Offline-Enable Your Android Application

1. Follow the instructions at Try Out the SAP BTP SDK Wizard for Android to create a new application using the SAP BTP SDK Wizard for Android and select Offline for the OData option on the Project Features tab. The push feature is not needed for this application.

   

2. Run the app. After the login process, a screen is displayed explaining that the offline store is opening. As the screen suggests, opening the offline store for the first time can take up to a few minutes. One technique to decrease this initial time is to only download data that is relevant to the user, such as customers that belong in their sales region.

   

3. When you get to the app’s home page, turn on airplane mode on your device, or disable Wi-Fi and data.

   

4. The entity list screen is populated based on the metadata.xml file retrieved when the application was created. Tap the Products list item.

   

   The Products screen makes a data request to display the available products. Notice that it succeeds without a working network connection. The data request is fulfilled from the offline store that was previously created and populated on the device. Tap the Accessories item to display the detail screen.

   

5. On the detail screen, tap the edit toolbar icon.

   

6. Make a change to the currency code and tap the save toolbar icon.

   

7. Navigate back to the app’s Home screen and tap Synchronize using the three-dot-menu in the top right of the title bar.

   

   The sync should fail because you haven’t turned airplane mode off yet.

   

8. Turn off airplane mode or re-enable Wi-Fi/data and attempt a sync again. You will see a notification that describes the sync action.

   

   When the sync completes, the change you made will have been applied to the back end.

To protect the data in offline store, you must enable offline store encryption by supplying an encryption key.

1. In Android Studio, on Windows, press Ctrl+N, or, on a Mac, press command+O, and type OfflineWorkerUtil, to open OfflineWorkerUtil.kt.

2. On Windows, press Ctrl+F12, or, on a Mac, press command+F12, and type initializeOffline, to move to the initializeOffline method. Call the setStoreEncryptionKey method of the OfflineODataParameters instance to encrypt the offline store. In single user mode, before you can get the encryption key using UserSecureStoreDelegate, you must generate and save an encryption key to UserSecureStore first.

   

For additional information about multiple user mode, see Enable Multi-User Mode for Your Android Application.

The offline store is populated based on objects called in using OfflineODataDefiningQuery. The defining queries are located in OfflineWorkerUtil.kt, in the initializeOffline method.

val customersQuery = OfflineODataDefiningQuery("Customers", "Customers", false)
val productsQuery = OfflineODataDefiningQuery("Products", "Products", true)

Defining queries tell the OfflineODataProvider (the class that manages the offline store) which entity sets to store on the device. In the case of the wizard-generated application, there is a defining query for each available entity by default, meaning that each entity is stored offline and available if the user doesn’t have an internet connection. For more information, see Defining Queries.

With an offline-enabled app, requests made against the entity sets that are included in the defining requests will always be fulfilled from the local offline store.

The application allows users to make changes against a local offline store and synchronize manually at any time. The sync operation is performed by a worker. There are three operations that must be implemented in order to use the offline store functionality: open, download, and upload. As their names suggest, the operations open the offline store, download server changes, and upload user changes, respectively. In the wizard-generated application, OfflineOpenWorker implements open during initialization and OfflineSyncWorker implements a sync operation, which requires download first, and then upload.

1. In Android Studio, on Windows, press Ctrl+N, or, on a Mac, press command+O, and type OfflineOpenWorker to open OfflineOpenWorker.kt and examine the open method.

   

   The worker’s work is to call the open method of the OfflineODataProvider class to perform the open operation and pass the given callbacks through.

   The OfflineOpenWorker class is called by the open method in OfflineWorkerUtil.kt, which is called by MainBusinessActivity when the user logs in to the application.

   
   

2. In Android Studio, on Windows, press Ctrl+N, or, on a Mac, press command+O, and type OfflineSyncWorker to open OfflineSyncWorker.kt and examine the download and upload methods.

   

   The worker’s work is to call the download and upload method of the OfflineODataProvider class to perform the sync operation and pass the given callbacks through.

   The OfflineSyncWorker class is called by the sync method in OfflineWorkerUtil.kt, which is called by EntitySetListActivity when the user wants to perform a sync. When an entity is created locally in the offline store, its primary key is left unset. This is because when the user performs an upload, the server will set the primary key for the client. An upload and a download are normally performed together because the download may return updated values from the server, such as a newly-created primary key.

   
   

For more information about how the offline store works, see the Working With Offline Stores.

When syncing changes made while offline, conflicts can occur. One example might be if two people attempted to update a description field for the same product. Another might be updating a record that was deleted by another user. The ErrorArchive provides a way to see details of any of the conflicts that may have occurred. The following instructions demonstrate how to use ErrorArchive.

