Internalize JSON chunk management to trace_event.h API.

chrome/browser/ui/webui/tracing_ui.cc

Index: chrome/browser/ui/webui/tracing_ui.cc
diff --git a/chrome/browser/ui/webui/tracing_ui.cc b/chrome/browser/ui/webui/tracing_ui.cc
index f13f15e3780c5aec12cf6afc5df99857b87335b6..76d6f11eaf8161b9d63d3c2dfbf855df68170e77 100644
--- a/chrome/browser/ui/webui/tracing_ui.cc
+++ b/chrome/browser/ui/webui/tracing_ui.cc
@@ -63,7 +63,7 @@ class TracingMessageHandler
 
   // TraceSubscriber implementation.
   virtual void OnEndTracingComplete();
-  virtual void OnTraceDataCollected(const std::string& json_events);
+  virtual void OnTraceDataCollected(const std::string& trace_fragment);
   virtual void OnTraceBufferPercentFullReply(float percent_full);
 
   // Messages.
@@ -419,7 +419,11 @@ void TracingMessageHandler::OnEndTracingComplete() {
 }
 
 void TracingMessageHandler::OnTraceDataCollected(
-    const std::string& json_events) {
+    const std::string& trace_fragment) {
+  std::string json_events;
+  base::debug::TraceResultBuffer trace_buffer_;

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

I wasn't expecting this. Was expecting the trace_buffer to be a member on the
message handler, and for us to only hand the the string to JS at the end. I
realize that might then blow-out JS' string size limits, but it'd be
better-matched to the design.

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

It's better for memory performance to pass these through, so let's keep it as
is.

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

Now that you have an output callback, use that instead. E.g. have a
trace_buffer_, and route the output callback to the ExecuteJavascript. You'll
have to tweak onTraceDataCollected to use strings instead of json objects, but
grab me and I can help you.

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

Since you've spent more time in the javascript side of this system, perhaps this
is a quick followup patch for you? The current code looks clean to me, but maybe
that's because I'm not familiar with the JS-side subtleties that you are
thinking about...

+  trace_buffer_.AddFragment(trace_fragment);
+  trace_buffer_.GetJSON(&json_events);
   DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
   std::string javascript = "tracingController.onTraceDataCollected("
       + json_events + ");";