[5.5] Add ability to pass callback to whenLoaded Resource method

src/Illuminate/Http/Resources/ConditionallyLoadsAttributes.php

@@ -111,18 +111,34 @@ protected function attributes($attributes)
     }
 
     /**
-     * Retrieve a relationship if it has been loaded.
+     * Retrieve a relationship or execute a callback
+     * if the given relationship has been loaded.
      *
      * @param  string  $relationship
+     * @param  mixed  $value
+     * @param  mixed  $default
+     *
      * @return \Illuminate\Http\Resources\MissingValue|mixed
      */
-    protected function whenLoaded($relationship)
+    protected function whenLoaded($relationship, $value = null, $default = null)
     {
-        if ($this->resource->relationLoaded($relationship)) {
+        if (func_num_args() < 3) {
+            $default = new MissingValue;
+        }
+
+        if (! $this->resource->relationLoaded($relationship)) {
+            return $default;
+        }
+
+        if (func_num_args() === 1) {
             return $this->resource->{$relationship};
         }
 
-        return new MissingValue;
+        if ($this->resource->{$relationship} === null) {
+            return null;
+        }
+
+        return value($value);
     }
 
     /**

tests/Integration/Http/Fixtures/PostResourceWithOptionalRelationship.php

@@ -10,6 +10,9 @@ public function toArray($request)
             'id' => $this->id,
             'comments' => new CommentCollection($this->whenLoaded('comments')),
             'author' => new AuthorResource($this->whenLoaded('author')),
+            'author_name' => $this->whenLoaded('author', function () {
+                return $this->author->name;
+            }),
         ];
     }
 }

tests/Integration/Http/ResourceTest.php

@@ -157,6 +157,7 @@ public function test_resources_may_load_optional_relationships()
             'data' => [
                 'id' => 5,
                 'author' => ['name' => 'jrrmartin'],
+                'author_name' => 'jrrmartin',
             ],
         ]);
     }
@@ -184,6 +185,7 @@ public function test_resources_may_shows_null_for_loaded_relationship_with_value
             'data' => [
                 'id' => 5,
                 'author' => null,
+                'author_name' => null,
             ],
         ]);
     }