1. Update a SalesOrderItem and change its quantity to be zero and save it. Update a second item and change its quantity to a different non-zero number and save it.

   
   
   

   Notice that the items are now marked with a yellow indicator to indicate that an item has been locally modified but not yet synced.

   

2. Attempt a sync, and you’ll notice that the sync completes, but if you examine the SalesOrderItems list, one item has a red mark beside it, indicating it is in an error state. This is because the back end has a check that SalesOrderItems cannot have zero for their quantity. This check does not exist in the local offline store, so the update succeeds locally but fails when the offline store is synced.

   

In this section we will create an Error Information screen that displays the details from the ErrorArchive.

1. Press Shift twice and type strings.xml to open res\values\strings.xml.

2. Add the following values.

   XML

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   <string name="error_header">Error Information</string>
   <string name="request_method">Request Method</string>
   <string name="request_status">Request Status</string>
   <string name="request_message">Request Message</string>
   <string name="request_body">Request Body</string>
   <string name="request_url">Request URL</string>
   

3. Create a new activity in the app/java/com.sap.wizapp/mdui folder by right-clicking, then selecting New > Activity > Empty Views Activity. Name the new activity ErrorActivity.

4. Press Shift twice and type activity_error to open res/layout/activity_error.xml.

5. Replace its contents with the following XML.

   XML

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   <?xml version="1.0" encoding="utf-8"?>
   <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
       xmlns:app="http://schemas.android.com/apk/res-auto"
       xmlns:tools="http://schemas.android.com/tools"
       android:layout_width="match_parent"
       android:layout_height="match_parent"
       tools:context=".mdui.ErrorActivity"
       android:orientation="vertical">
   
       <com.google.android.material.appbar.AppBarLayout
           android:id="@+id/app_bar"
           android:layout_width="match_parent"
           android:layout_height="wrap_content">
   
           <androidx.appcompat.widget.Toolbar
               android:id="@+id/toolbar"
               android:layout_width="match_parent"
               android:layout_height="?attr/actionBarSize"
               app:popupTheme="@style/AppTheme.PopupOverlay"
               app:titleTextColor="@color/colorBlack" />
   
       </com.google.android.material.appbar.AppBarLayout>
   
       <ScrollView
           android:layout_height="wrap_content"
           android:layout_width="match_parent">
   
           <LinearLayout
               android:layout_height="wrap_content"
               android:layout_width="match_parent"
               android:orientation="vertical">
   
               <TextView
                   android:layout_width="match_parent"
                   android:layout_height="wrap_content"
                   android:padding="@dimen/key_line_16dp"
                   style="@style/Test.ObjectCell.Headline"
                   android:text="@string/error_header"/>
   
               <View
                   android:layout_width="match_parent"
                   android:layout_marginTop="@dimen/key_line_16dp"
                   android:layout_height="1dp"
                   android:background="?android:attr/listDivider" />
   
               <TextView
                   android:layout_width="match_parent"
                   android:layout_height="wrap_content"
                   android:padding="@dimen/key_line_16dp"
                   style="@style/FioriTextStyle.OVERLINE"
                   android:text="@string/request_message"/>
   
               <TextView
                   android:id="@+id/requestMessageTextView"
                   android:layout_width="match_parent"
                   android:layout_height="wrap_content"
                   android:paddingLeft="@dimen/key_line_16dp"
                   android:paddingRight="@dimen/key_line_16dp"
                   style="@style/TextAppearance.Fiori.Subtitle1"
                   android:singleLine="false"
                   android:text="@string/request_message"/>
   
               <View
                   android:layout_width="match_parent"
                   android:layout_marginTop="@dimen/key_line_16dp"
                   android:layout_height="1dp"
                   android:background="?android:attr/listDivider" />
   
               <TextView
                   android:layout_width="match_parent"
                   android:layout_height="wrap_content"
                   android:padding="@dimen/key_line_16dp"
                   style="@style/FioriTextStyle.OVERLINE"
                   android:text="@string/request_body"/>
   
               <TextView
                   android:id="@+id/requestBodyTextView"
                   android:layout_width="match_parent"
                   android:layout_height="wrap_content"
                   android:paddingLeft="@dimen/key_line_16dp"
                   android:paddingRight="@dimen/key_line_16dp"
                   style="@style/TextAppearance.Fiori.Subtitle1"
                   android:singleLine="false"
                   android:text="@string/request_body"/>
   
               <View
                   android:layout_width="match_parent"
                   android:layout_marginTop="@dimen/key_line_16dp"
                   android:layout_height="1dp"
                   android:background="?android:attr/listDivider" />
   
               <TextView
                   android:layout_width="match_parent"
                   android:layout_height="wrap_content"
                   android:padding="@dimen/key_line_16dp"
                   style="@style/FioriTextStyle.OVERLINE"
                   android:text="@string/request_url"/>
   
