Unity C# <> Xcode Objective-C Bridge

A "simple" step-by-step tutorial on how to make a bridge between Unity's C# and Xcode's Objective-C. Either way.

Unity Scene set up

• First set up a new scene in Unity and add a plane with a simple sphere above it. So we can interact with it later.
• Make sure the purple Sphere has a RigidBody and Use Gravity is enabled.

💡 You can add some materials to it, so it stands out. And maybe move the Sphere a bit up (on the y-axis).

If you run the scene the purple sphere would drop on the white plane.

Unity GUI

Next up we're going to add a GUI button to send information to Objective-c

• Add a UI -> Button
• It should be aligned to the left-bottom
• Rename the button's text to your liking.

Unity Script

• Next stop, create a new C# Script, name it BehaviourScript or anything to your liking

Open the script and use this code:

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;
using System.Runtime.InteropServices;

public class BehaviourScript : MonoBehaviour {
    public GameObject ball;
    public Button button;

    [DllImport ("__Internal")]
    private static extern void calledFromUnity();

    void Start () {
        button.onClick.AddListener (Tap);
    }

    void Tap()  {
        calledFromUnity();
    }

    void Bounce(string direction) {
        ball.GetComponent<Rigidbody>().AddForce(transform.up * 300);
    }
}

Code blocks explained:

using System.Runtime.InteropServices;

Used for external communication.

[DllImport ("__Internal")]
private static extern void calledFromUnity();

This creates a reference to the c function calledFromUnity(), which will be implemented in your Xcode project later on.

void Start () {
    button.onClick.AddListener (Tap);
}

void Tap()  {
    calledFromUnity();
}

Would listen for a click event on the button. When a click occurs the c-function calledFromUnity() will be called and invoked in (Obj-)C.

void Bounce(string direction) {
    ball.GetComponent<Rigidbody>().AddForce(transform.up * 300);
}

This would make the ball bounce. Bounce(string) will be called from Obj-c / Xcode.

Now go back into Unity and add the BehaviourScript to your plane and connect Button and GameObject.

At this stage you would have a button which would do nothing (yet) and have a function which isn't called (yet).

iOS Plugin

• Create a new folder called Plugin in your project
• Create a new folder called iOS in Plugin
• Create a new file called plugin.mm in that iOS folder

⚠️ It's important to add these files to this specific folder. Adding them from Xcode would overwrite the plugin.mm once you (re)build the Unity project.

plugin.mm should contain the following code:

extern "C" {
    void calledFromUnity() {        
        [[NSNotificationCenter defaultCenter] postNotificationName:@"UnityNotification" object:nil];
    }
}

This is where the calledFromUnity() function we declared earlier would return. We make the default NSNotificationCenter post a notification with the name "UnityNotification", this way we can intercept it in our own UIViewController later on.

💡 There are a few options regarding dispatching methods from C to Objective-C's main view, but for now we'll stick to this.

Build the project

• Open Build Settings: File -> Build Settings or press ⇧⌘B.
• Use the following settings:
• Press Build
• Wait ...
• After a while your Xcode project should be created
• Open Unity-iPhone.xcodeproj

A UIViewController on top of Unity

• Create a new Objective-C class in the Classes folder name AppController.

💡 Make sure you have a .mm extension for the implementation file and it's linked to the Unity-iPhone target.

• Make sure AppController extends UnityAppController

The two files will look like this:

// .mm

#import "AppController.h"

@implementation AppController {
    UIButton *btn;
}
    
- (void)startUnity:(UIApplication *)application {
    [super startUnity:application];
    
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(notificationFired:) name:@"UnityNotification" object:nil];
    
    UIViewController *vc = [UIViewController new];
    [vc.view setFrame:self.window.bounds];
    [vc.view addSubview:self.rootViewController.view];    
    [vc addChildViewController:self.rootViewController];
    [self.window setRootViewController:vc];
    
    btn = [UIButton buttonWithType:UIButtonTypeSystem];
    [btn setTitle:@"To Unity" forState:UIControlStateNormal];
    [btn setBackgroundColor:[UIColor whiteColor]];
    [btn setTintColor:[UIColor orangeColor]];
    [btn setFrame:CGRectMake(10, 10, 100, 50)];
    [btn addTarget:self action:@selector(tapButton:) forControlEvents:UIControlEventTouchUpInside];   
    [vc.view addSubview:btn];
}

- (void)tapButton:(UIButton *)sender {
    UnitySendMessage("Plane", "Bounce", "Up");
}
    
- (void)notificationFired:(NSNotification *)notification {
    NSString *originalTitle = [btn titleForState: UIControlStateNormal];
    [btn setTitle:@"😎" forState:UIControlStateNormal];
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(1 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        [btn setTitle:originalTitle forState:UIControlStateNormal];
    });
}
@end

Lets have a look at each code blocks:

@implementation AppController {
    UIButton *btn;
}

A btn is declared so we can use it among the whole class instance.

- (void)startUnity:(UIApplication *)application {
    [super startUnity:application];

Since AppController extends UnityAppController and startUnity: is called when the Unity view is loaded, this method will be invoked once the Unity controller is started.

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(notificationFired:) name:@"UnityNotification" object:nil];

- (void)notificationFired:(NSNotification *)notification {
    NSString *originalTitle = [btn titleForState: UIControlStateNormal];
    [btn setTitle:@"😎" forState:UIControlStateNormal];
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(1 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        [btn setTitle:originalTitle forState:UIControlStateNormal];
    });
}

Listen for the "UnityNotification" notification to be fired and call the notificationFired: method. notificationFired: will change the title of the button and after 1 second set it back to the original one.

UIViewController *vc = [UIViewController new];
[vc.view setFrame:self.window.bounds];
[vc.view addSubview:self.rootViewController.view];    
[vc addChildViewController:self.rootViewController];
[self.window setRootViewController:vc];

This would create a new UIViewController and places the original unity viewcontroller in it. This way we can add our own UI on top of Unity.

btn = [UIButton buttonWithType:UIButtonTypeSystem];
[btn setTitle:@"To Unity" forState:UIControlStateNormal];
[btn setBackgroundColor:[UIColor whiteColor]];
[btn setTintColor:[UIColor orangeColor]];
[btn setFrame:CGRectMake(10, 10, 100, 50)];
[btn addTarget:self action:@selector(tapButton:) forControlEvents:UIControlEventTouchUpInside];   
[vc.view addSubview:btn];

Creates the actual button and places it on the newly created UIViewController

- (void)tapButton:(UIButton *)sender {
    UnitySendMessage("Plane", "Bounce", "Up");
}

The button tap will invoke tapButton: and will fire UnitySendMessage with 3 parameters:

• "Plane": The name of the GameObject in Unity
• "Bounce": The name of the function in the BehaviourScript.cs Unity C# Script.
• "Up": The parameter to be used in the Bounce(string) function. (For now this hasn't any effect, you can place any string in it)

main.mm

• Open main.mm (also in the Classes folder) and change this line:

const char* AppControllerClassName = "UnityAppController";

to

const char* AppControllerClassName = "AppController";

This way it will load the AppController class instead of the UnityAppController class.

Results