Internalize JSON chunk management to trace_event.h API.

base/debug/trace_event.h

Index: base/debug/trace_event.h
diff --git a/base/debug/trace_event.h b/base/debug/trace_event.h
index d5aa256dc15cb769c2de0a312eb5c2084f7f56ba..31819bc4b9a4ebb0295fcb0a1f8ebdc897f7d22e 100644
--- a/base/debug/trace_event.h
+++ b/base/debug/trace_event.h
@@ -515,6 +515,23 @@ class TraceEvent {
 };
 
 
+// TraceResultBuffer collects and merges trace fragments returned by TraceLog.
+// Add all the trace event fragments with AddFragment and then call GetJSON to
+// retrieve the full set of trace events in JSON format.
+class TraceResultBuffer {
+ public:
+  TraceResultBuffer();
+  ~TraceResultBuffer();
+
+  void AddFragment(const std::string& trace_fragment);

[nduca, 2011/10/20 00:28:03]

Brownie points: typedef TraceFragment so nobody sees the type except
TraceResultBuffer? 

[jbates, 2011/10/20 22:18:49]

I wanted to do this as well, but I didn't want to add the IPC struct Traits code
in this CL.

[nduca, 2011/10/20 23:20:05]

I realize its work, but we both agree its the right thing to do.

I'd love to pep-talk you into doing this, because I'm not gonna insist on it
from a procedural point of view. Its just silly to file a crbug for this, and it
smells funny.

[jbates, 2011/10/20 23:55:13]

Yup, we agree it's a good design, but low priority compared to the other issues
on my plate.

+
+  void GetJSON(std::string* json_trace_output);
+
+ private:
+  std::vector<std::string> fragments_;
+};
+
+
 class BASE_EXPORT TraceLog {
  public:
   // Flags for passing to AddTraceEvent.
@@ -549,7 +566,9 @@ class BASE_EXPORT TraceLog {
 
   // When enough events are collected, they are handed (in bulk) to
   // the output callback. If no callback is set, the output will be
-  // silently dropped. The callback must be thread safe.
+  // silently dropped. The callback must be thread safe. The string format is
+  // undefined. Use TraceResultBuffer to convert one or more trace strings to
+  // JSON.
   typedef RefCountedData<std::string> RefCountedString;
   typedef base::Callback<void(scoped_refptr<RefCountedString>)> OutputCallback;
   void SetOutputCallback(const OutputCallback& cb);