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Codename One Objective-C Bridge

A library to access the Objective-C runtime from Java code in Codename One, when running on iOS. This is based on the Java Objective-C Bridge library for Mac OS X, but has been modified significantly to run on iOS, and to provide an API that is convenient to use inside a Codename One application.

By its nature, this library is only useful when running on iOS. You should ensure that you don’t run this code on other platforms. You can accomplish this with:

if (Objc.isSupported()) {
   // Run some code that interacts with Objective-C


vs Native Interfaces

Since you can already access Objective-C in Codename One apps using native interfaces, it is reasonable to wonder why a bridge like this is necessary. Some advantages of this library over using native interfaces include:

  1. No Need For an Interface. Native interfaces involve a lot of structure. This bridge allows you to access all libraries in the Objective-C runtime (any linked framework or library), directly from Java.

  2. Blocks. Create blocks in Java that you can pass to objective-C APIs that require them for callbacks.

  3. Delegate objects. Register Java objects as delegates for your view controllers or views without any special setup.

  4. …​ more to come…​


Using Objc.eval()

The foundation of the Objective-C bridge is the eval() method, which allows you to send messages to the Objective-C runtime. It will return an ObjCResult object which is just a wrapper around the actual result, which can be a Pointer, String, or primitive value like a double.

eval() allows you to send messages to classes and objects in the objective-c runtime. This is not the same as evaluating arbitrary objective-c syntax expressions. I.e. you can’t just paste Objective-C source code into this method and expect it to work.

Calling Class Methods

// Get a UIColor blue instance
Pointer blue = eval("UIColor.blueColor").asPointer();
  // UIColor* button = [UIColor blueColor];

// Get reference to CodenameOne_GLViewController instance
Pointer cn1ViewController = eval("CodenameOne_GLViewController.instance").asPointer();
  // CodenameOne_GLViewController* cn1ViewController = [CodenameOne_GLViewController instance];

// Create a UIButton
Pointer button = eval("UIButton.buttonWithType:", 0).asPointer();
  // UIButton* button = [UIButton buttonWithType:0];

Calling Instance Methods

Pointer button = eval("UIButton.buttonWithType:", 0).asPointer();
eval(button, "setTitle:forState:", "Show View", 0);
    // [button setTitle:@"Show View" forState:0]

Chaining Messages

You can chain messages together using "dot" notation. E.g.

Pointer date = eval("NSDate.alloc.init").asPointer();
// Equivalent to:
// Pointer date = eval("NSDate.alloc").asPointer();
// eval(date, "init");

Getting and Setting Properties

You can get and set properties on objects using eval(). E.g.

Pointer customer = eval("Customer.alloc.init").asPointer();
eval(customer, "setFirstName:", "Steve");
String firstName = eval(customer, "firstName").asString();

However you can use the Objc.getProperty() and Objc.setProperty() methods to make this easier. E.g.

Pointer customer = eval("Customer.alloc.init").asPointer();
Objc.setProperty(customer, "firstName", "Steve");
String firstName = Objc.getProperty(customer, "firstName").asString();

This can cut down on typos as you don’t have to worry about the "setter" method naming conventions.

Boxed Properties

Currently structs are problematic. You can’t call methods that return a struct (pointers to structs: YES, structs themselves: NO). You can mitigate this problem slightly by using Objc.getBoxedProperty() and Objc.setBoxedProperty() to get and set these properties. These methods wrap Objective-C’s KVC getters and setters to automatically box structs (and primitives) in an NSValue pointer when they are retrieved, and unbox them when the are set.

For example, UIView.bounds is a CGRect which is a struct, so we can’t simply call button.bounds to get its bounds. We need to do:

Pointer bounds = Objc.getBoxedProperty(myButton, "bounds").asPointer();

For common struct types like CGRect, and CGPoint there are convenience methods to turn these into Rectangle2D and Point2D. But more work is required to provide better struct support in the future.

Runnables as Blocks

If you pass a Runnable as a parameter to eval() it will be wrapped in an Objective-C block.

            Log.p("This runs after animation is complete");

// [[CodenameOne_GLViewController instance] presentViewController:controller animated:YES completion:^{
//      NSLog(@"This runs after animation is complete");
// }]])

Create Callbacks

Use Objc.makeCallback() to generate a single-method anonymous Objective-C class that can be passed to methods that expect both a target object and a selector. The following example creates a callback that will receive touch events from a UIButton.

Pointer button = eval("UIButton.buttonWithType:", 0).asPointer();
CallbackMethod cb = Objc.makeCallback(()->{
    Log.p("Button was clicked");
eval(button, "addTarget:action:forControlEvents:", cb, cb.getSelector(), 1<<6);
  // Note: 1<<6 is the value of the UIControlEventTouchUpInside constant
1<<6 is the value UIControlEventTouchUpInside (See Apple docs).

See Apple’s documentation for the addTarget:action:forControlEvents: method.

The above example creates a callback method cb, which dynamically generates an instance of NSObject, and defines a single method on it. The addTarget:action:forControlEvents: method expects you to pass a target (you pass the CallbackMethod object), and a selector that it should call on that object. You can use the getSelector() method to retrieve this selector.

