MSE: Implement TrackDefault object

Source/modules/mediasource/TrackDefault.h

Index: Source/modules/mediasource/TrackDefault.h
diff --git a/Source/modules/mediasource/TrackDefault.h b/Source/modules/mediasource/TrackDefault.h
new file mode 100644
index 0000000000000000000000000000000000000000..34614e0e24d5689e813a2dc13725721fdfc1113f
--- /dev/null
+++ b/Source/modules/mediasource/TrackDefault.h
@@ -0,0 +1,56 @@
+// Copyright 2014 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.
+
+#ifndef TrackDefault_h
+#define TrackDefault_h
+
+#include "bindings/core/v8/ScriptWrappable.h"
+#include "wtf/text/WTFString.h"
+
+namespace blink {
+
+class ExceptionState;
+
+class TrackDefault final : public GarbageCollectedFinalized<TrackDefault>, public ScriptWrappable {
+    DEFINE_WRAPPERTYPEINFO();
+public:
+    enum TrackDefaultTypeCode {
+        TrackDefaultTypeAudio,
+        TrackDefaultTypeVideo,
+        TrackDefaultTypeText
+    };
+
+    static TrackDefault* create(const AtomicString& type, const String& language, const String& label, const Vector<String>& kinds, const String& byteStreamTrackID, ExceptionState& exceptionState)
+    {
+        return new TrackDefault(type, language, label, kinds, byteStreamTrackID, exceptionState);
+    }
+
+    static const AtomicString& audioKeyword();

[philipj_slow, 2014/11/14 12:03:37]

Unlike some keywords on TextTrack and VideoTrack, these don't need to be in the
header, as they're only used internally.

[wolenetz, 2014/11/17 21:51:07]

Done.

+    static const AtomicString& videoKeyword();
+    static const AtomicString& textKeyword();
+
+    virtual ~TrackDefault();
+
+    // Implement the IDL
+    String type() const { return m_type; }

[philipj_slow, 2014/11/14 12:03:37]

This is AtomicString internally, so return AtomicString?

[wolenetz, 2014/11/17 21:51:07]

Done.

+    String byteStreamTrackID() const { return m_byteStreamTrackID; }
+    String language() const { return m_language; }
+    String label() const { return m_label; }
+    Vector<String> kinds() const { return m_kinds; }

[philipj_slow, 2014/11/14 12:03:37]

const Vector<String>&, or the vector will be copied I think. (I'm not sure how
cheap AtomicString and String are to copy, it seems like return-by-value and
return-by-reference-to-const are both common.)

[wolenetz, 2014/11/17 21:51:07]

I'm not quite sure myself how the default v8 bindings work w.r.t. copy vs const
ref. Scanning other idl implementations that return Vector<String>, I see both
const-ref and just return-by-value.

Is it truly a safe assumption that any const ref of the retval Vector<String>
will not outlive the lifetime of a TrackDefault? If we cannot prove this
assumption, then we could introduce a use-after-free by returning const-ref
here.

What do you think?

[philipj_slow, 2014/11/18 10:47:31]

Lifetime isn't a concern here, because the bindings will copy the array into a
v8 array, see v8Array in V8Binding.h, which is what the generated
kindsAttributeGetter in V8TrackDefault.cpp uses.

There is a frustrating lack of consistency with how AtomicString and String are
returned, so I took a look in a debugger. Returning String by value invokes the
copy constructor, which is cheap enough because the underlying StringImpl is a
RefPtr. AtomicString has an internal String, so it boils down to the same thing.
The reference counting churn is unnecessary, but hardly a problem since
returning String by value is very common.

Returning Vector<String> by value also invokes the copy constructor, but unlike
String that needs to allocate on the heap. It doubt it will ever show up in a
benchmark, but it's just unnecessary.

So, return kinds() by reference-to-const and the AtomicString/String by value or
reference-to-const, whichever you think looks pretty.

[wolenetz, 2014/11/20 23:10:54]

Done as ref-to-const Vector<String>, and leaving the other property getters
as-is from PS9. Thanks for explanation and digging in a debugger :)

+
+    void trace(Visitor*) { }
+
+private:
+    TrackDefault(const AtomicString& type, const String& language, const String& label, const Vector<String>& kinds, const String& byteStreamTrackID, ExceptionState&);
+
+    AtomicString m_type;
+    String m_byteStreamTrackID;
+    String m_language;
+    String m_label;
+    Vector<String> m_kinds;
+};
+
+} // namespace blink
+
+#endif // TrackDefault_h