Page.EnqueueBackgroundTask Method

Version: Available or changed with runtime version 4.0.

Creates and queues a background task that runs the specified codeunit (without a UI) in a read-only child session of the page session. If the task completes successfully, the OnPageBackgroundTaskCompleted trigger is invoked. If an error occurs, the OnPageBackgroundTaskError trigger is invoked. If the page is closed before the task completes, or the page record ID on the task changed, the task is cancelled.

Syntax

[Ok := ]  Page.EnqueueBackgroundTask(var TaskId: Integer, CodeunitId: Integer [, var Parameters: Dictionary of [Text, Text]] [, Timeout: Integer] [, ErrorLevel: PageBackgroundTaskErrorLevel])

Parameters

Page
 Type: Page
An instance of the Page data type.

TaskId
 Type: Integer
Specifies the ID of the new page background task. The ID is assigned to the TaskId variable after the task is queued successfully. This parameter is passed by reference to the method.
CodeunitId
 Type: Integer
Specifies the ID of the codeunit to run when the task is started.
[Optional] Parameters
 Type: Dictionary of [Text, Text]
Specifies a collection of keys and values that are passed to the OnRun trigger of the codeunit that runs when the page background task session is started.
[Optional] Timeout
 Type: Integer
Specifies the number of milliseconds that the page background task can run before it is automatically cancelled.
[Optional] ErrorLevel
 Type: PageBackgroundTaskErrorLevel
Specifies the level of error handling on page background task level.

Return Value

[Optional] Ok
 Type: Boolean
true if the page background task is successfully queued for execution; otherwise false. If you omit this optional return value and the operation does not execute successfully, a runtime error will occur.

Remarks

The enqueued page background task stores the record ID of the current page. If the current record ID on the page changes, or the page is closed, the task is canceled. Typically, you call the EnqueueBackgroundTask method from a page trigger. The ID of the current record of the page must remain static after the call is made and while the background task is running. Otherwise, the task will be canceled. For this reason, we recommend that you don't enqueue the background task from the OnOpenPage trigger. Instead, use the OnAfterGetRecord or OnAfterGetCurrRecord triggers.

​The Child Session Max Concurrency setting of the Business Central Server controls how many page background tasks can be run simultaneously for a parent session. The setting has a default value of 5. If this number is exceeded, then they'll be queued and run when a slot becomes available as other tasks are finished. Enqueuing the task will fail if the total number of enqueued tasks exceed the Child Sessions Max Queue Length server configuration setting.​ For more information, see Configuring Business Central Server - Asynchronous Processing.

Timeout

When the value of the Timeout parameter is exceeded, the background task is canceled and an error with error code ChildSessionTaskTimeout occurs. On the page, the error will appear as a notification.

The Business Central Server instance includes two configuration settings related to the page back ground task timeout: PageBackgroundTaskDefaultTimeout and PageBackgroundTaskMaxTimeout.

  • The PageBackgroundTaskDefaultTimeout (which has a default value of 00:02:00) determines the timeout if the Timeout parameter isn't given a value.

  • The PageBackgroundTaskMaxTimeout specifies the maximum amount of time that a page background task can run. It doesn't matter what the Timeout parameter value is. If the Timeout value is greater than the PageBackgroundTaskMaxTimeout, which has a default value of 00:10:00, the PageBackgroundTaskMaxTimeout value determines the timeout.

For more information these settings, see Configuring Business Central Server.

It's possible to enqueue the task again in the completion trigger or error trigger, but this pattern isn't recommended as it can lead to an endless loop. For more information, see Page Background Tasks.

Example

The following code extends the Customer Card page with a page background task by using the EnqueueBackgroundTask method. For more information about this example, see Page Background Tasks.

pageextension 50100 CustomerCardExt extends "Customer Card"
{
    layout
    {
        addlast(General)
        {

            field(Before1; before1)
            {
                ApplicationArea = All;
                Caption = 'Before 1';
                Editable = false;
            }

            field(Duration1; duration1)
            {
                ApplicationArea = All;
                Caption = 'Duration 1';
                Editable = false;
            }

            field(After1; after1)
            {
                ApplicationArea = All;
                Caption = 'After 1';
                Editable = false;
            }
        }
    }

    var
        // Global variable used for the TaskID
        WaitTaskId: Integer;
        
        // Variables for the three fields on the page 
        before1: Text;
        duration1: Text;
        after1: Text;

    trigger OnAfterGetRecord();
    var
        //Defines a variable for passing parameters to the background task
        TaskParameters: Dictionary of [Text, Text];
    begin
        // Adds a key-value pair to the parameters dictionary 
        TaskParameters.Add('Wait', '1000');

        //Enqueues the page background task
        CurrPage.EnqueueBackgroundTask(WaitTaskId, 50100, TaskParameters, 10000, PageBackgroundTaskErrorLevel::Warning);
    end;

See Also

Page Background Tasks
Page Data Type
Getting Started with AL
Developing Extensions