fix(row-adding): implementing changes from review comments.

Author: ChronosSF

CHANGELOG.md

@@ -6,15 +6,18 @@ All notable changes for each version of this project will be documented in this
 
 ### New Features
 - `igxGrid`, `igxHierarchicalGrid`, `igxTreeGrid`
-    - Added two public methods that spawn the add row UI for an arbitrary record in the current data view. One that accepts a rowID to use as the row the UI spawns under and the other specifying the index at which to spawn it. Please, note that the new record is still added at the end of the data view, after the end-user submits it.
-    ```typescript
-    this.grid.beginAddRowById('ALFKI');         // spawns the add row UI under the row with PK 'ALFKI'
-    this.grid.beginAddRowById('ALFKI', true);   // spawns the add row UI to add a child for the row with PK 'ALFKI'
-    this.grid.beginAddRowById(null);            // spawns the add row UI as the first record
-    this.grid.beginAddRowByIndex(10);           // spawns the add row UI at index 10
-    this.grid.beginAddRowByIndex(10, true);     // spawns the add row UI to add a child for the row at index 9
-    this.grid.beginAddRowByIndex(0);            // spawns the add row UI as the first record
-    ```
+    - Added two public methods that spawn the add row UI for an arbitrary record in the current data view. One that accepts a rowID to use as the row the UI spawns under and the other accepting an index that has a distinct implementation for `IgxTreeGrid`. Please, refer to the official documentation for more information:[Grid Row Adding](https://www.infragistics.com/products/ignite-ui-angular/angular/components/grid/row-adding) and [Tree Grid Row Adding](https://www.infragistics.com/products/ignite-ui-angular/angular/components/treegrid/row-adding).
+
+        _Note:_ That the new record is still added at the end of the data view, after the end-user submits it.
+        ```typescript
+        this.grid.beginAddRowById('ALFKI');             // spawns the add row UI under the row with PK 'ALFKI'
+        this.grid.beginAddRowById(null);                // spawns the add row UI as the first record
+        this.grid.beginAddRowByIndex(10);               // spawns the add row UI at index 10
+        this.grid.beginAddRowByIndex(0);                // spawns the add row UI as the first record
+        this.treeGrid.beginAddRowById('ALFKI', true);   // spawns the add row UI to add a child for the row with PK 'ALFKI'
+        this.treeGrid.beginAddRowByIndex(10, true);     // spawns the add row UI to add a child for the row at index 10
+        this.treeGrid.beginAddRowByIndex(null);         // spawns the add row UI as the first record
+        ```
 
 ## 12.1.3
 

projects/igniteui-angular/src/lib/grids/grid-base.directive.ts

@@ -6029,22 +6029,17 @@ export abstract class IgxGridBaseDirective extends DisplayDensityBase implements
      *
      * @remarks
      * Accepted values for index are integers from 0 to this.grid.dataView.length
-     * @remarks
-     * When adding the row as a child, the parent row is the one at the previous index. You cannot add a child at index 0.
      * @example
      * ```typescript
-     * this.grid.beginAddRowByIndex(10);
-     * this.grid.beginAddRowByIndex(10, true);
      * this.grid.beginAddRowByIndex(0);
      * ```
      * @param index - The index to spawn the UI at. Accepts integers from 0 to this.grid.dataView.length
-     * @param asChild - Whether the record should be added as a child. Only applicable to igxTreeGrid.
      */
-    public beginAddRowByIndex(index: number, asChild?: boolean): void {
+    public beginAddRowByIndex(index: number): void {
         if (index === 0) {
-            return this.beginAddRowById(null, asChild);
+            return this.beginAddRowById(null);
         }
-        return this.beginAddRowById(this.gridAPI.get_rec_id_by_index(index - 1, this.dataView), asChild);
+        return this.beginAddRowById(this.gridAPI.get_rec_id_by_index(index - 1, this.dataView));
     }
 
     protected beginAddRowForIndex(index: number, asChild: boolean = false) {

projects/igniteui-angular/src/lib/grids/tree-grid/tree-grid.component.ts

@@ -605,6 +605,31 @@ export class IgxTreeGridComponent extends IgxGridBaseDirective implements GridTy
         this.notifyChanges();
     }
 
+    /**
+     * Enters add mode by spawning the UI with the context of the specified row by index.
+     *
+     * @remarks
+     * Accepted values for index are integers from 0 to this.grid.dataView.length
+     * @remarks
+     * When adding the row as a child, the parent row is the specified row.
+     * @remarks
+     * To spawn the UI on top, call the function with index = null or a negative number. In this case trying to add this row as a child will result in error.
+     * @example
+     * ```typescript
+     * this.grid.beginAddRowByIndex(10);
+     * this.grid.beginAddRowByIndex(10, true);
+     * this.grid.beginAddRowByIndex(null);
+     * ```
+     * @param index - The index to spawn the UI at. Accepts integers from 0 to this.grid.dataView.length
+     * @param asChild - Whether the record should be added as a child. Only applicable to igxTreeGrid.
+     */
+    public beginAddRowByIndex(index: number, asChild?: boolean): void {
+        if (index === null || index < 0) {
+            return this.beginAddRowById(null, asChild);
+        }
+        return this.beginAddRowById(this.gridAPI.get_rec_id_by_index(index, this.dataView), asChild);
+    }
+
     /**
      * @hidden
      */