net: Allow URLFetcher users to inject custom URLFetcherResponseWriter

net/url_request/url_fetcher_response_writer.h

Index: net/url_request/url_fetcher_response_writer.h
diff --git a/net/url_request/url_fetcher_response_writer.h b/net/url_request/url_fetcher_response_writer.h
index 72a8c520aeac9cb9db0423277ed99d34cfb20602..0032a4a8b934785e9fffa607edb0ea1ab35f654a 100644
--- a/net/url_request/url_fetcher_response_writer.h
+++ b/net/url_request/url_fetcher_response_writer.h
@@ -11,6 +11,7 @@
 #include "base/memory/scoped_ptr.h"
 #include "base/memory/weak_ptr.h"
 #include "net/base/completion_callback.h"
+#include "net/base/net_export.h"
 
 namespace base {
 class TaskRunner;
@@ -26,7 +27,7 @@ class URLFetcherStringWriter;
 
 // This class encapsulates all state involved in writing URLFetcher response
 // bytes to the destination.
-class URLFetcherResponseWriter {
+class NET_EXPORT URLFetcherResponseWriter {
  public:
   virtual ~URLFetcherResponseWriter() {}
 
@@ -54,7 +55,7 @@ class URLFetcherResponseWriter {
 };
 
 // URLFetcherResponseWriter implementation for std::string.
-class URLFetcherStringWriter : public URLFetcherResponseWriter {
+class NET_EXPORT URLFetcherStringWriter : public URLFetcherResponseWriter {
  public:
   URLFetcherStringWriter();
   virtual ~URLFetcherStringWriter();
@@ -76,7 +77,7 @@ class URLFetcherStringWriter : public URLFetcherResponseWriter {
 };
 
 // URLFetcherResponseWriter implementation for files.
-class URLFetcherFileWriter : public URLFetcherResponseWriter {
+class NET_EXPORT URLFetcherFileWriter : public URLFetcherResponseWriter {

[wtc, 2013/10/14 19:57:23]

Nit: I understand why we need to export the base class URLFetcherResponseWriter.
Do we also need to export the derived classes URLFetcherStringWriter and
URLFetcherFileWriter?

[hashimoto, 2013/10/15 01:00:35]

By exporting these derived classes, URLFetcher user code can implement custom
URLFetcherResponseWriter easily, without implementing the whole class from
scratch. (e.g. Our Google Drive code will use a flavored URLFetcherFileWriter
with OnURLFetchDownloadData() equivalent functionality.)

  public:
   // |file_path| is used as the destination path. If |file_path| is empty,
   // Initialize() will create a temporary file.