This document discusses how to create a native interface in Codename One to allow calling native OS code from a Codename One app. It explains that native interfaces allow invoking native OS code and wrapping it in a library. It describes how native code differs across platforms like Android, iOS, Windows, and JavaScript. It provides examples of generating native interface stubs, implementing them for different platforms, and using the native interface code from a Codename One app. Build hints can be used to inject additional native behavior by modifying the app manifest or plist.

1. How do I?

2. Create a Native Interface

3. What’s a Native Interface?
✦Native interfaces allow us to invoke OS native code
✦We can use it directly in the app or wrap it in a library
(cn1lib) for a common abstraction
✦Native in Codename One is different from native in Java.
In Java native is always C, in Codename one it’s
something else
© Codename One 2017 all rights reserved

4. Native in Codename One
✦Android - Java (Dalvik/Art) code, you can still wrap
NDK code with AAR
✦iOS - Objective-C code and native libraries
✦Windows - C# code
✦JavaScript - JavaScript code
✦Desktop Port - Java SE code
© Codename One 2017 all rights reserved

5. package com.codename1.app.nativedemo;
import com.codename1.system.NativeInterface;
public interface CallNativeCode extends NativeInterface {
public String doThisInNative(String arg1, int arg2);
}
Hello World Native Interface

6. Generate Native Interface
✦The Generate Native Access action automatically
generates required native code stubs
✦Stubs will appear under the “native” directory
© Codename One 2017 all rights reserved

7. package com.codename1.app.nativedemo;
public class CallNativeCodeImpl {
public String doThisInNative(String param, int param1) {
return null;
}
public boolean isSupported() {
return false;
}
}
Android Stub

8. package com.codename1.app.nativedemo;
public class CallNativeCodeImpl {
public String doThisInNative(String param, int param1) {
return "Dummy";
}
public boolean isSupported() {
return true;
}
}
Android Implementation

9. #import "com_codename1_app_nativedemo_CallNativeCodeImpl.h"
@implementation com_codename1_app_nativedemo_CallNativeCodeImpl
-(NSString*)doThisInNative:(NSString*)param param1:(int)param1{
return nil;
}
-(BOOL)isSupported{
return NO;
}
@end
iOS Stub

10. #import "com_codename1_app_nativedemo_CallNativeCodeImpl.h"
@implementation com_codename1_app_nativedemo_CallNativeCodeImpl
-(NSString*)doThisInNative:(NSString*)param param1:(int)param1{
return @"Dummy";
}
-(BOOL)isSupported{
return YES;
}
@end
iOS Implementation

11. namespace com.codename1.app.nativedemo{
public class CallNativeCodeImpl : ICallNativeCodeImpl {
public string doThisInNative(String param, int param1) {
return null;
}
public bool isSupported() {
return false;
}
}
}
Windows Stub

12. (function(exports){
var o = {};
o.doThisInNative__java_lang_String_int = function(param1, param2, callback) {
callback.error(new Error("Not implemented yet"));
};
o.isSupported_ = function(callback) {
callback.complete(false);
};
exports.com_codename1_app_nativedemo_CallNativeCode= o;
})(cn1_get_native_interfaces());
JavaScript Stub

13. CallNativeCode cn = NativeLookup.create(CallNativeCode.class);
if(cn != null && cn.isSupported()) {
String returnVal = cn.doThisInNative("String", 1);
}
Using the Code

14. Implementation
✦Code doesn’t compile on the client & won’t have code
completion
✦A neat trick is using “include source” and then opening
the native source in xcode or Android Studio and
implementing the native code there
✦You can’t use all types. You can’t pass objects!
✦Use PeerComponent to represent a native UI widget
✦You can use callbacks from the native code
© Codename One 2017 all rights reserved

15. Build Hints
✦You can use build hints to inject many native interface
behaviors
✦Native code on Android often requires a 3rd party library
thru gradle, native code on iOS often uses cocoapods
we support both thru build hints
✦You can inject values into the manifest or plist to make
use of this
✦You can look thru existing cn1libs for sample code
© Codename One 2017 all rights reserved

16. Thank You!