               <TextView
                   android:id="@+id/requestURLTextView"
                   android:layout_width="match_parent"
                   android:layout_height="wrap_content"
                   android:paddingLeft="@dimen/key_line_16dp"
                   android:paddingRight="@dimen/key_line_16dp"
                   style="@style/TextAppearance.Fiori.Subtitle1"
                   android:singleLine="false"
                   android:text="@string/request_url"/>
   
               <View
                   android:layout_width="match_parent"
                   android:layout_marginTop="@dimen/key_line_16dp"
                   android:layout_height="1dp"
                   android:background="?android:attr/listDivider" />
   
               <View
                   android:layout_width="match_parent"
                   android:layout_height="1dp"
                   android:background="?android:attr/listDivider" />
   
               <TextView
                   android:layout_width="match_parent"
                   android:layout_height="wrap_content"
                   android:padding="@dimen/key_line_16dp"
                   style="@style/FioriTextStyle.OVERLINE"
                   android:text="@string/request_status"/>
   
               <TextView
                   android:id="@+id/requestStatusTextView"
                   android:layout_width="match_parent"
                   android:layout_height="wrap_content"
                   android:paddingLeft="@dimen/key_line_16dp"
                   style="@style/TextAppearance.Fiori.Subtitle1"
                   android:text="@string/request_status"/>
   
               <View
                   android:layout_width="match_parent"
                   android:layout_marginTop="@dimen/key_line_16dp"
                   android:layout_height="1dp"
                   android:background="?android:attr/listDivider" />
   
               <TextView
                   android:layout_width="match_parent"
                   android:layout_height="wrap_content"
                   android:padding="@dimen/key_line_16dp"
                   style="@style/FioriTextStyle.OVERLINE"
                   android:text="@string/request_method"/>
   
               <TextView
                   android:id="@+id/requestMethodTextView"
                   android:layout_width="match_parent"
                   android:layout_height="wrap_content"
                   android:paddingLeft="@dimen/key_line_16dp"
                   style="@style/TextAppearance.Fiori.Subtitle1"
                   android:text="@string/request_method"/>
   
               <View
                   android:layout_width="match_parent"
                   android:layout_marginTop="@dimen/key_line_16dp"
                   android:layout_height="1dp"
                   android:background="?android:attr/listDivider" />
           </LinearLayout>
       </ScrollView>
   </LinearLayout>
   

6. Replace the ErrorActivity.kt generated activity code with the following code.

   In the package statement and the import, if needed, replace com.sap.wizapp with the package name of your project.

   Kotlin

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   package com.sap.wizapp.mdui
   
   import androidx.appcompat.app.AppCompatActivity
   import android.os.Bundle
   import android.view.MenuItem
   import android.view.View
   import android.widget.TextView
   
   import com.sap.wizapp.R
   
   class ErrorActivity : AppCompatActivity() {
   
       override fun onCreate(savedInstanceState: Bundle?) {
           super.onCreate(savedInstanceState)
           setContentView(R.layout.activity_error)
           setSupportActionBar(findViewById(R.id.toolbar))
           supportActionBar!!.setDisplayHomeAsUpEnabled(true)
           val errorCode = intent.getIntExtra("ERROR_CODE", 0)
           val errorMethod = intent.getStringExtra("ERROR_METHOD")
           val requestURL = intent.getStringExtra("ERROR_URL")
           val errorMessage = intent.getStringExtra("ERROR_MESSAGE")
           val body = intent.getStringExtra("ERROR_BODY")
           (findViewById<View>(R.id.requestStatusTextView) as TextView).text = "".plus(errorCode)
   
           errorMethod?.let {
               (findViewById<View>(R.id.requestMethodTextView) as TextView).text = it
           }
           requestURL?.let {
               (findViewById<View>(R.id.requestURLTextView) as TextView).text = it
           }
           errorMessage?.let {
               (findViewById<View>(R.id.requestMessageTextView) as TextView).text = it
           }
           body?.let {
               (findViewById<View>(R.id.requestBodyTextView) as TextView).text = it
           }
       }
   
       override fun onOptionsItemSelected(item: MenuItem): Boolean {
           finish()
           return super.onOptionsItemSelected(item)
       }
   }
   

7. On Windows, press Ctrl+N, or, on a Mac, press command+O, and type EntitySetListActivity to open EntitySetListActivity.kt.

