Develop Reference

Query InApp purchases within my Android app

I have an Android app where users can buy 1 inapp product to unlock some features.
I've read extensively the guide at: https://developer.android.com/google/play/billing/billing_library_overview#java
I understand that in order to let users buy the inapp product I have to:
Retrieve the list of available SKUs (in this case, my only 1 inapp product) using the querySkuDetailsAsync() call. This is just to double check the user's device is capable of managing inapp products.
Show the BUY button if the SKU appears in the result of the previous call (which means the local Google Play instance in the user's device can handle inapp products).
Call launchBillingFlow() passing the SKU of my inapp product, to initiate the Google payment process flow (Google UI, popup asking for card details etc..)
Listen to the callback onPurchasesUpdated() to get the return code (basically payment denied, payment successful or payment cancelled) and act accordingly. In case of payment successful proceed to verify the purchase token signature either locally (using a local copy of the Play developer's RSA public key) or remotely on my secure server with the same key.
unlock the paid features(s) on my app
That's where things get confusing. From the Google documentation perspective the job is done, they explained to you how to retrieve/purchase/and verify a user payment. However, nowhere it is explained how to remember the payment and unlock the paid feature during the app startup.
The documentation states:
To retrieve information about purchases that a user makes from your
app, call the queryPurchases()
So it seems like the app doesn't need to remember anything, just call the queryPurchases() at startup and check if the SKU is present (user already paid for it) or not (user still hasn't bought the paid version of the app).
So my app is doing just that, calling queryPurchases() at startup and check if the SKU is present or not.
This method works very well, even when the app starts offline. However some users are lamenting the fact that sometimes the app doesn't start in paid mode, because (I debugged the code) the function queryPurchases() fails (sometimes) when the device is offline. Could it be that the queryPurchases() is calling the local Google Play cache which can lose track of previous purchases for some reason? (cache purging, etc...)
What's the appropriate method to remember user purchases and enable paid features at app startup?

You could keep your own "cache" (SharedPreferences or a DB) with the results of onPurchasesUpdated and use queryPurchaseHistoryAsync. When the app first starts you can show paid content if your cache is telling that the user purchased the product and call queryPurchaseHistoryAsync at the same time to get the most recent purchase made by the user for each SKU, when onPurchaseHistoryResponse you can update your cache and hide the paid content if the purchase expired.
Take also into account that it's recommended for security purposes to go through purchases verification on your backend.
If you don't want to manage your own server, it may be worth using a tool like RevenueCat, that offers a purchase/subscription backend-as-a-service.

How often does it happen? My take is that Google Play client is taking care of caching, so there is no direct way how to handle that if you are just relying on this service. This can cause troubles if users re-install or change their devices.
Another approach is to build your own back-end, which will be necessary if you want to grow your app anyway.

How to Restore inApp purchase in Android?

How do I restore inApp purchase if the user has already bought the app?
The problem is that if a user first purchases then uninstalls the app then re-installs the app, the user must have to buy that product again.

InApp purchases made for Managed products is always managed by google.
Once a user buys it, it remains in his purchase list unless it is consumed by making InApp SDK API consume call.
So make the inapp products as Managed product and restore the purchases by making getPurchases() call. Even if the payment dialog is invoked it will show "User already own this item".

What should do for this Scenerio in-app billing

I have an application and I am trying to understand in-app billing. I want to do this : If a user buys an item with in-app billing. user can use this item with same account in different device without paying again. So my scenerio is this:
Assume that I have a application and it has in-app billing V3 service. And then a user purchases an "Managed Product" item and the user has an another device then the user wants to install this purcashed item in other device without paying again. But I read here, people talk about this problem:
BILLING_RESPONSE_RESULT_ITEM_ALREADY_OWNED
At this point I am thinking to consume this product as soon as it is purchased. with this code:
mHelper.consumeAsync(inventory.getPurchase(SKU_MY_ITEM), mConsumeFinishedListener);
if it right way. what will the user see after clicking buy button in another device.(is it possible free purchase)
Also I am not sure what will happen, if the user uninstall and install again my app. what will user see if click a buy button. (purchase again or free purchase or a ERROR)
Can you give advice about in app-billing service for 1 account and many device and also about what will see a user if try to purchase in another device even everything goes right.

From here http://developer.android.com/google/play/billing/api.html :
Managed In-app Products
Managed in-app products are items that have their ownership
information tracked and managed by Google Play. When a user purchases
a managed in-app item, Google Play stores the purchase information for
each item on a per-user basis. This enables you to later query Google
Play at any time to restore the state of the items a specific user has
purchased. This information is persistent on the Google Play servers
even if the user uninstalls the application or if they change devices.
What you want to do is ( like in the example app ) query the inventory on successful setup of your in app service and consume any managed item thus granting access to it.

Does In App Billing support multiple accounts?

Does InApp billing (V2 or V3) works for a user who uses several accounts and makes the purchase with an account that isn't the primary one?
PD: Some explanation required, no simple Yes/No

In App Billing support multiple accounts reported as a bug in code.google, have a look at it
According to Mr.Sergej (posted in G+ android dev community) in-app products purchased with the first account won't be active anymore while using multiple user accounts!.

InApp billing just charges the user and tells your app whether it was successful or not. It is up to you to figure out how to give the user what they purchased.
You can...
toggle the SharedPreferences of the app to save their purchase. Obviously multiple people can use the same app on the same device with the purchase.
provide login functionality to save a purchase to a server, in case the user deletes the app or gets a new device.
Use one-time purchases to check whether the user has purchased previously. This won't carry over to several accounts on different devices/multiple-accounts-per-device.

Android In-app Billing network access

I would like to know if the in-app billing api needs to have internet access when checking if the user have purchased an item or not.
I want to sell game levels using the in-app billing but don't want to access the internet everytime the user starts playing the level.
Does market have any cache mechanism to avoid calling the server everytime? Do I need to implement it on my app?

There are two types of purchases. Managed and unmanaged. Managed products can only be bought once, however unmanaged products can be bought any number of times.
There is no caching as such by the market app. Say for example, you have a managed product ID setup, and you a user has already bought it. The next time you request the market app to buy the same item, it will return with a response, saying "already purchased".
In case of unmanaged items, it just carries out the transaction.
In both the cases, internet is required.
It's upto you how you save the transactions locally, and initiate a purchase by checking if you really need to do another purchase.

You don't even need the Internet permission to use in-app billing. Market will take care of that for you.