bcquality/community/knowledge/ui/page-background-tasks.md

bc-version	domain	keywords	technologies	countries	application-area
all	ui	enqueuebackgroundtask async-calculation child-session factbox cue-tile onaftergetcurrrecord responsive-page read-only	al	w1	all

Offload Slow Read-Only Page Calculations To Background Tasks

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Description

Pages that compute statistics, aggregates, or external lookups inline block the page from rendering until the calculation finishes, producing a visible freeze on FactBoxes, cue tiles, and calculated fields. Business Central provides page background tasks: CurrPage.EnqueueBackgroundTask runs a dedicated codeunit in a read-only child session and returns values via OnPageBackgroundTaskCompleted, so the page opens immediately and fills in computed values as they arrive. This matters because users should never wait on a calculation they may not need. The mechanism has specific rules that are easy to get wrong, which is why it warrants an explicit pattern.

Best Practice

Move any noticeable read-only computation off the synchronous render path into a background task. Enqueue from OnAfterGetCurrRecord so the task is tied to the currently focused record, and pass small payloads through the Dictionary of [Text, Text] input/output, converting types with Format and Evaluate. Keep each task focused on one value or a small related set rather than one large task, and show a placeholder until results land. Because tasks auto-cancel when the page closes, the record changes, or a same-ID task is re-enqueued, always supply sensible defaults and handle the timeout path in OnPageBackgroundTaskError — never let critical functionality depend on completion. For tests, drive the task synchronously with RunPageBackgroundTask.

Anti Pattern

Enqueuing from OnAfterGetRecord on a list page fires the task for every row, and each cancels the instant the selection moves to the next row — pure wasted child-session churn; a reviewer spots EnqueueBackgroundTask called from OnAfterGetRecord (or from OnOpenPage, where the record context is not yet stable). The other tell is a task codeunit attempting a database write or Modify: background tasks run read-only and the write fails at runtime. Inline heavy calculation directly in OnAfterGetCurrRecord with no task at all is the baseline smell — it reintroduces the page freeze the feature exists to remove.