Implement evaluation of the configuration

Sources/OpenVPNAdapter/library/OpenVPNAdapter.h

@@ -126,13 +126,24 @@ NS_SWIFT_NAME(openVPNAdapter(_:handleEvent:message:));
  */
 @property (nonatomic, readonly) OpenVPNTransportStats *transportStatistics;
 
+/**
+ Evaluate the given configuration object and determine needed credentials.
+
+ @param configuration The configuration object.
+ @param error If there is an error applying the configuration, upon return contains an error object that describes the problem.
+ @return An object describing the configuration which has been evaluated.
+ */
++ (nullable OpenVPNConfigurationEvaluation *)evaluateConfiguration:(OpenVPNConfiguration *)configuration
+                                                             error:(NSError **)error
+NS_SWIFT_NAME(evaluate(configuration:));
+
 /**
  Applies the given configuration object.
  Call this method prior to connecting, this method has no effect after calling connect.
 
  @param configuration The configuration object.
  @param error If there is an error applying the configuration, upon return contains an error object that describes the problem.
- @return A properties object describing the configuration which has been applied.
+ @return An object describing the configuration which has been applied.
  */
 - (nullable OpenVPNConfigurationEvaluation *)applyConfiguration:(OpenVPNConfiguration *)configuration
                                              error:(NSError **)error

Sources/OpenVPNAdapter/library/OpenVPNAdapter.mm

@@ -50,6 +50,24 @@
 
 #pragma mark - OpenVPNClient Lifecycle
 
++ (nullable OpenVPNConfigurationEvaluation *)evaluateConfiguration:(OpenVPNConfiguration *)configuration error:(NSError **)error {
+    ClientAPI::EvalConfig eval = OpenVPNClient::eval_config_static(configuration.config);
+    
+    if (eval.error) {
+        if (error) {
+            NSString *message = [NSString stringWithUTF8String:eval.message.c_str()];
+            *error = [NSError ovpn_errorObjectForAdapterError:OpenVPNAdapterErrorConfigurationFailure
+                                                  description:@"Failed to evaluate OpenVPN configuration."
+                                                      message:message
+                                                        fatal:YES];
+        }
+        
+        return nil;
+    }
+    
+    return [[OpenVPNConfigurationEvaluation alloc] initWithEvalConfig:eval];
+}
+
 - (OpenVPNConfigurationEvaluation *)applyConfiguration:(OpenVPNConfiguration *)configuration error:(NSError * __autoreleasing *)error {
     ClientAPI::EvalConfig eval = self.vpnClient->apply_config(configuration.config);
     
@@ -57,9 +75,9 @@
         if (error) {
             NSString *message = [NSString stringWithUTF8String:eval.message.c_str()];
             *error = [NSError ovpn_errorObjectForAdapterError:OpenVPNAdapterErrorConfigurationFailure
-                                          description:@"Failed to apply OpenVPN configuration."
-                                              message:message
-                                                fatal:YES];
+                                                  description:@"Failed to apply OpenVPN configuration."
+                                                      message:message
+                                                        fatal:YES];
         }
         
         return nil;