Develop Reference

How can an age restriction be applied to an Android app?

I'm working on an Android app where a user must be 18 years or older to apply.
If there will be people under 18, the app would have negative public view which I do not want to deal with.
Is it somehow possible to be sure that my users are older than 17? Is there any service which could possibly accomplish this?
Ideal fictional solution: in my registration form the user writes his/her id number and I would check if he/she is old enough with fictional service.
One not very good solution which I found. There is an app called Muzmatch. It is dating app for Arabs. When the user register he have to use device camera to take picture of himself and send it to server.
I can do the same in my app and check if the user seems old enough but I think it's stupid idea.

Yes this is possible through the Google Play content rating system.
Check out the link: https://support.google.com/googleplay/android-developer/answer/188189?hl=en-GB
Scroll down to the Take your app’s rating questionnaire section and it'll run you through the steps to apply an age rating to your app.
In addition to this, maybe you could create a free in app purchase, that the user will have to buy when first entering the app. They would then need either a password or finger print in order to pay via Google Play.

Offer free features to earlier buyers

I have a paid app in the Google Play Store. I'm considering reducing the price of that app (somewhat; not all the way to free) and offering one of the features as a separate in-app purchase.
If I did that, I wouldn't want to yank the feature away from anybody who's already bought it.
Is there any way to figure out either the date that the user bought my app, or the original version of the app that they bought, or something like that? I'd like to say something like, "If the app was before the price change (either by date or by version), they should have the feature for free; otherwise, require IAP to unlock the feature."
For example, iOS does have a feature like this; the app receipt includes an "originalVersion" field which can be used to control access to features.

Unfortunately for your customers, this is impossible. There is no API call or anything else to Google Play where you can get the time on which the app was bought.
I know there is an android-publisher API in existance, however it doesn't offer any feature to check that.
The functions you want to use are not public availible and only used by the Playstore internally!
Workarounds which you could do:
1. Get the time the app was installed
On the first start you could check that and unlock the features.
Warning: This system could be abused by changing the time on the device
long installed = context
.getPackageManager()
.getPackageInfo(context.getPackag‌​eName(), 0)
.firstInstallTime;
2. Give users free keys
You could give every user who's using the app atm a free key via mail or push notification
3. Unlock the inapp purchase now
Publish an app update which unlocks the inapp purchase for free. After a few weeks you could pusblish your new version with the lower price and just unlock the features as if your current customers had bought your extension.
You might be able to hack your way around this if you're using some sort of persistent storage.
For SharedPreferences, on the first run, do a check for one of your preferences using SharedPreferences.contains(). If it contains it, the app must have already been installed. If not, set another preference that marks the user as new, and set yet one more so it doesn't do the check every time.
That might only work if the preference doesn't have a "default" value, I'm not entirely sure if setting a default in xml will mark it as contained.
You could do something similar if you have any assets that get transferred to SD, or any similar one-time setup. Just check to see if it's already done before doing it the first time.
If you're using an SQLite DB, you could increment the DB version and mark as "paid" in onUpgrade() if coming from the current version(or earlier).
There are some pitfalls here, though. For instance, if a previous paid customer completely uninstalls before installing the new version, or if it's on a new device. For that reason you should:
4. Provide Support
In the about or FAQ section of your app and on first run of your new version set a support mail adress which customers can use if they have any problems because the new features were not unlocked for them.
They could provide any proof (bill) for their purchase.
Like I said, those ideas are workarounds, but I don't know of any "official" way to check to see an app install is an upgrade or an initial install.
Your best option may be a combination of those four.
FYI: I've opened a feature request/idea in Google Cloud Connect for work which you could vote: https://connect.googleforwork.com/ideas/9392 (You can only vote if you have a paid Google Buisiness Account)
I hope this helps you at least a bit.

As far as i know, the best you can do is find the date it was installed. http://developer.android.com/reference/android/content/pm/PackageInfo.html#firstInstallTime

App once paid, now free, give benefits to people who paid?

I have an app right now that I am planning to put onto Google play as a paid app. I know that one can decide at a later date to make the app free. I'm thinking that I might one day want to make the app free, selling some of the features that are now included in the paid version. Is there a way that I can tell if a user paid for an app?

