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Enable offline sync for your Android mobile app

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Overview

This tutorial covers the offline sync feature of Azure Mobile Apps for Android. Offline sync allows end users to interact with a mobile app—viewing, adding, or modifying data—even when there is no network connection. Changes are stored in a local database. Once the device is back online, these changes are synced with the remote backend.

If this is your first experience with Azure Mobile Apps, you should first complete the tutorial Create an Android App. If you do not use the downloaded quick start server project, you must add the data access extension packages to your project. For more information about server extension packages, see Work with the .NET backend server SDK for Azure Mobile Apps.

To learn more about the offline sync feature, see the topic Offline Data Sync in Azure Mobile Apps.

Update the app to support offline sync

With offline sync, you read to and write from a sync table (using the IMobileServiceSyncTable interface), which is part of a SQLite database on your device.

To push and pull changes between the device and Azure Mobile Services, you use a synchronization context (MobileServiceClient.SyncContext), which you initialize with the local database to store data locally.

  1. In TodoActivity.java, comment out the existing definition of mToDoTable and uncomment the sync table version:

     private MobileServiceSyncTable<ToDoItem> mToDoTable;

  2. In the onCreate method, comment out the existing initialization of mToDoTable and uncomment this definition:

     mToDoTable = mClient.getSyncTable("ToDoItem", ToDoItem.class);

  3. In refreshItemsFromTable comment out the definition of results and uncomment this definition:

     // Offline Sync
 final List<ToDoItem> results = refreshItemsFromMobileServiceTableSyncTable();

  4. Comment out the definition of refreshItemsFromMobileServiceTable.

  5. Uncomment the definition of refreshItemsFromMobileServiceTableSyncTable:

     private List<ToDoItem> refreshItemsFromMobileServiceTableSyncTable() throws ExecutionException, InterruptedException {
     //sync the data
     sync().get();
     Query query = QueryOperations.field("complete").
             eq(val(false));
     return mToDoTable.read(query).get();
 }

  6. Uncomment the definition of sync:

     private AsyncTask<Void, Void, Void> sync() {
     AsyncTask<Void, Void, Void> task = new AsyncTask<Void, Void, Void>(){
         @Override
         protected Void doInBackground(Void... params) {
             try {
                 MobileServiceSyncContext syncContext = mClient.getSyncContext();
                 syncContext.push().get();
                 mToDoTable.pull(null).get();
             } catch (final Exception e) {
                 createAndShowDialogFromTask(e, "Error");
             }
             return null;
         }
     };
     return runAsyncTask(task);
 }

Test the app

In this section, you test the behavior with WiFi on, and then turn off WiFi to create an offline scenario.

When you add data items, they are held in the local SQLite store, but not synced to the mobile service until you press the Refresh button. Other apps may have different requirements regarding when data needs to be synchronized, but for demo purposes this tutorial has the user explicitly request it.

When you press that button, a new background task starts. It first pushes all changes made to the local store using synchronization context, then pulls all changed data from Azure to the local table.

Offline testing

  1. Place the device or simulator in Airplane Mode. This creates an offline scenario.

  2. Add some ToDo items, or mark some items as complete. Quit the device or simulator (or forcibly close the app) and restart. Verify that your changes have been persisted on the device because they are held in the local SQLite store.

  3. View the contents of the Azure TodoItem table either with a SQL tool such as SQL Server Management Studio, or a REST client such as Fiddler or Postman. Verify that the new items have not been synced to the server

    + For a Node.js backend, go to the [Azure portal](https://portal.azure.com/), and in your Mobile App backend click **Easy Tables** > **TodoItem** to view the contents of the `TodoItem` table.
+ For a .NET backend, view the table contents either with a SQL tool such as *SQL Server Management Studio*, or a REST client such as *Fiddler* or *Postman*.

  4. Turn on WiFi in the device or simulator. Next, press the Refresh button.

  5. View the TodoItem data again in the Azure portal. The new and changed TodoItems should now appear.

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