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Offline Enable Your Android Application

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Offline Enable Your Android Application
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Offline Enable Your Android Application

Enable offline OData in your Android application resulting in an application that can be used without a network connection and one 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

Step 1: Generate and run an offline app

Follow the instructions at Try Out SAP Cloud Platform SDK for Android Wizard to create a new application using the SAP Cloud Platform SDK for Android wizard and select Offline for the OData option. The push feature is not needed for this application.

Choose Offline OData

Run the app.

After logging in and setting a passcode, you’ll be met by a screen 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.

Offline store opening

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

Turn on Airplane mode

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

Entity list screen

The products screen makes a data request to show 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.

Select the second product

On the detail page, tap the edit toolbar icon.

Select product edit button

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

Change currency code

Navigate back to the app’s home screen tap on Synchronize using the three dot menu in the top right of the title bar.

Attempt a sync

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

Sync fails

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.

Syncing notification

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

Which of the following are true the first time an offline enabled app is opened?
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Step 2: Examine the defining queries

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

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?
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Step 3: Examine the offline service and service manager

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 Foreground Service. In the wizard-generated application, the OfflineODataSyncService is the foreground service. There are three operations that must be implemented in order to use the offline store functionality, namely openStore, downloadStore and uploadStore. As their names suggest, the operations open the offline store, download server changes, upload user changes respectively. In Android Studio, on Windows press Ctrl+N or on a Mac press command+O and type OfflineODataSyncService to open OfflineODataSyncService.java and examine the three methods.

Offline store methods

The service is simply a wrapper for the corresponding methods of the OfflineODataProvider class. The methods perform the open/download/upload operations and pass the given callbacks through.

Implementation of open and download store

The openStore method is called by LogonActivity.java, when the user logs in to the application and binds the OfflineODataSyncService. For more information, see Creating a bound service.

LogonActivity calls openStore

The uploadStore and downloadStore methods are called by SAPServiceManager 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.

SAPServiceManager performs sync

For more information about how the offline store works, see the Offline API.

What does the method synchronize do?
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Step 4: Introduce a synchronization error

When syncing changes made while offline, conflicts can occur. One example might be if two people attempted to update a description field of 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 conflicts that have occurred. The following instructions will demonstrate this.

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.

Create SalesOrderItem ButtonEdit SalesOrderItem ButtonCreate with zero quantity

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

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.

Sync Error
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Step 5: Display ErrorArchive details

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.

<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 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 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">

    <ScrollView
        xmlns:android="http://schemas.android.com/apk/res/android"
        android:layout_height="wrap_content"
        android:layout_width="match_parent">

        <LinearLayout
            xmlns:android="http://schemas.android.com/apk/res/android"
            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 below code.

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

package com.sap.wizapp.mdui;

import android.support.v7.app.AppCompatActivity;
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);
        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("" + 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(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 synchronize method, find the line that performs the sapServiceManager.synchronize method call.

SapServiceManager synchronize

Between the progressBar.setVisibility(View.INVISIBLE); line and the syncCompleteHandler.call(); line in the success callback, add the following code that queries the error archive and displays information to the user about the first error encountered.

SAPServiceManager serviceManager = ((SAPWizardApplication)getApplication()).getSAPServiceManager();
OfflineODataProvider provider = serviceManager.retrieveProvider();
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/c2d571df73104f72b9f1b73e06c5609a/Latest/en-US/docs/user-guide/odata/Offline_OData_Handling_Failed_Requests.html#reverting-an-error-state
        // provider.deleteEntity(errorEntity, null, null);
        startActivity(errorIntent);
        break; //For simplicity, only show the first error encountered
    }
} catch (OfflineODataException e) {
    e.printStackTrace();
}

Run the app again, and re-attempt the sync. When the sync fails, you should be brought to the following error screen.

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 which did not produce errors, 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 visit 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 Cloud Platform SDK for Android — Part 6 — Offline OData.

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

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