Develop Reference

Is there a way to create iOS and Android libraries (maven and cocoapds) from one source code (eg. via Flutter)?

We are looking for a way to create a simple mobile SDK for both Android as well as iOS.
The SDK is super simple and will only make API calls and open page in WebView.
Is there a tool or language (eg. Flutter/React Native) where we would be able to create the small SDK and then export it for both Android and iOS and add in their respective repositories?
Or are we going to have to create 2 projects in for example Swift and Kotlin?
Looking for advices and best practices. Thank you.

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.

Multiplatform app development with individual guis

I am currently planning the development of a multiplatform app, and I am not sure wich way to do it fits my requirements best, since all possibilties I could think of failed to satisfy me.
First I want to write an app for android, which should feel like a normal typical android app. So I want to use the standard actionbar and android design look and feel in my other gui elements. After finishing the android app I am planning on developing an ios app, which should have a different design, so i am going to redevelop the gui for this anyway.
But I don´t want to rewrite the other code wich represents the intelligence of the app, independent from the gui. I came up with the following possibilities:
1. Java GUI With native library
Here I abstract all the code of my app in a C++ library (since as far as I know ios supports the usage of c++ libraries too) and develop the gui android typically in java. The library would than have a function to start and would inform the gui about every change via callback functions.
Pro
I can reuse all the code that would be the same on both platforms. I just would implement the gui seperately
The design of the android gui is straightforward as I want it to be. It looks like typical android because it is typical android.
Cons
I dislike the usage of JNI very much. Especially the signature and names of the callback functions (calling java functions from c++) are not checked at compile time and require a lot of manual work. If I rename a function and forget to rework the native part I only notice this mistake at runtime.
2. Build the GUI on the native side
Here I had difficulties finding out what is possible, especially for 2.2
2.1 Use Qt
I have only a few first step experiences with Qt in general but as far as I understood i would have the following pros and cons:
Pro
Reuse most parts of the code for ios and Android. I would than redesign the gui for each platform to make them feel natural. I can´t evaluate how much qt may even assists me at doing that
Cons
I have to copy the android gui by using other qt widgets. This is more effort and I don´t know if one can replicate the android gui elements (like the actionbar) so that the user wouldn´t notice it.
2.2 Using the android framework from the native side
I dont know if this is possible at all, I wasn´t able to find this information. Can I use the class "NativeActivity" and use the android framework to build the gui and use e.g. the actionbar? If this is possible somehow it would have the pros from 1. and maybe wouldn´t have it cons?
Do you have any feedback to my ideas or maybe even new approaches I didn´t think of? How do other multiplatform apps like WhatsApp solve this problem? Do they have redundant code for each platform?
Thank you,
Tobi

I would say that it depends very much on your application requirements. By my opinion, a better solution is to develop a separate application for each platform using recommended SDK's for that platform, and implement in native C++ only the time consuming data processing algorithms.
Application runtime on mobile platforms is not so straightforward as on desktop platforms. You should take into account background and foreground processing, specific application life cycling, accessing system resources such as network, file system, etc. And all these issues do differ on iOS and Android.
Regarding possibilities that you listed.
Qt/QML is ok only in case two requirements are met:
1.1 Your application is a foregroud application without any background operations.
1.2 You purchase a commercial Qt license because only commercial Qt can be submitted to Apple iTunes app store (even GPL apps are under
question).
Using NDK Native Activities on Android with cross-platform C/C++ backend. Android NDK API offers much less API then Android Java SDK, so a lot of things you will have to implement or wrap manually. It is a hard road.
Mixing Java code and C/C++ using JNI gives you more of Android SDK API. But you should remember that an Android activity's life cycle is not somewhat that you're used to deal with when developing on C/C++.
Approach that we are using
We've been developing an application with a huge amount of cross platform functionality that should work on Windows/Linux/Mac OSx/Android/iOS. We're using the approach as follows.
Cross-platform core is written in pure C++.
We have adaptors to GUI interface for each platform.
On desktop we use Qt as it reaches all desktop platforms with minor adapting to each platform.
On iOS the GUI is built using iOS SDK with Objective-C and C++ core is linked as a Framework. Still, we had to patch our core in some way for iOS background requirements and so.
On Android we wrap our C++ core in a background process and build all the GUI using only Android Java SDK. Foreground GUI activities interact with the core via local sockets, so we don't need to bother with activities life cycling in our C++ core. But the adaptor is a litle bit complicated.
Nevertheless, both mobile platforms often require workarounds and adaptations in C++ core which add a number of #ifdef'ed branches in code for each platform.

Xamarin.forms (or) xamarin.ios/xamarin.android (or) Native

We are developing an app in our organization where we have to implement charts,maps,GPS, If we Develop in Xamarin.ios/xamrin.android can developers able to share business logic of the app? since both android/ios projects will be in c#.

I'm doing exactly the same thing.
Do not use Xamarin Forms. It is not designed for nor capable of doing heavily customised UIs.
If you are going to require integration with Maps and also producing any kind of non-standard rendering or "User Controls" Xamarin.Forms is not going to work for you.
Go down the route of using a framework like MvvmCross to further abstract away the platforms and keep your core code / logic and user experience flow within the realms of a Core.Pcl type library.
Also I wouldn't try doing this on Windows 7 based bachines if you are doing this on Windows. Most of the components you will find yourself wanting to use will be targetting Profile259 and since Windows 7 doesn't support Windows Phone 8 you'll have all manner of oddities, even if you do the Profile259 hack.
Go with MvvmCross, Windows 8 and ignore Xamarin Forms.
I wrote a bit of it up on my blog.
Read this from Xmarin, they clearly state that Forms is for run of the mill business apps or for prototyping. It's not mature enough yet.

Yes, you can write your core app logic (services, data, domain, etc) in a PCL library that is shared by both your Android and iOS apps.

From my experience, you can build most apps in Xamarin Forms. The main hurdle is that doing anything not included in X.Forms requires that you write Custom Renderers. The thing is, writing custom renderers is much more straightforward if you have a solid grasp of vanilla Xamarin (Xamarin.iOS & Xamarin.Android).
What I think happens is that developers getting into Xamarin go directly to Xamarin.Forms and then become frustrated when they are unable to write custom renderers to accomplish some custom requirements.
So my recommendation is that if you have experience building apps using Xamarin.iOS and Xamarin.Android, Xamarin.Forms is awesome for getting an application developed rapidly for multiple platforms. Being able to use MVVM + XAML + Data Binding is a huge win.
If you are new to Xamarin or even mobile development, build the app using Xamarin.iOS and Xamarin.Android.

Can I use MVVMCross outside Xamarin?

can I use MVVMCross library as MVVM platform outside Xamarin With any Android Native project?
if no , is there any native MVVM framework for android applications ?

Currently MvvmCross relies on C# - so using it on Android requires Xamarin.Android.
You could possibly port it to Java, or could try the dot42.com platform which compiles C# to Dalvik.
For pure Java alternatives, you could also try a couple of library answers from Android data-binding (similar to WPF)?

Hi I would like to recommend RoboBinding to you, A data-binding Presentation Model(MVVM) framework for the Android platform. There are a minimal sample app AndroidMVVM and an album sample, which is an android translation of Martin Fowler's original sample created to explain the Presenation Model(called MVVM in microsoft community) pattern. Both samples can be imported into Android Studio with no extra config. The pattern is an involvement from old MVC pattern and its major motivation is to further decouple UI state and logic to a PURE POJO presenation model, which allows to be easily unit tested. If you like RoboBinding, i encourage you to have a look into its source code and have a feel of its code quality.