Offline-Enable Your Android Application

Beginner

30 min.

SAP BTP SDK for Android, Beginner, Tutorial, SAP Business Technology Platform, Android, OData, Mobile

Enable offline OData in your Android application, resulting in an application that can be used without a network connection and that performs data requests with less latency.

You will learn

How the offline feature works, through demonstration
How the synchronization code works
How to handle errors that occur while syncing

Bruce MengMarch 22, 2022

Created byOctober 14, 2018

Contributors

Step 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.

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.

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

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.

On the detail screen, tap the edit toolbar icon.

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

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.

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.

Which of the following is true the first time an offline-enabled app is opened?
The app opens instantly
Extra time is needed to create the offline store and populate it with data
A network connection is required
A network connection is not required
Step 2
Make sure you are selecting the right language above.
To protect the data in offline store, you must enable offline store encryption by supplying an encryption key.

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

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.
Step 3
The offline store is populated based on objects called in using OfflineODataDefiningQuery. The defining queries are located in OfflineWorkerUtil.java, in the initializeOffline method.
Java
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OfflineODataDefiningQuery customersQuery = new OfflineODataDefiningQuery("Customers", "Customers", false); OfflineODataDefiningQuery productsQuery = new 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.
What is the purpose of a defining query?
They specify the indexes of the entities
They provide a way to specify which data of the OData service should be included in the offline store
To provide permission to upload the offline store
They control the theme of the app
Step 4
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.

In Android Studio, on Windows, press Ctrl+N, or, on a Mac, press command+O, and type OfflineOpenWorker to open OfflineOpenWorker.java 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.java, which is called by MainBusinessActivity when the user logs in to the application.

In Android Studio, on Windows, press Ctrl+N, or, on a Mac, press command+O, and type OfflineSyncWorker to open OfflineSyncWorker.java 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.java, 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.
What does the method synchronize do?
Sends local changes to the OData backend, and updates the local offline store with changes from the backend
Downloads changes from the server. Does not upload local changes.
Sends a notification to the appropriate user to reorder items with low stock
Uploads changes to the backend. Does not download changes from the server
Step 5
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.

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.

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.

Step 6

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

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

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>

Create a new activity in the app/java/com.sap.wizapp/mdui folder by right-clicking, then selecting New > Activity > Empty Activity. Name the new activity ErrorActivity.
Press Shift twice and type activity_error to open res/layout/activity_error.xml.

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"
        android:theme="@style/AppTheme.AppBarOverlay">

        <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/colorWhite" />

    </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>

Replace the ErrorActivity.java 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.

Java

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package com.sap.wizapp.mdui;

import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;
import androidx.appcompat.widget.Toolbar;
import android.os.Bundle;
import android.view.MenuItem;
import android.widget.TextView;

import com.sap.wizapp.R;

public class ErrorActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_error);
        Toolbar toolbar = findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        getSupportActionBar().setDisplayHomeAsUpEnabled(true);
        int errorCode = getIntent().getIntExtra("ERROR_CODE", 0);
        String errorMethod = getIntent().getStringExtra("ERROR_METHOD");
        String requestURL = getIntent().getStringExtra("ERROR_URL");
        String errorMessage = getIntent().getStringExtra("ERROR_MESSAGE");
        String body = getIntent().getStringExtra("ERROR_BODY");
        ((TextView) (findViewById(R.id.requestStatusTextView))).setText(String.valueOf(errorCode));

        if (errorMethod != null) {
            ((TextView) (findViewById(R.id.requestMethodTextView))).setText(errorMethod);
        }
        if (requestURL != null) {
            ((TextView) (findViewById(R.id.requestURLTextView))).setText(requestURL);
        }
        if (errorMessage != null) {
            ((TextView) (findViewById(R.id.requestMessageTextView))).setText(errorMessage);
        }
        if (body != null) {
            ((TextView) (findViewById(R.id.requestBodyTextView))).setText(body);
        }
    }

    @Override
    public boolean onOptionsItemSelected(@NonNull MenuItem item) {
        finish();
        return super.onOptionsItemSelected(item);
    }
}

On Windows, press Ctrl+N, or, on a Mac, press command+O, and type EntitySetListActivity to open EntitySetListActivity.java.
In the updateProgressForSync method, find the line progressBar.setVisibility(View.INVISIBLE);.

