped()
check in hasPendingActivity().

Is it really okay to collect the wrapper even though the execution context can
be resumed in the future?

[yhirano, 2016/02/04 01:31:28]

haraken@, are you talking about activeDOMObjectsAreSuspended? IIUC "stop" is
irrevocable.

+        return false;
+
+    // This will return false within a finite time period _assuming_ that
+    // consumers use the controller to close or error the stream.
+    // Browser-created readable streams should always close or error within a
+    // finite time period, due to timeouts etc.
+    return m_controller && m_controller->isActive();
+}
+
+DEFINE_TRACE(UnderlyingSourceBase)
+{
+    ActiveDOMObject::trace(visitor);
+    visitor->trace(m_controller);
+}
+
+} // namespace blink