initial import of Chrome's trace_event into skia framework

include/utils/SkEventTracer.h

+/*
+ * Copyright (C) 2014 Google Inc. All rights reserved.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef SkEventTracer_DEFINED
+#define SkEventTracer_DEFINED
+
+// The class in this header defines the interface between Skia's internal
+// tracing macros and an external entity (e.g., Chrome) that will consume them.
+// Such an entity should subclass SkEventTracer and provide an instance of 
+// that event to SkEventTracer::SetInstance.
+
+// If you're looking for the tracing macros to instrument Skia itself, those
+// live in src/core/SkTraceEvent.h
+
+#include "SkTypes.h"
+
+// This will mark the trace event as disabled by default. The user will need
+// to explicitly enable the event.
+#define TRACE_DISABLED_BY_DEFAULT(name) "disabled-by-default-" name
+
+typedef uint32_t SkTraceEventHandle;

[mtklein, 2014/01/30 14:18:26]

Suggestion: make this SkEventTracer::Handle ?

[humper, 2014/01/30 16:03:38]

I like that a lot better; done.

+
+class SK_API SkEventTracer {
+public:
+
+    static SkEventTracer* GetInstance();
+
+    static void SetInstance(SkEventTracer* tracer) { 
+        SkDELETE(SkEventTracer::gInstance);
+        SkEventTracer::gInstance = tracer;
+    }
+
+    virtual ~SkEventTracer() { }

[mtklein, 2014/01/30 14:18:26]

Sorry, somehow I'd brainfarted and not realized it was an abstract class. 
Keeping the destructor here is not currently necessary but good practice.

+
+    // The pointer returned from GetCategoryGroupEnabled() points to a
+    // value with zero or more of the following bits. Used in this class only.
+    // The TRACE_EVENT macros should only use the value as a bool.
+    // These values must be in sync with macro values in trace_event.h in chromium.

[mtklein, 2014/01/30 14:18:26]

Sorry, I hadn't realized these have to stay in sync with something else.  Is
there a way to eliminate that with a direct dependency?  Can we have this file
#include SkTraceEvent.h?  Or have them canonically defined here instead of in
SkTraceEvent.h?

If not, let's make an exception and go back to having the enum value names match
the #defines exactly.

[humper, 2014/01/30 16:03:38]

I believe the design of the Chrome tracing system is such that these values are
locked down; changing them would break tracing integration with Blink and V8
also so I think the chances of that are extremely low.

+    enum CategoryGroupEnabledFlags {
+        // Category group enabled for the recording mode.
+        kEnabledForRecording_CategoryGroupEnabledFlags = 1 << 0,
+        // Category group enabled for the monitoring mode.
+        kEnabledForMonitoring_CategoryGroupEnabledFlags = 1 << 1,
+        // Category group enabled by SetEventCallbackEnabled().
+        kEnabledForEventCallback_CategoryGroupEnabledFlags = 1 << 2,
+    };
+
+    virtual const unsigned char* getCategoryGroupEnabled(const char* name) = 0;
+    virtual const char* getCategoryGroupName(
+      const uint8_t* category_group_enabled) = 0;
+    
+    virtual SkTraceEventHandle 
+        addTraceEvent(char phase,
+                      const uint8_t* categoryEnabledFlag,
+                      const char* name,
+                      uint64_t id,
+                      int32_t numArgs,
+                      const char** argNames,
+                      const uint8_t* argTypes,
+                      const uint64_t* argValues,
+                      uint8_t flags) = 0;
+    
+    virtual void 
+        updateTraceEventDuration(const uint8_t* categoryEnabledFlag, 
+                                 const char* name, 
+                                 SkTraceEventHandle) = 0;
+private:
+    static SkEventTracer *gInstance;
+};
+
+#endif // SkEventTracer_DEFINED