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Applies only to Mobile Apps and Reactive Web Apps
Service Studio version:

Implement asynchronous data fetching using Aggregates

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  • Sometimes you want to update only a part of the Screen, and you can do it by loading data in chunks. For example, you may have a list with employees where clicking an employee name loads the details.

    This is often called a master-detail pattern, and you can implement it by using two Aggregates with different Fetch properties, where the first "master" Aggregate fetches data immediately, and the second "detail" Aggregate only when needed. The "details" are loaded asynchronously, as the request takes place in the background and updates the UI when the data is received.


    Before we move on to the steps of implementing the master-detail pattern and showing how the asynchronous data fetching works, let's prepare our app:

    • Create a new Reactive Web App. You can check the instructions in the article Create a Reactive Web App.
    • In the newly created Reactive App, check if you have OutSystemsSampleDataDB under Data > Entities > Database. If not, follow the steps in Referencing sample data in apps, as we need it to for the example in this article.


    We're going to create a Screen with the employee list. Clicking the name shows the employee details. The employee list loads when the Screen loads, but the details are fetched only after the click, asynchronously.

    Here's the overview of our implementation:

    1. Get the list of all employees
    2. Get the details of one employee
    3. Create logic to update the UI

    Get the list of all employees

    Let's create a "master" UI populated by the records from the Aggregate that fetches the list of all Employees once the Screen loads. We set the Fetch property of this Aggregate to At start.

    1. Start by creating a blank Screen and naming it MasterDetailTutorial.

    2. Right-click on the Screen in Interface > UI Flows > MainFlow, and select Fetch data from database from the help menu. The Aggregate details open.

    3. With the Aggregate details open, click the Data tab, navigate to Entities > Database > OutSystemsSampleDataDB, and drag the Entity Sample_Employee to the Aggregate details screen. Sample data populates the view while the name of Aggregate changes to "GetEmployees".

    4. Go back to the Interface tab, and Set the Fetch property of the GetEmployees Aggregate to At start. This ensures that the app requests the data from the database when the Screen loads.

      Fetch property of the Aggregate

    5. Open your MasterDetailTutorial Screen and drag a Columns Medium Left Widget to the editor.

    6. Drag a List Widget to the left column of the Columns Medium Left Widget. Service Studio now shows that a source of data is needed for the List. In the Properties of the List Widget, locate the Source property and select GetEmployees.List.

    7. Drag a Tag Widget to the List Widget. At this point, your Screen should look similar to this:

      Load all items in master detail

      You can adjust the tag style in the properties:

      Properties of the Tag Widget

    8. Drag an Expression Widget to the Tag Widget. Expression Value editor opens. Enter GetEmployees.List.Current.Sample_Employee.FirstName + " " + GetEmployees.List.Current.Sample_Employee.LastName and click Done. The tags now show the employees' first and last names. Run the app, and you should see a Screen with the list of employees.

    Get the details of one employee

    Let's now create the "detail" UI that loads and shows data only after a click. The Fetch property of this Aggregate is set to Only on demand. Additionally, we set a Filter in this Aggregate with the ID of the record, so that the app fetches one record only. This results is a very responsive user interface.

    1. Add a new the Aggregate for the Screen, for the same Sample_Employee Entity. Name the Aggregate "GetEmployeeDetails".

    2. Still in the Aggregate details pane, add a Filter to GetEmployeeDetails Aggregate with the following condition: Sample_Employee.Id = GetEmployees.List.Current.Sample_Employee.Id. This forces the Aggregate to return only a single record, a record that matches selection in the list of employees.

      Filter that ensures only one record is returned

    3. Set the Fetch property of the GetEmployeeDetails Aggregate to Only on demand. With this setting, the Aggregate queries the database only upon a request.

    4. Open your MasterDetailTutorial Screen and drag an If Widget to the Column2 of the Columns Medium Left Widget. In the Condition property of the If Widget, enter GetEmployeeDetails.List.Empty. This evaluates if an employee's name is clicked.

    5. Select the True branch of the If Widget and type "Select an employee to see the details." This is a blank state for this part of the Screen, when no employee name is selected.

    6. Drag GetEmployeeDetails Aggregate to the False branch of the If Widget. This "scaffolds" an interface with the employee details and picture. At this point you should have a Screen with two Aggregates that update different parts of UI. The screenshot contains additional overlay to show how Aggregates fetch data for different parts of the Screen.

      Load all items in master detail

    7. Publish the app. If you open it in the browser, you should see the message "Select an employee to see the details.". In the next section we implement the logic to refresh the UI asynchronously.

    Note that the scaffolding from Entity in this section works only after you add a Filter to the Aggregate, as instructed.

    Create logic to update the UI

    The "detail" UI that loads data only when requested must be triggered. In this example, the app loads the data after you click a name in the list. Thanks to the architecture of the app, the UI refreshes automatically after the data bound to it changes.

    1. Select the Expression within the Tag. In the "Events" section of the properties in the Events list, select onclick. In the Handler list select New Client Action. The OnClick Client Action opens for editing.

      Adding events to trigger async loading

    2. Drag an Assign node to the flow. In the Variable property, enter GetEmployeeDetails.List.Current.Sample_Employee.Id, and in Value enter GetEmployees.List.Current.Sample_Employee.Id. This tells the app which employee is selected in the list.

    3. Drag a Refresh Data to the flow below the Assign. When the Select Data Source dialog shows, select GetEmployeeDetails. This refreshes the UI where the data is used.

      Flow with Assign and Refresh Data

    4. Publish the app and try clicking on the employee names. You should see the details load on the right side of the screen.

    Example from Screen Templates

    To see a working example, you can also create a Screen from one of the Screen Templates that are built around the master-detail pattern.

    Master-Detail Pattern in Screen Template dialog

    In the created Screen, check the Fetch properties of the Aggregates and related logic.

    Master-Detail Pattern in Service Studio