DOC Explain how to implement a custom QueryPermissionChecker

docs/en/02_Developer_Guides/19_GraphQL/01_Verifying_CanView_Permission.md

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+---
+title: Controlling CanView access to DataObjects returned by GraphQL 
+summary: Your GraphQL service should honour the CanView permission when fetching DataObjects. Learn how to customise this access control check.   
+icon: cookie-bite
+---
+
+# Controlling who can view results in a GraphQL result set
+
+The [Silverstripe ORM provides methods to control permissions on DataObject](Developer_Guides/Model/Permissions). In 
+most cases, you'll want to extend this permission model to any GraphQL service you implement that returns a DataObject.
+
+## The QueryPermissionChecker interface
+
+The GraphQL module includes a `QueryPermissionChecker` interface. This interface can be used to specify how GraphQL 
+services should validate that users have access to the DataObjects they are requesting.
+
+The default implementation of `QueryPermissionChecker` is `CanViewPermissionChecker`. `CanViewPermissionChecker` directly calls the `CanView` method of each DataObject in your result set and filters out the entries not visible to the current user.
+
+Out of the box, the `CanView` permission of your DataObjects are honoured when Scaffolding GraphQL queries.
+
+## Customising how the results are filtered
+
+`CanViewPermissionChecker` has some limitations. It's rather simplistic and will load each entry in your results set to perform a CanView call on it. It will also convert the results set to an `ArrayList` which can be inconvenient if you need to alter the underlying query after the _CanView_ check.
+
+Depending on your exact use case, you may want to implement your own `QueryPermissionChecker` instead of relying 
+on `CanViewPermissionChecker`.
+
+Some of the reasons you might consider this are:
+* the access permissions on your GraphQL service differ from the ones implemented directly on your DataObject
+* you want to speed up your request by filtering out results the user doesn't have access to directly in the query
+* you would rather have a `DataList` be returned.
+
+### Implementing your own QueryPermissionChecker class
+
+The `QueryPermissionChecker` requires your class to implement two methods:
+* `applyToList` which filters a `Filterable` list based on whether a provided `Member` can view the results
+* `checkItem` which checks if a provided object can be viewed by a specific `Member`.
+
+#### Filtering results based on the user's permissions 
+
+In some context, whether a user can view an object is entirely determined on their permissions. When that's the
+case, you don't even need to get results to know if the user will be able to see them or not.
+
+```php
+<?php
+use SilverStripe\GraphQL\Permission\QueryPermissionChecker;
+use SilverStripe\ORM\ArrayList;
+use SilverStripe\ORM\Filterable;
+use SilverStripe\Security\Member;
+use SilverStripe\Security\Permission;
+
+/**
+* This implementation assumes that only users with the ADMIN permission can view results.
+*/
+class AdminPermissionChecker implements QueryPermissionChecker
+{
+
+    public function applyToList(Filterable $list, Member $member = null)
+    {
+        return Permission::check('ADMIN', 'any', $member) ?
+        $list :
+        ArrayList::create([]);
+    }
+
+    public function checkItem($item, Member $member = null)
+    {
+        return Permission::check('ADMIN', 'any', $member);
+    }
+}
+```
+
+#### Filtering results based on the user's permissions 
+
+Some times, whether a user can view an object is determined by some information on the record. If that's the case,
+you can filter out results the user can not see by altering the query. This has some performance advantage, because 
+the results are filtered directly by the query.
+
+```php
+<?php
+use SilverStripe\GraphQL\Permission\QueryPermissionChecker;
+use SilverStripe\ORM\Filterable;