8. In the updateProgressForSync method, in the WorkInfo.State.SUCCEEDED block, add the following code, which queries the error archive and displays information to the user about the first error encountered:

   Kotlin

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   val provider = OfflineWorkerUtil.offlineODataProvider
   
   try {
       val errorArchive = provider!!.errorArchive
   
       for (errorEntity in errorArchive) {
           val requestURL = errorEntity.requestURL
           val method = errorEntity.requestMethod
           val message = errorEntity.message
           val statusCode = errorEntity.httpStatusCode ?: 0
           val body = errorEntity.requestBody
   
           LOGGER.error("RequestURL: $requestURL")
           LOGGER.error("HTTP Status Code: $statusCode")
           LOGGER.error("Method: $method")
           LOGGER.error("Message: $message")
           LOGGER.error("Body: $body")
   
           val errorIntent = Intent(this@EntitySetListActivity, ErrorActivity::class.java)
   
           errorIntent.putExtra("ERROR_URL", requestURL)
           errorIntent.putExtra("ERROR_CODE", statusCode)
           errorIntent.putExtra("ERROR_METHOD", method)
           errorIntent.putExtra("ERROR_BODY", body)
   
           try {
               val jsonObj = message?.let { JSONObject(it) }
               jsonObj?.let {
                   errorIntent.putExtra(
                       "ERROR_MESSAGE",
                       jsonObj.getJSONObject("error").getString("message")
                   )
               }
           } catch (e: JSONException) {
               e.printStackTrace()
           }
   
           // Reverts all failing entities to the previous state or set
           // offlineODataParameters.setEnableIndividualErrorArchiveDeletion(true);
           // to cause the deleteEntity call to only revert the specified entity
           // https://help.sap.com/doc/f53c64b93e5140918d676b927a3cd65b/Cloud/en-US/docs-en/guides/features/offline/common/handling-failed-requests/reverting-error-state.html
           // provider.deleteEntity(errorEntity, null, null);
           startActivity(errorIntent)
           break //For simplicity, only show the first error encountered
       }
   } catch (e: OfflineODataException) {
       e.printStackTrace()
   }
   

   After modification, the WorkInfo.State.SUCCEEDED block should look like this:

   Kotlin

   Copy

   WorkInfo.State.SUCCEEDED -> {
       LOGGER.info("Offline sync done.")
       val provider = OfflineWorkerUtil.offlineODataProvider
   
       try {
           val errorArchive = provider!!.errorArchive
   
           for (errorEntity in errorArchive) {
               val requestURL = errorEntity.requestURL
               val method = errorEntity.requestMethod
               val message = errorEntity.message
               val statusCode = errorEntity.httpStatusCode ?: 0
               val body = errorEntity.requestBody
   
               LOGGER.error("RequestURL: $requestURL")
               LOGGER.error("HTTP Status Code: $statusCode")
               LOGGER.error("Method: $method")
               LOGGER.error("Message: $message")
               LOGGER.error("Body: $body")
   
               val errorIntent = Intent(this@EntitySetListActivity, ErrorActivity::class.java)
   
               errorIntent.putExtra("ERROR_URL", requestURL)
               errorIntent.putExtra("ERROR_CODE", statusCode)
               errorIntent.putExtra("ERROR_METHOD", method)
               errorIntent.putExtra("ERROR_BODY", body)
   
               try {
                   val jsonObj = message?.let { JSONObject(it) }
                   jsonObj?.let {
                       errorIntent.putExtra(
                           "ERROR_MESSAGE",
                           jsonObj.getJSONObject("error").getString("message")
                       )
                   }
               } catch (e: JSONException) {
                   e.printStackTrace()
               }
   
               // Reverts all failing entities to the previous state or set
               // offlineODataParameters.setEnableIndividualErrorArchiveDeletion(true);
               // to cause the deleteEntity call to only revert the specified entity
               // https://help.sap.com/doc/f53c64b93e5140918d676b927a3cd65b/Cloud/en-US/docs-en/guides/features/offline/common/handling-failed-requests/reverting-error-state.html
               // provider.deleteEntity(errorEntity, null, null);
               startActivity(errorIntent)
               break //For simplicity, only show the first error encountered
           }
       } catch (e: OfflineODataException) {
           e.printStackTrace()
       }
   }
   

9. Run the app again, and re-attempt the sync. When the sync fails, you should see the following error screen.

   

   You can see that the HTTP status code, method, and message are included. When the application attempted a sync, the entity being updated didn’t pass the backend checks and produced a DataServiceException and is now in the error state. All entities that did not produce errors are successfully synced. One way to correct the exception would be to change the quantity from 0 to a valid positive number. Another would be to delete the ErrorArchive entry, reverting the entity to its previous state. For more information on error handling, see Handling Errors and Conflicts and Handling Failed Requests.

Further information on using the offline feature can be found at Step by Step with the SAP BTP SDK for Android — Part 6 — Offline OData.

Congratulations! You have created an offline-enabled app using the SAP BTP SDK Wizard for Android and examined how the ErrorArchive can be used to view synchronization errors!