dart2js: fetch http Resources

sdk/lib/_internal/js_runtime/lib/js_helper.dart

Index: sdk/lib/_internal/js_runtime/lib/js_helper.dart
diff --git a/sdk/lib/_internal/js_runtime/lib/js_helper.dart b/sdk/lib/_internal/js_runtime/lib/js_helper.dart
index 6692d2723c426a4e4f984899aac1ac55fb05e77b..70d744b0f9591e99ebf5576dbb3a89bea6696c29 100644
--- a/sdk/lib/_internal/js_runtime/lib/js_helper.dart
+++ b/sdk/lib/_internal/js_runtime/lib/js_helper.dart
@@ -3903,6 +3903,57 @@ Future<Null> _loadHunk(String hunkName) {
   return completer.future;
 }
 
+// Performs an HTTP GET of the given URI and returns the response. The response
+// is either a String or a ByteBuffer.
+Future<dynamic> readHttp(String uri) {
+  Completer completer = new Completer();
+
+  void failure([error, StackTrace stackTrace]) {
+    completer.completeError(
+        new Exception("Loading $uri failed: $error"),
+        stackTrace);
+  }
+
+  enterJsAsync();
+  completer.future.whenComplete(() => leaveJsAsync());

[Siggi Cherem (dart-lang), 2015/08/25 00:43:27]

I guess we can also remove the explicit closure :)

  completer.future.whenComplete(leaveJsAsync);

[Harry Terkelsen, 2015/08/25 17:45:05]

Done.

+
+  var xhr = JS('var', 'new XMLHttpRequest()');
+  JS('void', '#.open("GET", #)', xhr, uri);
+  JS('void', '#.addEventListener("load", #, false)',
+     xhr, convertDartClosureToJS((event) {
+    int status = JS('int', '#.status', xhr);
+    if (status != 200) {
+      failure("Status code: $status");
+    }
+    String responseType = JS('String', '#.responseType', xhr);
+    var data;
+    if (responseType.isEmpty || responseType == 'text') {
+      data = JS('String', '#.response', xhr);
+      completer.complete(data);
+    } else if (responseType == 'document' || responseType == 'json') {
+      data = JS('String', '#.responseText', xhr);
+      completer.complete(data);
+    } else if (responseType == 'arraybuffer') {
+      data = JS('var', '#.response', xhr);
+      completer.complete(data);
+    } else if (responseType == 'blob') {
+      var reader = JS('var', 'new FileReader()');
+      JS('void', '#.addEventListener("loadend", #, false)',
+          reader, convertDartClosureToJS((event) {
+            data = JS('var', '#.result', reader);
+            completer.complete(data);
+          }, 1));
+    } else {
+      failure('Result had unexpected type: $responseType');
+    }
+  }, 1));
+
+  JS('void', '#.addEventListener("error", #, false)', xhr, failure);
+  JS('void', '#.addEventListener("abort", #, false)', xhr, failure);
+  JS('void', '#.send()', xhr);
+  return completer.future;

[Siggi Cherem (dart-lang), 2015/08/25 00:43:27]

Question: I see this is also how it was done in _loadHunk. I was wondering
whether we need to chain things to avoid "forking" the future. In other words,
if you have:

  f = readHttp(...);
  f.then(doSomething);

do we care if doSomething executes before leaveJsAsync?

From a quick look at leaveJsAsync, it seems that we should be OK leaving it as
you have it now, but I just wanted to check

[Harry Terkelsen, 2015/08/25 17:45:06]

That's a good question. I think we're OK since all leaveJsAsync does is
decrement a counter, it should be OK if another async operation kicks off before
we decrement the counter.

+}
+
 class MainError extends Error implements NoSuchMethodError {
   final String _message;