Add support for iOS with RoboVM.

retrofit/src/main/java/retrofit2/Platform.java

@@ -20,6 +20,7 @@ import android.os.Handler;
 import android.os.Looper;
 import java.lang.invoke.MethodHandles.Lookup;
 import java.lang.reflect.Constructor;
+import java.lang.reflect.InvocationTargetException;
 import java.lang.reflect.Method;
 import java.util.concurrent.Executor;
 import org.codehaus.mojo.animal_sniffer.IgnoreJRERequirement;
@@ -44,6 +45,11 @@ class Platform {
       return new Java8();
     } catch (ClassNotFoundException ignored) {
     }
+    try {
+      Class.forName("org.robovm.apple.foundation.NSObject");
+      return new IOS();
+    } catch (ClassNotFoundException ignored) {
+    }
     return new Platform();
   }
 
@@ -98,4 +104,46 @@ class Platform {
       }
     }
   }
+
+  static class IOS extends Platform {
+    @Override CallAdapter.Factory defaultCallAdapterFactory(Executor callbackExecutor) {
+      if (callbackExecutor == null) {
+        callbackExecutor = new MainThreadExecutor();
+      }
+      return new ExecutorCallAdapterFactory(callbackExecutor);
+    }
+
+    static class MainThreadExecutor implements Executor {
+      private static Object queue;
+      private static Method addOperation;
+
+      static {
+        try {
+          // queue = NSOperationQueue.getMainQueue();
+          Class<?> operationQueue = Class.forName("org.robovm.apple.foundation.NSOperationQueue");
+          queue = operationQueue.getDeclaredMethod("getMainQueue").invoke(null);
+          addOperation = operationQueue.getDeclaredMethod("addOperation", Runnable.class);
+        } catch (Exception e) {
+          throw new AssertionError(e);
+        }
+      }
+
+      @Override public void execute(Runnable r) {
+        try {
+          // queue.addOperation(r);
+          addOperation.invoke(queue, r);
+        } catch (IllegalArgumentException | IllegalAccessException e) {
+          throw new AssertionError(e);
+        } catch (InvocationTargetException e) {
+          Throwable cause = e.getCause();
+          if (cause instanceof RuntimeException) {
+            throw (RuntimeException) cause;
+          } else if (cause instanceof Error) {
+            throw (Error) cause;
+          }
+          throw new RuntimeException(cause);
+        }
+      }
+    }
+  }
 }