bug 309525 Add a canConvert to nsIStreamConverterService that allows determining

whether a conversion from one type to another is possible.
r=darin sr=bz

netwerk/streamconv/public/nsIStreamConverterService.idl

@@ -55,11 +55,17 @@
  * @author Jud Valeski
  * @see nsIStreamConverter
  */
-[scriptable, uuid(e086e1e2-40ff-4193-8b8c-bd548babe70d)]
+[scriptable, uuid(f2b1ab53-f0bd-4adb-9365-e59b1701a258)]
 interface nsIStreamConverterService : nsISupports {
+    /**
+     * Tests whether conversion between the two specified types is possible.
+     * This is cheaper than calling convert()/asyncConvertData(); it is not
+     * necessary to call this function before calling one of those, though.
+     */
+    boolean canConvert(in string aFromType, in string aToType);
 
     /**
-     * <b>SYNCRONOUS VERSION</b>
+     * <b>SYNCHRONOUS VERSION</b>
      * Converts a stream of one type, to a stream of another type.
      *
      * Use this method when you have a stream you want to convert.
@@ -81,7 +87,7 @@ interface nsIStreamConverterService : nsISupports {
                            in nsISupports aContext);
 
     /**
-     * <b>ASYNCRONOUS VERSION</b>
+     * <b>ASYNCHRONOUS VERSION</b>
      * Retrieves a nsIStreamListener that receives the original/raw data via its
      * nsIStreamListener::OnDataAvailable() callback, then converts and pushes 
      * the data to aListener.

netwerk/streamconv/src/nsStreamConverterService.cpp

@@ -53,6 +53,7 @@
 #include "nsStreamConverterService.h"
 #include "nsIServiceManager.h"
 #include "nsIComponentManager.h"
+#include "nsIComponentRegistrar.h"
 #include "nsString.h"
 #include "nsReadableUtils.h"
 #include "nsIAtom.h"
@@ -472,7 +473,42 @@ nsStreamConverterService::FindConverter(const char *aContractID, nsCStringArray
 
 
 /////////////////////////////////////////////////////
-// nsIStreamConverter methods
+// nsIStreamConverterService methods
+NS_IMETHODIMP
+nsStreamConverterService::CanConvert(const char* aFromType,
+                                     const char* aToType,
+                                     PRBool* _retval) {
+    nsCOMPtr<nsIComponentRegistrar> reg;
+    nsresult rv = NS_GetComponentRegistrar(getter_AddRefs(reg));
+    if (NS_FAILED(rv))
+        return rv;
+
+    nsCAutoString contractID;
+    contractID.AssignLiteral(NS_ISTREAMCONVERTER_KEY "?from=");
+    contractID.Append(aFromType);
+    contractID.AppendLiteral("&to=");
+    contractID.Append(aToType);
+
+    // See if we have a direct match
+    rv = reg->IsContractIDRegistered(contractID.get(), _retval);
+    if (NS_FAILED(rv))
+        return rv;
+    if (*_retval)
+        return NS_OK;
+
+    // Otherwise try the graph.
+    rv = BuildGraph();
+    if (NS_FAILED(rv))
+        return rv;
+
+    nsCStringArray *converterChain = nsnull;
+    rv = FindConverter(contractID.get(), &converterChain);
+    *_retval = NS_SUCCEEDED(rv);
+
+    delete converterChain;
+    return NS_OK;
+}
+
 NS_IMETHODIMP
 nsStreamConverterService::Convert(nsIInputStream *aFromStream,
                                   const char *aFromType,