Add lazy props feature

src/LazyProp.php

@@ -0,0 +1,20 @@
+<?php
+
+namespace Inertia;
+
+use Illuminate\Support\Facades\App;
+
+class LazyProp
+{
+    protected $callback;
+
+    public function __construct(callable $callback)
+    {
+        $this->callback = $callback;
+    }
+
+    public function __invoke()
+    {
+        return App::call($this->callback);
+    }
+}

src/Response.php

@@ -54,9 +54,15 @@ public function toResponse($request)
 
         $props = ($only && $request->header('X-Inertia-Partial-Component') === $this->component)
             ? Arr::only($this->props, $only)
-            : $this->props;
+            : array_filter($this->props, function ($prop) {
+                return ! ($prop instanceof LazyProp);
+            });
 
         array_walk_recursive($props, function (&$prop) use ($request) {
+            if ($prop instanceof LazyProp) {
+                $prop = App::call($prop);
+            }
+
             if ($prop instanceof Closure) {
                 $prop = App::call($prop);
             }

src/ResponseFactory.php

@@ -54,6 +54,11 @@ public function getVersion()
         return (string) $version;
     }
 
+    public function lazy(callable $callback)
+    {
+        return new LazyProp($callback);
+    }
+
     public function render($component, $props = [])
     {
         if ($props instanceof Arrayable) {

tests/LazyPropTest.php

@@ -0,0 +1,27 @@
+<?php
+
+namespace Inertia\Tests;
+
+use Illuminate\Http\Request;
+use Inertia\LazyProp;
+
+class LazyPropTest extends TestCase
+{
+    public function test_can_invoke()
+    {
+        $lazyProp = new LazyProp(function () {
+            return 'A lazy value';
+        });
+
+        $this->assertSame('A lazy value', $lazyProp());
+    }
+
+    public function test_can_resolve_bindings_when_invoked()
+    {
+        $lazyProp = new LazyProp(function (Request $request) {
+            return $request;
+        });
+
+        $this->assertInstanceOf(Request::class, $lazyProp());
+    }
+}

tests/ResponseFactoryTest.php

@@ -6,6 +6,7 @@
 use Illuminate\Session\Middleware\StartSession;
 use Illuminate\Support\Facades\Route;
 use Inertia\Inertia;
+use Inertia\LazyProp;
 use Inertia\ResponseFactory;
 use Inertia\Tests\Stubs\ExampleMiddleware;
 
@@ -69,4 +70,14 @@ public function test_shared_data_can_be_shared_from_anywhere()
             ],
         ]);
     }
+
+    public function test_can_create_lazy_prop()
+    {
+        $factory = new ResponseFactory();
+        $lazyProp = $factory->lazy(function () {
+            return 'A lazy value';
+        });
+
+        $this->assertInstanceOf(LazyProp::class, $lazyProp);
+    }
 }

tests/ResponseTest.php

@@ -12,6 +12,7 @@
 use Illuminate\Support\Collection;
 use Illuminate\Support\Fluent;
 use Illuminate\View\View;
+use Inertia\LazyProp;
 use Inertia\Response;
 
 class ResponseTest extends TestCase
@@ -182,4 +183,40 @@ public function test_xhr_partial_response()
         $this->assertSame('/user/123', $page->url);
         $this->assertSame('123', $page->version);
     }
+
+    public function test_lazy_props_are_not_included_by_default()
+    {
+        $request = Request::create('/users', 'GET');
+        $request->headers->add(['X-Inertia' => 'true']);
+
+        $lazyProp = new LazyProp(function () {
+            return 'A lazy value';
+        });
+
+        $response = new Response('Users', ['users' => [], 'lazy' => $lazyProp], 'app', '123');
+        $response = $response->toResponse($request);
+        $page = $response->getData();
+
+        $this->assertSame([], $page->props->users);
+        $this->assertObjectNotHasAttribute('lazy', $page->props);
+    }
+
+    public function test_lazy_props_are_included_in_partial_reload()
+    {
+        $request = Request::create('/users', 'GET');
+        $request->headers->add(['X-Inertia' => 'true']);
+        $request->headers->add(['X-Inertia-Partial-Component' => 'Users']);
+        $request->headers->add(['X-Inertia-Partial-Data' => 'lazy']);
+
+        $lazyProp = new LazyProp(function () {
+            return 'A lazy value';
+        });
+
+        $response = new Response('Users', ['users' => [], 'lazy' => $lazyProp], 'app', '123');
+        $response = $response->toResponse($request);
+        $page = $response->getData();
+
+        $this->assertObjectNotHasAttribute('users', $page->props);
+        $this->assertSame('A lazy value', $page->props->lazy);
+    }
 }