Pattern added Delegate API operation

content/docs/patterns/api-delegate-operation/index.md

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+---
+title: "Delegate API Operation"
+tags: ["AL","API"]
+categories: ["Pattern"]
+---
+
+_Created by Arend-Jan Kauffmann, Described by Arend-Jan Kauffmann_
+
+## Abstract
+The goal of this pattern is to delegate data operations from the API page to a codeunit. The codeunit can implement its own logic for inserting, modifying or deleting data.
+
+## Context
+API pages implement a specific pattern for insert, modify and delete operations. An API page uses delayed insert behavior, which means that all fields will be validated before the record is inserted or modified.
+This is different from the standard behavior on a UI card page, where a record is first inserted, followed by a modify when field values are updated.
+
+## Problem
+The standard behavior can result in a number of challenges:
+
+* Code behind the OnValidate triggers and its event subscribers may expect that a record already exists when a field is being validated.
+* Usage of temporary buffer tables can become really complex when fully handled from the API page.
+* Applying default data to records before they are created does not work well with delayed inserts.
+
+## Description
+To mitigate these problems, we can delegate the data operation to a codeunit while canceling the data operation inside the API page.
+
+```al
+codeunit 50000 "Item API Operations"
+{
+    internal procedure InsertItem(var Item: Record Item)
+    begin
+    end;
+
+    internal procedure ModifyItem(var Item: Record Item)
+    begin
+    end;
+
+    internal procedure DeleteItem(var Item: Record Item)
+    begin
+    end;
+}
+```
+
+It is important that the record parameter is updated with the final result. This allows the API page to return the result of the API operation to the caller.
+
+The API page implements this in the page triggers as follows:
+
+```al
+    var
+        ItemAPIOperations: Codeunit "Item API Operations";
+
+    trigger OnInsertRecord(BelowxRec: Boolean): Boolean
+    begin
+        ItemAPIOperations.InsertItem(Rec);
+        exit(false);
+    end;
+
+    trigger OnModifyRecord(): Boolean
+    begin
+        ItemAPIOperations.ModifyItem(Rec);
+        exit(false);
+    end;
+
+    trigger OnDeleteRecord(): Boolean
+    begin
+        ItemAPIOperations.DeleteItem(Rec);
+        exit(false);
+    end;
+```
+
+The triggers must return false in order to cancel the operation in the page.
+
+## Benefits
+Inside the functions in the codeunit you have full control over the steps that are performed for the specific operation.