具体代码

以下是一个简单的示例代码，演示iOS端如何发送消息给Unity：

在Unity中的C#脚本：

using UnityEngine;
using System.Collections;

public class MessageReceiver : MonoBehaviour {

    void OnMessageReceived(string message) {
        Debug.Log("Received message from iOS: " + message);
    }
}

在iOS中的Objective-C类：

#import "UnityAppController.h"

@interface iOSUnityBridge : UnityAppController<UIApplicationDelegate>

@end

@implementation iOSUnityBridge

- (BOOL)application:(UIApplication*)application didFinishLaunchingWithOptions:(NSDictionary*)launchOptions {
    // Initialize Unity
    UnityInit([[[NSBundle mainBundle] bundlePath] UTF8String], "AppController", [self getUnityAppCommandLine]);

    // Add Unity view to the current view controller
    UIView* unityView = (__bridge UIView*)UnityGetGLView();
    UIViewController* rootViewController = [UIApplication sharedApplication].keyWindow.rootViewController;
    [rootViewController.view addSubview:unityView];

    return YES;
}

- (void)sendMessageToUnity:(NSString*)message {
    UnitySendMessage("MessageReceiver", "OnMessageReceived", message.UTF8String);
}

@end

在需要发送消息给Unity的地方，调用sendMessageToUnity方法即可：

iOSUnityBridge* bridge = [[iOSUnityBridge alloc] init];
[bridge sendMessageToUnity:@"Hello, Unity!"];

需要注意的是，以上代码只是一个简单的示例，实际实现中还需要考虑更多的细节和异常情况。