The best I can suggest is to put some SharedPreference in your app marks it as "paid", then release a new version of your app which does not have this (when you make it free). Then, you can check the flag to see if it was there before it went free.
The downside to this is that root users may be able to change that value, if they were to discover it. Another downside is that clearing the data would clear the flag.
To avoid those downsides, you could store the data in a webserver database, but this seems like a fair bit of extra work for something like this.

Use a SharedPreference, as Eric suggests, but encode your value; instead of using true or yes, use a hash of the device ID so that no one can post that value for use on other devices. Then you can just recompute the hash when testing this indicator, to ensure it is your own value and not one entered by a user.

How to check if user paid on android?

I would like to use in-app billing in my android application (instead of creation 2 versions of application - free and pro). If user paid, then additional options in Preferences should be available.
My application synchronizes data with website (not my).
Each time synchronization happens, I would like to check if user paid or not.
How should I do it?

I think the easiest means for you would be to use Managed purchase. More on it is at http://developer.android.com/guide/market/billing/billing_admin.html#billing-purchase-type
The "manage by user account" purchase
type is useful if you are selling
items such as game levels or
application features. These items are
not transient and usually need to be
restored whenever a user reinstalls
your application, wipes the data on
their device, or installs your
application on a new device.
Update: website and API updated refer this now https://developer.android.com/google/play/billing/api.html

There are to many ways to do that, i will try and give you some of the most used
what about user accounts? can your application support users? if it does, its practically solved with 1 user per 1 account. just add a flag to your DB for each user. this is the most secure way, its very easy to know if someone is stealing from you thats why all MMO's (like WOW) use this type of check. you can easily know if theres more then one user on an account as well
the downside is that it requires more processing and some support is case someone's account gets stolen or other user support of that kind.
you can save a sort of key inside your application. and some other key on the server. if the key matches (either 1 to 1 or after some manipulation) then you have yourself a paid user, other you dont.when someone buys the application then change the key on the device.
just do what you dont want to do and have 2 applications for free and paid it will probably be less work then the other 2 list above.
there are many ways of doing what you want. but i'll tell you this:
most applications use method 2 but on the opposite direction, meaning serial key.
i dont think your gonna create an mmo so theres no need for option 1 unless you really dont want anyone hacking your application (which i assume no matter what you do it will be hacked in todays world. i'll advice you to trust those who pay and accept those who dont)
method 3 is just easy in my opinion especialy when you want to just remove menu options, but it also creates duplicate repositories for code.
good luck. what ever you decide is good, make a theft protection, if someone wants to steal it, they will, dont fight it (even PS3 got hacked in the end)

make android application free for some users

I would like to make my application free to install and upgrade for some users only (for example translators, developers and friends, to which else I would have to send the package at every release).
I thought the new licensing would allow that, but it seems not. Since I can't find the answer to whether it is even possible, I am asking right away:
Is it possible to put a paid application on the market and have it either freely istallable or paid, let's say based on the user id (account)?

The easiest thing to do would be to just send them an apk that they can install. They won't get auto updates, but should be able to use it.

I think I saw somewhere in the documentation for in-app billing that it is possible to have "unmanaged" purchases, where it is up to you to manage which user purchased what, either on the device or on a separate server. Maybe it's possible to implement your own purchase server to run in the "Google cloud" using AppEngine?
Anyway, in-app billing is, AFAIK, not available yet, and it seems a tad complicated just to let your friends try your application for free. I've been considering an alternative approach to implementing a try-before you buy scheme:
a) Implement a free App with basic functionality
b) Implement a paid but otherwise empty "unlock" App
c) When users activate "paid" functionality, use the PackageManager to look for the "unlock" app. If it is installed then activate the requested feature, otherwise show a dialog asking the user to go to the market and buy the unlock app.
If you did something similar, you could tell your friends and your testers to download and/or upgrade the unlocked version, and just send them the .apk for the "unlock" app. Furthermore, you would only need to send them the unlock app once.
I think I've seen such "unlock" apps on the market, but I haven't actually tried the approach myself (yet), so I can't guarantee that it will work. Can't see why it shouldn't, though.

For this, if it's possible, you need to have a database for the userId (if you want to manage that)
Or if you want a freely application.. it will need to be a Freeware (a beta version) from the complete application
Else, i don't think it's possible