ENHANCEMENT GridFieldDetailForm->setItemEditFormCallback()

forms/gridfield/GridFieldDetailForm.php

@@ -147,6 +147,20 @@ public function getItemRequestClass() {
 			return 'GridFieldItemRequest_ItemRequest';
 		}
 	}
+
+	/**
+	 * @param function $cb Make changes on the edit form after constructing it.
+	 */
+	public function setItemEditFormCallback($cb) {
+		$this->itemEditFormCallback = $cb;
+	}
+
+	/**
+	 * @return function
+	 */
+	public function getItemEditFormCallback() {
+		return $this->itemEditFormCallback;
+	}
 }
 
 class GridFieldDetailForm_ItemRequest extends RequestHandler {
@@ -296,6 +310,10 @@ function ItemEditForm() {
 			// e.g. page/edit/show/6/ vs. page/edit/EditForm/field/MyGridField/....
 			$form->Backlink = $toplevelController->Link();
 		}
+
+		$cb = $this->component->getItemEditFormCallback();
+		if($cb) $cb($form, $this);
+
 		return $form;
 	}
 

tests/forms/gridfield/GridFieldDetailFormTest.php

@@ -133,6 +133,25 @@ function testCustomItemRequestClass() {
 		$component->setItemRequestClass('GridFieldDetailFormTest_ItemRequest');
 		$this->assertEquals('GridFieldDetailFormTest_ItemRequest', $component->getItemRequestClass());
 	}
+
+	function testItemEditFormCallback() {
+		$category = new GridFieldDetailFormTest_Category();
+		$component = new GridFieldDetailForm();
+		$component->setItemEditFormCallback(function($form, $component) {
+			$form->Fields()->push(new HiddenField('Callback'));
+		});
+		// Note: A lot of scaffolding to execute the tested logic,
+		// due to the coupling of form creation with request handling (and its context)
+		$request = new GridFieldDetailForm_ItemRequest(
+			GridField::create('Categories', 'Categories'),
+			$component,
+			$category,
+			new Controller(),
+			'Form'
+		);
+		$form = $request->ItemEditForm();
+		$this->assertNotNull($form->Fields()->fieldByName('Callback'));
+	}
 }
 
 class GridFieldDetailFormTest_Person extends DataObject implements TestOnly {