Use async contract calls when a callback is passed

lib/web3/function.js

@@ -67,21 +67,47 @@ SolidityFunction.prototype.signature = function () {
     return web3.sha3(web3.fromAscii(this._name)).slice(2, 10);
 };
 
-/**
- * Should be used to call function
- * 
- * @method call
- * @param {Object} options
- * @return {String} output bytes
- */
-SolidityFunction.prototype.call = function () {
-    var payload = this.toPayload.apply(this, Array.prototype.slice.call(arguments));
-    var output = web3.eth.call(payload);
+
+SolidityFunction.prototype.unpackOutput = function (output) {
+    if (output == null) {
+        return;
+    }
+
     output = output.length >= 2 ? output.slice(2) : output;
     var result = coder.decodeParams(this._outputTypes, output);
     return result.length === 1 ? result[0] : result;
 };
 
+/**
+ * Calls a contract function.
+ *
+ * @method call
+ * @param {...Object} Contract function arguments
+ * @param {function} If the last argument is a function, the contract function
+ *   call will be asynchronous, and the callback will be passed the
+ *   error and result.
+ * @return {String} output bytes
+ */
+SolidityFunction.prototype.call = function () {
+    var this_ = this;
+    var args = Array.prototype.slice.call(arguments);
+    var callback;
+    if (typeof(args[args.length - 1]) === 'function') {
+        callback = args.pop();
+    }
+
+    var payload = this.toPayload.apply(this, args);
+
+    if (typeof(callback) === 'undefined') {
+        var output = web3.eth.call(payload, callback);
+        return this.unpackOutput(output);
+    } else {
+        web3.eth.call(payload, function (error, output) {
+            callback(error, this_.unpackOutput(output));
+        });
+    }
+};
+
 /**
  * Should be used to sendTransaction to solidity function
  *