What is the best way to make apps portable across mobile platforms?

I am working on an app for Android, but would like to have it ported in the future to iPhone and (if it seems to gain significant market share in the future) WP7/8.

I know nothing about a开发者_如何学编程ny mobile platform other than Android. Should I be using C++ (NDK), Java, or something else to get as much portability as possible? I assume I will have to rewrite (or have someone rewrite) at lest part of it, but I would like to make as much of the codebase as possible portable.

Edit: It is a simple app that "scans" a barcode (camera) or has the number typed in, sends some information to a server, gets back some more info (probably in XML, but could be whatever works best) and displays it in a few simple screens.

Edit 2: The 4 parts I have, and would like to ideally make them portable are:

1. Scan a barcode (probably the hardest to make portable)
2. Allow entering a barcode number manually
3. Send and receive some custom data
4. Display a few screens (This can be in HTML, if I have an option to make it look "native", not like the browser window displaying a webpage it is)

For a non-trivial app that isn't a game, there are currently no mature solutions to this problem.

Tools like Titanium Mobile and PhoneGap are meant to help you do exactly what you want, but impose limitations on what your app can do and are so far at the cutting edge of development that they often introduce bugs of their own that you won't be able to fix. Additionally, you're at their developer's mercy when it comes to support for alternative platforms like WP7 or Blackberry. In some cases, these might be worth the risk, or you may get lucky, but you need to be wary before rushing to use them.

Often, the best technique is still just to develop independent codesbases. You can model them after each other in architectural terms, and if you do so it isn't typically that hard to translate between different platforms.

Another technique to aid in portability for many apps is to use embedded web browsers where it makes sense. This allows you design some of your views in HTML/CSS/Javascript rather than with native UI tools, and then just load them into the aforementioned embedded web browsers on each platform.

Depends on how many platforms you are aiming for.

I have some experience doing iPhone + Android. Honestly, I wouldn't spend too much time trying to learn a portability tool on top. I have done couple of apps (simple communication stuff + a light weight game) and my experience is so far - assumed you are familiar with both - it is far faster + easier to just write your app and then port it.

Besides the normal coding work, the most difficult tasks are to develop the app structure including the main classes and data structures - this you can copy basically 1 to 1 in terms of structure of course - not code.

Also since you will keep getting updates on different platforms like iOS (Android + Eclipse are a little slower it seems to come up with updates/upgrades), which make life easier all the time, you would most probably have a big disadvantage when relying on someone elses portability development toolkit.

So far - certainly not to be taken as a general rule - while development on iPhone took 100%, porting it on Android took at most 30-40%

This question is way too broad, it depends on what kind of app you're doing.

Generally speaking, if you're just as good off doing a web app, do that. It'll work across different platforms.

MonoTouch / MonoDroid by Xamarin (previously owned by Novell) allow you to write C# (the language used by WP7) for iOS/Android, respectively. It is not free, however.

I don't believe the frameworks go so far as to include cross-platform UI, but I'd say that's probably a good thing. At least your application logic can remain the same across platforms.

I've not used the framework myself, so I cannot vouch for it.

We do use MonoTouch/MonoDroid and it helps us to simultaneously support WM/WP7/Android/iOS. The common denominator of all these platforms (after employing Xamarin products) is .Net.

The policy of our company is to maintain toolkits for all platforms that provide similar UI. Then the apps can be written in a nearly platform-agnostic way. This isn't, however, a general statement as the independency refers to the specific types of applications.

I don't advise using our toolkits for your case as it looks like an overkill. However, investing into C#/.Net might not be a bad idea.

U should definitely go with the easiest route: Java. You will still have to rework a lot while porting, but if you were using NDK, the overhead for Android itself might be similar. (In an average app. As you havent provided too much information about the project, the answer couldnt be more specific. If you are developing a performance-critical app, like a 3D game, you should consider NDK for the high porfile program parts).

Edit: for this scanning solution, I would go with Java on android and wont bother with portability. For a project with such a clean application logic it wont worth the effort to make it portable (except if the aim of the project is to experiment with application portability, but as far as I got it, it isnt).