Right after the progressBar.setVisibility(View.INVISIBLE); line, in the SUCCEEDED case of sync work state, add the following code, which queries the error archive and displays information to the user about the first error encountered:

Java

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OfflineODataProvider provider = OfflineWorkerUtil.getOfflineODataProvider();
try {
    List<OfflineODataErrorArchiveEntity> errorArchive = provider.getErrorArchive();

    for (OfflineODataErrorArchiveEntity errorEntity : errorArchive) {
        String requestURL = errorEntity.getRequestURL();
        String method = errorEntity.getRequestMethod();
        String message = errorEntity.getMessage();
        Integer statusCode = errorEntity.getHttpStatusCode() != null  ?  errorEntity.getHttpStatusCode() : 0;
        String body = errorEntity.getRequestBody();

        LOGGER.error("RequestURL: " + requestURL);
        LOGGER.error("HTTP Status Code: " + statusCode);
        LOGGER.error("Method: " + method);
        LOGGER.error("Message: " + message);
        LOGGER.error("Body: " + body);

        Intent errorIntent = new Intent(EntitySetListActivity.this, ErrorActivity.class);
        errorIntent.putExtra("ERROR_URL", requestURL);
        errorIntent.putExtra("ERROR_CODE", statusCode);
        errorIntent.putExtra("ERROR_METHOD", method);
        errorIntent.putExtra("ERROR_BODY", body);
        try {
            JSONObject jsonObj = new JSONObject(message);
            errorIntent.putExtra("ERROR_MESSAGE", jsonObj.getJSONObject("error").getString("message"));
        } catch (JSONException e) {
            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 (OfflineODataException e) {
    e.printStackTrace();
}

After modification, the SUCCEEDED block should look like this:

Java

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case SUCCEEDED:
  LOGGER.info("Offline sync done.");
  OfflineODataProvider provider = OfflineWorkerUtil.getOfflineODataProvider();
  try {
      List<OfflineODataErrorArchiveEntity> errorArchive = provider.getErrorArchive();

      for (OfflineODataErrorArchiveEntity errorEntity : errorArchive) {
          String requestURL = errorEntity.getRequestURL();
          String method = errorEntity.getRequestMethod();
          String message = errorEntity.getMessage();
          Integer statusCode = errorEntity.getHttpStatusCode() != null  ?  errorEntity.getHttpStatusCode() : 0;
          String body = errorEntity.getRequestBody();

          LOGGER.error("RequestURL: " + requestURL);
          LOGGER.error("HTTP Status Code: " + statusCode);
          LOGGER.error("Method: " + method);
          LOGGER.error("Message: " + message);
          LOGGER.error("Body: " + body);

          Intent errorIntent = new Intent(EntitySetListActivity.this, ErrorActivity.class);
          errorIntent.putExtra("ERROR_URL", requestURL);
          errorIntent.putExtra("ERROR_CODE", statusCode);
          errorIntent.putExtra("ERROR_METHOD", method);
          errorIntent.putExtra("ERROR_BODY", body);
          try {
              JSONObject jsonObj = new JSONObject(message);
              errorIntent.putExtra("ERROR_MESSAGE", jsonObj.getJSONObject("error").getString("message"));
          } catch (JSONException e) {
              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 (OfflineODataException e) {
      e.printStackTrace();
  }
  break;

Run the app again, and re-attempt the sync. When the sync ends, 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, 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!

Generate and run an offline app
Examine the encryption key
Examine the defining queries
Examine the offline service and service manager
Introduce a synchronization error
Display ErrorArchive details

Offline-Enable Your Android Application

Which of the following is true the first time an offline-enabled app is opened?

What is the purpose of a defining query?

What does the method synchronize do?

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