Delegate Objects

Another common pattern in Objective-C is to provide a delegate that conforms to a protocol. The delegate would generally implement a handful of methods which would be called in response to certain events, when the object is set as a ViewController’s delegate.

DelegateObject delegate = Objc.makeDelegate()

    // - (void) overlayController:		(nonnull SBSOverlayController *) 	overlayController
    //        didCancelWithStatus:		(nullable NSDictionary *) 	status
    .add("barcodePicker:didScan:", Method.create(ArgType.Void, new ArgType[]{ArgType.Object, ArgType.Object}, args->{
        Pointer picker = Method.getArgAsPointer(args[0]);
        Pointer session = Method.getArgAsPointer(args[1]);

        Log.p("Scanning ocurred");

        return null;

    // - (void) overlayController:		(nonnull SBSOverlayController *) 	overlayController
    //        didCancelWithStatus:		(nullable NSDictionary *) 	status
    .add("overlayController:didCancelWithStatus:", Method.create(ArgType.Void, new ArgType[]{ArgType.Object, ArgType.Object}, args-> {

        Log.p("Scanning was cancelled");

        return null;

Objc.setProperty(picker, "scanDelegate", delegate);
    // picker.scanDelegate = delegate;
Objc.setProperty(overlayController, "cancelDelegate", delegate);
    // overlayController.cancelDelegate = delegate;

The above example creates an object with two methods, barcodePicker:didScan:, and overlayController:didCancelWithStatus: which comply with protocols for the API in question. We use Method.create() to create the methods themselves. Method.create takes 3 args

  1. Return type. The return type of the method.

  2. Arg types. The parameter types for the method.

  3. A MethodBody object that defines the actual code that will run. This is most conveniently provided as a lambda.

Methods as Blocks

Above you saw that Runnable parameters are converted to Objective-C blocks by eval(). The resulting block will by a no-arg block with void return. If the use case calls for a block with a parameter, you can use a Method instead. E.g. Using the speech recognition API requires us to call the requestAuthorization: method with the following signature:

+ (void)requestAuthorization:(void (^)(SFSpeechRecognizerAuthorizationStatus status))handler;

I.e. it takes a block as a parameter, which takes a single argument. The docs indicate that this argument is a Swift enum, which is exposed to Objective-C as an int. So we require a block that takes an int as a parameter. The objective-c for this call would be something like:

[SFSpeechRecognizer requestAuthorization:^(int status){
    if (status == SFSpeechRecognizerAuthorizationStatusAuthorized) {
        NSLog(@"We are authorized for speech recognition");

To do this in Java, we would do

eval("SFSpeechRecognizer.requestAuthorization:", Method.create(ArgType.Int, args->{
    if (Method.getArgAsInt(args[0]) == 0) { // the status for authorized
      Log.p("We are authorized for speech recognition");

Creating a Native Component

Use the Objc.createPeerComponent() method to create and wrap a UIView inside a Codename One PeerComponent so that it can be used seamlessly in your UI. This method takes a callback in which you should define your "builder" method, which builds and returns the UIView. This builder method is run on the main thread (not the Codename One EDT) which is generally necessary for interaction with iOS native views. The following example creates a UIButton and wraps it in a PeerComponent so that it can be added to the UI.

import static com.codename1.objc.ObjC.eval;
// We're on the EDT
PeerComponent cmp = Objc.createPeerComponent(()->{
    // This callback runs synchronously (inside dispatch_sync()) on app main thread so that we
    // can create and interact with UIKit safely

    // NOTE:  This block is wrapped in an autorelease pool.  You should autorelease
    // any objects you create here to prevent memory leaks.

    Pointer button = eval("UIButton.buttonWithType:", 0).asPointer();
    eval(button, "setTitle:forState:", "Show View", 0);
    eval(button, "setTitleColor:forState:", eval("UIColor.blueColor"), 0);
    CallbackMethod cb = Objc.makeCallback(()->{
        Log.p("Button was clicked");
    eval(button, "addTarget:action:forControlEvents:", cb, cb.getSelector(), 1<<6);

    // The result is passed back to the EDT
    // You don't need to retain this reference -- createPeerComponent() handles that
    return button;

Mixing in your Own Objective-C Classes

Working directly in Java is nice, but you run into situations where you would prefer to work directly in Objective-C for parts of your app. For example, if you need to call methods that take structs as parameters, or return structs as parameters, you may need to create your own wrapper that you intend to call from Java. Alternatively, you might prefer to keep a certain module in pure objective-C to make it easier to debug in Xcode - or to make it easier to incorporate snippets of code you find online.

Well this is easy.

Just add your Objective-C code into the "native/ios" folder of your project, and it will be automatically and fully accessible through the bridge. You don’t need to do anything special.

E.g. Create a file named "HelloWorld.m" into native/ios.

#import <Foundation/Foundation.h>

@implementation HelloWorld : NSObject {

  +(void) hello {
      NSLog(@"Hello From objective-c");


Then you can call this from Java: