Simplify PaymentSession state and lifecycle management (#2165)

Summary
- Create `PaymentSessionViewModel` and integrate in `PaymentSession`
- Remove `savedInstanceState` argument from `PaymentSession#init()`
- Remove `PaymentSession#savePaymentSessionInstanceState()`
- Remove `PaymentSession#onDestroy()` - the `listener` instance
  is now released automatically
- In `PaymentSession#init()`, immediately call the `listener` when
  `shouldPrefetchCustomer` is false
- Move `CustomerSession` initialization in example app to
  `Application` subclass

Motivation
Simplify `PaymentSession` state management and API. It is
no longer necessary to pass a state `Bundle` and manage
lifecycle events.

Testing
Add unit tests
Manually verify with both `Activity` and `Fragment`

example/src/main/java/com/stripe/example/ExampleApplication.kt

@@ -4,9 +4,8 @@ import android.os.StrictMode
 import androidx.multidex.MultiDexApplication
 import com.facebook.stetho.Stetho
 import com.stripe.android.CustomerSession
-import com.stripe.android.EphemeralKeyProvider
-import com.stripe.android.EphemeralKeyUpdateListener
 import com.stripe.android.PaymentConfiguration
+import com.stripe.example.service.ExampleEphemeralKeyProvider
 import kotlinx.coroutines.CoroutineScope
 import kotlinx.coroutines.Dispatchers
 import kotlinx.coroutines.launch
@@ -38,12 +37,10 @@ class ExampleApplication : MultiDexApplication() {
             Stetho.initializeWithDefaults(this@ExampleApplication)
         }
 
-        // initialize empty CustomerSession
-        CustomerSession.initCustomerSession(this,
-            object : EphemeralKeyProvider {
-                override fun createEphemeralKey(apiVersion: String, keyUpdateListener: EphemeralKeyUpdateListener) {
-                }
-            }
+        CustomerSession.initCustomerSession(
+            this,
+            ExampleEphemeralKeyProvider(this),
+            false
         )
     }
 }

example/src/main/java/com/stripe/example/activity/CustomerSessionActivity.kt

@@ -11,7 +11,6 @@ import com.stripe.android.model.Customer
 import com.stripe.android.model.PaymentMethod
 import com.stripe.android.view.PaymentMethodsActivityStarter
 import com.stripe.example.R
-import com.stripe.example.service.ExampleEphemeralKeyProvider
 import kotlinx.android.synthetic.main.activity_customer_session.*
 
 /**
@@ -28,11 +27,6 @@ class CustomerSessionActivity : AppCompatActivity() {
         super.onCreate(savedInstanceState)
         setContentView(R.layout.activity_customer_session)
         setTitle(R.string.customer_payment_data_example)
-        CustomerSession.initCustomerSession(
-            this,
-            ExampleEphemeralKeyProvider(this),
-            false
-        )
 
         progress_bar.visibility = View.VISIBLE
         CustomerSession.getInstance().retrieveCurrentCustomer(

example/src/main/java/com/stripe/example/activity/FragmentExamplesActivity.kt

@@ -28,7 +28,6 @@ import com.stripe.example.R
 import com.stripe.example.StripeFactory
 import com.stripe.example.module.BackendApiFactory
 import com.stripe.example.service.BackendApi
-import com.stripe.example.service.ExampleEphemeralKeyProvider
 import io.reactivex.android.schedulers.AndroidSchedulers
 import io.reactivex.disposables.CompositeDisposable
 import io.reactivex.schedulers.Schedulers
@@ -50,8 +49,8 @@ class FragmentExamplesActivity : AppCompatActivity() {
 
         val newFragment = LauncherFragment()
 
-        val ft = supportFragmentManager.beginTransaction()
-        ft.add(R.id.root, newFragment, LauncherFragment::class.java.simpleName)
+        supportFragmentManager.beginTransaction()
+            .replace(R.id.root, newFragment, LauncherFragment::class.java.simpleName)
             .commit()
     }
 
@@ -275,10 +274,6 @@ class FragmentExamplesActivity : AppCompatActivity() {
         }
 
         private fun createCustomerSession(): CustomerSession {
-            CustomerSession.initCustomerSession(
-                requireContext(),
-                ExampleEphemeralKeyProvider(requireContext())
-            )
             val customerSession = CustomerSession.getInstance()
             customerSession.retrieveCurrentCustomer(
                 object : CustomerSession.CustomerRetrievalListener {

example/src/main/java/com/stripe/example/activity/PaymentSessionActivity.kt

@@ -20,7 +20,6 @@ import com.stripe.android.view.BillingAddressFields
 import com.stripe.android.view.PaymentUtils
 import com.stripe.android.view.ShippingInfoWidget
 import com.stripe.example.R
-import com.stripe.example.service.ExampleEphemeralKeyProvider
 import kotlinx.android.synthetic.main.activity_payment_session.*
 import java.util.Currency
 import java.util.Locale
@@ -45,9 +44,7 @@ class PaymentSessionActivity : AppCompatActivity() {
         super.onCreate(savedInstanceState)
         setContentView(R.layout.activity_payment_session)
 
-        progress_bar.visibility = View.VISIBLE
-
-        paymentSession = createPaymentSession(savedInstanceState)
+        paymentSession = createPaymentSession(savedInstanceState == null)
 
         btn_select_payment_method.setOnClickListener {
             paymentSession.presentPaymentMethodSelection()
@@ -57,21 +54,17 @@ class PaymentSessionActivity : AppCompatActivity() {
         }
     }
 
-    private fun createCustomerSession(): CustomerSession {
-        CustomerSession.initCustomerSession(
-            this,
-            ExampleEphemeralKeyProvider(this),
-            false
-        )
-        return CustomerSession.getInstance()
-    }
-
     private fun createPaymentSession(
-        savedInstanceState: Bundle?,
-        shouldPrefetchCustomer: Boolean = true
+        shouldPrefetchCustomer: Boolean = false
     ): PaymentSession {
+        if (shouldPrefetchCustomer) {
+            disableUi()
+        } else {
+            enableUi()
+        }
+
         // CustomerSession only needs to be initialized once per app.
-        val customerSession = createCustomerSession()
+        val customerSession = CustomerSession.getInstance()
 
         val paymentSession = PaymentSession(
             activity = this,
@@ -96,8 +89,7 @@ class PaymentSessionActivity : AppCompatActivity() {
                 .build()
         )
         paymentSession.init(
-            listener = PaymentSessionListenerImpl(this, customerSession),
-            savedInstanceState = savedInstanceState
+            listener = PaymentSessionListenerImpl(this, customerSession)
         )
         paymentSession.setCartTotal(2000L)
 
@@ -159,22 +151,12 @@ class PaymentSessionActivity : AppCompatActivity() {
         paymentSession.handlePaymentData(requestCode, resultCode, data ?: Intent())
     }
 
-    override fun onDestroy() {
-        paymentSession.onDestroy()
-        super.onDestroy()
-    }
-
-    override fun onSaveInstanceState(outState: Bundle) {
-        super.onSaveInstanceState(outState)
-        paymentSession.savePaymentSessionInstanceState(outState)
-    }
-
     private fun onPaymentSessionDataChanged(
         customerSession: CustomerSession,
         data: PaymentSessionData
     ) {
         paymentSessionData = data
-        progress_bar.visibility = View.VISIBLE
+        disableUi()
         customerSession.retrieveCurrentCustomer(
             PaymentSessionChangeCustomerRetrievalListener(this)
         )
@@ -186,6 +168,12 @@ class PaymentSessionActivity : AppCompatActivity() {
         btn_start_payment_flow.isEnabled = true
     }
 
+    private fun disableUi() {
+        progress_bar.visibility = View.VISIBLE
+        btn_select_payment_method.isEnabled = false
+        btn_start_payment_flow.isEnabled = false
+    }
+
     private fun onCustomerRetrieved() {
         enableUi()
 
@@ -239,10 +227,10 @@ class PaymentSessionActivity : AppCompatActivity() {
                 BackgroundTaskTracker.onStart()
             }
 
-            listenerActivity?.progress_bar?.visibility = if (isCommunicating) {
-                View.VISIBLE
+            if (isCommunicating) {
+                listenerActivity?.disableUi()
             } else {
-                View.INVISIBLE
+                listenerActivity?.enableUi()
             }
         }