Develop Reference

Create custom SDK for React Native

What i want to achieve is something like creating my own SDK like what react-native-fbsdk or rn-onesignal-sdk provides , so that those functionalities can be implemented in the react native project.
Basic questions :
Where to start from? Like what are the pre-requisites?
Native coding is required i believe for this functionality?
Any explanation or any such experience in such would be of great help .

Where to start from? Like what are the pre-requisites?
You can use a template like this one. It includes an example project which is basically an app that you can use to test integration of your native module and to quickly iterate on it.
Native coding is required i believe for this functionality?
Only if you need to use platform-specific functionality, or provide bindings to some pre-existing native library. Both examples you mentioned do exactly that – because they have native SDKs that can be used in non-RN apps, and they provide React Native bindings for them.
If your library is just business logic + some networking, and you don't intend it to be used in non-RN apps, you can do it all in JS. No need for native code. If you choose to go with the template that I have linked above, there are instructions how to use it to create a pure JS React Native library.
I'll be happy to update my answer if you can share some concrete requirements of what you want your SDK to do.

using "reactive native" with our existing code

I wrote my app using native method, currently I want to use reactive native in our further development to avoid writing two copies of code for iOS and Android.
The problem is we can't completely rewrite all the code, we want to replace the project piece by piece.
So I am wondering if I can write some views of my project using reactive native and others remain native code that we have used a long time.
If this is possible, is there any existing tutorial about how to do this?

You can keep your originally written native code via native modules. You can port your current project pieces by pieces to native modules, then let React-Native have access to it.
I guess best option is, start a new React-Native project and make it access your native modules, you won't have to rewrite everything then.
Here is more info for Android and iOS

There is official documentation to this. Checkout:
https://facebook.github.io/react-native/docs/integration-with-existing-apps.html

Does Android Studio supports AngularJS for Android UI design?

I have a question about Android Studio: Does AS supports AngularJS code while designing XML files, e.g. for small animations or effects?

SUMMARY : NO
There is no way you can write Angular JS code while developing Android Native app. You can use Angular in a WebView (using js/html so), that's all.

AngularJS is a completely different technology than what is used in native Android apps. Native Android apps use Java and/or C for their code, which is compiled when you build the app. AngularJS is a framework for web development, running specifically on Javascript (a completely different technology than Java).
You can build apps without using native Java or C code, but there are tradeoffs. As noted in other answers, you can display webpage views as apps (using technologies like Cordova]), or use frameworks like React Native to write code in Javascript that then gets compiled to a native app. You could use AngluarJS in the webview instance, since it just displays a webpage, however not in something like React Native. I highly encourage you to check both of these (and other options) out, but keep in mind that they have their own limitations and tradeoffs – no one way is the "right" way to build an app.
In summary: No, AngularJS is a web technology, not a native app technology.

new answer = Yes... Sort of, you can do it with:
https://www.nativescript.org/
It translates javascript, angular and typescript code into native components!
Apparantly it's possible to use nativescript in Android Studio with:
https://docs.nativescript.org/runtimes/android/getting-started/hello-world

Android Does not support AngularJs. but still if you want to use you can use it in webview.

Customizing android NuPlayer for inclusion in Android app

Question:
What is the suggested way to customize native components of Android for inclusion in an Android app? By "native components" I mean components for which the native source code is available in AOSP - but which use API's not exposed in the NDK documentation.
Details:
I want to use a simplified version of the Android media framework (actually, all I need is the NuPlayer to play an RTSP stream for my app). I know this sort of customization involves using the native code (frameworks/base/media) from AOSP, modifying what I need; and building this to generate the .so file as per my needs. I then do a System.loadLibrary() plus JNI.
My question is - what is the suggested way to do this?
I cannot use the NDK (rather, I am not supposed to use NDK) since many of the API's used in the media framework are not exposed to the NDK. But then, how do I build my modified media framework code? I do see Android.mk files in there. What do I use to build these?

Your question is self-contradictory: you want to customize native components of Android, without use the NDK. NDK stands for Native Development Kit: I have heard no one developing native components without using Native Development tools. And the NDK is the easiest one to use...
IMHO, your real problem lies in this statement: I cannot use the NDK (rather, I am not supposed to use NDK). Here I ask: why?

C++ library for mobile application

I have my own C++ library with source code. It contains functions like this: CreateDvice, FillDevice, CloseDevice and etc. There is no dependency to any third-party libraries.
There is requirement to create application for mobile platform:
Blackberry
iPhone
Android
The application has to use logic provided by this library.
Is there chance to use existing library in mobile application or at least some of them?
Or does it require to re-implement library code for each platform?

For iOS (iPhone/iPad) you can directly compile your library and use it from a regular iOS app written in Objective-C++ and/or C++.
For Android you can directly compile your library using the NDK, then either write your app in Java and call your library via JNI, or write the whole app in C++ using the NDK.
I believe you are out of luck on Blackberry, for this platform you'll need to rewrite your library in Java, as neither apps or libs can be written in C++.
Edit: See my other answer for a completely different approach that may work for you.

Blackberry:
It's technically possible to have ASM on BlackBerry (or Android, iPhone, etc.) but 3rd-party developers are often not allowed (or not able in the case of BlackBerry) to do so.
iPhone:
Absolutely. You can statically link a C++ library. Of course it will have to be compiled with the right instruction set. There are a host of examples out there on how to do this. Translate - you will need the code.
Android:
Absolutely. There is a good book on this by Mark Murphy. Introductory material here:
http://www.androidguys.com/2009/10/14/android-beyond-java-part-one/

Your question is unclear. Do you need a cross-platform library/engine to create a mobile application?
If it is so, Cocos2D would be the best choice. Originally it's a game engine, but it is suitable for applications too. And it supports all the platforms written above.

Instead of compiling your C++ library on each target device that you intend to support, you could opt for creating a service that packs your library. You can install this library on a host you have control, then from each platform the only thing you need to do is invoke this service.
I'm not sure if this thing makes sense for the kind of library that you have, but this would be a way to maintain a single version of your library, and you'll have a guaranteed same behavior on all devices.
Good luck.

Android is not natively Java, it's natively C++. And iOS is also natively C++. So why not just leave the C++ code untouched and drop RIM's current platform (since they are switching to BBX which does support C++ as well).

For Blackberry you can use the C++/Qt Cascades; for iOS you can use C/C++ & Objective-C (a superset of C) and Android can use the C++ NDK. You can use Java on all platforms as long as the Java apps are standalone and the JRE is pre-packaged with the app (iOS). You can interface with C/C++ libs using JNI

If you want to use a Java library on all platforms, that would work.
Android and Blackberry are natively Java.
You can use a tool called XMLVM to cross-compile your Java library to Objective C for use on iOS.
http://xmlvm.org/overview/
It is not 100% perfect, but pretty darn close. We use it extensively to port common Java library code to iOS. Port the C++ library to Java and you are good to go.