Feature/add additional checker

README.md

@@ -122,6 +122,21 @@ Simply call static method `BillingProcessor.isIabServiceAvailable()`:
         // continue
     }
 ```
+Please take a note that calling to `BillingProcessor.isIabServiceAvailable()` (only checks Play Market app installed or not) is not enough because there might be a case when it returns true but still payment won't success. Therefore, it's better to
+call to method `isOnTimePurchaseSupported()` after initializing `BillingProcessor`:
+```java
+    boolean isOneTimePurchaseSupported = billingProcessor.isOnTimePurchaseSupported();
+    if(isOneTimePurchaseSupported) {
+        // launch payment flow
+    }
+```
+or call to method `isSubscriptionUpdateSupported()` for checking update subcription usecase:
+```java
+    boolean isSubsUpdateSupported = billingProcessor.isSubscriptionUpdateSupported();
+    if(isSubsUpdateSupported) {
+        // launch payment flow
+    }
+```
 
 ## Consume Purchased Products
 

library/src/main/java/com/anjlab/android/iab/v3/BillingProcessor.java

@@ -74,6 +74,7 @@ public interface IBillingHandler {
 	private BillingCache cachedSubscriptions;
 	private IBillingHandler eventHandler;
 	private String developerMerchantId;
+	private boolean isOneTimePurchaseSupported;
 	private boolean isSubsUpdateSupported;
 
 	private class HistoryInitializationTask extends AsyncTask<Void, Void, Boolean>
@@ -233,6 +234,19 @@ public boolean subscribe(Activity activity, String productId, String developerPa
 		return purchase(activity, productId, Constants.PRODUCT_TYPE_SUBSCRIPTION, developerPayload);
 	}
 
+	public boolean isOnTimePurchaseSupported(){
+		if (isOneTimePurchaseSupported)
+			return true;
+
+		try {
+			int response = billingService.isBillingSupported(Constants.GOOGLE_API_VERSION, contextPackageName, Constants.PRODUCT_TYPE_MANAGED);
+			isOneTimePurchaseSupported = response == Constants.BILLING_RESPONSE_RESULT_OK;
+		} catch (RemoteException e) {
+			e.printStackTrace();
+		}
+		return isOneTimePurchaseSupported;
+	}
+
 	public boolean isSubscriptionUpdateSupported() {
 		// Avoid calling the service again if this value is true
 		if (isSubsUpdateSupported)