+use SilverStripe\Security\Member;
+
+/**
+ * This implementation assumes that the results are assigned an owner and that only the owner can view a record.
+ */
+class OwnerPermissionChecker implements QueryPermissionChecker
+{
+
+    public function applyToList(Filterable $list, Member $member = null)
+    {
+        return $list->filter('OwnerID', $member ? $member->ID : -1);
+    }
+
+    public function checkItem($item, Member $member = null)
+    {
+        return $member && $item->OwnerID === $member->ID;
+    }
+}
+```
+
+### Using a custom QueryPermissionChecker implementation
+
+There's three classes that expect a `QueryPermissionChecker`:
+* `SilverStripe\GraphQL\Scaffolding\Scaffolders\ItemQueryScaffolder`
+* `SilverStripe\GraphQL\Scaffolding\Scaffolders\ListQueryScaffolder`
+* `SilverStripe\GraphQL\Pagination\Connection`
+
+Those classes all implement the `SilverStripe\GraphQL\Permission\PermissionCheckerAware` and receive the default
+`QueryPermissionChecker` from the Injector. They also have a `setPermissionChecker` method that can be use to provide a custom `QueryPermissionChecker`.
+
+#### Scaffolding types with a custom QueryPermissionChecker implementation 
+
+There's not any elegant way of defining a custom `QueryPermissionChecker` when scaffolding types at this time. If 
+you need the ability to use a custom `QueryPermissionChecker`, you'll have to build your query manually.
+
+#### Overriding the QueryPermissionChecker for a class extending ListQueryScaffolder
+
+If you've created a GraphQL query by creating a subclass of `ListQueryScaffolder`, you can use the injector to
+override `QueryPermissionChecker`.
+
+```yaml
+---
+Name: custom-graphqlconfig
+After: graphqlconfig
+---
+SilverStripe\Core\Injector\Injector:
+  SilverStripe\GraphQL\Permission\QueryPermissionChecker.my-custom:
+    class: App\Project\CustomQueryPermissionChecker
+  App\Project\CustomListQueryScaffolder:
+    properties:
+      permissionChecker: '%$SilverStripe\GraphQL\Permission\QueryPermissionChecker.my-custom'
+```
+
+#### Manually specifying a QueryPermissionChecker on a Connection
+
+If you're manually instantiating an instance of `SilverStripe\GraphQL\Pagination\Connection` to resolve your results,
+you can pass an instance of your own custom `QueryPermissionChecker`.
+
+```php
+$childrenConnection = Connection::create('Children')
+    ->setConnectionType($this->manager->getType('Children'))
+    ->setSortableFields([
+        'id' => 'ID',
+        'title' => 'Title',
+        'created' => 'Created',
+        'lastEdited' => 'LastEdited',
+    ])
+    ->setPermissionChecker(new CustomQueryPermissionChecker());
+```
+
+## API Documentation
+
+* [CanViewPermissionChecker](api:SilverStripe\GraphQL\Permission\CanViewPermissionChecker)
+* [Connection](api:SilverStripe\GraphQL\Pagination\Connection)
+* [ItemQueryScaffolder](api:SilverStripe\GraphQL\Scaffolding\Scaffolders\ItemQueryScaffolder)
+* [ListQueryScaffolder](api:SilverStripe\GraphQL\Scaffolding\Scaffolders\ListQueryScaffolder)
+* [PermissionCheckerAware](api:SilverStripe\GraphQL\Permission\PermissionCheckerAware)
+* [QueryPermissionChecker](api:SilverStripe\GraphQL\Permission\QueryPermissionChecker)

docs/en/02_Developer_Guides/19_GraphQL/index.md

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+---
+title: GraphQL
+summary: Learn how to create and customise GraphQL services on your Silverstripe CMS project.
+introduction: Learn how to create and customise GraphQL services on your Silverstripe CMS project.
+icon: cookie
+---
+
+[CHILDREN]
+
+## Additional documentation
+
+A substantial part of the GraphQL documentation for Silverstripe CMS still 
+reside on the GraphQL module's _Readme_ file. We are in the process of folding
+this information back into the main Silverstripe CMS documentation.
+
+* [silverstripe/graphql](https://github.com/silverstripe/silverstripe-graphql/)
+
+## API Documentation
+
+* [GraphQL](api:SilverStripe\GraphQL)