NEW: Add GridField_StateProvider interface

This interfaces provides a mechanism by which components can reliably
specify default state.

src/Forms/GridField/GridField.php

@@ -156,8 +156,6 @@ public function __construct($name, $title = null, SS_List $dataList = null, Grid
 
         $this->setConfig($config);
 
-        $this->state = new GridState($this);
-
         $this->addExtraClass('grid-field');
     }
 
@@ -407,13 +405,31 @@ public function getManipulatedList()
      */
     public function getState($getData = true)
     {
+        // Initialise state on first call. This ensures it's evaluated after components have been added
+        if (!$this->state) {
+            $this->initState();
+        }
+
         if ($getData) {
             return $this->state->getData();
         }
 
         return $this->state;
     }
 
+    private function initState()
+    {
+        $this->state = new GridState($this);
+
+        $data = $this->state->getData();
+
+        foreach ($this->getComponents() as $item) {
+            if ($item instanceof GridField_StateProvider) {
+                $item->initDefaultState($data);
+            }
+        }
+    }
+
     /**
      * Returns the whole gridfield rendered with all the attached components.
      *

src/Forms/GridField/GridFieldPaginator.php

@@ -140,14 +140,15 @@ public function handleAction(GridField $gridField, $actionName, $arguments, $dat
      */
     protected function getGridPagerState(GridField $gridField)
     {
-        $state = $gridField->State->GridFieldPaginator;
+        return $gridField->State->GridFieldPaginator;
+    }
 
-        $state->initDefaults([
+    public function initDefaultSate(GridState_Data $data)
+    {
+        $data->GridFieldPaginator->initDefaults([
             'currentPage' => 1,
             'itemsPerPage' => $this->getItemsPerPage()
         ]);
-
-        return $state;
     }
 
     /**

src/Forms/GridField/GridFieldSortableHeader.php

@@ -18,7 +18,7 @@
  *
  * @see GridField
  */
-class GridFieldSortableHeader implements GridField_HTMLProvider, GridField_DataManipulator, GridField_ActionProvider
+class GridFieldSortableHeader implements GridField_HTMLProvider, GridField_DataManipulator, GridField_ActionProvider, GridField_StateProvider
 {
 
     /**
@@ -281,9 +281,11 @@ public function getManipulatedData(GridField $gridField, SS_List $dataList)
      */
     private function getState(GridField $gridField): GridState_Data
     {
-        $state = $gridField->State->GridFieldSortableHeader;
-        $state->initDefaults(['SortColumn' => null, 'SortDirection' => 'asc']);
+        return $gridField->State->GridFieldSortableHeader;
+    }
 
-        return $state;
+    public function initDefaultState(GridState_Data $data): void
+    {
+        $data->initDefaults(['SortColumn' => null, 'SortDirection' => 'asc']);
     }
 }

src/Forms/GridField/GridField_StateProvider.php

@@ -0,0 +1,21 @@
+<?php
+
+namespace SilverStripe\Forms\GridField;
+
+/**
+ * A GridField component that provides state, notably default state.
+ *
+ * Implementation of this interface is optional; without it, no default state is assumed.
+ * The benefit of default state is that it won't be included in URLs, keeping URLs tidier.
+ */
+interface GridField_StateProvider extends GridFieldComponent
+{
+    /**
+     * Initialise the default state in the given GridState_Data
+     *
+     * We recommend that you call $data->initDefaults() to do this.
+     *
+     * @param $data The top-level sate object
+     */
+    public function initDefaultState(GridState_Data $data): void;
+}