How to call method and return its value with UnityPlayer.UnitySendMessage

Calling Methods and Returning Values with UnityPlayer.UnitySendMessage

Understanding UnityPlayer.UnitySendMessage

UnityPlayer.UnitySendMessage is a crucial function in Unity’s Android and iOS plugins that allows you to interact with your Unity game from external native code (Java or Objective-C). It acts as a bridge, facilitating communication between the native code and the Unity game.

The Challenge of Returning Values

While UnityPlayer.UnitySendMessage can effectively call methods in your Unity game, retrieving a return value from these methods isn’t straightforward. The function itself doesn’t provide a mechanism to directly retrieve the return value.

Strategies for Value Retrieval

Here are several approaches to overcome this limitation and retrieve values from your Unity methods called using UnityPlayer.UnitySendMessage:

1. Using Global Variables

• Concept: Modify the Unity method to store the return value in a global variable accessible from both the Unity game and the native code.
• Steps:
  • Declare a public static variable in your Unity script.
  • Within your Unity method, assign the calculated value to this global variable.
  • Access the variable from your native code to obtain the result.

2. Implementing a Callback System

• Concept: Set up a mechanism where the Unity method calls a function in the native code (the callback) after it has finished execution, passing the desired value as a parameter.
• Steps:
  • Define a callback function in your native code. This function will be called from Unity.
  • In your Unity method, use UnityPlayer.UnitySendMessage to call the callback function in the native code, passing the desired return value as an argument.

3. Employing Shared Memory (Advanced)

• Concept: Utilize shared memory regions accessible by both the native code and the Unity game to transfer data between them.
• Steps:
  • Set up a shared memory region (e.g., using memory mapping or inter-process communication).
  • In your Unity method, write the return value to the shared memory region.
  • Read the value from the shared memory region in your native code.

Comparison of Methods

Code Examples

Example 1: Global Variables

// Unity Script
public static class MyScript {
    public static int result;

    public static int AddNumbers(int a, int b) {
        result = a + b;
        return result;
    }
}

// Java Code
public class MyPlugin {
    public void callAddNumbers(int a, int b) {
        UnityPlayer.UnitySendMessage("MyGameObject", "AddNumbers", a + "," + b);
        int returnValue = MyScript.result;
        // ... use returnValue ...
    }
}

Example 2: Callback System

// Unity Script
public class MyScript {
    public static void CallBackResult(int value) {
        // Handle the returned value
        Debug.Log("Result from native code: " + value);
    }

    public static int AddNumbers(int a, int b) {
        int sum = a + b;
        UnityPlayer.UnitySendMessage("MyPlugin", "ReceiveResult", sum.ToString());
        return sum;
    }
}

// Java Code
public class MyPlugin {
    public void ReceiveResult(String result) {
        int value = Integer.parseInt(result);
        // ... use value ...
    }
}

Conclusion

Choosing the appropriate method for returning values from Unity methods called using UnityPlayer.UnitySendMessage depends on your project’s requirements and complexity. Global variables offer simplicity, callbacks provide flexibility, and shared memory excels in performance. Carefully consider your use case and select the strategy that best aligns with your project’s architecture and needs.