Allow advanced users to enforce JSON response types in XHRs.

Add support for setting the XHR `responseType` property to `"json"`.  This
defines the response data as JSON and prevents the browser from handling other
types of data.  It allows JSON parsing to be done off of the main thread,
reducing CPU contention that can happen with large responses and low-power
devices; this can yield smoother animations during navigation, for example.
However, it prevents processing chunked responses on-the-fly (i.e.
pseudo-streaming), effectively eliminating most of the benefits of multipart
responses.  Guard this behavior behind an "advanced" config flag and provide
no default to prevent accidental usage.  Setting `advanced-response-type-json`
to `true` will enable the feature.

Closes youtube#317.

src/client/nav/request.js

@@ -163,6 +163,14 @@ spf.nav.request.send = function(url, opt_options) {
       onDone: handleComplete,
       onTimeout: handleComplete
     };
+    // As an advanced option, allow XHR requests to enforce JSON responses.
+    // This can make response parsing more efficient by reducing contention on
+    // the main thread (especially for very large responses), but as a
+    // side-effect, it removes the ability to parse chunked multipart responses
+    // on-the-fly.
+    if (spf.config.get('advanced-response-type-json')) {
+      xhrOpts.responseType = 'json';
+    }
     var xhr;
     if (options.method == 'POST') {
       xhr = spf.net.xhr.post(requestUrl, options.postData, xhrOpts);
@@ -293,8 +301,10 @@ spf.nav.request.handleChunkFromXHR_ = function(url, options, chunking,
  */
 spf.nav.request.handleCompleteFromXHR_ = function(url, options, timing,
                                                   chunking, xhr) {
-  spf.debug.debug('nav.request.handleCompleteFromXHR_ ',
-                  url, {'extra': chunking.extra, 'complete': xhr.responseText});
+  spf.debug.debug('nav.request.handleCompleteFromXHR_ ', url,
+                  xhr.responseType == 'json' ?
+                      xhr.response :
+                      {'extra': chunking.extra, 'complete': xhr.responseText});
   // Record the timing information from the XHR.
   if (xhr['timing']) {
     for (var t in xhr['timing']) {
@@ -347,22 +357,43 @@ spf.nav.request.handleCompleteFromXHR_ = function(url, options, timing,
                                           xhr, '', true);
     }
   }
-  // Always attempt a full parse with error handling.
-  // A multipart response parsed on-the-fly via chunking may be invalid JSON if
-  // the response is truncated early.  (If truncated just after a token,
-  // the chunking.extra value will be empty and no additional chunk parsing
-  // will be done, but the overall response will stil be invalid.)
+
   var parts;
-  try {
-    var parsed = spf.nav.response.parse(xhr.responseText);
-    parts = parsed.parts;
-  } catch (err) {
-    spf.debug.debug('    JSON parse failed');
-    if (options.onError) {
-      options.onError(url, err);
+  if (xhr.responseType == 'json') {
+    // If using the JSON `responseType`, parsing is complete and no chunking
+    // has been handled on-the-fly.
+    if (!xhr.response) {
+      spf.debug.debug('    JSON parse failed');
+      if (options.onError) {
+        options.onError(url, new Error('JSON response parsing failed'));
+      }
+      return;
+    }
+    parts = (typeof xhr.response.length == 'number') ?
+        xhr.response :
+        [xhr.response];
+    if (spf.config.get('experimental-parse-extract')) {
+      parts = spf.nav.response.extract(parts);
+    }
+  } else {
+    // Otherwise, parsing may need to be done.  Always attempt a full parse with
+    // error handling. A multipart response parsed on-the-fly via chunking may
+    // be invalid JSON if the response is truncated early.  (If truncated just
+    // after a token, the chunking.extra value will be empty and no additional
+    // chunk parsing will be done, but the overall response will stil be
+    // invalid.)
+    try {
+      var parsed = spf.nav.response.parse(xhr.responseText);
+      parts = parsed.parts;
+    } catch (err) {
+      spf.debug.debug('    JSON parse failed');
+      if (options.onError) {
+        options.onError(url, err);
+      }
+      return;
     }
-    return;
   }
+
   if (options.onPart && parts.length > 1) {
     // Only execute callbacks for parts that have not already been processed.
     // In case there is an edge case where some parts were parsed on-the-fly

src/client/nav/response.js

@@ -89,28 +89,17 @@ spf.nav.response.parse = function(text, opt_multipart, opt_lastDitch) {
       }
     }
     if (spf.config.get('experimental-parse-extract')) {
-      for (var i = 0; i < parts.length; i++) {
-        parts[i] = spf.nav.response.extract(
-            /** @type {spf.SingleResponse} */(parts[i]));
-      }
+      parts = spf.nav.response.extract(parts);
     }
     return {
       parts: /** @type {Array.<spf.SingleResponse>} */(parts),
       extra: extra
     };
   } else {
     var response = JSON.parse(text);
-    var parts;
-    if (typeof response.length == 'number') {
-      parts = response;
-    } else {
-      parts = [response];
-    }
+    var parts = (typeof response.length == 'number') ? response : [response];
     if (spf.config.get('experimental-parse-extract')) {
-      for (var i = 0; i < parts.length; i++) {
-        parts[i] = spf.nav.response.extract(
-            /** @type {spf.SingleResponse} */(parts[i]));
-      }
+      parts = spf.nav.response.extract(parts);
     }
     return {
       parts: /** @type {Array.<spf.SingleResponse>} */(parts),
@@ -506,23 +495,27 @@ spf.nav.response.completeAnimation_ = function(data) {
 /**
  * Extracts all resources from HTML in a SPF response.
  *
- * @param {spf.SingleResponse} response The SPF response object to extract.
- * @return {spf.SingleResponse} The response, updated to have resources
- *     extracted from HTML strings.  This does not create a new object and
- *     modifies the passed response in-place.
+ * @param {T} response The SPF response object to extract.
+ * @return {T} The response, updated to have resources extracted from HTML
+ *     strings.  This does not create a new object and modifies the passed
+ *     response in-place.
+ * @template T
  */
 spf.nav.response.extract = function(response) {
   spf.debug.debug('spf.nav.response.extract', response);
-  if (response['head']) {
-    response['head'] = spf.nav.response.extract_(response['head']);
-  }
-  if (response['body']) {
-    for (var id in response['body']) {
-      response['body'][id] = spf.nav.response.extract_(response['body'][id]);
+  var parts = (typeof response.length == 'number') ? response : [response];
+  for (var i = 0, l = parts.length; i < l; i++) {
+    if (parts[i]['head']) {
+      parts[i]['head'] = spf.nav.response.extract_(parts[i]['head']);
+    }
+    if (parts[i]['body']) {
+      for (var id in parts[i]['body']) {
+        parts[i]['body'][id] = spf.nav.response.extract_(parts[i]['body'][id]);
+      }
+    }
+    if (parts[i]['foot']) {
+      parts[i]['foot'] = spf.nav.response.extract_(parts[i]['foot']);
     }
-  }
-  if (response['foot']) {
-    response['foot'] = spf.nav.response.extract_(response['foot']);
   }
   return response;
 };