[WIP] Working prototype of swift model integrated into TuriCreate.

This adds an XCode project that demonstrates how to effectively write a
swift ML model while allowing it to be called from C++ or python.

The key idea is to expose the swift class to objective-C (add @objc
modifier), then write a simple Objective-C++ class that is registered
using the new model registration mechanism.  The type convesion is handled by
overloading the variant_type converter to handle native Objective-C
types.

See src/xcode_swift_toolkit/TC_Swift_Demo/TC_Swift_Demo.xcodeproj for a
proof of concept.

src/model_server/lib/variant.hpp

@@ -15,6 +15,7 @@
 #include <core/data/flexible_type/flexible_type.hpp>
 #include <core/storage/sframe_data/dataframe.hpp>
 #include <core/storage/serialization/serialization_includes.hpp>
+#include <model_server/lib/variant.hpp>
 
 namespace turi {
 class model_base;
@@ -308,13 +309,6 @@ template <typename T>
 inline variant_type to_variant(const T& f) {
   return variant_converter<typename std::decay<T>::type>().set(f);
 }
-
-/** Overload for the case when we're trying to wrap a void return 
- *  type in a template.
- */
-inline variant_type to_variant() {
-  return variant_type();  
-}
 } // namespace turi
 
 namespace turi {

src/xcode_swift_toolkit/TC_Swift_Demo/Objective-C Wrapper/Wrapper.mm

@@ -0,0 +1,61 @@
+//
+//  Wrapper.m
+//  TC_Swift_Demo
+//
+//  Created by Hoyt Koepke on 12/23/19.
+//  Copyright © 2019 Hoyt Koepke. All rights reserved.
+//
+
+#import <Foundation/Foundation.h>
+
+#import "TC_Swift_Demo/TC_Swift_Demo-Swift.h"
+
+#include <model_server_v2/model_base.hpp>
+#include <model_server_v2/model_server.hpp>
+#include <model_server_v2/registration.hpp>
+
+
+namespace turi {
+
+    // TODO: Make an Objective-C variant converter library.
+    template <>
+    struct variant_converter<NSString*, void> {
+      static constexpr bool value = true;
+        NSString* _Nonnull get(const variant_type& val) {
+            return @(variant_get_ref<flexible_type>(val).get<flex_string>().c_str());
+        }
+        variant_type set(NSString* val) {
+            return to_variant([val UTF8String]);
+        }
+    };
+
+class Wrapper : public turi::v2::model_base {
+public:
+
+    SwiftMLClass* m;
+
+    Wrapper() {
+        m = [[SwiftMLClass alloc] init];
+
+        register_method("append_string", &Wrapper::append_string, "s");
+        register_method("get_string", &Wrapper::get_string);
+    }
+
+    const char* name() const override {
+        return "SwiftWrapperTest";
+    }
+
+    void append_string(NSString* _Nonnull s) {
+        [m append_stringWithX:s];
+    }
+
+    NSString* get_string() const {
+        NSString* _Nonnull r = [m get_string];
+        return r;
+    }
+
+};
+
+REGISTER_MODEL(Wrapper);
+
+}

src/xcode_swift_toolkit/TC_Swift_Demo/Objective-C Wrapper/WrapperTests.mm

@@ -0,0 +1,40 @@
+//
+//  WrapperTests.m
+//  TC_Swift_DemoTests
+//
+//  Created by Hoyt Koepke on 12/23/19.
+//  Copyright © 2019 Hoyt Koepke. All rights reserved.
+//
+
+#import <XCTest/XCTest.h>
+
+#include <model_server_v2/model_server.hpp>
+
+using namespace turi;
+using namespace turi::v2;
+
+@interface WrapperTests : XCTestCase
+
+@end
+
+@implementation WrapperTests
+
+
+
+- (void)testWrapping {
+    // This is an example of a functional test case.
+    // Use XCTAssert and related functions to verify your tests produce the correct results.
+
+    auto m = model_server().create_model("SwiftWrapperTest");
+
+    m->call_method("append_string", "This");
+    m->call_method("append_string", " is");
+    m->call_method("append_string", " Fun!");
+
+    std::string result = variant_get_value<std::string>(m->call_method("get_string"));
+
+    XCTAssert(result == "This is Fun!");
+}
+
+
+@end

src/xcode_swift_toolkit/TC_Swift_Demo/TC_Swift_Demo/Info.plist

@@ -0,0 +1,24 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<dict>
+	<key>CFBundleDevelopmentRegion</key>
+	<string>$(DEVELOPMENT_LANGUAGE)</string>
+	<key>CFBundleExecutable</key>
+	<string>$(EXECUTABLE_NAME)</string>
+	<key>CFBundleIdentifier</key>
+	<string>$(PRODUCT_BUNDLE_IDENTIFIER)</string>
+	<key>CFBundleInfoDictionaryVersion</key>
+	<string>6.0</string>
+	<key>CFBundleName</key>
+	<string>$(PRODUCT_NAME)</string>
+	<key>CFBundlePackageType</key>
+	<string>$(PRODUCT_BUNDLE_PACKAGE_TYPE)</string>
+	<key>CFBundleShortVersionString</key>
+	<string>1.0</string>
+	<key>CFBundleVersion</key>
+	<string>$(CURRENT_PROJECT_VERSION)</string>
+	<key>NSHumanReadableCopyright</key>
+	<string>Copyright © 2019 Hoyt Koepke. All rights reserved.</string>
+</dict>
+</plist>

src/xcode_swift_toolkit/TC_Swift_Demo/TC_Swift_Demo/TC_Swift_Demo.swift

@@ -0,0 +1,29 @@
+//
+//  SwiftMLClass.swift
+//  TC_Swift_Demo
+//
+//  Created by Hoyt Koepke on 12/23/19.
+//  Copyright © 2019 Hoyt Koepke. All rights reserved.
+//
+
+import Foundation
+
+
+@objc public class SwiftMLClass : NSObject {
+    var s: String
+
+    public override init() { // Constructor
+        self.s = ""
+         super.init()
+    }
+
+    @objc public func append_string(x: String) {
+        self.s += x
+        print("New string: \(self.s)")
+    }
+
+    @objc public func get_string() -> String {
+        return self.s
+    }
+}
+